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Boiler size to minimize shoulder month cycling

In_New_England
In_New_England Member Posts: 48
edited April 22 in THE MAIN WALL
Once we figure out design day size requirement for a boiler, what factor do we use to estimate the lowest heat load requirement to minimize cycling during shoulder months?

Asking because I'm considering a modulating boiler. I have one zone, 87 ft of fin tube baseboard. I averaged 28000 BTU/h in the winter months.

Is it half , a third, a quarter?

Thanks!

Comments

  • hot_rod
    hot_rod Member Posts: 16,118
    Maybe look at your smallest zone load, from a room by room load calc, and look for a turndown close to that. If you have micro zones or loads, say under 8000 btu/hr you can still expect cycling, even on design days.

    Other options:
    Combine the smallest zone with another zone
    Buffer tank, or high water content type boiler, Viessmann and HTP have some options
    properly apply ODR
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    In_New_England
  • Jamie Hall
    Jamie Hall Member Posts: 18,699
    How big is a box? I don't think, realistically, that one could come up with a factor. The load -- and hence the power input -- will vary down to zero. If the boiler can modulate its power, that's one thing. If it can't, that's another problem. Any heating design will be a compromise between the power output of the boiler and the power requirement of the load -- and the degree to which conditions, particularly temperature, can be allowed to vary --sometimes, for temperature, referred to as the swing.

    If the boiler can modulate, and you are using outdoor reset preferably with an indoor temperature trim, one is limited by how low the boiler can go. Many newer ones can modulate down to 10%. Below that -- whatever it is -- the boiler is going to have to cycle on and off. If the boiler can't modulate, then it will have to cycle on an off for any load less than full peak output. How long the cycles will be depends on how much thermal inertia there is in the system --mass of radiation, volume of water, and characteristics of the structure -- and how much tolerance there is between a low temperature where the boiler will start and a high temperature where it will stop.

    There are a lot of design tradeoffs which will ultimately determine how long a boiler firing cycle will be relative to the off time.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    In_New_England
  • In_New_England
    In_New_England Member Posts: 48
    hot_rod said:
    Maybe look at your smallest zone load, from a room by room load calc, and look for a turndown close to that. If you have micro zones or loads, say under 8000 btu/hr you can still expect cycling, even on design days.

    Other options:
    Combine the smallest zone with another zone
    Buffer tank, or high water content type boiler, Viessmann and HTP have some options
    properly apply ODR
    Interesting. We effectively have one zone with 87 feet of baseboard. The other zone is tiny and will never be on without the other.

    My estimate for average BTU/hr during Jan/Feb (Boston) is 28000 if that means anything.
  • hot_rod
    hot_rod Member Posts: 16,118
    Another way to look at it is by fin tube output. If you know the brand and model you can get close numbers. Call it 550 Btu/ ft of actual fin.

    550 x 87'= 47,850. so that is all the heat emitters can deliver to the space.

    You could run the home through the free SlantFin heat load calculator to see how numbers match.

    Does the home maintain a comfortable condition now on your coldest day? If so the fin tube is correct.

    Ideally it would have enough fin tube to run at low water supply conditions to maximize a condensing boilers efficiency.

    Various places to find hours of occurrence data for your location and help predict the required output across the heating season. That would correlate to boiler output required at those varying conditions. See the NY example below

    Here is some good info on running low temperature conditions.
    Of course the more you insulate and tighten the building envelop, the smaller the boiler= lower operating costs :)


    https://www.caleffi.com/sites/default/files/file/idronics_25_na.pdf
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    In_New_England
  • In_New_England
    In_New_England Member Posts: 48
    I agree with your upper output computation. I used a slant/fin chart to estimate an output of 20000 at 130f which I use as the lower bound, and which matches the lower end of the NKC199 I'm looking at.

    The house is nice and warm on the coldest days, driven by a 140000 BTU WM oil boiler set at 180f.

    I'm confident a NKC199 won't short cycle in winter, though it won't be condensing. I'm thinking if the NKC150 would be better for the shoulder months re: short cycling.
  • Jamie Hall
    Jamie Hall Member Posts: 18,699
    Is short cycling really a problem, though? If it is, it's not a matter of boiler size. You need a certain boiler output on the coldest days. You need much less power on warmer days, so either the boiler will modulate its combustion, or it will turn on an off. Now if the turning on and off is a problem for you for some reason, your only feasible approach is to increase the heat capacity of the system, so it can be off for longer periods without the system cooling too much -- and, correspondingly, run longer to get the system back up to the maximum temperature.

    Or... to put it simply, a buffer tank.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    pecmsg
  • hot_rod
    hot_rod Member Posts: 16,118
      If you are confident with your 28,000 design load 
    You have 87’ of fin tube
    Expect 300 btu/‘ at 135 swt
    So you have 26,000 btu/hr at a temperature that will condense, call it 120F return on design days

    The 120 Nobel has a lower turn down, covers the load, and 3 gpm DHW

    Not sure why you would consider a 199?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    In_New_England
  • In_New_England
    In_New_England Member Posts: 48
    edited May 8
    hot_rod said:

      If you are confident with your 28,000 design load 
    You have 87’ of fin tube
    Expect 300 btu/‘ at 135 swt
    So you have 26,000 btu/hr at a temperature that will condense, call it 120F return on design days

    The 120 Nobel has a lower turn down, covers the load, and 3 gpm DHW

    Not sure why you would consider a 199?

    I agree! It's a combi, so I don't want to go to the 110 because I'm not sure that'll be enough for hot water, but from all the rough calculations I've done, the 150 will do enough for our single shower house. I just have to convince the contractor.

    That said, at 120F I estimate my base board to be putting out 18270 BTU/h which is around the min fire of the NKC199.

    Do people go below 120F on regular baseboard? I see the slant/fin chart goes to 110F.

    Thanks.
  • In_New_England
    In_New_England Member Posts: 48

    Is short cycling really a problem, though? If it is, it's not a matter of boiler size. You need a certain boiler output on the coldest days. You need much less power on warmer days, so either the boiler will modulate its combustion, or it will turn on an off. Now if the turning on and off is a problem for you for some reason, your only feasible approach is to increase the heat capacity of the system, so it can be off for longer periods without the system cooling too much -- and, correspondingly, run longer to get the system back up to the maximum temperature.

    Or... to put it simply, a buffer tank.

    Hi Jamie,

    I'd like to understand further why you think cycling is not an issue. If it isn't then I won't worry about it at all.

    From what I've read excessive cycling of the boiler reduces its life.

    I estimate my 87' of baseboard will put out 18270 BTU/h @120F.

    This is not a terrible match for the low fire of the NKC199, which is 18.9 MBH but I was thinking I can drive the base board at 110F (or lower) on the shoulder months (like now - May in New England has a bunch of 45F days) and avoid those chilly in between days.

    But then the NKC199 will have to cycle, since the output will be 13920 BTU/h which is closer to the min fire of the NKC150.

    Now, if you all tell me that the NKC199, when pumping out 13920 BTU/h will not cycle frequently enough to worry about, I'll just chill and stop losing hair :smiley:

    Thanks!




  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 2,983
    Here is a thought. A separate water heater. If the appliance fails, and it is a Combi that handles both heat and hot water, then you loose both! If you size the Combi for DHW demand and find that the size is 5 times larger (oversized) than needed for space heating then you are 5 times farther from your goal.

    A 199K boiler for DHW and 120K modulating (or smaller off/on) for space heating will provide you with something more reliable and longer lasting.

    There are some reasonable 199K DHW units available and some very small input boilers available. The combination of 2 separate systems may be higher up front, but if you plan on this home being long term, the simple boiler will last two times longer (or more)

    Just the rambling thoughts of an old heating contractor that installed and serviced all kinds of stuff.


    Mr. ED
    Edward Young
    Retired HVAC Contractor from So. Jersey.
    Services first oil burner at age 16
    P/T trainer for EH-CC.org
  • In_New_England
    In_New_England Member Posts: 48

    Here is a thought. A separate water heater. If the appliance fails, and it is a Combi that handles both heat and hot water, then you loose both! If you size the Combi for DHW demand and find that the size is 5 times larger (oversized) than needed for space heating then you are 5 times farther from your goal.

    This is an important consideration. By my calcs, I'm oversized by 1.4x for the shoulder months:

    The NKC199 low fire is 18.9 MBH and, if I can run my baseboard at 110F during those months, it will be putting out 13920 BTU/h.

    I suppose that is close enough?

    I want to try out a combi. I have a friend who has had a combi for a while and is fine with it. We can do without showering in the winter :smiley: but heat is a must.





  • Jamie Hall
    Jamie Hall Member Posts: 18,699
    At some risk of pointing out the obvious, if your boiler is sized correctly for the coldest times, either it must be able to modulate the burner or it will be oversized at any other time. What you are interested in is how far down it will modulate. Using outdoor rest and modulation, you should be able to get a long way into the shoulder months before it starts to cycle on and off.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    In_New_England
  • In_New_England
    In_New_England Member Posts: 48
    edited May 10

    At some risk of pointing out the obvious, if your boiler is sized correctly for the coldest times, either it must be able to modulate the burner or it will be oversized at any other time. What you are interested in is how far down it will modulate. Using outdoor rest and modulation, you should be able to get a long way into the shoulder months before it starts to cycle on and off.

    Hi Jamie, thank you. I believe I have grasped this. My question to you then is this: would a 1.5-ish oversizing lead to a lot of cycling or acceptable cycling?

    I get my 1.5x number by looking at the low fire (18.9 MBH) and my smallest estimate for baseboard output (14000 BTU/h @ 110F)

    Thanks!

  • Hot_water_fan
    Hot_water_fan Member Posts: 436
    What you need to know is your heat loss - that’s not what you average, it’s how much you use on the coldest day (roughly). For example: say your heat loss is 30kbtu/hr on a day with an average temperature of 5 degrees. That means at 35 degrees you’ll need about 15kbtu. If your minimum is 19kbtu, you’ll be cycling whenever the daily average is about 27, which is probably 75%+ of the time. 
    In_New_England
  • In_New_England
    In_New_England Member Posts: 48
    edited May 10

    What you need to know is your heat loss - that’s not what you average, it’s how much you use on the coldest day (roughly). For example: say your heat loss is 30kbtu/hr on a day with an average temperature of 5 degrees. That means at 35 degrees you’ll need about 15kbtu. If your minimum is 19kbtu, you’ll be cycling whenever the daily average is about 27, which is probably 75%+ of the time. 

    Excellent point. Here are my heat loss calcs based on a Weil-McLain worksheet.

    0F 41000
    9F 35000 (Design day for where I am)
    32F 20500
    45F 14500

    Now, that said, I read somewhere that boiler cycling is driven by the radiator heat load.

    In my case, with 87' of copper tube and fin baseboards:

    @ 110F = 160 * 87 = 13920 BTU/h
    @ 120F = 210 * 87 = 18270 BTU/h (chart) 1 GPM
    @ 140F = 320 * 87 = 27840 BTU/h
    @ 180F = 580 * 87 = 50460 BTU/h (Max load)

    With this estimate, the boiler cycling, which is distinct from thermostat cycling, will happen if I drop the baseboard temp to 110F, but not lower. Is this correct?
  • Hot_water_fan
    Hot_water_fan Member Posts: 436
    You’re halfway right. Boiler cycling can also happen if the heat load is lower than the lowest modulation (which will be very often in your case, since the heat loss at 32 is only 20500, which is just above your minimum). The situation you describe is a bit different - if you put 110 degree water into the system, yes you could only output 13920 btu/h, but that’s if the ambient air temperature is around 70. However, if your heat loss is > 13920 at that point in time, the ambient air temperature won’t be 70, it’ll drop and the amount of heat output would increase. 
    In_New_England
  • Steve Minnich
    Steve Minnich Member Posts: 2,716
    If it were my home, I’d use an 055 mod con, with a 40 gallon (?) indirect prioritized, full outdoor reset, and I’d start out aggressive with the reset curve. 
    Steve Minnich
    Minnich Hydronic Consulting & Design, LLC
    [email protected]
    In_New_England
  • Steve Minnich
    Steve Minnich Member Posts: 2,716
    edited May 11
    The sole purpose of doing a heat loss load calculation is so that you size your boiler properly. I can’t think of any good reason to oversize if your goal is energy efficiency and equipment that will last for its intended life. Short cycling not only causes unnecessary wear and tear, it’s also going to affect efficiency due to cycling losses because the burner is going to ramp up to 100% firing rate on most boilers on each new call for heat. 
    Steve Minnich
    Minnich Hydronic Consulting & Design, LLC
    [email protected]
    In_New_EnglandScottSecorEdTheHeaterMan
  • In_New_England
    In_New_England Member Posts: 48
    Based on the discussions here I'll be going with an NKC150 which has a 3.6 GPM hot water rating and with a 13.9 MBH low fire. I believe this should be a good balance.

    I hope I can get this installed for this season, and can report back in Dec about the shoulder month performance and next March about the winter performance and if I regret not going with the NKC199 because of the DHW.

    I might even have it together enough to put together a boilertron like @fentonc

    The most helpful input here was from people who have this series of boilers and a similar hot water load.
  • In_New_England
    In_New_England Member Posts: 48
    If it were my home, I’d use an 055 mod con, with a 40 gallon (?) indirect prioritized, full outdoor reset, and I’d start out aggressive with the reset curve. 
    I think we have a 30gal right now and it has been excellent. The contractor had a proposal for a 45gal tank and I don't know why it was upsized. The lowfire of the proposed boiler was also around 20k.

    Interestingly, the boiler + indirect had a lot of extra rebates, including a large one from the gas company. That struck me as unusual.
  • flat_twin
    flat_twin Member Posts: 317
    If it relieves your worries about not having enough btu for your DHW.... I have our 110 btu Weil Mclain mod con dialed down to 70k btu max fire for heating and DHW. Min fire is 21k btu. Our 40 gallon indirect recovers in about 11 minutes @ 160 degrees supply temp. Shortest heating cycle is 9 minutes in shoulder season. Below 35 degrees it runs nearly continuously.
    Hot_water_fanScottSecor
  • In_New_England
    In_New_England Member Posts: 48
    @flat_twin thank you for that data. I can't make a direct comparison to my house because it's an indirect.

    For heat, what is your heat loss or location and square footage?
  • hot_rod
    hot_rod Member Posts: 16,118
    You need to size to the largest load. In low heat load homes, the DHW becomes the load you need to size to.
    The largest mod con, with the lowest turndown is a good starting point. A small difference between 50 ad 100K mod con low end. I have not seen any that turndown below the 8,000 BTU/ hr. range
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 2,983
    edited May 13
    There is a lot of discussion here over the potentially lowest operating cost of a heating system. Will there be a substantial difference in operating cost and maintenance cost? I recall the words of Gil Carlson stated many times herein, on HH.com "a difference to be a difference it has to make a difference". I'm also thinking "you can try to beat a dead horse, but what difference would it make?"

    Put in the 199K or the 120K or even smaller and the home will still get heaterd and you will still be able to take a shower... so what is the difference?

    Finally, if the smallest is not going to offer sufficient hot water, then do what the Europeans do. Use less hot water. There are shower heads that provide excellent force to feel good and properly clean and rinse using only 1 GPM (or less).

    I remember telling my customers about tankless water heaters, when they first became popular. If you want a tankless in order to save $$$, you have to also adjust how you use hot water. Europeans have been doing it for years. Military personnel also take showers differently that we wasteful Americans have become accustom to.

    Just more ramblings from an old man in a wheelchair.

    Mr.Ed
    Edward Young
    Retired HVAC Contractor from So. Jersey.
    Services first oil burner at age 16
    P/T trainer for EH-CC.org
    In_New_England
  • flat_twin
    flat_twin Member Posts: 317

    @flat_twin thank you for that data. I can't make a direct comparison to my house because it's an indirect.

    For heat, what is your heat loss or location and square footage?

    Central Ohio winters aren't much different from much of New England. Design day is 5 degrees. 1740 sq ft house built in 1835, 58k btu heat loss per Slant Fin app, cast iron rads on a single zone which produce 102k btu @ 180 degree boiler supply water. Our outdoor reset curve is charted below. An extreme cold snap forced me to extend the curve to minus 20. Having far more radiation than is required is the key to low supply temps and staying in the condensing range taking full advantage of the mod con. This is true for any hydronic heating system but my home's specs are like apples and oranges compared to your home's 28k heat loss and fin baseboards.



    In_New_England
  • In_New_England
    In_New_England Member Posts: 48
    edited May 16
    @flat_twin That is a cool plot to see, thank you. One question I had was if anyone ran their baseboard at 100F and I see you do for the shoulder season. We have fin and tube, and my calculations show that I'll hit the low fire of the NKC150 (13.9 MBH) at 110F.

    It'll be fun to adjust the curve to my heat loss estimates and see how it stacks up to reality - the worst that will happen is that I'll be in the basement with my overcoat fat fingering the controls to raise the curve one cold night.


  • flat_twin
    flat_twin Member Posts: 317
    Cast iron radiators and finned baseboards don't respond the same way. Mass has a lot to do with it, and speaking of mass, my rads are fed with large diameter piping which essentially acts like a buffering tank adding to the flywheel effect. I don't know how your finned baseboards will work with sub 100 degree supply water but your heat loss is much less than mine.

    One method I read about here on this website to dial in outdoor reset is to raise the thermostat to say 80 degrees and simply let the ODR dictate the interior temperature. I've done this and could see the effect of solar gain from a sunny day vs inside temps dropping due to a cloudy windy day. When I got to that point I bumped the SWT up just a bit to cover those windy cloudy days and it's worked out very well. Our NG mod con boiler was installed in 2016. I added the indirect water heater the following year. Very happy with the whole system. This replaced a fuel oil fired standard boiler that was damaged in a basement flood.
    In_New_England
  • pedmec
    pedmec Member Posts: 109
    i always set my low boiler temperature no lower than 110 degrees for baseboard on my modcons. just too many callbacks from the older customers and the ladies. can't get the thermal mass of the property warm enough for the chill to be taken out during the shoulder season. the temperature drops too fast from day to night in the boston area that i found myself going back and resetting my curve. its a little warmer than you need but still condensing.
    it really depends on the property. i have a lot of old brick structure apartments. newer well insulated properties would work with lower temps.

    btw, love the graphs.
    In_New_England
  • Canucker
    Canucker Member Posts: 710
    flat_twin said:
    Cast iron radiators and finned baseboards don't respond the same way. Mass has a lot to do with it
    Finned tube heats mostly with convection which is why its output falls off so much below 120 F. The surface area of the radiators is part of the reason they keep heating well at low temperatures
    You can have it good, fast or cheap. Pick two
    In_New_England
  • hot_rod
    hot_rod Member Posts: 16,118
    This Idronics issue will show you how to derate the various emitters, for low temperature operation.

    https://www.caleffi.com/sites/default/files/file/idronics_25_na.pdf
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    In_New_England