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Boiler Sizing

Crissie
Crissie Member Posts: 84
If I need 81,500 BTU's of heat and am looking at a 92% efficient boiler, do I size for a 90,000 BTU boiler (81500 / .92)? If I really get 92% AFUE. I'll probably get more like 86%. Do 90,000 BTU boilers provide more heat than 90K?

Comments

  • Jamie Hall
    Jamie Hall Member Posts: 16,514
    Don't look at the input BTU. They will always also have rated output, and that's what you need to look ar.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    Rich_49Crissie
  • Robert O'Brien
    Robert O'Brien Member Posts: 3,363
    Where did the 81.5K number come from?
    To learn more about this professional, click here to visit their ad in Find A Contractor.
  • Crissie
    Crissie Member Posts: 84
    edited October 13

    Where did the 81.5K number come from?

    I used the Slant Fin app to do a room by room heat loss calculation. It is the total of that. Used 1 degree and the cold design out temp. Not far from Ohare airport, it is actually I think -4 but the app would not accept negatives.
  • JUGHNE
    JUGHNE Member Posts: 8,650
    In case no one else has mentioned it, your coldest design degree day used for the calculation, perhaps 0 or 10 degrees, will only occur a few times of the year.

    And that coldest time is just before sunrise and the temp typically will rise that morning.
    So in choosing a boiler size that is something to keep in mind.
    It should run full throttle on the coldest day of the year and maintain comfort.

    If you feel cold in your house on that extra cold day, say -15 degrees, then go outside for awhile and come back in. It will feel great. ;)

    I believe you were considering baseboard and/or radiant panels.
    Any radiant heat will feel warmer at a lower air temp than baseboard heaters.
    BB is actually mostly hot air convection and although comfy, will not be as good as a panel radiator. IMO.....I have both methods in my house in a little colder climate you have.
    Crissie
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 9,616
    @Crissie

    As @Jamie Hall mentioned

    Now that you have the BTUs needed to heat the home (81500) that is all you need to know. Look at the boiler OUTPUT or NET rating and ignore the input.

    The boiler manufacturer makes a boiler and comes up with a BTU input, then they deduct for the boilers efficiency and they deduct for "piping and pick up factor" 15%) and thy come up with a NET rating which will tell you how much baseboard it will heat in total BTUS

    Working this backwards with a load of 81550 your looking for something with about an input of 108,000 roughly using a 90% boiler which the boiler efficiency will be roughly 85% when heating baseboard
  • Crissie
    Crissie Member Posts: 84
    edited October 14

    @Crissie

    As @Jamie Hall mentioned

    Now that you have the BTUs needed to heat the home (81500) that is all you need to know. Look at the boiler OUTPUT or NET rating and ignore the input.

    The boiler manufacturer makes a boiler and comes up with a BTU input, then they deduct for the boilers efficiency and they deduct for "piping and pick up factor" 15%) and thy come up with a NET rating which will tell you how much baseboard it will heat in total BTUS

    Working this backwards with a load of 81550 your looking for something with about an input of 108,000 roughly using a 90% boiler which the boiler efficiency will be roughly 85% when heating baseboard

    @EBEBRATT-Ed So if am looking at a 84,000 BTU boiler, I should be fine, yes? (Because the 84,000 BTU label is on actual output, yes?) Thx!
  • Hot_water_fan
    Hot_water_fan Member Posts: 137
    Whoa @Crissie, your heat loss is ~40k. Do not install an 84k Btu boiler unless it modulates down to ~8-12k. 
    Rich_49
  • Crissie
    Crissie Member Posts: 84

    Whoa @Crissie, your heat loss is ~40k. Do not install an 84k Btu boiler unless it modulates down to ~8-12k. 

    Wouldn't and outdoor temp reset address that? I was looking at the Weil McCann GV90
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 9,616
    @Crissie

    For whatever boiler you pick you need to look at the AHRI rating which = the maximum amount of radiation you can install with that boiler.

    See the attached brochure.

    A GV-90-4 will heat 84,000 btu of baseboard (max) which is perfect if your heat loss is 81,500.
    Crissie
  • Hot_water_fan
    Hot_water_fan Member Posts: 137
    Wouldn't and outdoor temp reset address that? I was looking at the Weil McCann GV90
    Nope, outdoor reset lowers water temperature, but does not lower boiler output (it will lower baseboard output). That boiler is too big and doesn’t modulate. 
    CrissieRich_49
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 9,616
    @Hot_water_fan

    From what I am getting her heat loss is 81500 and the GV90-4 has a net rating of 84000. I don't see an issue.

    Maybe I am misunderstanding something
  • Hot_water_fan
    Hot_water_fan Member Posts: 137
    @EBEBRATT-Ed there’s a couple different posts related to this question: the fuel usage shows heat loss of 40ish, the slant-fin shows 81500. The old furnace was 72 with poorly installed ducts. 
    Rich_49
  • Crissie
    Crissie Member Posts: 84

    @EBEBRATT-Ed there’s a couple different posts related to this question: the fuel usage shows heat loss of 40ish, the slant-fin shows 81500. The old furnace was 72 with poorly installed ducts. 

    Fuel usage showed an annual average heat loss of 46K BTU. From what I gather, that is an average and would not support extreme temps (below zero). My current furnace is 115K BTU and is operating at 88% efficiency, rated for 96%. Way oversized. Old furnace was 90K BTU. House is very tight.
    This is sort of confusing. If the Manual J = around 81,000 BTU, shouldn't I use that to size the boiler?
  • Hot_water_fan
    Hot_water_fan Member Posts: 137
    The fuel usage is the heat loss at below design temperature (I used -15) it is not an average. It is how much you’d use at -15. The manual J has a lot of moving parts and assumptions. The fuel usage takes all of that and throws it out the window. It describes what is. 
    Crissie
  • KC_Jones
    KC_Jones Member Posts: 4,686
    Crissie said:

    @EBEBRATT-Ed there’s a couple different posts related to this question: the fuel usage shows heat loss of 40ish, the slant-fin shows 81500. The old furnace was 72 with poorly installed ducts. 

    Fuel usage showed an annual average heat loss of 46K BTU. From what I gather, that is an average and would not support extreme temps (below zero). My current furnace is 115K BTU and is operating at 88% efficiency, rated for 96%. Way oversized. Old furnace was 90K BTU. House is very tight.
    This is sort of confusing. If the Manual J = around 81,000 BTU, shouldn't I use that to size the boiler?
    What did you use for the infiltration rate on the rooms? I saw what you did on the first one, and IMHO I think that might be high. I dropped the infiltration on the one I checked for you and it reduced the load ~17%, if we apply that to your 81k, that would drop it to ~67k. I only mention this because you had a blower door test and they told you the house was tighter than average, but the number you plugged in (for the first room) would indicated the room is average or below average.
    2014 Weil Mclain EG-40
    EcoSteam ES-20 Advanced Boiler Control
    Boiler pictures updated 2/21/15
  • Crissie
    Crissie Member Posts: 84

    The fuel usage is the heat loss at below design temperature (I used -15) it is not an average. It is how much you’d use at -15. The manual J has a lot of moving parts and assumptions. The fuel usage takes all of that and throws it out the window. It describes what is. 
    @Hot_water_fan I think the variance is my current furnace does not adequately heat my lower level. Split house. At -15 degrees, my main level would be 70 degrees, lower level would ranges from 62-67 degrees, upper level might be in the 73 degree range, and furnace would cycle non-stop. So I never got the heat I needed at the temps of -15, or even at 5 degrees.

  • KC_Jones
    KC_Jones Member Posts: 4,686
    Crissie said:

    The fuel usage is the heat loss at below design temperature (I used -15) it is not an average. It is how much you’d use at -15. The manual J has a lot of moving parts and assumptions. The fuel usage takes all of that and throws it out the window. It describes what is. 
    @Hot_water_fan I think the variance is my current furnace does not adequately heat my lower level. Split house. At -15 degrees, my main level would be 70 degrees, lower level would ranges from 62-67 degrees, upper level might be in the 73 degree range, and furnace would cycle non-stop. So I never got the heat I needed at the temps of -15, or even at 5 degrees.



    You just perfectly described a distribution problem not a size problem. The size of your equipment basically has nothing to do with the issue you described. This is why system design is so important.
    2014 Weil Mclain EG-40
    EcoSteam ES-20 Advanced Boiler Control
    Boiler pictures updated 2/21/15
    Rich_49
  • Hot_water_fan
    Hot_water_fan Member Posts: 137
    At -15 degrees, my main level would be 70 degrees, lower level would ranges from 62-67 degrees, upper level might be in the 73 degree range, and furnace would cycle non-stop. So I never got the heat I needed at the temps of -15, or even at 5 degrees.


    That's a separate distribution issue. Especially if the highest level is greater than set point, there's not 40k Btu of variance. The manual J calcs can be great, but they're a lot of effort and it's easy to make a mistake.
  • Crissie
    Crissie Member Posts: 84
    You just perfectly described a distribution problem not a size problem. The size of your equipment basically has nothing to do with the issue you described. This is why system design is so important.

    @KC_Jones @Hot_water_fan Yes, it is my entire reason for looking at boiler hot water heat. I do not have enough room to run the needed ductwork from the furnace throughout the lower level. I can barely fit the ductwork needed for the upper level. The entire system is a mess. Still won't the fact I never completely heated the lower level impact sizing on actual fuel use?
    Regarding the Manual J / Slant Fin app, I still need to do and use that on a room by room basis or I will never get it right.

  • Hot_water_fan
    Hot_water_fan Member Posts: 137
    Still won't the fact I never completely heated the lower level impact sizing on actual fuel use?


    Yes, but not significantly. It's also offset by the upper floor overheating. The room by room is important, but you can get around that through zoning and piping, one of hydronic's heatings greatest strengths. You have a well insulated, tight house! It's a good thing.

  • Steve Minnich
    Steve Minnich Member Posts: 2,659

    Wouldn't and outdoor temp reset address that? I was looking at the Weil McCann GV90
    Nope, outdoor reset lowers water temperature, but does not lower boiler output (it will lower baseboard output). That boiler is too big and doesn’t modulate. 
    If the target boiler supply water temperature is lowered via ODR, it will deliver less BTU/h (output), which gets passed along to the heat emitters which then do the same.
    Steve Minnich
    Minnich Hydronic Consulting & Design, LLC
    [email protected]
  • Hot_water_fan
    Hot_water_fan Member Posts: 137
    If the target boiler supply water temperature is lowered via ODR, it will deliver less BTU/h (output), which gets passed along to the heat emitters which then do the same.


    The heat emitters will emit less heat at lower temps, but why would a non-modulating boiler output fewer BTUs? That would imply a radiant floor run at a lower temp needs a larger boiler than a system using just baseboard for the same heat loss.
  • Steve Minnich
    Steve Minnich Member Posts: 2,659
    If you have a cast iron boiler using an ODR control that fires to meet a 180* Setpoint at design conditions and then fires to meet a Setpoint at 160* in warmer weather, which example is going to provide more BTU/h, output? 
    Steve Minnich
    Minnich Hydronic Consulting & Design, LLC
    [email protected]
  • Hot_water_fan
    Hot_water_fan Member Posts: 137
    Boiler output? The exact same. Say it’s oil: the GPH doesn’t change based on the set point. Imagine a house with baseboards needing 180 degree water at design day. If they were switched for a radiant floor needing 90 degree water, you would double the boiler size? 
  • Steve Minnich
    Steve Minnich Member Posts: 2,659
    You can vary the output in other ways than firing rate. You can vary flow, setpoint temperature, etc. 
    Steve Minnich
    Minnich Hydronic Consulting & Design, LLC
    [email protected]
  • Crissie
    Crissie Member Posts: 84
    edited October 14
    It seems there might be some different opinions even among the pros. If my total heat loss was really 81,000, then the 84,000 Weil McCann GV90 will work. If my heat loss is really more like 55,000, are you all saying that the 84K WM GV90 will not work because it doesn't modulate?
  • Hot_water_fan
    Hot_water_fan Member Posts: 137
    You can vary the output in other ways than firing rate. You can vary flow, setpoint temperature, etc.


    The boiler output will not change, it will be on for the same amount of time. The firing rate is the lever to control boiler output (unless it's a condensing boiler, in which output will increase if the ODR is asking for temps in the condensing range). Why would they make modulating boilers with ODR if you could change the output using just ODR? They could also make every boiler the same size if ODR changed output.

    However, ODR will reduce heat loss from the boiler itself through by operating at lower temps. That's a big deal!

    It seems there might be some different opinions even among the pros. If my total heat loss was really 81,000, then the 84,000 Weil McCann GV90 will work. If my heat loss is really more like 55,000, are you all saying that the 84K WM GV90 will not work because it doesn't modulate?


    It won't work *well*. That said, the GV90 one size smaller would work better, especially with thermal purge and/or a buffer tank.
  • Steve Minnich
    Steve Minnich Member Posts: 2,659
    I think we’re making the same point. I’m not talking about what you will see on an AHRI certificate. I’m talking about the amount of heat (BTU/h) being delivered to the system by the boiler. 180 degree water will absolutely delivered more heat (output) than 160* water. There’s no way around that. 
    Steve Minnich
    Minnich Hydronic Consulting & Design, LLC
    [email protected]
    Hot_water_fanRich_49
  • pecmsg
    pecmsg Member Posts: 2,200
    Crissie said:
    It seems there might be some different opinions even among the pros. If my total heat loss was really 81,000, then the 84,000 Weil McCann GV90 will work. If my heat loss is really more like 55,000, are you all saying that the 84K WM GV90 will not work because it doesn't modulate?
    You need roughly 15 minutes of run time for the boiler to stabilize and reach its max efficiency. By over sizing it will only reach the max efficiency 2 or 3 days a rear. Less then 1%. All other days it WILL short cycle. Waisting if fuel and shorting it’s life. 
  • sunlight33
    sunlight33 Member Posts: 352
    edited October 14
    pecmsg said:

    All other days it WILL short cycle. Waisting if fuel and shorting it’s life. 

    This. That's why there are modulating boilers, lower end output to match the heating load in spring/fall to minimize short cycling, and higher end exceeds the worse case in the winter, leave the rest to ODR and electronic controls.
  • Rich_49
    Rich_49 Member Posts: 2,589
    edited October 14
    Crissie is very frustrated as she has listened to so called professionals on different occasions and been disappointed . That said , now she is here trying to get help in something she has been researching for about 2 months . She has been looking for something that can vent in plastic but has been steered to believe she needs what we all consider Hi temp for baseboard that she was thinking about . Apparently too many PROFESSIONALS ( LMAO ) believe that you require hot water for baseboard and have no clue that when there is DeltaT there will absolutely be heat transfer . She also has looked at panel rads now . I spoke with her and offered assistance but realized quickly that I do not want to be involved in fighting much misinformation in my quest to help her .

    Her heat loss is very low as the building is well insulated and based on her blower door number her home has an approximate point 2 ACH Nat , you heard that right .2 . She needs a comprehensive audit of the house to identify and fix a couple possible shell issues , she is taking steps toward those I believe and have furnished her with the contact information for 2 separate HVAC 2.0 process guys in her immediate area , these are guys who will not mislead her , for certain .

    She has been able to come up with the GV90+ as you all know not wanting a mod con for reasons I cannot grasp besides contractors have told her they are a maintenance / longevity nightmare . I suggest a room by room heat loss done by a competent professional and not the slant fin app . ACH numbers must be taken into account , her home is more than likely in the 40K range if not less . She can purchase an HTP Pioneer for just a couple hundred more dollars than the GV90 and have 55 gallons of mass , ODR , modulating capability and very few headaches . That can be connected without P/S piping and run on constant circulation with panel rads w/ bypass valves and TRVs from a manifold and she'd be done .

    She will certainly need some ducts for A/C and ventilation . That is where the guys I referred would come in . Winter aint gettin no further away Crissie , You have my number of you'd like , the guys I referred also are perfectly capable of doing the hydronic work . Too bad our friend Steve Minnich is no longer available , he could also design for you I believe . Steve ?


    https://www.htproducts.com/pioneer.html



    You didn't get what you didn't pay for and it will never be what you thought it would .
    Langans Plumbing & Heating LLC
    732-751-1560
    Serving most of New Jersey, Eastern Pa .
    Consultation, Design & Installation anywhere
    Rich McGrath 732-581-3833
  • Crissie
    Crissie Member Posts: 84
    edited October 15
    Rich_49 said:


    She will certainly need some ducts for A/C and ventilation . That is where the guys I referred would come in . Winter aint gettin no further away Crissie , You have my number of you'd like , the guys I referred also are perfectly capable of doing the hydronic work . Too bad our friend Steve Minnich is no longer available , he could also design for you I believe . Steve ?

    Rich, I did call Adam, Ralph called me back. I explained I needed ductwork, he is going to get back to me. I have had both of those guys in my home at some point 4-5 yrs ago.
  • Crissie
    Crissie Member Posts: 84
    @Rich_49 My building 'Shell'. Would this be impacted by the poly vapor we put over rockwool exterior insulation, per manufacturer's spec? Otherwise nearly every exterior wall was gutted down to studs and upgraded insulation. I don't think it is the poly, I think it is crap in ductwork.