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Turning outdoor reset advice

adk0212 Member Posts: 18
Now that I'm past the "hammering at zone turn-on" issue I can move on to more erudite subjects like tuning the system for efficiency. Background: Solo 110 boiler, 2 CH zones + DHW. Each CH zone heats ~1000 sq ft thru ~90 ft of finned copper baseboard. That's it. No in-floor, in-slab, etc. Just baseboards.

I suspect I'm a bit short on baseboard for my heat loss, but I haven't done a proper heat loss calc yet, so I realize we're going to be doing some quesswork here.

I'm used to my old boiler that always ran 180 deg water at all times, meaning zones heated up quickly. I run with a 5 deg setback on the zone thermostats at night and during the workday when the house is empty. In the evening, making up that 5 degrees with 180 deg CH water takes ~15 mins or less on an average day. But with a modest reset curve programmed in the Solo it takes more like 60-90 minutes. (In this case the "modest" curve was calling for 120 deg water.)

So my first question is, what should I be shooting for in terms of make-up time for that 5 deg setback? Clearly 15 mins is well faster than needed. But 60-90 feels too long to me...it seems like cranking the boiler for that long, even at low fire, has to be less efficient than a few minutes at high fire.

But I'm completely green here, so looking to be pointed in the right direction.


  • Zman
    Zman Member Posts: 7,330
    edited February 2012
    Boost mode

    Your baseboards were never intended to to run at 120 degree (that's the bad news). The good news is that you have a ton of length for your square footage. The issue is whether the heaters will convect (move air) well enough with the low temp water to heat effectively. It sounds like you have figured out how to set perimeters 4 and 10. I would experiment with them at 160 and 110. I would also set the boost feature (parameter 19) This will automatically boost the temp 18 degrees if the load exists for the minutes programmed. Try setting it for 30 minutes? I always set #43 DHW timeout to 90minutes if applicable. Keep in mind these are SWAGS that you can try because you live there and like messing with it. if you want to know the real numbers do a heat loss and then use the manufactures data on the heater output at design and typical loads to determine the appropriate temps.

    If the reset curve is causing the boiler to "short cycle" the first 30 minutes, I would say it should be raised for efficiency's sake.

    Depending how you are piped you are likely getting 40-50K Btu's at 180 degrees. That is a lot for 1000 square feet.This makes me think you can run lower temps.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Gordan
    Gordan Member Posts: 891
    There's another way to skin this cat

    Boost is fine if you're not planning on constant circulation, but that somewhat hampers your ability to dial in the reset curve. If you ARE planning on constant circulation, there's a setting to change the heating mode to constant heat call. The thermostat inputs are not used to "enable" the boiler's central heating mode then, but you can still use them if you set an offset parameter that will shift the reset curve by that number of degrees if the thermostat contacts are open. I think that this would be a good way to avoid short cycling at low loads, as well as recover from shallow setbacks - set your curve somewhat higher than it needs to be and then let the thermostat shift it down when your setpoint is reached. It's like widening the differential on the boiler some, based on crude indoor feedback.
  • CMadatMe
    CMadatMe Member Posts: 3,086
    Stop Setting Back

    Your running a high efficiency boiler that is using setback as if it a dumb boiler. Leave the thermostat alone and let the boiler do its thing.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • Ironman
    Ironman Member Posts: 6,868

    The most efficient use of a mod/con is to adjust the reset curve to match the load.

    ".it seems like cranking the boiler for that long, even at low fire, has to be less efficient than a few minutes at high fire.

    But I'm completely green here, so looking to be pointed in the right direction."

    Let me help you in the right direction: The boiler will be the most efficient when it runs the longest it can at the lowest firing rate. Properly adjusting the reset curve will achieve this. Take a close look at Gordon's suggestion of constant circ.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • kcopp
    kcopp Member Posts: 3,990
    I agree...

    find the right setting/ temp and leave it.
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    Boost and setback.

    I have two zones, one radiant slab, and a smaller one with baseboard.

    The baseboard zone needs only 6500 BTU/hr when it is 14F below design temperature outside. The boiler, that has outdoor reset, will modulate down to only 16,000 BTU/hr, so if I did nothing, it would cycle too rapidly. I have put in oversized baseboard, so I get all the heat I need if I set the reset curve to go up to 135F when it is 8F below design temperature, and will not get enough if it goes below that. At the warm end, when it is more than 50F outside, I put 110F water into the baseboard. I would like to go lower, but then it cycles too rapidly. The reset curve is almost exactly right when it is below 50F outside. It tends to run the circulator continuously, It never does as well as the radiant zone, but it does pretty well.

    Since the thermostat for the baseboard zone has the setback capability, I tried setting 4F setback at night, but that does not work out too well. I take 4 hours or more to recover from the setback because the reset curve is just right for maintaining the temperature, but too low to recover from that much setback.

    This year, I enabled the boost feature for my boiler for the baseboard zone. After an interval, if the thermostat has not been satisfied, the controllor boosts the supply water 10F. If it still is not satisfied after that time interval again expires, it boosts the supply another 10F, rinse and repeat. It never goes over the maximum, though.

    This helped recovery from setback. I picked two hours for the interval because I wanted the zone to run for at least two hours at a time when things are normal, and did not want boost then. With those settings, it recovers from the setback in a little less than 4 hours, but more than 3. I am almost done with this experiment, and have reduced the setback to 2F. That works pretty well, but I question whether it is worth the bother.
  • adk0212
    adk0212 Member Posts: 18

    for all the ideas and advice. I appreciate the education.

    Boost setting is interesting, I will try setting that. It should help reduce the make up time but still let me run lower water temperatures when maintaining.

    I'm currently running [email protected]/[email protected] reset curve. With my max at 180 am I in danger of the boost kicking in and producing 198 deg water? Or is the boost capped at the max setpoint?

    I think I was unclear on the sq footage. I have 2000 sq ft total, divided in 2 zones equally. Roughly 90 ft of baseboard per zone, so ~180 ft total. One zone is heated primarily. The second zone is only heated (by which I mean brought out of setback) a couple times a week.

    I never planned to run the system as constant circ. I didn't realize that was even an option for baseboards. I assumed it required a large thermal capacitor like in-slab radiant.

    As for abandoning setbacks completely, I see my setbacks are old school thinking. But I'm drawn to them for two reasons: (1) comfort...when we're up in the evening watching TV or working at the desk we like it 65 deg or maybe a little warmer. But at night we much prefer 60 or a little lower for sleeping. Much warmer than that at night and neither of us can sleep well. (2) Although I understand continuous firing helps my mod-con achieve peak efficiency, I can't see the point in heating a house completely when nobody is home. The boiler may do so with great efficiency, but isn't it just very efficiently burning fuel it doesn't need to burn in the first place? Or does making up the 5 degree setback every evening completely negate the savings of running the boiler less during the day?
  • Ironman
    Ironman Member Posts: 6,868

    Depending on some variables, your 5* setback probably is not saving anything and may indeed be making your system less efficient. Still, if you're using it for comfort, that's an understandable trade-off.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Gordan
    Gordan Member Posts: 891
    ANY emitter is capable of constant circulation

    So, using the parameters I described, you could do something like this: set the min temperature so that the burner is operating constantly at low modulation until the thermostat cuts out. At that temperature, you will be delivering more heat to your living space than you're losing, but you'll still be efficient because 1) you're at minimum modulation, 2) supply temperatures will still be relatively low, and 3) longer burner cycles will result.

    Once the thermostat cuts out, the shift parameter will lower the setpoint enough to prevent the burner from firing, and allow the water in the pipes to gradually cool until you drop below your setpoint and the thermostat kicks in again. Your shift should be set to drop you down to a water temperature that's enough to maintain a setback indoor temperature at the mildest outdoor temperature. This should ensure that you get a wider differential, and therefore longer cycles, during mild weather when your heat loss is below minimal boiler modulation.

    Your high limit should be set so that it, minus the shift, is enough to maintain your design indoor temperature at the design outdoor temperature. This accomplishes another useful thing: your setbacks will automatically get shallower and shallower as the outdoor temperature (and hence your ability to recover from setbacks) decreases. You also get a little built-in boost capacity in case there's any extra loss due to wind, and what have you.
  • NRT_Rob
    NRT_Rob Member Posts: 1,013
    hold on

    "boost" is the "don't call me back to fix this" setting. it's not a good setting if you are actually trying to maximize your efficiency. Don't use it.

    Setbacks reduce comfort. You should be able to find a comfort balance at a more consistent temperature, but if not, you need a programmable thermostat and you can experiment with different on times, or use one with an "optimum start" feature. if you must set back don't worry about it taking a long time to get out of setback.

    I would start with 95/65 and I don't remember where you are located... not sure if 16 is the right design temp for you. but if it is, I would scream it down to 140 based on what you're saying for footage and square footage... let it go for a couple of days and see if you fall short. If not, I'd turn it down to 130. If you do, turn it up to 150.

    trial and error is easy when you're on site and motivated.
    Rob Brown
    Designer for Rockport Mechanical
    in beautiful Rockport Maine.
  • zacmobile
    zacmobile Member Posts: 211

    I use the setback on my vitodens 200 which is pushing panel radiators and I am satisfied with how it performs, there is no boost function on the built-in control (boost is not a very energy concious function, only comfort) I just have it programmed to setback at night only from 10:00 PM to 5:00 AM. I have the curve dialled in perfectly so after a particularly cold night it can take about 2 hours to recover (the reason for the 5:00 AM start)

    Just play around with it and find your optimum start.
  • CMadatMe
    CMadatMe Member Posts: 3,086
    Reduced Room Temp

    Zac how come you don't program in the heating circuit to correspond with the setback on the thermostats? In coding 2 you can address a boost coming out of reduced room temp via FA:20 and Fb:30.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
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