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Forced Air furnace sizing

QuintonS
QuintonS Member Posts: 34
in THE MAIN WALL

I did an in depth heat load calc. It said that without wall insulation that I need a 100k btu furnace for my house. But once I add insulation I’d only need a 70k Btu furnace. I plan on adding insulation next year to the walls. Which size furnace would be worth getting?

Comments

  • Hot_water_fan
    Hot_water_fan Member Posts: 2,162

    https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler


    is the house built? If so, use the fuel usage.

  • pecmsg
    pecmsg Member Posts: 5,848

    100K is large

    Where is this house and how many sq ft?

    mattmia2
  • LRCCBJ
    LRCCBJ Member Posts: 969

     I’d only need a 70k Btu furnace.

    This is not a difficult question.

  • ILikeEmOlder
    ILikeEmOlder Member Posts: 49

    Total heat load seems high. Also, a 30K BTU load just for uninsulated walls seems unbelievably high.

    If 70K BTU is the actual load at design then consider an 80K BTU modulating furnace.

    My bet is that the load after insulating the walls (with attention to detail) is low enough where a 60K BTU modulating furnace will get the job done without issue.

    Existing ductwork is usually too restrictive for larger equipment, so “downsizing” of equipment is super helpful in this respect (and others).

    Swinging hammers and fitting pipe…bringing the dream to life

    QuintonS
  • QuintonS
    QuintonS Member Posts: 34
    https://forum.heatinghelp.com/discussion/comment/1846231#Comment_1846231

    Yes it's built. Here's the info I gathered based on that article:

    Dec 21st to Jan 22nd usage: 600CCF = 623 Therms based on an online calculator

    Boiler Efficiency: 82.2%

    65F Heating degree days: 1,308.2

    60F Heating degree days: 1,143.2

    Math:

    623 * 82.2% * 100,000 = 51,210,600 BTU

    60F:

    51,210,600 BTU / 1,143.2 degree days = 44,796 BTU per degree-day

    44,796 BTU per degree-day / 24 hours * 45F = 83,991 BTU/HR

    ASHRAE sizing factor 1.4 x 83,991 BTU/HR = 117,587 BTU/HR

    65F:

    51,210,600 BTU / 1,308.2 degree days = 39,145 BTU per degree-day

    39,145 BTU per degree-day / 24 hours * 45F = 73,397 BTU/HR

    ASHRAE sizing factor 1.4 x 73,397 BTU/HR = 102,756 BTU/HR

    Is my math correct? So you think I should get something around 100,000 BTU's still? Even though I am insulating walls soon?

  • QuintonS
    QuintonS Member Posts: 34
    edited March 3

    Northeast PA and 2,500 SQFT

  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 17,454

    70k for 2500 sq feet sound right.

    QuintonS

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