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Boiler size for Wirsbo garage heat 1000sq ft

amons Member Posts: 3

I am looking for advise on sizing a boiler for wirsbo garage heat. 

1000sq ft attached garage
r-19 garage doors. 16’, 9’, 7’. All 7’ high
r-60 ceiling r-20 walls. 
Well sealed. 
Minneapolis MN
this is our only garage so the doors will open a few times per day. 

looking to heat air temp to 50-55 degrees

there are 5 loops about 160 ft each 1/2” pex.
boiler will only be used for the floor heat. 

What variables am I missing? Google searches are putting the btu requirements all over the map. 

Based on my research it looks like I should be purchasing a modulation condensing boiler. Any reason I shouldn’t?



  • amons
    amons Member Posts: 3
    Forgot to mention. There is 2” foam under the slab and around the entire foundation 
  • Zman
    Zman Member Posts: 6,530
    In theory, you do a heat loss calc for the building and figure out the boiler size.
    In reality, your heat loss will be less than any boiler on the market.
    A small Mod/Con would be a good choice.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • mattmia2
    mattmia2 Member Posts: 2,842
    Probably want to run glycol in it in case it isn't heated at some point for some reason.
  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,256
    I know you said the boiler will only heat the attached garage....but what heats the house now? Forced air I presume.

    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
  • amons
    amons Member Posts: 3
    This was an addition to the existing home. We tore off the existing garage and slab. Added a mud room and reconfigured the garage and made it way larger. The home has forced air. I am sure I will want to run Pex for heat everywhere once I have warm toes in the garage but I will have to cross that bridge another day. :)

    I will run the glycol for sure. 

    Sadly my neighbor did something similar and had a 120btu boiler installed. I was think that
    must be way too large based on what I have read. 
  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,256
    Complete stab in yue dimly lit room guess is your heatloss will be in the 15,000-20,000 btu/hr range. May even be lower. Smallest mod/con will cover the load without issue. If you do decide to heat the whole house with water someday, you will likely still have enough capacity left from this mod/con to do that as well. 

    Again, to make the real determination we would need to do a proper heatloss calc on the house etc. 
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
  • GroundUp
    GroundUp Member Posts: 1,163
    I'm in Minneapolis (Loring Park area) every day and would be happy to perform a heat loss calc for you if you'd like, but as these fellas said, any mod/con on the market today would be plenty. I wouldn't guess that load to be over 15-20k given the info you provided. I could even set you up with a boiler, feel free to send a PM if you want to discuss options.
  • STEVEusaPA
    STEVEusaPA Member Posts: 4,907
    Unless you want to add some snow melt too
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