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How to use outdoor reset with setback?

Every year I have the same dilemma. Do I run the (NG) mod-con boiler at a set temp on a low curve, or do I setback, running on a higher curve to make-up? I have never found a clear answer to this question.

My set-up is a Viessmann Vitodens 100 mod-con with weather comp (ODR) - not the best, I know (fixed curves), but it's still got life in it. The system is controlled by Tado (the German smart 'stat that uses geofencing to know when you're home). The house is old (1905) and not well-insulated (solid walls, some original single glazing). Panel radiators (sized 35 years ago - not greatly oversized (if at all) given envelope not greatly (or at all) improved in some rooms. TRVs throughout. Single zone.

If I set the stat to hold a steady temperature (effectively, over-riding the smart aspects of the stat), i can crank the curve right down and the house holds temp at very low flow temps, but if I set back at all on these curves, the temp will never recover.

Alternatively, I can set a higher curve, setback at night and when everyone is out, and then the smart stat will ensure the house is back up to temp for when everyone is home. However, the stat is then in control and will relay the boiler on and off to keep the temperature stable, effectively over-riding the ODR.

Either model effectively heats the house. If I have a preference, it's probably no set-back, because I like the boiler holding the temp rather than the stat (and the pump over-run is such an annoyance). However, which is more efficient in a relatively poorly insulated house - running higher curve heating for 8 hours/day, or low curve heating 24/7?

I've spent hours researching this and have never found a definitive answer either way. Taking my own readings hasn't proven particularly helpful either, as even over a long period, the weather (in the UK) is just too inconsistent.

Anyone got experience of which control mechanism actually works most efficiently?


  • Zman
    Zman Member Posts: 7,130
    unclejohn said:

    IMO if you have ODR you do not need night setback. End of story, stop worrying about. Grab a beer and just think how all that deliciousness got it that bottle.

    I wish you could LOL and agree with a comment. Set it and forget it.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Ironman
    Ironman Member Posts: 6,703
    Setback is counterproductive to ODR and the logic of a modulating boiler. Leave it set at one temp.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • dm9321287
    dm9321287 Member Posts: 36
    Thanks all for taking the time to respond. Are you able to explain why, in your view, ODR is going to be more efficient in my set up?

    Presumably, the savings from the lower curve would need to offset the higher heat loss from running the system 24/7? That might well be the case in a modern insulated building, but in an old house with solid walls, no floor insulation and some single glazing? I’m not so sure. That said, it’s rare we ever get much below 0c /32f during the day here. At night we do in the winter.

    Thanks for any advice.
  • DZoro
    DZoro Member Posts: 1,048
    Your boiler when set on odr is designed to operate like a car on cruise control set at 25mph. It will get to the same place as a young kid in a car racing and hitting every stoplight a block away. He will burn up all his gas, and grandma on cruise will miss every stop light and pass him up at the gas station while he is filling his tank for more gas.
    The trick is finding the lowest set point possible for your home. That may require some time and experiments with temperature adjustments. Keep lowering the water temp until set temp starts to fall, with the boiler almost on constant fire. Then bump it up a bit from there. Allow 24 hours in between adjustments and outside air temp differences.
  • dm9321287
    dm9321287 Member Posts: 36
    Thanks. I've heard that analogy before, but momentum and thermodynamics are surely different?

    I can't help thinking keeping a leaky house at temp for 16 hours (ODR) when the heating would otherwise be off (set-back) has to use more fuel, even accounting for the higher curve required for recovery, but I'm no expert in these things, particularly physics.

    I guess there are just too many variables for a one-size-fits-all answer. Just because in some climates, with particular systems in particular buildings, one will work better, probably doesn't mean it will work better in different circumstances. If I had a tent to heat, I'd probably put the heat on high only when I needed it. Otherwise, I'm heating the field it's in. In a passive house, I'd never need to switch it on. Horses for courses.

    My own experience (and hence the query), is that whichever method I use, I still use a lot of gas to heat the house. ODR is smoother and quieter, but I never can get my head around leaving the heating on all night when everyone's tucked up in bed, or when nobody is home. Then again, I grew up in the 1970s when we all ran cast iron monsters kicking out 200C. Nobody left those babies running all the time. The house would be up in the 40s!

    My beermat calculations suggest set back should be way more efficient, but they're probably wrong:

    ODR 24/7 = 24 'units' of fuel (for simplicity)

    Fuel savings from ODR are what, 1% per 3F lower temp?

    On that basis, if I ran 24/7 on a curve 33F higher (on curve used for set back), I would use 11% more fuel. For simplicity let's say thats 24 x 111% = 26.6 'units'.

    However, I can use setback on that higher curve, and run the heating for 8 hours, not 24, so I need to divide my 26.6 by 3 (8 being 1/3 of 24). Total 'units' used would be 8.9.

    So ODR is way more comfortable and quiet, but on these calculations, it's also way more inefficient.

    I must be missing something here, right?
  • Jamie Hall
    Jamie Hall Member Posts: 17,810
    @dm9321287 -- you are missing something, but it's very hard to quantify, which is why -- as you say -- one size does not fit all! The something is the heat capacity of the building and everything in it. If you run a setback, the building (not the air) and its contents cools off. Which takes time. Then when you come out of the setback, you not only have to heat the air, but also the contents.. Which takes more time and fuel. For forced air heat, usually one is really only interested in the air, so deep setbacks make some sense and, since most folks have forced air heat, setbacks get a very good press. The air temperature comes up very quickly, and the cold furnishings and walls don't bother people (so much). Other systems -- or even other buildings -- not so much. Over the years there has been a lot of debate on how big a setback is too much -- if any -- with no really good conclusions. I have run the main building I care for with both a 3 degree night setback and no setback at all -- and the difference in fuel use was too small to be statistically significant. Your mileage, as they say, may vary.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • dm9321287
    dm9321287 Member Posts: 36
    Thanks again to everyone for their comments. @Jamie Hall , your point about thermal mass is particularly helpful, I had forgotten that the air can get up to temp, but it can still feel cold if the walls and contents are cold. @DZoro, I've also always been too impatient to allow much time between adjustments, which I'm going to try now.

    If anyone has ever seen any real world numbers for the savings ODR and setback produce, I'd be interested to see them. As far as I can tell, there's a lot of supposition ("well, it has to work, doesn't it?"), which is logical, but has no scientific basis. Either one in isolation is obviously better than neither, but if like me, you have to try to balance them, or choose one over the other, it would be great to cut through to some proof that one works better than the other. Or maybe, as @unclejohn @Zman and @Ironman essentially point out, if you have a mod-con with ODR, set-back probably isn't going to work very well. If you have a big-old cast-iron monster that's massively over-sized, it is.

    If I ever do work out which one is best, I'll share the revelation. I see from this forum alone, people have been having this debate for more than 15 years, and I'm not convinced anyone has ever figured it out. Most people seem to just choose one and, as @unclejohn says, do something more interesting with their time instead.
  • ChrisJ
    ChrisJ Member Posts: 12,112
    edited October 2019

    I understand your point but the problem is no one here has multiple buildings that are all built identical close together and unoccupied to do testing in.

    Anything else would give false results.

    A lot of this is guesses and assumptions. It's the best we've got.

    That said, I have a single pipe steam system running an ODR which operates totally different than yours, but I do not do a setback simply because I pay to be comfortable.

    My bedrooms and kitchen are always cool, living room and bathrooms are always warm and everyone is happy.

    That's what I came up with in the end. I know it's not really an answer, but it's the reasoning I used to set my system up.
    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • Brewbeer
    Brewbeer Member Posts: 616
    If you approach the ORD vs set-back discussion as a thought problem, set-back has a slight edge in fewer BTUs consumed because buildings at lower temps lose heat slower. But those BTU reductions come with a comfort cost, and our heating systems are all about comfort.
    Hydronics inspired homeowner with self-designed high efficiency low temperature baseboard system and professionally installed mod-con boiler with indirect DHW. My system design thread:
    System Photo:
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    Back when Storm Sandy took out my power, and therefore, my heat, for six and a half days, that amounted to a severe setback. My pretty-well insulated house dropped from 69F to about 58F in that time. (It did not go below freezing outdoors, but pretty close).

    My mod-con with outdoor reset is set very tight, so after about a day with the power restored, the house did not get past 60F. The boiler was just not putting out enough heat to recover from, say, 10F of setback. It seemed as though it never would (even though my boiler is somewhat oversized).

    My solution was to disable the ODR and let the system supply temperature go up to 130F for my radiant slab zone and 150F for my baseboard zone. It still took a few days for my radiant slab zone to recover.

    Of course with a radiant slab at grade zone, setbacks make no sense at all. It takes 24 hours or more for a change in thermostat setting to have much effect.
  • dm9321287
    dm9321287 Member Posts: 36
    Thanks for the further comments, which are helpful.

    Unfortunately, my particular set up has fairly limited ODR capabilities. The curve can be shifted (up or down) but the slope cannot be adjusted. This results in the SWT either running too cool on mild days (cannot maintain temp), or too warm on cooler days (indoor temp keeps climbing). Ideally, I would set a flatter curve to address this, but the basic ODR on the Viessmann 100 does not allow this.

    With these limitations, it seems I inevitably have to go with the lowest curve that can keep the house warm on milder days (min temp of around 30C/86F to put any heat through the rads), and then use the room stat to stop the temperature climbing too high on cooler days (when the SWT's up at 50C/122F at freezing ODT).

    This likely reduces the efficiency of the system vs a correctly dialled curve, but does also allow some setback without a comfort impact, because the temperature can recover (it not being possible to set the perfect ODR curve on my system).

    Coupled with the smart stat, I can then (a) turn the heating off at night (bonus - I really hate the pump hum) and it can have the house back up to temp for the morning; and (b) set-back the temp a bit when we're out (the geofencing stat knows how far away we are (from our phones) and therefore reduces the temperature accordingly).

    In this way, I probably recover any inefficiencies in not having the perfect ODR curve. I maybe even run the system more efficiently because I can also set back (bearing in mind, it's still always running with ODR, albeit slightly imperfectly, and at these temps, it's always condensing).

    Time for that cold beer @unclejohn suggested.

    Thanks again for sharing your views.
  • Dave H_2
    Dave H_2 Member Posts: 491
    We start looking at diminishing results and then compromising comfort.
    We can tweak, adjust, coax the most efficiency out of the equipment, but it ultimately comes down to is everyone comfortable at home.
    I, myself, do a little of both.
    Yes, I have ODR on my modulating condensing boiler
    Then I have ODR on the load with a couple of modulating mix valves
    Then I have a variable speed circulators with delta T
    And I have a couple of setback thermostats.

    Then ........ it depends on the occupants. It took a while and alot of complaining....its too cold,.....its too hot.....

    As others have said, how do we compare all of the adjustments?
    My only comparison, as flawed as it it, is the closest I can do.
    Previous home - 1700 square with on/off boiler, on/off tstat
    Remodeled same home with additional square footage, now about 2900 square. Two family, two kitchens with gas appliances, two laundry. Same boiler, same hot water. Updated insulation and some of the windows.
    Same Fuel bill !!!

    Dave H.
    Dave H
  • dm9321287
    dm9321287 Member Posts: 36
    Spot on, @dave H_2. Is it efficient? Yes. Could it be another 2% efficient? Probably. Should I care? Definitely not.

    I think the problem is probably that we swapped out the boiler and stat at about the same time, upgraded the insulation where we could, and replaced all but the ground floor windows. Dropped the fuel bills by 50%, but still left me wondering whether I could squeeze out a bit more if I could just get a handle on the ODR - but you know, sometimes, it's important to remember life's too short!
  • chiamac
    chiamac Member Posts: 5
    I'll post my results on a new discussion, but here is my experience and thoughts. 2020 made me into a mix of Tim the Toolman and Red Green, so your milage may vary.

    My house is a stucco 1909 1450sqf victorian in Minneapolis. I have right sized cast iron radiators, 50 gallons in the system, with a 59k btu slant fin boiler, and my pump is about 8gpm or so.

    My boiler is the ES2 from Burnham and outdoor reset is an option card. I can also control the boiler temp, differential to fire (or whatever it is when max temp is reached and how low it gets to refire) to a min of 10 degrees. Min boiler temp, and the amount of pump time to 20 min that the boiler will run above min temp.

    All that means I have options. 

    I don't have celing fans, but will put some small 50cfm fans on the first floor radiators this spring.

    So for me I saved about 20-25% the first year I went to a pretty aggressive setback without outdoor reset. This was 55 at night, 65 at home day, and 60 away for work. Iirc I didn't change the original boiler settings of 180max, 140 min, 15 differential, and 5 to 10 min pump run.

    Fast forward to this year, and actually having attic insulation (but no heat in the space), and a warmer house due to old kitty and working from home. Now it's 60 at night, 68 day, with no day setback due to working from home.

    I have outdoor reset setup to respond slowly to the smaller setback, so it's not as aggressive as it could be. 

    On warmer months it looked like I saved between 5-10% with odr over only the setback. This makes sense due to me not having fans and how long 50 gallons takes to heat. Outdoor reset holding the water temp to something reasonable (my min temp is 140 iirc) is more efficient than it going to 180. 

    On colder months I'm up 10% gas use. This makes sense due to the house being warmer now, and when it's 10 to 20 out my system is around 165f to 170f anyway. 

    I'm indifferent on how critical odr is for my system, since I have other means for boiler efficiency. It would make more sense on a older boiler with set temperatures.

    I do have a "turbo mode" that raises the temp 10 degrees if a heat call isn't answered in 30. That may be the answer, I'm not sure, however my system takes a while to recover and in the morning it would basically be set back to max. I'd have to get a stop watch out sometime to time it, but at a glance it seems it would run excessively at a higher temp, rather than now running it slowly, but I may be wrong. 

    Bottom line though for me I found a sweet spot that allows me to recover from the 8 degree setback in around 2 hours. I think that still saves vs keeping a steady heat overnight. It did take a ton of tinkering to get that, and any person who has one needs to know how to adjust it themselves. Or else it'd be a nightmare of service calls.
  • chiamac
    chiamac Member Posts: 5
    edited February 2021
    Try two, I'll blame trying to edit a post with my "smart" phone.

    I do realize my reply is a year after the discussion started. Also please realize about me that 2020 and being forced to work and stay at home has turned me into a cross between Tim the Toolman and Red Green. So take what I say with a grain of salt, it's only my opinion, and your mileage may vary.

    My house is in Minneapolis Minnesota, where we normally see low temps (well and sometimes highs for a few days) down to -15f to -20f. My house was built in 1909, 1450sqf, has a stucco exterior, no wall insulation, and now a well insulated attic (but not heated yet, I'm in the middle of converting it into livable space). The radiators are normal sized, not oversized as some later old homes, and they don't keep up when the temperature stays a consistent -15. I do not have any fans to speak of, but am going to experiment with a small 50cfm fan under some radiators this spring. My boiler is a modern Burnham ES3 that's the small 59k btu model, put in by those who renovated the house years before I bought it. I have 50 gallons of water in the system, and my pump manages around 8gpm or so. I get about 9-12 degrees of heat between the input and output of the boiler, depending on which way you look at the gauges. There is a single programmable thermostat on the main floor.

    I know that's a lot, but it really helps go through the rest of this, and please correct me if I'm too far wrong, I don't want to mislead people. I just feel what I went through, and am going through may be help to some.

    So for the first winter or season we had the house (I'll use we and I interchangeably I'm not an English major) we kept the thermostat at a constant 65 degrees. This gave a baseline to measure further improvements. Which leading up to outdoor reset were really better weather stripping and putting plastic up in the attic to stop airflow.

    My boiler also at this point was setup with the defaults. If I recall correctly it was 180 max temp, 140 min temp, a 15 degree differential, and a post purge (I think that's the term) that ran the pump an additional 5 min if the water was above 140 and there was a heat call. I'll get into more of these settings later.

    Putting in an aggressive set back on the thermostat saved about 20% on the heating bill. At the time this was 55 at night, 65 in the morning and return from work, and 60 during the day. 50 gallons is a lot to keep hot, and the response rate for the system meant that within 45 min to an hour it'd be up to temp. To explain (someone correct me if I'm wrong) it was cheaper to run the boiler extra at times to heat the system, than to keep everything at a constant temp.

    I got an outdoor reset because it seemed like a fun idea, and it would keep me busy, and my partner liked the idea of me going to the basement to "operate" the boiler rather than drive her crazy. It was more as an experiment rather than anything to save a bunch of money. Keeping in mind, even at a 20% savings, it would take at least a handful of years to pay off the cost of the card.

    When I got the outdoor reset card in the fall it took the first month to really start to get it set for that style of heat. I found out through trial and error that my cast iron radiators and lack of fans needs a boiler temp of at least 140 to do anything serious. So basically at one end I have 140, and the other is the same 180, although I'm going to try 190 if things get down below -10 to see if that helps.

    So my savings are going to be whatever the run time is between 140 and 180. My system right now, with 8gpm takes a short 10 min per 10 degrees, so in this case an extra 30 min of run time. Also my system doesn't always stay up to temp, even now in Jan my water temp on a 20f day gets below 100 as the heat isn't always called for, so I'm potentially saving some time throughout the day by not running over the needed amount of heat.

    All that said, with outdoor reset I'm also running the temperatures a little warmer in the house, due to Old Kitty, and us working from home. The setback now is 60 at night, and 68 day. In the fall I was saving a little money, I believe around 10%, with outdoor reset (actually maybe 20% gas use due to a price increase this year over last), and now in winter I'm only using 10% more gas. This makes sense because the reset will have more impact on the over running in the fall and spring, when the boiler temp is lower, than in winter when I'm usually around 160f to 170f anyway.

    My boilers outdoor reset card does offer a "turbo mode" that will increase the temp by 10 degrees every 30 min a heat call isn't answered. My issue with this is that with as slow as my system is I'd go from the 140 min temp (actually 135) to 160 or 180 very quickly. I'm taking the bet that, with a 15 degree differential, that I'm running the boiler as much or less in the extra hour it takes to get to temp, that it would take getting up to and keeping a higher temp before the thermostat turns off. In the fall that bet seemed to have paid off, in the winter it's looking like I may be losing, time and more tinkering with it will tell - and the future fans will help.

    Depending though I still think a setback is helpful although I may be at the extreme at 8 degrees. At some point I'll get the fancy touchscreen for my boiler which I believe tracks times better, so I'll be able to do more research than waiting for the bill.

    A few more thoughts...

    I was a bit of fun tinkering and cold mornings to setup. This would not work well with a hands off home or building owner. I have a modern boiler that I can already tune pretty well. More than likely I could have got the same savings with a 20 min pump pre purge (pump runs if the water temp is over 140 at the start of a heat call) and a 20 degree differential from max heat to ignition. Some boilers don't offer that level of tuning, and those systems may benefit more from reset.

    Also, and again, even a 20% savings isn't going to pay this off in a reasonable amount of time, so at least for my house it may not be worth the hassle. Although, in todays green world it is nice to know I'm saving some gas, and at least when it's super cold I'm not using as much as potentially I could.

    I do think though that with a modern boiler, that has outdoor reset as an integrated option, that it would be something an involved homeowner gets. It's a great tool, and even in my case it seems to be helping even if I'm not running it in a traditional sense.

    I expect more "savings" in the future as I move to a zone system for the house. Although that system cost will far outweigh any money saved, at least for the time I own the house. Which is why for me it comes down to being more of a hobby and something to tinker with than a way to really drive some savings.

    IMO, I'm not an expert, I know just enough to sound like I know what I'm talking about, and please read the other great replies here that talk about constant temp.
  • dm9321287
    dm9321287 Member Posts: 36
    Hi @chiamac, always a thread I'm happy to pick up, no matter the interim period.

    Your US heating system's very different to mine here in the UK, and your outdoor winter temperatures are literally off our scale, but it sounds like your house, like mine, is old and ideally, in need of more insulation.

    Since my original post on this, I've come to think of my basic ODR as more of an efficiency tweak than an overall controller. What I mean, is that I set up my condensing gas boiler to run at the necessary flow temperature (which is around 60C/140F at 0C/32F) and then all the ODR does, is crank the flow temps up or down in real time based on whether it's warmer or colder outside. In all other respects, I run the system in a more traditional way, with a daytime set temp (21C/70F) and a setback of 18C/64F, which it very rarely drops to.

    In practice, this results in the heating being on continuously except for the night-time set backs (and when everyone is out, based on the stat geolocation, which also sets it back). This is because I also use TRVs to throttle the radiators to keep the house just under the set temperature. In other words, I have continuous flow (when we're home and awake) and the stat is never switching the heating on and off all day.

    To my original question, is this the most efficient set-up? Well, this winter is colder than last year, so right now, my bills are higher. That said, earlier in the season, I did try leaving the system on 24/7 and turning the flow temperature down by about 10C (that's about 20F, right?), which resulted in more gas being used.

    So I've concluded that set-back uses less gas than continuous flow on ODR in my house. If I had a modern house (which is definitely the plan next, after 20 odd years in 'period properties'!), I suspect the opposite may be true, because the heat loss would be much lower.

    In summary, for basic ODR like mine, set the flow temp needed for design day. Run it as normal (and out at work (remember that?) set-back). Let the ODR automatically reduce the gas consumption on milder days. Spend more time and energy insulating, or moving to a modern house.
  • Big Ed_4
    Big Ed_4 Member Posts: 1,829
    I run a outdoor reset and set back with no boost on my system . Works find in the milder weather . When the outdoor temperture drops closer to my design temperture ( coldest day of the year ) I switch off the setback for comfort ..
    I have enough experience to know , that I dont know it all
  • chiamac
    chiamac Member Posts: 5
    edited February 2021
    Thanks for the reply! I'm making it a slower day at work so I'll give it a go. I do understand that there are differences between yourself and myself, but that also this website and discussion showed up on a google search, so I think there is value adding my experience here.
    dm9321287 said:

    Your US heating system's very different to mine here in the UK, and your outdoor winter temperatures are literally off our scale, but it sounds like your house, like mine, is old and ideally, in need of more insulation.

    So I had a energy audit at one point, and it was just shy of useless (for various reasons) but they said that at some point the walls are losing more heat than the attic. From the seat of my pants this summer, the attic did cool a little better with wall insulation, and I have a feeling the little windows are letting through a bunch of heat. On the flip side, they are small single pane home made windows, so I'm sure they leak like a sieve, and they are on the list to get replaced.

    That said, at some point there are diminishing returns to old houses. From the way it seems for me R60 in the attic isn't going to make much more of a difference than R30, since at some point my walls are going to lose more heat than through the 2nd floor ceiling. So we can do what we can, and some of these updates are livability so they make financial sense that way, but I'm not going to make back the expense from putting in wall insulation via the savings it would create.

    Also what's really annoying is the floor sits on top of the foundation, and I'd never be able to deal with that. Thankfully natural gas prices aren't too crazy, and this past bill was something like $160, and with gas prices being more than last year I think I'll peak out around $170 or so. Then it goes down under $100 and down to $60. So in reality it's costing me around $500ish to heat the house a year, and unless the savings are extreme I'm not really going to see much of a return on really expensive insulation options.
    dm9321287 said:

    To my original question, is this the most efficient set-up? Well, this winter is colder than last year, so right now, my bills are higher. That said, earlier in the season, I did try leaving the system on 24/7 and turning the flow temperature down by about 10C (that's about 20F, right?), which resulted in more gas being used.

    I'll be able to answer this question if/when I put in the fancy LCD touch screen on my boiler. It keeps better track of usage, or at least it's an easy but expensive way to do keep better track of it.

    My boiler, and I believe most small consumer ones like we're talking about, are either on or off. So my 59k BTU an hour boiler uses .59 Therms per hour. My last heating bill had me at 6.6 therms per day on average, the energy company says that I used 11 therms total for the month of July. Seat of the pants math tells me then that I use .35 therms a day for water heating and cooking, so I'm at 6.3 for an average temperature of 25f. Since my boiler uses .59 therms an hour, and I used 6.3, that means the boiler was running 10.7 hours a day.

    So what a person would need to do, even to check math, is to put an hour counter on the boiler and then try it both ways.

    I may have to do that sometime because it sounds like fun.

    Another thing a person should do, and it's easy with (at least my) outdoor reset, is at what temperature does the system hold steady. For me right now it's about 5f degrees less than where it's set, and that the 5f degree difference gives me heating of about 3 to 4 degrees per hour for the house. Mind you that time is also taking into consideration that it takes my system that short 10 min to heat the total water 10 degrees. I'm willing to bet when I put in the zone system and cut the water basically in half, that I'm going to see less run time and thus more savings. But really, all the outdoor reset is doing for me is taking an hour longer to heat up from the 8f degree setback at night, and allowing me to run a lower water temperature.

    At some point, and an hour meter on the boiler would sort this out, it takes less time to heat the water in the morning after the setback than the amount of time spent keeping a steady temperature.

    I'd be willing to bet again that for me adding a fan or some type of means to make the radiators more responsive will help.

    Maybe this week I'll go through the math that gives me the BTU ratings for my radiators and then walk things back. Which is what is interesting to me about all this. It's all mostly math and can be sorted out or figured out, and then applied.
    dm9321287 said:

    In summary, for basic ODR like mine, set the flow temp needed for design day. Run it as normal (and out at work (remember that?) set-back). Let the ODR automatically reduce the gas consumption on milder days. Spend more time and energy insulating, or moving to a modern house.

    Well let's talk about that for a second...

    I apparently enjoy talking about myself and seeing myself type, so I'll use me as an example. Keeping in mind we're mostly happy with where we live now, and are currently employed in town and can't really move outside of our area. Retirement or completely working from home is a different ballgame.

    We bought our house at the right time in the market, and our total mortgage payment is something around $800 a month. If we bought our house again it'd be a mortgage payment of around $1200 a month, at least, maybe a little more. A more modern house, in our suburbs would be closer to $250k and a payment of $1500+ taxes not withstanding. That's an increase of $4800 per year for our house on the current market, to about $8400 for a modern regular cheap suburban house.

    At the old house extreme, and again, heating maybe costs around $500 a year, using round numbers let's say $600, and cooling to a somewhat acceptable level is somewhat $400 a year. So basically in my seat of the pants example it's costing on average $100 a month to heat and cool the old house.

    My parents built a modern double insulated home in the early 1980s. From what they said their heating bill is something like $40 a month over the winter, and I'd imagine their cooling is going to be less than ours at well, so let's ballpark it at $100 a year. In any event, they are more than likely spending let's say $300 a year to my $1300. Yes, that is a big difference, but it's a lot less than buying a new house based on energy consumption.

    Which is why I said what I did about savings before, and others said about chasing down every single efficiency. Although it is a decent amount of money, and it does add up, I can afford that extra $1000 per year over my parents house, especially when compared to moving. Windows and adding in wall insulation sound expensive, and would never pay for themselves. The attic was easy, and affordable if all a person does is blow in a foot or two of insulation. Likewise outdoor reset kits are around $300, and we all know programmable thermostats are affordable. Those things are a larger bang for the buck, even adding door trim or plastic on the windows makes a decent difference.

    That's also why I said outdoor reset may have a large benefit on an older boiler that's not as adjustable. Even if all a owner did was leave it at 180f on cold days and bring it down to 140f on the fall and spring months. That's still going to save someone a little bit, and add to it an adjustable thermostat and they could easily see some benefit. Just as an example my first winter heating bill peaked at $220 keeping a steady 65f and having sawdust for attic insulation. That's down to the $150 now with a setback, outdoor reset, insulation. The $220 went down to $180ish just by using a setback overnight (on a colder month though, so my current bill doesn't compare as well).

    I do like to type. Long story short we pay for gas, my boiler is on or off, and I'd have to add a detailed run timer and compare either way to see how well setback works. Which I may do since it sounds like fun. =)
  • chiamac
    chiamac Member Posts: 5

    I guess thinking about home renovation and heating costs keep me up at night, and my mind occupied.

    I do have to add though, that I'm not sure how well some systems (just talking about single family homes) would do if they just meet the bare minimums. For instance here in MN we may see some pretty drastic temperature swings in winter. As well as due to my cast iron radiators I find that they hold their heat a little more, and that sometimes when heated my boiler is starting from 80f, which is basically from scratch. At that point my radiators really start to show heat at around 140f, which at my gpm is going to be about 40 min later, plus another 20 or so min to get up to 160+. Which means that my heat call potentially may go unmet for at least 30-40 min as the water is heating up.

    I know in the real world I wouldn't be heating a super cold house from scratch, or that there would be a need for 160f water if the room is already hot. However, my point is that in a system like that, where there is lag due to the radiators and then lag due to heating the system, that I would want to keep the settings I have now that recover from a setback at 3 to 4 degrees per hour.

    Generally and advice to some setting this all up if I may, although I don't know exactly since I never built a curve on paper or spreadsheet for the house or for my reset. I found that bumping up the water temp on the reset curve by 4 or 5 degrees usually was enough to add a degree or two per hour recovery or enough to stop a stalemate if the recovery heating call wasn't met for a few hours. So I'm guessing that if I ran at a constant temp I'd be able to lower it about that much, but maybe not more.

    Reading other comments here, it seems there is a big difference between something completely unresponsive like radiant heat, or a main boiler in an apartment building, to the middle ground like I have, to something completely responsive like baseboard heat or maybe a heat exchanger with a fan.

  • dm9321287
    dm9321287 Member Posts: 36
    @chiamac the point you make about insulation is important in terms of the bigger picture. The costs of upgrading the house will often outstrip the savings. There's a 'hierarchy' with loft insulation and draft stripping being really cost-effective, but replacement windows taking decades to return their investment, so only really worth it if new windows are needed anyway (not to save money).

    That said, it's of course important to understand how different systems work with different envelopes. Poorly insulated, older properties lose heat faster than modern/upgraded properties. Even more so, when the delta T between inside and outside is greater (on colder days). Therefore, it can be less efficient to run an ODR-based, constantly circulating system in an older house than it is to setback and return the heat when needed. This is the conundrum I've been working with for a few years now.

    As I mentioned, my house (built 1905) is about 2300ft2, solid 9" brick walls, generally original single-glazed windows on the ground floor, but mostly double or secondary glazed on the upper 2 floors. No underfloor insulation. No basement. Roof pretty well insulated now. We run a 29kw condensing natural gas boiler with basic ODR on a single zone. TRVs on all but one radiator. Degree day temp here is probably around the -1C/30F level, so pretty temperate. However, it's damp here in the UK (rains most of the winter).

    I tend to alternate between setback and constant circulation, as I can never find one I'm happy with. If I do setback, I crank the boiler setpoint up and then effectively switch the heating off at night and when we're out. The (Tado) smart 'stat does a pretty good job of getting it back up to temp for the morning, but oddly never comes in quite where it should. And here's the key point. The air temp is probably about 1-1.5C off the target, but the house feels colder because everything in it cooled down overnight, and actually takes most of the day to heat back up again, before I switch it off and the cycle repeats.

    Alternatively (and actually currently now, since you got me thinking again), I 'set and forget' the system, at 21C/70F and can turn the boiler set point down pretty low (it's currently 10C/45F outside and the boiler is running at 43C/110F. If I used set back, the boiler would be running at about 55C (around 130F). The house is absolutely stable at 20C, with the TRVs ensuring every room stays at target. (The 21C is deliberately high to allow for heat gain from occupants, solar, etc.).

    For comfort and quietness, constant circulation wins hands down. However, from past experience, I can tell you it tends to be a bit more expensive to run this way in my house. In a better insulated house, it would be less expensive to heat constantly (less heat escaping) and that difference in cost could be reduced, if not eliminated.

    At the end of the day, it all depends what we're trying to achieve. I want to run my system as efficiently as possible, whatever the current envelope. At one extreme, that's not using the heating at all, but that's not viable, so I seek to find the lowest acceptable comfort point. What I've discovered is that using setback tends to cause noticeable discomfort (despite the 'smart' stat) unless I crank the boiler setpoint up to a level where using constant circulation (and a lower setpoint) is only slightly more expensive, and a lot quieter (no system expansion) and more comfortable (as the temperature never changes). The question then becomes, am I prepared to have the system be slightly less efficient (or at least more costly), for the benefits of constant circulation? I think I probably am, but with a colder week forecast, I may find it's actually more expensive than I've anticipated.

    Key differences for you to bear in mind:
    - I have a NG condensing boiler, which when used with ODR can be efficient into the upper 90s% (as it's always returning less than 55C and therefore constantly condensing).
    - I have wall-mounted steel radiators. They're quick response and can cope with low temperature flows (cast iron can't). Unfortunately, they're not particularly oversized, but some are due replacement anyway.
    - TRVs throughout, which gives me room-by-room control. They actually work really well together, as they sort of 'pass around' the flow based on their real time needs. Plus, when they're all at temperature, more heat returns to the boiler, so less gas needed to get to the setpoint. (A key benefit of constant circulation).
    - While I have a smart stat, it's actually pointless when I'm running constantly. It's just a high limiter.

    All of this results in an old, relatively poorly insulted house using about 130% of the average gas usage in the UK, where the average house size is less than half of ours. In other words, we're using 65% of the per ft2 gas to heat this leaky old sieve. I'll take that as a reasonable outcome, notwithstanding NG is pretty pricey (and rising) here.

    Keep mulling it over, and find what works best for you. I found this article enlightening:

    Don't spend too much time on it though. I did, and I probably could have used the time more wisely!

  • chiamac
    chiamac Member Posts: 5

    So I really have to wonder out loud if your use of TRV's is counter productive with outdoor reset. Again I'm not nearly an expert but it seems that they are also trying to save energy, or apply just the energy needed based on the ambient temp change. The issue is though that with outdoor reset you're only really giving the system the energy needed, and trying not to give more. For instance on my system a difference of 4f is a huge deal and would take an hour to recover, a difference of that on one of those valves may only mean it's half open and/or it's not overly alarmed and interested enough to fully open.

    Maybe it has no impact though, dunno. Personally I like the idea of manually throttling radiators by regular valves.

    Also, and dumb question, but the upper radiators will need to be closed off more. Do you have regular valves there that are a quarter open for the higher floors? All of your heat may be going to the upper floors first, and only getting in focus on the main floor once those TRV's are all satisfied.

    Then we had a single pane window on a door to the 2nd floor balcony. Adding a storm door helped a lot. Likewise installing the storm window to a 2nd floor bedroom made a real difference. So even putting up plastic around the first floor windows may help, or at least be a good test.

    With the constant circulation link. My boiler is a modern Burnham Series 3 and has something like that with pump pre-purge or whatever it is called. I have it set to run for 20 min if there is a heating call and if there is water in the system above 135f or 140f - which is when my radiators start working well anyway. This can also somewhat be done with the differential between high set point and when the boiler refires, I have mine set to 15 degrees now and I think that's adjustable to over 25. Combine all of them, although with a larger boiler and less water in the system, and a person could really start to control run times without something like you described. Or at least how I see it anyway.

    My old studio building and apartment had TRV's, since that was easier for the building management than a zone and the subsequent plumbing for each given space. However, for a regular sized house, I think it may be easier with standard valves, just IMO.

  • dm9321287
    dm9321287 Member Posts: 36
    Thinking aloud about the TRVs is an interesting point. I'm fortunate enough to have two heating engineers available who both know their onions. One said the TRVs shouldn't be used with the reset, but the other, like me, couldn't understand why that would be. All they do is close the valve once the TRV setpoint is reached, and keep that space steady. In theory at least, if the ODR was giving heat authority of exactly 1.0, they would never close, but it's unlikely any system would be so perfectly balanced, or immune from heat gain, that they would never be used. IMO, they're an essential addition to constant circulation. Otherwise, some rooms will overheat and others will struggle, notwithstanding best efforts balancing of the system. Crucially, they also allow temperature differential across the different rooms (so allowing constant circulation overnight, when everyone is in bed and not wanting it too warm in those rooms). What I do tend to find once the house has heated up and running constantly, is that many of the radiators are off thanks to the TRVs, so I'm essentially heating a much smaller space at any given time.

    I guess as with most things in life, we try stuff out and find what works best for us. The TRVs work for me because I want some spaces cooler than others. It may well be that not using them would make the boiler run more efficiently and I'd be interested in any further thoughts you have on that. Like I said though, I can't myself fathom why that would be, engineer 1 didn't seem too bothered about trying to explain it to me, and Engineer 2 (they work together, BTW), couldn't come up with a reason why that would be the case either. Engineer 2 is ex-Veismann, so possibly knows how ODR works. Also interesting that the constant circulation set-up advocates TRVs too, in conjunction with ODR.

    I should also be clear again, that my ODR is basic (Viessmann 100). I can't change the angle of the curve, only move it up or down, so it has its limitations and will never give me a heat authority of 1.0 across the curve. Oh, and when my HW comes on, it shuts the heating down until the HW is hot (HW priority), so I need a more than 1.0 to bring the heat back up. That being the case, the house would eventually just keep heating up on CH, until either (i) TRVs kick in, or (ii) room stat hits high point. The latter would undermine constant circulation (being noisier and stopping the boiler modulating).

    All in all, it's the best I can get to at the moment. Next house will be better insulated with proper ODR. Not worth the expense on this place as we won't be staying for too more many years. As you've said, bigger picture expenses.

    Have fun tweaking ;-)
  • dm9321287
    dm9321287 Member Posts: 36
    @Big Ed_4 , as a matter of interest, is your ODR system set to a constant indoor temperature with no setback?
  • PC7060
    PC7060 Member Posts: 573
    edited February 2021
    RE the use of TRV with ODR; I've got a system configured with ODR and TRV and the system works well. The ODR optimizes the temperature based on overall house heat loss while the TRV enable fine tuning of heat on room by room basis. Some radiators have TRV wide open, most are set to mid-range.

    I've also learned that use of setback is counter productive with ODR. I recently lowered temp setting by 2 degrees during a recent cold stretch while I was away for a few days and the systems had a hard time recovering back to standard temp. The house is well insulated for a 100 YO home but the plaster walls have lots of mass and suck up lots of BTU to raise the temps.
  • dm9321287
    dm9321287 Member Posts: 36
    Cheers @PC7060, that's exactly how I would it expect it to work. Constant circulation or do you generally use a room stat to cut it off?
  • PC7060
    PC7060 Member Posts: 573
    dm9321287 said:

    Cheers @PC7060, that's exactly how I would it expect it to work. Constant circulation or do you generally use a room stat to cut it off?

    I have a tstat located in the largest room with lots of window exposures that controls master call for heat. My ODR / Supply temperature has been tweaked to closely match house loss so the system routinely runs for hours on end when outside temps are below 45F especially on overcast days with no sun load. On very cold nights (low 20's / teens) I'll see the temp drop below target temp by a degree or two but the is currently under radiated because 8 radiators located in a new addition are offline.
  • dm9321287
    dm9321287 Member Posts: 36
    Thanks @PC7060, that sounds very similar to my current set up - I have the stat running as a master in the room with highest heat demand. I do have a TRV in there too though, to reduce/stop to the flow to that rad when the room is occupied in the evenings, and stop it cutting off the heating to the rest of the house. So mine is currently running constantly unless the external temperature means there's no need to heat the property.

    I've tried a few different variations to controlling the system over the years. @chiamac's recent response piqued my interest to try constant again. Yet to see how it pans out, but currently most rads are off due to rooms being at temp (not in the master room), and it's currently 5C/41F outside. The comfort is extraordinary. I'd like to see some efficiency in the gas burn too, but I have a feeling this approach can only use more fuel. How much more will probably determine whether I bottle it and revert to setback again, and heating the house like it's the 1970s!
  • PC7060
    PC7060 Member Posts: 573
    edited February 2021
    @dm9321287 - agree the low and long heatings cycle are very comfortable; my experience show reduced cost too.  My gas bill went down 30% last month over the old CI unit same month cost from 2 years ago and 50% from last year.  The old boiler was 148k BTU and spent a crazy amount on energy just heating internal tank to 180F. Only downside to replacement so far is the basements not as warm since there’s little wasted heat coming off new 80K Modcon 
  • Big Ed_4
    Big Ed_4 Member Posts: 1,829
    edited February 2021
    I run my ODR all the time with a set back , I also have a room sensor which I can bypass the set back with a push of a button . I am more concern about my comfort . I like sleeping when its cool but do not enjoy making coffee in it :)
    I have enough experience to know , that I dont know it all
  • moallen
    moallen Member Posts: 10
    I have a Buderus G115 oil-fired hot water heating system with baseboard radiators (fin–tube convectors). I have the aquastat set for 180/160 in the winter and lower it to 160/140 in the warmer months. My 1970 house is about as energy efficient as I can get it in terms of insulation, windows, doors, etc.

    I monitor my oil consumption by calculating the number of seconds the oil pump is pushing oil through its .65 gal/hour nozzle. I've been doing this for the 5 years we have lived here. I also have a weather station and calculate my own heat degree days (HDD). That makes it easy to compare apples to apples as far as one day's oil consumption to another.

    This was my first hot water heating system, so I'm no expert on these things. Until two years ago I was using a nighttime setback thermostat like I've done for probably 40 years thinking that would save fuel like it always did before. Not so with a hot water heating system! By trying all setbacks between 68 and 58, I discovered setting mine back to 58 at night from 68 during the day was costing me $10 per month. In fact, I found the best savings, which wasn't much, happened with a 67 degree setback at night from 68 during the day.

    The 2nd part of my post has to do with chimney condensation relative to boiler temperature. Do you think a higher boiler temperature would result in more or less condensation? We had some moisture problems until we had a stainless steel line installed. Bases on other situations, I know stainless steel doesn't last forever, especially the stuff they call stainless steel these days.

  • dm9321287
    dm9321287 Member Posts: 36
    @moallen, being in the UK, I'm not familiar with the kit you're using. I'm guessing that's not a mod con boiler like mine and your question is therefore to avoid rather than encourage condensation, as I do?

    As I understand it, condensation increases on a mod con when the return temp is below 55C and the cooler the better. Logically speaking, this must be due to cooler water hitting the heat exchanger. Therefore, on a mod con, you get LESS condensation with a higher flow temperature. I also understand that using ODR with non-condensing boilers is an issue because the lower flow temperatures from ODR cause more condensation. On that basis, I would conclude that a higher temp results in less condensation, which if you have a non-condensing boiler, is probably a good thing.

    On your setback, I suspect your results reflect your work insulating your house. The better the insulation, the smaller the savings from setback.
  • ryanwc
    ryanwc Member Posts: 40
    This may or may not be the best place for this question, but I'm wondering whether someone could point to a tutorial on mod/cons and setting the curve. I feel like I could better understand these threads if I first saw the basics laid out clearly, rather than trying to glean the basics by listening to a higher level discussion. Thanks.
  • ryanwc
    ryanwc Member Posts: 40
    It seems like with outdoor reset, my thermostat functions as a max temp rather than a minimum. In other words, in a traditional system, one wants the heat source to turn on only when temps fall too low. But with outdoor reset, you want the boiler always on unless temps get too high. Is that accurate?
  • dm9321287
    dm9321287 Member Posts: 36
    100% accurate @ryanwc - your room stat effectively becomes the high limit controller. It can work alongside, or instead of warm weather shut down. I've found WWSD (think outdoor thermostat) doesn't work so well in my system, so use the room stat as the high limit, on the basis that when it's actually warm enough outside, it'll be warm enough inside. Up to that point, and the boiler's in charge, adjusting the flow temp.

    As for tutorials, I found this a great place to start for an overview of the principles for setting up the whole system: - scroll down @Mike T., Swampeast MO

    I don't have 'true' ODR, so can't advise you on curve setting details. As far as I can tell, it's trial and error - go as low as you can until the house won't hold heat, then back up a fraction.
  • ryanwc
    ryanwc Member Posts: 40
    I'm starting to internalize some of this thought. Thanks, DPM.

    I'm going to read that link.

    I also think I figured something out that I'm going to post in my original thread from a few days ago.
  • dm9321287
    dm9321287 Member Posts: 36
    Thought I'd post an update on this, lest it be useful for others trying to figure this out in future.

    I've realised that my ODR isn't really ODR at all (and to be fair, Viessmann call it weather compensation, not ODR). This is key for anyone trying to figure out how to use a Vitodens 100 fitted with the legacy 'dumb' weather compensation (i.e. the one that shifts the curve up and down, but on which the angle of the curve can't be adjusted - on the older 100s, not the 200s).

    Thinking of this system as ODR is always going to cause problems. It doesn't behave like ODR because the curve doesn't match the property. Instead, the flow temp will be right at a certain outdoor temperature and then too high/low at other points on the fixed angle curve. Therefore, you have to start using the room stat to get acceptable temperatures across the curve (i.e. set it high for warmer days and use the stat to collar it when it's colder).

    The better way of thinking of this system is automation of the turning the boiler up and down to reflect the seasons that folks used to do back in the day. I've set mine to the 'factory condition' curve (5), which gives a flow temp of 70C at -2.5C. This is necessary to give a radiator delta T of 40C (rad sizing design was probably delta T 50, but I'm looking for as much con in the mod-con as I can get without the house being cold).

    When it's warmer than -2.5C (about 350+ days of the year here in the UK), the flow temp automatically drops. At 5C, it'll run at about 60C, giving me a radiator delta T 30, which is fine at 5C.

    I get a flow and return delta T of about 20C, so I'm condensing all the way, unless it's exceptionally cold for UK land.

    So with the weather compensation (not ODR) set up to give the design flow temp (or near as dammit) when it's normally cold, I ignore the boiler and run the system the old-fashioned way. Programmer, room stat, TRVs. Set back in the day and at night. I have a smart stat (tado), with optimum start and geofencing. Pretty effective, as long as I don't try to run it with aggressively low ODR strategy and constant circulation - then I may as well disconnect it.

    All this is not to say ODR isn't great. If I had a boiler that ran it (e.g Vitodens 200), I'm sure it would be great. That said, my 120 year old house isn't the best at keeping the heat in, and so actually, set back is the more efficient strategy in this property. I don't need to heat the street when I'm working/asleep. In a modern house (which will be the next one), I'd go ODR all the way.

    In summary, unless weather compensation is actually fully adjustable ODR, don't run it as ODR. It doesn't work. Instead, think of it as an automated boiler knob turner, and carry on using the system as before weather compensation, knowing it's more efficient than before, albeit not so much as the manufacturers claim.