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Garage heater sizing

mattmia2 Member Posts: 3,991
I have a garage that I want to heat.  I have looked

at the heat loss in a couple ways and get very different answers.  When I

look at volume and rise though various heater manufacturers', the

calculations say that i need about 50 mbtu/hr.  When I put it in web

calculators that are more complicated heat loss calculators that use

wall, roof, and floor construction and window/door area, I get a

something around 100 mbtu/hr.

The garage is 16'w x 24' deep x 7.5'h.  The peak of the roof

is 5.5' above the walls and it is a gable roof with wood walls at the

gables.  The garage walls themselves are cinderblock.  It is a single

car garage door, it has an uninsulated wood side door and 2 3'x4' steel

single pane windows.

It is in ann arbor michigan so the max rise is ~70 degrees f.

The volume is 3936 cubic feet.


calculation is correct?  Is the calculation from the heater sites only

calculating what is needed to heat the air and not accounting for the

heat that contact and radiation to the outer shell materials is removing

from the air?




  • icesailor
    icesailor Member Posts: 7,265

    That's not really enough information.

    Are you going to live in this garage or park your car.

    Is the garage connected to a heated building or does it stand alone?

    Garages and stand alone structures like garages are complicated because there are usually code issues to cover,
  • mattmia2
    mattmia2 Member Posts: 3,991
    Additional info

    It is a detached garage.  Primarily I want to park a car in there.  I want to install a unit heater to heat it intermittently when I need to either work on the car or do outdoor projects in the winter.

    I did not think of a gas supply to an outbuilding being a separate service in the way an electrical feeder is unless it is under the single circuit exception.


  • MikeG
    MikeG Member Posts: 169
    It's a heat sink

    Block walls, single pane glass with steel frames, all exposed exterior walls, it's probably going to suck the heat up and out especially in that location.  If you are only looking at short term heatiing, go with some type of what I call spot heating.  Use a LP infrared heater off a 20# tank to heat you while you work.  If it's 40 degrees you might get some actual building heat.  Use a salamander type kerosene heater, they have blowers on them if you have electric.  They can really heat up the air in a short order.  If you have 240V, go with a good sized electric heater with a long cord and heat you while you work.  I have a 30'x40' x10' high Morton pole building, concrete floor with 1" extruded foam 2' wide under the floor around the perimeter and also about 1.5' deep horizontal outside to help with frost intrusion.  12" of cellulose in the ceiling, 6" of fiberglass in the walls, OSB on the walls metal ceiling, 2 garage doors, entry door and three windows.  Located in NW Ohio.  Up untill we hit the -18 in Jan, and most years it does not freeze in there.  But this year due to extended sub zero, -11 last night I had to plug in a 4000 watt 240V heater to keep it at 40-45 degrees.  If I'm working out there I just put it closer to me.  If I had been smarter back in 97 when I had it built I would have insullated the whole floor and put in some radiant floor.  I tried using the kerosene heater but they put off too much moisture.  In the spring when it warms up nothing sweats in there, and it stays cool all summer.  If you have electric and big enough service the easiest way to go.  Right now for me electic is cheaper per 1 million BTUs than burning LPG in my boiler.  Just my thoughts.
  • Jamie Hall
    Jamie Hall Member Posts: 17,132
    Methods forr determining heater size

    which rely only on the volume of the space and the desired rise make a number of assumptions as to the construction of the space which they don't bother to mention.  And which your building doesn't come close to meeting.

    The methods which take into account wall construction and area are considerably more accurate; therefore the second estimate is more likely to be correct.

    But for such a use, I'd very much agree with MikeG as to how to heat the place.  Spot heaters, and used only when you need them.  The only downside to spot heating -- no matter whether it's LP or electric -- is the need to be very careful about fire, particularly if you are working on vehicles.  A heater is an ignition source; treat it accordingly.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • HC7151
    HC7151 Member Posts: 1

    You are looking at 384 sq. ft. even at 75 btu's a sq. ft. you are under 30,000 btu's.

    I would get a get no bigger than a 45,000 btu unit heater. Quick heat , but not over kill.  Insulate your walls with r-11 if 2x4.  some r-19 on top of the drywall between the rafters if it isn't their already and you are good to go.  You probably don't need it 70 degrees in there at -20. But that garage will heat up in a half hour in February.  Check out your fleet supply store for one of these units.  Don't buy used goods.  Get a 1/2" gas line brought in & enjoy!
  • mattmia2
    mattmia2 Member Posts: 3,991

    So where do you get 75 btu/hr/ft^2 from, especially when we are dealing with a 3 dimensional space?


  • mattmia2
    mattmia2 Member Posts: 3,991
    gas vs electric vs lpg vs oil and other stuff

    I buried polyethylene (and tracer) to the garage last summer when i was

    installing a gas grill.  I just need to run another line from the meter

    to get gas out to the garage.   If I use electricity i'm looking at

    ~100a at 240v so natural gas is more feasible.  I'm mot really using it

    enough that the cost of the fuel is a major factor other than I don't

    want to have to go out and get it, i want it plumbed in.  I have very

    limited space so reddy heater type heaters don't really have a safe

    place to be, not to mention the lack of venting.  I have more room

    hanging a unit heater from the collar ties. 

    With 12000 btu/hr

    of space heaters i got a rise from about 30 outdoor to 40 indoor

    fahrenheit so estimating with that method I would need about 85,000

    btu/hr.  It doesn't seem that it should be linear but some research

    shows that it more or less is.


  • icesailor
    icesailor Member Posts: 7,265
    3 dimensional spaces:

    I never used any Sq. Ft. method. I always used the IBR Methods. For spaces like a garage that had no mid walls, I always divided the space into some short bus number (divide in half) and calculated the two. Then added the two together. For example, the tables only cover three outside walls, not four. For exposure purposes. Now, all walls only have three walls.

    Do what you think. But if you insulate and sheetrock the walls and ceilings, you will have a really nice workspace. You can probably use a through the wall gas heater like  Rannai that comes on when you want it to, and take up little space. They really heat well. They're quiet, and vent through the wall.

    If it's February, you're still not going out there to futz around in shorts and a tee shirt. You'll be well dressed for the trips outside. If you put up a ceiling heater, it will circulate hot air at the ceiling and the floor will be cold. A wall heater near the floor will be heating the cold air near the floor and the hotter air will rise to the top.

    Look seriously to the direct vented wall heaters. They look nice and work well. I installed quite a few where I used to work.

    You won't believe how noisy and what gas hogs those gas ceiling fan heaters are.
  • mattmia2
    mattmia2 Member Posts: 3,991
    So I went for about 50,000 btu/hr from the heat loss clacs(with a hydronic unit heater). That is what the math says anyhow, I need a way to measure flow to confirm, but it should be in that ball park. I will need to measure and calculate and see if slowing down some flow would increase xfer in the unit heater or the hx.
    It is a little undersized for the recovery rate i would like(assuming the performance is close to that which was calculated), but it is clear the full heat loss calculation was correct and the calculators from those selling "garage heaters" aren't even close and are grossly undersizing the heat loss. According to them i should be heating a 5 car garage on a mcmansion when in fact i am barely heating a 1.5 car garage.
    EBEBRATT-Ed Member Posts: 10,356
    50,000 should do it you don't need to heat it to 70 deg do you? Any insulation in the ceiling?
  • mattmia2
    mattmia2 Member Posts: 3,991
    edited November 2019
    Cinderblock walls and the roof is just the rough framing. i pasted screenshots of the hx transfer calculations in to the spreadsheet and they have turned in to broken links so i no longer know what i calculated it at. Once i figure out how to measure differential pressures without spending $1000 I'll have a better idea of what is going on. It will get to design temp which was 60 f but it might take several hours when outdoor is at design temp.
  • GroundUp
    GroundUp Member Posts: 1,283
    Why not insulate it? 10k should heat a building that size to 70 all day every day. My shop and garage are both 4x the size and have heat losses of 36k and 17k at -20F. The shop has 14ft sidewalls and a whole lot of doors and windows
  • mattmia2
    mattmia2 Member Posts: 3,991
    It isn't worth it to insulate it for the couple days a month that I will want to heat it especially when you account for all the assorted nastiness that rodents and larger animals and even water getting in to the insulation and the materials to support the insulation will cause.
  • pecmsg
    pecmsg Member Posts: 2,345
    > @mattmia2 said:
    > It isn't worth it to insulate it for the couple days a month that I will want to heat it especially when you account for all the assorted nastiness that rodents and larger animals and even water getting in to the insulation and the materials to support the insulation will cause.

    When you see the fuel usage you might reconsider.