NEW! Faster Search Option


How to size shop heater

SteveC.'s picture

I have a 22x20 shop with 9 foot ceilings off of an unheated garage. I would consider my insulation to be average at this point. I am planing on using a propane direct vent unit from Modine (Hot Dawg) or an Empire unit for heat. I have used 7 online BTU calculators and get estimates of 22K to 70K BTU, with most in the range of the mid 20Ks. I am looking for some guidance from people with real world experience. I live on the coast of Maine and plan to keep the shop at 45 duirng the nights with 3 or 4 days a week of full usage during the winter. Is is better to have somethign too big (say 45K BTU) rather  than something that is just meeting the BTU estimates (say 30K BTU)? Any advice woudl be welcome.



PSeverin's picture

Be Green (post #157441, reply #1 of 6)

I think it safe to say that you don't need to oversize the unit and doing so might be a waste.   On the other hand, asking someone who's business it is, what they would recommend.  I would think any of your local heating companies who sell the equipment (whether gas, propane, oil or electric)  would tell you what capacity unit you would need.   And they, unlike the web, should have knowledge and experience with local conditions and rates.

In the upper midwest, using off-peak power it is now cheaper to use electricity than propane to heat a house in the country.    


JohnWW's picture

Steve, How average is (post #157441, reply #2 of 6)


How average is average on the insulation?  A mouse eaten 4 inches of fiberglass is not the same as a well installed 6".   Ceiling insulation is critical, a foot or more is considered the minimum these days.  Poor air sealing, especially in the ceiling, will draw off heat as fast as you can pay for it.  

A small heater, even if theoretically adequate, will take forever to heat up a shop that has been cold for several days.  Having to go out to start the heat at 6:00 AM so you can start work at 11:00 cuts down on spontaneity or starting rush jobs.  

Most propane dealers would rather sell you a big heater rather than a small one, and will be using the same widely diverging calculators you have been using, so I would take their suggestions with a grain of salt also.

Bottom line is go larger rather than small.  A  larger heater will just turn itself off when the shop gets warm, and use no more fuel, than a small one that can't get the shop warm on the really cold days.  

This is coming from a Vermonter that has had some pretty hard to heat shops.

John White Shop Manager for FWW Magazine, 1998 to 2007

SteveC.'s picture

Insulation details (post #157441, reply #3 of 6)

I called my insulation "average". What I have is R21 in the walls with poly between the insulation and the sheetrock. I have R38 in the 8' wide ceiling areas that  are not floored over where the useable space is upstairs and R21 plus 1" rigid foam board at R7 in the 12' wide space where the floor is. I sealed all of the joints in the solid foam board, between the foam and the walls and around all windows and doors with tape so I think it is pretty tight.  I agree that I do not want to get just the "minimum" sized heater inc ase it is cold for a long spell. Thanks for your input.



PSeverin's picture

Sometimes sales people actually know a bit more than we do. (post #157441, reply #4 of 6)


As someone who relies on local mechanical sub contractors to design and install mechanical system in houses and commercial structures, I have faith that they have more knowledge about the systems they install than I do.   A good mechanical contractor will not generally install something miss sized and inappropriate for the application.  There have limited options for heater sizing, and if you fall within the capacity of the heater, I can't believe that doubling the size of the unit does anything more than waste materials.   When I suggested being green, I meant that sometimes it is wasteful to squander resources unnecessarily.  

Also, when I suggested using electricity to heat, it was because programs in Minnesota and Wisconsin that make heating with electricity cheaper using off peak power than propane.  I understand that the cost per KWH is cheaper here than in New England, but has anybody looked to see how close the cost of electric heat is getting to propane where you live?  I mean there are certain hazards in sawdust and propane flames too.

SteveC.'s picture

Final recomendation (post #157441, reply #5 of 6)

Thanks for all of your input. I had a heating guy come over and based on locations for the tank (I didn't realize there were so many restrictions for tank and heater locations) and my desire not to take up too much wall space, he recommended a Hot Dawg 30K unit that will vent and draw in outside air for combustion. Now I am on to air filtration.



roc's picture

How to stay warm in the shop on cold winter days : (post #157441, reply #6 of 6)

I usually go by what I am going to do in the shop on that day :

  • Sit and do detail work = two pair thermal under wear, down coat, snow mobile boots, balaclava, couple of scarves etc.
  • Stand and finish plane or medium sawing, or time at the white board designing = one pair  thermal under wear, long sleeve mock turtle neck shirt ( the exact same brand  Steve Jobs wears; I find that important to my creativity and general mood ) , Birkenstock shoes/no socks of course ( none of that other stuff ).
  • Scrub plane and flatten = just shorts and a T - shirt
  • Resaw by hand = no cloths at all and a fan going ( I don't do that much now thanks to Laguna ).

Thanks for listening.

Carry on.



Give me six hours to chop down a tree and I will spend the first four sharpening the axe. Abraham Lincoln ( 54° shaves )