Oversizing a Modulating Boiler with 10 to 1 turndown
I found 3 options.
1. Safely reduce the output of my boiler by plugging orifices of the burners and readjust air flow accordingly. The net result is like having half the burners. I surmise that cast iron manifold will tolerate the temperature gradients. This solution poses the cheapest, quickest fix, but highest risk since nobody else seems to have tried it. I scoured the internet, and did not find a single engineering oriented discussion on the subject.
2. If I kept the same oversized boiler, I can increase the heat dissipation by Increasing the lengths of my existing finned baseboard emitters. After checking on the price of finned baseboards, and the cost of plumbing new baseboards to existing ones, I decided this is not something economically viable (baseboards and A/C contractors are not cheap)
3. Convert to a Weil McLain AquaBalance with a 10 to 1 turndown ratio.
Regardless of how big a boiler is, the max amount of heating is determined by the amount of radiant baseboard you have in your home It is easy to find out what heat dissipation you system has, as engineers has decided that a finned baseboard radiator can dissipate 600 BTU @ 180 F and 200 @ 120F). Just walk around your house, measure all the baseboard registers, add them up, and multiple the total length with those dissipation numbers. Assuming you are not going to add more baseboard or replace them all with low temperature baseboards, it is obvious that the boiler output should not exceed the maximum heat dissipation ability of your emitters. Anything bigger contributes only to constant start stopping. Something slightly smaller will actually be more efficient in a multi-zone house as not all zones will be “on" at all times. If you leave some rooms colder than others, those zones will be closed more often than zones where you want maximum warmth. Therefore a slightly UNDERSIZED boiler will be most energy efficient if you are replacing your existing boiler with a non modulating new boiler (cast iron or otherwise).
With a boiler that has some level of modulation (up to 5 to 1) , it is still wise NOT to oversize a boiler in order to get the maximum benefit of the modulation. I’ll give an example: If you doubled the size of a modulated boiler with a 5 to 1 turndown ratio, you will only be running at 2 or 3 at the coldest days. At best, you are left with only 2 levels to modulate. It will be ruining at Modulation Level 1 on an average cold day when it should be running at ¼ or less.
But, with the advent of Weil McLain’s AquaBalance that has a game changing turndown ratio of 10 to 1, you can OVERSIZE the modulating boiler by 25-30%. Why 25%? With the AquaBalance you can set the water set point to 135 F instead of a normal 145 degrees. That way the boiler can run at 70% capacity even on the coldest days. The lower temperature will lengthen the life of the boiler since the boiler is subjected to less heat. The other factor which increases the life span of the boiler is that it can run with less start-stop to fulfill the maximum heating demand. Since you are modulating at Level 7 (out of 10), you have 6 levels left on milder days. On each of the 6 levels, the system is working the heat exchanger less hard than on a perfectly sized system. With 6 levels left, this system is MORE EFFICIENT than a perfectly sized modulating boiler that only has 5 levels of turndown. Assume you have 6 Zones, and only 1 Zone is open. The AquaBalance can run at 10% (Level 1). Ideally, the pumps will run more and burners will cycle less. Depending on how well insulated or air tight your house is, you can adjust the OTR (Outdoor Temperature Reset), to further tune the system.
With a lower water temperature set point, the return water can be closer to the ideal 110 degrees or lower for maximum energy efficiency.
But what if you messed up on the heat requirements of your home and the boiler is too small? All a DIY homeowner has to do is to crank up water temperature set point in small increments until a comfortable level is reached.
Comments
-
Add a buffer tank.
Also make sure the 2 minute cycling is really because it doesn't have a system to dissipate the heat to, 2 minutes is really short if water is actually moving out of the boiler.
2 -
I think you have a fundamental misunderstanding of what modulation is for. All things the same and you take a burner that is on/off and one that modulates, you will use the same amount of fuel to heat a given space to a given temperature. Modulation is there primarily to eliminate short cycling, which generally lowers the life of the equipment. Longer cycles can also increase comfort level as there are less swings in the room temperature
If you have undersized a boiler, making a change to the water temperature will not increase the boiler capacity, it will however change how many BTU/h your emitters can output. Water temp does not equal heating capacity of a heating appliance.
In your situation you have a few options to lower fuel usage. you can lower the thermostat temperature, increase the insulation values of your home (windows walls etc), or you can increase the fuel efficiency of your heating equipment. This may be where you get the idea that modulation is strictly more efficient, you are comparing 80% equipment to 95% equipment. That being said you will only get that efficiency if you are able to keep your system temps in condensing ranges.
I would also add that whatever you think is happening when you oversize the boiler and turn down the setpoint, that is not how it works. If you have baseboard heating, you will get the same BTU/h from the emitters at a given setpoint temperature and flow rate, whether the btu input of the equipment is properly matched, or oversized by double. Oversizing a boiler absolutely does not allow you to turn down the setpoint temperature, other factors may allow this, but boiler size is not one of them
2 -
How old is the home?
Have any improvements been made, insulation, air sealing, windows, doors upgraded?
Have a new Room by room heat load loss performed to determine exactly what size boiler you need.1 -
Interestingly enough, I think the short cycling = less efficiency is pretty overstated once you deploy some sort of "thermal purge" (obviously a new boiler should be sized as well as it can be, it's free/potentially cheaper to do it the right way). This old study shows some of the benefits of pumping all heat out of the boiler, regardless of size, once the heating call is over. Seems like most manufacturers now include this simple tech - the circulator simply runs even after the boiler stops firing.
https://www.bnl.gov/isd/documents/41399.pdfBut, with the advent of Weil McLain’s AquaBalance that has a game changing turndown ratio of 10 to 1, you can OVERSIZE the modulating boiler by 25-30%.
This is just marketing fluff- there's nothing game changing about a 10:1 turndown as that seems to be the industry's table stakes for mod-cons. A true game changing boiler might not have a larger turndown than 10:1, but would have a lower minimum firing rate than 8,000 btu. That seems to be about the floor.0 -
10-1 is common, a few 12 and at least one 15-1 turndown.
Any boiler over 10-1 they fudge a bit. I don't think they ramp down the blower any lower, they add an air shutter
My steps would be:
Room by room load calc. One or all the rooms may have lower loads then when built? Really no downside to knowing and identifying the high load areas. Reduce load, whenever and wherever possible!
Measure the fin tube in each room. How does it match the load and at what temperature.
If in fact there is more then enough fin tube, then look at output/ ft at lower temperatures.
Numerous studies have shown reducing SWT lowers boiler operating cost, especially in condensing temperatures.
I believe both modulation and condensing add to boiler efficiency.
If the boiler is a bit oversized and it is a mod con, the lower the firing rate in condensing mode the more efficient it operates. The concept is you expose the boiler HX surface to low firing and you get addition condensation, more surface area.
The holy grail is the 120F SWT or lower
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
2 -
Not sure if you are going to change emitters, but will add my recent experience any way..sorry for the rambling..hope it helps. For sure, oversize the emitters and go with a smaller boiler, as it provides flexibility if you can afford the space. I have a 3K SF 1955 brick and block rambler with little/no insulation on the main floor. The old boiler was rated about 110K BTU with fin-tubes. I replaced it with a Lochinvar WHB110 mod/con (same size) and 10:1 turndown, with 15 panel rads throughout on 3 zone manifolds. From the manifolds, I ran HePEX to Runtal radiators throughout. The Lochinvar is a great boiler so far, but it is far more than I need. Replacement occurred during COVID when boilers were in very short supply so a smaller one wasn't an option.
Certainly, the heat loss calc should NOT be THE only consideration to size the emitters. I finally had a heat-loss completed by an experienced contractor who began to size the system strictly on that output, but I didn't understand the result any better than I understand Modern Monetary Theory. For instance the recommended radiators had a lower BTU output than the baseboard I was replacing and I already knew those weren't adequate from a comfort standpoint running 180 degree water. I channeled my inner-geek and built an EXCEL spreadsheet comparing my old fin tube output at 180 degree water to the various panel size options at various water temps in increments of 20 degrees (120,140, 160 etc) and based on a per SF heat load. In the end, I came to the conclusion that I needed the largest standard panels that would fit under the windows in each room in order to get to lower water temps.
My goal for lower SWT was shaped by reading/studying from Siegnthalers book, Modern Hydronic Heating and his YouTube videos. I also watched and listened to Coffee with Caleffi and Taco pod casts and came here to ask several specific questions. Very helpful. I learned that Hot Rod, besides knowing more about this subject than I could ever know, likes fuzzy slippers and lattes and that... as a universal truth, size matters. So as he suggests, planning for lower water temps (larger emitters) will, I believe serve you well. You can adjust water temps and use/adjust variable speed pumps on a mod/con using outdoor reset. The down side to low temps: I didn't realize that the thermostats on my kickplate heaters were set at 130 degrees...which shows you how low I am running.
The final result is that my house is very comfortable, even in the current 0-10 degree weather and my water temps are very low even with panel radiators (relatively speaking (110-140 degrees, no kidding) and it has extra capacity should the zombie apocalypse take out the sun. The boiler generally operates between 20-50% in 20 degree weather and about 40-60% in the current cold snap and I have seen it operating at 10% in the shoulder seasons. I'm very confident that given the larger emitters, a 50-70K BTU boiler would have been a little more efficient and just as effective, if it were available. My gas bill runs about 30% lower even with an oversized boiler, so I won't be losing sleep over it now.
Recommend designing a system with a specific goal in mind from the start and plan in max flexibility. Minimize any adjustments or changes to those in the boiler room. Perform a heat-loss, but more importantly consider your current comfort. In my mind there is not one right answer, but the biggest risk is trying to plan a system around the exact right number or to have 'just enough' heat from the emitters at max water temps.
0 -
OK you engineers out there….. let me try this on you. I am proposing the KISS method (Keep it Simple Stupid) to determine the optimal boiler size when REPLACING an older non-condensing boiler (NOT a new home). Please comment if you think I missed something.
- Most existing homes have whatever emitters they have. It is not cheap to increase the capacity. So any discussion of increasing or changing emitters are usually irrelevant.
- Most homes can be heated to 73 degrees whatever the outside temperatures. Therefore, I conclude the emitters are enough for heating the home. More is always better, but for most homes, they are enough.
- Based on 1 and 2, I submit that the maximum boiler size should NOT be determined by square footage calculations of roof, walls, windows, doors etc, etc. but by a method that is far simpler and more precise. It should be determined by the EXISITNG emitters. How?
- Try this experiment on the coldest (or near coldest) day of the year. Let the home cool to 50 degrees (or whatever temperature your vacation setting is). Open ALL the zone valves (if you have them) and turn the temperature up to 74 degrees. This is the MAXIMUM amount of heat your emitters will EVER dissipate. Proceed to 5.
- Stay next to the boiler and time how long the burners run before it turns off. Then time the amount of time the burners stay off. If the on time is 2 minutes and off time is 2 minutes, the duty cycle is 50%. In other words, the boiler is 2X bigger than you need. If the boiler runs for an hour before it starts cycling, you have perfect sizing. My 45 year old boiler runs for 2 minutes and turns off for 4 minutes, so I believe my builder is 3X oversized.
NEXT TOPIC
- We all know that short cycling is not not good for longevity or for efficiency.
- However, cast iron boilers are usually oversized and short cycles, yet they don't fail.
- I use a Time Off relay to keep my circulators running for thermal efficiency, longevity and non frozen pipes. But that is another discussion.
- As for the thermal inefficiency of short cycling, I've read numbers like 6% - 15%, but how do we calculate those numbers? There is a HUGE difference between 6% and 15%. It may be the determining factor between downsizing my reliable cast iron boiler to a smaller one. Assuming annual gas bill of $2,000. A 6% savings is merely $120 — nowhere close to justifying a $7000 (labor and materials ??? ) for smaller boiler. At 15%, savings is $300/yr, still not enough to justify $7000. But here is another angle — since older boilers are 65% efficient when new, throw in 15% for short cycling and another 10% for old age, we have a 40% efficient boiler. Today's cast iron boilers are 80%-85% efficient. So a properly sized, newer, cast iron boiler may save up to 50% in fuel savings ($1000/ye). So the ROI is 7 years.
- Thoughts?
0 -
calculate the run fraction
for example a 2 minutes run , followed by 10 minute off Gives you an elapsed time of 12 minutes
2/12=0.167
Graph developed by Brookhaven Lab
16% on the bottom axis, up to the curve, read cycle efficiency on left
Additionally it may be condensing that entire 2 minutes run
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 -
As a simple sanity check on your overall heating efficiency, I would suggest starting by doing a simple heat loss calculation on your house to estimate. You can enter all the relevant info here to get a reasonably good estimate:
Enter your energy unit cost, and enter your boiler as 100% efficient. Now you know how much it would cost to heat your house if your boiler were 100% efficient, which of course it isn't.
Now change the boiler efficiency number until it matches your actual seasonal heating costs. You'll probably have to get down to 40%-50% or even lower to match your actual cost.
If your heat loss estimate was very accurate, this method would tell you exactly how efficient your boiler really is on a seasonal average basis. But your heat loss estimate won't be entirely accurate. It depends in part on your air changes per hour, which you don't know for sure. But you should be able to get within 10-20% of your actual heat loss.
Obviously plus or minus 10 or 20 percent is a fairly wide range, so this is not a precise method. But if you are consuming, say, twice as much fuel as you should be, this method will tell you that, and you will then know roughly how bad your season-average efficiency really is.
Or, you can simply tell us where you live (so we know your climate zone), the square footage of your house, when it was built, what type of windows you have, type of construction and how much insulation you have in the walls and attic. We can make an educated estimate of your house's heat loss to within 25% or so.
Also, one less-expensive approach to increasing efficiency (vs. replacing the boiler) is to add a large buffer tank. The boiler runs, say 15-30 minutes at a time every few hours to raise the water temp in the tank. Meanwhile, your thermostat controls a circulator drawing hot water from the buffer tank to feed your radiators. This effectively de-couples the boiler from the system and lets it make longer, more efficient runs.
0 -
Ah… I beg to differ, at least slightly. As I read your comment, @jesmed1 , you seem to imply — or at least I think it could be taken that way — that if a boiler is only running — let's say half the time that therefore the seasonal efficiency for the boiler is 50%.
Not true. What is true is that the boiler is twice as large as it would need to be if those average conditions applied throughout the heating season — which they don't.
Calculating based on duty cycle is an excellent method of determining just how much too big (it at all) a boiler is for a given set of conditions. It says nothing at all about the efficiency of the boiler itself. That can only be determined by knowing what the actual output power is and dividing that by the input energy consumption rate.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England2 -
@Jamie Hall I agree there are two entirely different questions: how efficient the boiler is, and how oversized it is.
I was trying to address the OP's main concern, which apparently is that he's spending $600+/mo in heating costs. Evidently his alarm over that cost is what brought him here. This then raises the question of what should he be spending to heat his envelope? And we can figure that in a ballpark way by (1) estimating his envelope heat loss, and (2) his cost per BTU.
If, for example, the OP is spending $3000/yr in heating costs, while his estimated envelope heat loss would be costing, say $2000/yr given his unit energy cost assuming a 100% efficient boiler, that tells us that there may only a potential savings of less than $1000/yr available from improving heating system efficiency (by replacing it, or changing settings, or adding buffer tank, etc). Because obviously 100% efficiency will never be reached, he'll never get more than $1000 savings per season. And that ROI limit helps inform choices about making improvements.
It seems we often get people here saying "my heating bills are huge," and they have no idea what the "right" size heating bill should be.
2 -
Oh quite so, @jesmed1 ! I'm a bit of a nut case about terminology, and it sometimes gets in my way…
And you are quite right — until one has some idea as to what one's heat loss is, it really isn't realistic to speculate on how much energy one should be consuming — and even less realistic to think of that in terms of "it's too expensive" — except when one is contemplating changing something. Now mind you, for most of us the question, when one gets a bill for electricity or gas or oil or whatever, isn't "how much energy is that" but "can I pay this thing?". And that's entirely understandable!
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
1 -
As a reference point, our heating cost for January was $970 (based on actual boiler run time at known gallons per hour) for a 4800 sq ft residence in Boston area. (That's 277 gallons of oil at $3.50/gal) .
This is for a 100-yr-old 2-story house divided into 4 apartments, with upgraded windows and some insulation, reasonably airtight, heat loss around 15-20 BTU/hr/sq ft at zero degrees outdoor air temp. Heating system is 2 very oversized cast iron oil boilers, probably similar to the OP's situation.
It may be worth mentioning that our January heating cost was about 25% of our entire expected season heating bill. So if the OP is in the Boston area and reports a $600+ heating bill for January, I would estimate his season total at about $2400+ (multiply by 4).
Depending on OP's unit fuel price, house location, construction and square footage, that may or may not unreasonable.
0 -
#JamieHall You are absolutely correct in saying that determining boiler efficiency has nothing to do with trying to figure out optimal boiler size. The point of my post was to help Homeowners with existing oversized boilers figure out what the optimal size a replacement boiler should be. As I mentioned in my post, it has nothing to do with square footage, roof size, walls insulation, roof insulation etc. It has everything to do with how much the emitters are capable of.
Using the existing paradigm of sf calculations, you can miss a lot of important detail that cannot be completely distort the results —- especially I an ol house. How much air leakage is there? Is there a gap in the roof insulation where some has fallen off the rafters? Has the attic insulation compressed? HAs a squirrel make an. air tunnel between the eaves and the attic? Are the windows and electrical outlets leaky? Is there a wall in an older house that has no insulation, where other walls have the insulation? Using the duty cycle observation I proposed eliminates all the guesswork.
The point of my post is to solicit critical discussion from you guys on whether II have missed something in my analysis. If my analysis is not sound, let's discuss it. If it is sound, it can radically simplify the effort to figure out how to size a replacement boiler. It will be a paradigm change.
0 -
what is the purpose of getting something that you’ll never use?
0 -
With such a wide range of btu output, it would be hard to miss with a mod con sizing.
With combis the BTU size is more about the DHW production.
Running any mod con say a 150K at it's lowest modulation, call it 15,000 BTU/hr can actually increase efficiency as a large surface are is now exposed to a small burner flame, enhancing condensation.
So no worries running on low fire. Now, I wouldn't purposely over-size the boiler, get as close as you can, step up when in doubt.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream1 -
OK, I'll bite. I'm not a heating pro, just a homeowner who is also a mechanical engineer.
We also have boilers that are roughly 3x times the size of our building's actual heat loss on a zero degree day, and I've done a lot of run time analysis and tweaking to maximize our efficiency. So I can comment on your step #5 above, where you said:
"5. Stay next to the boiler and time how long the burners run before it turns off. Then time the amount of time the burners stay off. If the on time is 2 minutes and off time is 2 minutes, the duty cycle is 50%. In other words, the boiler is 2X bigger than you need. If the boiler runs for an hour before it starts cycling, you have perfect sizing. My 45 year old boiler runs for 2 minutes and turns off for 4 minutes, so I believe my builder is 3X oversized."
I'm going to take issue with the words in bold. For example, our cast boilers are 3x oversized for our building's heat loss. I've determined this by extensive run time analysis during zero-degree days.
However, because our boilers start cold and are heating large water volume converted gravity systems with cast iron radiators, the thermal mass of the water and rads take a long time to heat up. We could run the boilers for 90+ minutes from a cold start without hitting the aquastat high limit of 180. So they would, in fact, run for at least an hour before cycling, and they would still be 3x oversized.
So your test method may work for some other situations, but it wouldn't work for cold-start cast iron boilers, which in some cases (like ours) can run for 1+ hours without cycling, even if they are 3x oversized.
0 -
Actually using real measured fuel usage — together with a reasonable estimate of boiler efficiency, so that one can figure the net BTUh of the boiler — is a very good way of determining the actual heat loss of a structure. There are at least two booby traps, though. One is a reasonable estimate of boiler efficiency — and that can be determined if there has been a competent tech. tuning and cleaning the boiler with instruments — and measuring the fuel usage on a cold day, when the boiler is doing nice long runs. There is a hit on efficiency of the boiler in starting up from cold. — but how much it is is very much open to debate. My personal opinion, based on almost no hard measurement, is that for longer runs it's not likely to be more than 5% overall. Some beg to differ… there is a potential problem with modulating boilers. With fixed firing rate boilers — no problem. With modulating ones, though, you really need to measure real time fuel usage, as run time won't help, and return temperature, as that will affect condensation and hence efficiency.
The other is a bit trickier, especially for older houses. Be conservative! Ideally — cold, night, windy. And accurate indoor and outdoor temperatures…
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England0 -
@mwong00 : I like reading these kind of posts because I can learn so much other stuff that people post about in their discussion to help with the problem.
But could you perhaps give us a little bit about the personality of the system—it would be more fun and perhaps better help would ensue. Boiler make and model/, perhaps an old octopus coal burner that has been re-fitted? Those tend to be the kind that end up extremely inefficient 60% or lower! Old drafty house or one that has been really tightened up? etc. How many square ft? Size of the tapings and the near boiler piping? Was it originally gravity fed? What kind of circulator do you have? and where is it located in the piping?
It doesn't sound right to me that your boiler comes on and off every 2 minutes; boilers don't usually satisfy the thermostat in 2 minutes—-so I think it has something else to do with perhaps your aquastat or high limit—do you have a low limit control? perhaps a mixing valve that is broken? do you have a loop that sends (could send) the hot supply water back through the boiler? It could be that the anticipator setting is too short/low. Maybe you could add an anticipator?
There are solutions to deal with an oversized non-condensing boiler.
You haven't had an answer to the question of plugging up the ports.
If you can keep the beast working, you may be able to vault the current technology and when you replace, perhaps replace it with something terrific coming down the pike. Heatpump?— depending on where you live.
My understanding is that the start and stop problem reduces efficiency but only in a very minor way and fixing it will not do much to alleviate your $600 burn rate. And fixing it with a new mod con boiler is not going to be cheap. Can you even vent it out a side wall?
I'm a newbee, so listen to the pros —but I think you could fix the boiler-I don't think it is working right.
0 -
@dronic123. Thanks for your comments. My specific situation may not be interesting to anyone else, but here it is. My house and the original cast iron boiler are 45 years old. House is 4700 sf with mostly triple pane windows and some double panes. I did a spreadsheet and came up with 940 sf of windows. 2000 sf roof and 3000 sf of walls. I don't know if I have 2X6 or 2X4 walls. My furnace is a Burnham 805B. 264,000 BTU Input and 178,000 BTU output (65% efficiency). I suspect it is much less than that since it is so old. The original house had 7 zones which I determined was highly inefficient because it reduced the heat dissipation. So I forced all of them open in order to maximize heat dissipation. Anytime I ask for heat in any of the zones, the heated water goes to all the rooms. I also added a power off relay which makes the circulators run for 6 hrs after the furnace turns off. I did this to release all the heat trapped in the huge cast iron boiler into the house instead of going up the chimney. The other benefit is that I have some underground pipe in the basement that froze during a particularly cold spell. Luckily for me, it was not frozen solid, just enough to make me sweat bullets. With the water constantly circulating, there is no way the pipes will ever freeze. I have a TACO 007 circulator, which uses 60 watts, so the electric cost is merely $3/month — a small price for all the benefits I mentioned.
I have an old HW heater which I don't need. So I added it in series with one of the zones to increase thermal mass to the system. I just completed it today. I have not studied the results yet.
I keep my house at 61 F to keep my energy cost low. Last month's gas bill was still $400, I would like to eventually be able to get it up to 68 if I can get my old boiler to be more efficient. I have toyed with the idea of blocking half the jets, but Burnham tech support told me not to do so. I am also afraid that with a n 45 year old boiler, I might strip the threads trying to remove them and then I will really be in deep trouble.
My boiler at its best goes on for 2 minutes before shutting off —— not because of the thermostat, but because the aquastat is telling the boiler the water has reached 180 degrees. My digital thermometer with 4 sensors tells me that the circuit board is working correctly in that the furnace comes back on after the water drops 10 degrees. That tells me that the boiler is grossly over sized given that the emitters cannot dissipate the heat fast enough.
The controller board is Honeywell L7248. The default setting ibetween the Hi and Lo temp is 15F, but my measurements show 10F. I want to change the setting so that the lower setting is 20 or even 30 degrees to reduce the short cycling. I am not sure Honey will allow that wide a differential. That is the next step in my journey. Anyone who is willing to share how to modify the circuit is greatly appreciated.
0 -
@jesmed1. You are absolutely right about the initial time it stakes to heat a system from 60F to 74 due to the thermal mass. Thank you for catching my error. I did the rest of the test NEARLY exactly as I described, but I did not report the first step, and you caught it. The test was — coldest day of the year outside, 60 degrees inside, thermostat turned up to 74 degrees. The system took around 12 minutes (maybe 20 mins, I forget). to have the water reach 180F , but then cycled 2 mins on, 4 mins off, as it ramped up from 60F to target temp of 74F. The duty cycle is 33%. Would you agree my test sets conditions to maximize how fast your emitters dissipate heat on the coldest day, regardless of the boiler, and regardless of the thermal mass. I submit that by measuring the duty cycle AFTER the water (and by extension, the rest of the thermal mass) reaches 180F, it takes the thermal mass out of the equation.
If uoi agree to that assertion, then AFTER THE WATER REACHES 180F. it turns off for awhile. Then when it restarts, and if it runs 1 hr on and say 10 mins off to ultimately ramp up to 74F room temperature, I submit that your boiler is properly sized. It is basically running near 100% duty cycle to keep up with the emitters. In my house, I submit my boiler is 3X oversized. Not 1.5X. Not 2X, It is 3X. (Actually a little more than 3X oversized because of the inefficiency of startups.). Thoughts?
0 -
@mwong00 The most important things you didn't tell us are (1) where you're located, and (2) what your gas costs per therm.
I'm in a 4800 sf old house in the Boston area, with original cast iron rads. We have about 1000 sf of radiator EDR. With that amount of radiation, we could radiate 150 BTU/hr x 1000 sf EDR = 150,000 BTU/hr at 170 degrees F water temp.
But we never get close to that water temp for heating, because our heat loss on design day is under 100,000 BTU/hr, and most days much less. So we can heat with low water temps. 120-130F is typical.
Since your house was built circa 1980, your envelope should be better than ours at 20 BTU/hr/sq ft heat loss, so you should be under 94,000 BTU/hr heat loss. But you also probably have less radiation. This is common in newer houses.
And with your 178,000 MBTU/hr output, you should be running no more than a 50% duty cycle on design day, and less than that 99% of the time.
Most of the time you're probably running 25%-33% or less.
For best efficiency, your boilers should be cold start, should run maybe 30 minutes to heat your rads, then should shut down while your circulator does your thermal purge, and should stay idle for at least 2-3 hours most days.
If your boiler cannot run 30 minutes or more from cold start without hitting the high limit, you may have a flow problem. What type of radiators do you have, and what diameter piping?
Having said that, a $400 heating bill for January is quite good for a 4700 sf house. Here in Boston that would have cost you almost $1000 if your heat loss was 20 BTU/hr/sq ft, or $750 for 15 BTU/hr/sq ft heat loss. (at $2.50/therm which is the current price here).
So either you live farther south, or your gas is much cheaper, or your house is well insulated, and possibly all three.
0 -
So much here to unpack. Oh dear.
First, if the boiler runs only two minutes before shutting off on the high temperature limit — you have way too little flow for the boiler.
Now having said that, what is missing — and no fault, it often is — is that the heating system is… a system. The various components must be matched to each other. In a hot water system such as we are looking at here, there's no point in having the boiler bigger than what the radiation can use at the maximum temperature of the water. If the boiler is grossly too much bigger, use a buffer tank to give the boiler a reasonable run time.
Somewhere up there there was a comment about downfiring the boiler by blocking some orifices. Don't. Just plain don't. On some boilers it is possible to downfire, but it may involve adding baffles in the firebox and in any case it requires the use of good test instruments and someone who really knows how to use them.
There was also a comment about the boiler efficiency being lower than specified just because it is old. That is simply not true. It may be lower than spec., but not because of age — because of inadequate maintenance.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England0 -
@jesmed1.
You are right I have less radiation. I also have way less thermal mass. It takes a lot less time to raise the water temp from 170 to 180. My house has mostly baseboard registers (¾" pipe with copper fins) and some underfloor heating in the kitchen, dinette and a large entry. A primary Taco 007 pump drives the whole system and a secondary Taco 007 drives the underfloor pipes. ¾" copper piping to the registers and ¾" plastic (not PEX, unfortunately) to the underfloor. "The rest of the house has a total of 180 feet of baseboard registers. (That's measly compared with your 1000 sf of radiator EDR). The underfloor heating in the kitchen and dinette are GREAT,. But under the entry is totally messed up because the original builder put down 2 layers of ¾" chip board, so the heat doesn't come through. I am not going to tear up 250sf of tile and underlayment.
My gas cost ranges from $0.80/Therm normally to $1.66/Therm during end of 2021. My usage is around 300 Therms, which sounds ok, but that is because I keep my house at 61F . I imagine it will double if I kept it at 71F.
I live in Colorado. We have 300 days of sunshine every year. Most days when the sun is out, the boiler doesn't run at all from 10am - 3pm. It runs after the sun goes dowm, at this ridiculous 2 on 4 off (or worse). Same on cloudy and cold days. Yesterday, I installed a 40 gallon buffer tank with an old unused HW heater.
I was not able to run the entire boiler output into the buffer thank because of space and piping , so I just ran it to 1 of the 7 zones. I know that is not optimal, but given the space/pipe constraints, I didn't have much of a choice. For the first few hours, the duty cycle changed to 2 on 2 off. The buffer tank never got up to full temperature. That is an incomplete test, and I need to monitor it after the tank gets up to temperature. This HW heater provides me with another data point — I plan to run the HW heater on high, but not the boiler. The HW heater is 34,000 BTU input 28,000 BTU Output. If this setup keeps my house at 60F on a 25F outdoor temperature, it will give more correlation to my assertion that 60,000 BTU output is all I need for really cold days.
By the way, thank you for your calculations on my heat loss. It is close to the calculation I discussed a at the begging on my thread . But I believe I need only ⅓ of 178,000 BTU on the coldest day, or 60,000 BTU output. This is academic, because I did a cost benefit analysis and it would take decades to break even on a replacement. My goal at this point is to reduce the short cycling at low cost.
0 -
@jamiehall. Thanks for your comment.
I don't think I have insufficient flow. Please read my previous post to @jesmed1. I have 2 Taco 007 pumps. Using all the pump specs, I calculated there is sufficient flow. But if you think there are other factors affecting flow, I am all ears.
You are absolutely correct that the system when I bought the house is incorrectly designed — I believe the boiler is oversized by a factor of 3X. But I did the cost of a new system, and it will take way too long to recover the investment. Besides, a ModCon system has its own disadvantages — more complexity, new vents, more expensive parts, parts that are hard to get, and inevitably fail in the middle of the coldest days, acidic effluent that corrodes sewer lines if the calcium carbonate is not replaced soon enough, shorter equipment lifespan (12 years instead of 40 +years). Add to that is the fact that local contractors will not replace just the boiler, they will only replace the entire manifold — over $20,000.
Your comment "There was also a comment about the boiler efficiency being lower than specified just because it is old. That is simply not true. It may be lower than spec., but not because of age — because of inadequate maintenance.". I think it gets less efficient because there is calcification and corrosion INSIDE the cast iron water jacket. Since it is a sealed system, it is impossible to remove that. Calcification and crud reduces the heat transfer, so more heat is lost up the chimney instead. I know there is a lot of corrosion because every time I replace a component — pressure regulator, air bleed valve, expansion tank, I see lots of crud. If it forms at those places, it must form inside the tank too.
0 -
OK, there's a lot going on here. Let's review:
You have a 178,000 BTU/hr output old Burnham cast iron gas boiler, but your envelope heat loss is probably only round 60,000 BTU/hr on design day.
Your radiation consists of 180 feet of baseboard registers, plus two smaller zones of underfloor radiant.
The system is divided into 7 zones. 5 of those are the basboard registers, and the other 2 are the underfloor radiant.
All those zones are fed by 3/4" copper or plastic.
One Taco 007 feeds all 7 zones, plus one Taco 007 feeding the 2 underfloor radiant zones.
You have one thermostat, and on a call for heat, you run all 7 zones to try to prevent short cycling.
Because of limited flow, limited water volume and limited radiation, when the thermostat calls for heat, the boiler quickly heats up from cold start (if it hasn't been running during the day when solar gain means the Tstat stays satisfied without the boiler).
So the boiler heats quickly (within 10-15 minutes) to the aquastat high limit (180), then begins short-cycling between 170 and 180 (for how long typically) until the Tstat satisfies.Typically the cycles are 2 minutes on, 4 minutes off. Meanwhile, I assume, the circulators are still running.
In an effort to reduce short cycling, you then plumbed one zone into an unused (?) gas water heater as a buffer tank. Then, with one zone plumbed into the buffer tank, you ran the boiler again and found the short cycling reduced somewhat, 2 minutes on and 2 minutes off (as opposed to 4 minutes off previously). (Presumably this is because your piping run to the water heater is shorter, reducing head loss in that one zone and increasing flow through the boiler.)
But still, the boiler short cycles 2 minutes on, 2 minutes off, during calls for heat (which typically last how long until the Tstat satisfies?) And you want a low-cost solution to minimize short cycling.
You really need an expert like @hot_rod or @EdTheHeaterMan to suggest a low-cost solution. Because I am not a heating expert.
However, until the experts get here, my non-expert first thought is that you need a much wider "deadband" for your boiler aquastat control. It's ridiculous to be bouncing between 170 and 180 when most of the time you could heat with much colder water.
So my guess is the experts will tell you how to either reset your existing aquastat or add some strap-on aquastat that will have a low limit of, say 130 degrees, so that when your boiler hits the 180 high limit, it stops while the circulators continue to run, until the return water temp reaches, say, 130 degrees. Then the boiler starts running again. And now instead of running for 2 minutes, it runs for 5x as long (because your deadband is now 50F vs the previous 10F), which would be 10 minutes. Then the "off" time will be 5x as well, or 10 minutes.
So you'll still have the 50% duty cycle, but at least it will be 10 minutes on, 10 minutes off, vs. 2 minutes on, 2 minutes off.
I'm sure the experts will have more (and better) thoughts for you, but I think at least we now understand your setup and why it's short cycling.
1 -
@jesmed1. Hahaaa. You are a smart guy. You understand exactly my installation and my goal. I also have been trying to set the LL temp on my aquastat. Unfortunately I have a hard time finding manuals for something as old as mine. For what I have been able to determine, my old aquastat does not have an adjustable LL setting. But we are on the same page. Setting the LL to 139F is exactly the right thing to do. I don't know if I can replace my existing aquastat with a modern one with the kind of LL adjustability you mention.
0 -
@EdTheHeaterMan will know what to do about the aquastat. It might help if you can post the model # of your existing aquastat.
1 -
@RayWohlfarth posted a video recently discussing use of delay timers to reduce short cycling
0 -
@jesmed1. Thanks! I will give @EdtheHeaterMan a try. First, I will have to find out where the Aquastat is at. Meanwhile I managed to make my boiler stop turning on, while dorking around with the HW tank. I might have shut off too many zones and created too much back pressure to the boiler. Murphy's Law is real!
0 -
@mwong00 It is easier to reply to our query here.
After reading @jesmed1 's summary
I have a pretty clear idea of what you are looking for. The only thing that you added in the messages you sent me is that you have a defective L8148J control now. How did you determine that the control is bad?
The next question I have is: What is the white control in this picture and where is it connected to on your boiler's wiring
Finally, this started out 3 years ago, as a question about selecting a Weil McLain AquaBalance and turned into some unusual theoretical discussion about selecting an oversized ModCon boiler on purpose, which by the way, is not a real good idea.
Now you have everything in @jesmed1 comment with the addition that a new boiler is out of the question and you need a replacement for your L8148J
Does that just about sum it up?
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
2 -
we haven't asked for pictures yet ? really?
a couple wide angle shots, floor to ceiling, showing boiler, circs, and buffer tank, all in one, from a couple angles,
and don't we need a 3rd circ now that a buffer tank is added
known to beat dead horses0 -
Look for the 4 jpeg attachments about 10 posts up.
0 -
ok,kinda dark, and hard to follow, and can't find 2nd circ, don't we still need a 3rd circ?
assuming the left tank is buffer and right is still domestic?
if left tank is buffer, and if burner is disabled, consider sealing off top of tank at exhaust vent, or you're chimneying buffer heat up that stack, that burner is disabled, correct?
known to beat dead horses0 -
agree that @EdTheHeaterMan is very knowledgeable. And a funny guy. 🥸 Also like the delay recommendation; @RayWohlfarth discussed delays in his YouTube channel a couple weeks ago.
looking at the item Ed circled in his earlier post you appear to have some type of delay device already connected. Can you tell us how it’s connected and the part number?0 -
To @EdTheHeaterMan and @jesmed1
Sorry, I have been dealing with the boiler for the past week including some obvious things I had overlooked. But I finally have everything working PERFECTLY. I thank @EdTheHeaterMan for his help, and #jesmed1 for referring him to me.
It turns out 2 of the Honeywell Zone Valves that I installed 8 years ago have gone bad. As I mentioned before, in order to maximize emitter heat dissipation, I open up all the ZV by using the manual override lever. I taped it with the aluminum tape. However, the ZV manual override lever was bent totally out of shape (I don't know why, but I think it is Honeywell using soft metal for the lever), and the ZV valve was closed even with the manual override. I only found that out when I tried to flush the system from air, and realized I couldn't with those zones. The trickle of water running out was not ice cold , so I knew the pipe for that Zone wasn't frozen. I replaced the valves, and the water is now running through as it should. The two Taco 007 are pumping fine. Secondly, the unused HW heater I had added (to increase thermal mass) to one of the 7 zones is doing a spectacular job, reducing the short cycling completely. I haven't measured it yet, but it is so much longer than before that I didn't bother to wait.
The only reason I was looking to replace the 8148J Aquastat was not because it was failing, but because the temperature differential was only 8F. I wanted something wider. However ,I did not even need to replace my Honeywell L8148J aquastat after all. In my research, I leaned something very useful —- I found out that the Honeywell H8124A1007 thermostat would be totally compatible . The L8124A1007 has a temperature differential setting of 20F instead of the stock H8148J which has a non adjustable 8F temperature differential. So that would have been a good upgrade, even though the thermal mass from the 40 gallon HW heater made it unnecessary. I found that out only because @EdTheHeaterMan was kind enough to tell me that the temp bulb size is universal, so I didn't have to worry about the physical issue of fitting the temp bulb into the existing cavity. And when Ed referred found a generic Aquastat that was less expensive, I realized that the electronics part of it should be universal too, as long as the new aquastat is 24V. By that I mean 24V output to Ignitor, and 110AC to circulator and the power vent. Again, many thanks to you Ed!
1 -
@pc7060. The relay I have is a "GEYA Delay Off Single Function Time Relay 1 Output 16A Electronic Power Off Timer Delay Relay(GRT8-B1,AC230V)".
The relay allows me to provide power even after the Honeywell aquastat tells the circulators to turn off. The timer can be set from a few seconds to days to "ON".
Please do not confuse that with a Power On relay. Amazon has one that is labeled as Power Off relay, and it is not. I doubted myself for a long time, and finally, I tested the darn thing on a bench and proved conclusively that Amazon has messed up. The part I show above is the right part.
The far more useful thing using this relay is that I have solved the problem with frozen pipes when the weather gets super cold. I seldom use my basement in the winter, and so I leave the temp at 50F. Well, on a particularly cold snap that lasted for days, the pipe under the basement floor froze up. The only reason I found out was because I the basement temperature dropped below my setting of 50F. I touched the pipe next to the boiler, at it was cold. That meant that the water has frozen somewhere inside the piping. Terror struck me. the cost of figuring out where the freeze was, and to bust through the concrete to fix it would be super expensive. I used my wood stove at full flame to raise the temperature as high as I could to get heat to the floor. Luckily the pipe was not frozen solid,
as the next day, the weather warmed up, and I felt the hot water running through it again. As you know, the temperature in a room may be set at 50 F, but if the zone is not asking for heat, the water in the pipes can be below 32F if the pipe is close to an outside wall, or if the pipe is buried under the basement floor. But, as long as the water is running inside the pipes, it will never freeze. Now, with the zone valve always open, I do not worry about it freezing any more. I use the shutoff lever to neck down the flow so that I don't waste heat in an unused part of the home.
By the way, when you go on vacation, you can leave your thermostat at 45F if you open up all the valves and run the circulator continuously.
1
Categories
- All Categories
- 86.8K THE MAIN WALL
- 3.1K A-C, Heat Pumps & Refrigeration
- 56 Biomass
- 423 Carbon Monoxide Awareness
- 106 Chimneys & Flues
- 2K Domestic Hot Water
- 5.6K Gas Heating
- 104 Geothermal
- 160 Indoor-Air Quality
- 3.6K Oil Heating
- 69 Pipe Deterioration
- 953 Plumbing
- 6.3K Radiant Heating
- 385 Solar
- 15.3K Strictly Steam
- 3.4K Thermostats and Controls
- 54 Water Quality
- 43 Industry Classes
- 47 Job Opportunities
- 18 Recall Announcements