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Manual J and semi-conditioned spaces

gmcinnesgmcinnes Posts: 45Member
Hi all:

I have a 3 bay garage, with 2 unit heaters. Above that there are two radiator heated bonus rooms that are rarely used. They are separated from the main house by a breezeway, and are on two separate zones, respectively.

I only heat the garage to as low as I can get it to keep the pipes from freezing, but the Manual-J that was done has the inside dry bulb temp set to 70F, resulting in a delta of 68F.

Is there provision in code to allow a lower inside target temp for semi occupied spaces like garages? Surely they're not expected to be constantly heated to the same temp as the house, are they?

If so, it means I need to oversize the boiler output by at least 40K to account for that space.

Thanks in advance


  • Jamie HallJamie Hall Posts: 10,302Member
    Manual J isn't code as such. It's a way of calculating heat loss. You may find it easier, though, to use Slant/Fin's calculator -- just set the interior temperature to what you need.

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • EBEBRATT-EdEBEBRATT-Ed Posts: 5,641Member
    The garage if it needs to be heated to prevent freezing you would select a temperature for the garage to accomidate that 40 deg or whatever margin of error to keep it above freezing you can tolerate.

    The bonus rooms if insulated and finished off should be capable of being heated to your normal inside design temp 70 degrees in not just for resale value of your home
  • gmcinnesgmcinnes Posts: 45Member
    @Jamie Hall @EBEBRATT-Ed

    You guys are both amazing. Thanks for your help. (You helped me in the spring with issues with my oversized HB Smith boiler. Unfortunately my hand has been forced on a new boiler now - a section cracked a couple of days ago)

    I assume there should be no problems if I submit a Manual J to the inspector with design temp of 40 F for the garage? They don't typically expect it to be 70F, do they?
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