Need advice installing a propane shop furnace

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Seems likely to me that you're going to get substantial heat stratification - heat rises, you're on the floor - so installing some fans may be helpful as well. Could be that enough cold happens at the roof that the air in the place is similar from man-height to higher; also a fan moving air while keeping the temperature more consistent it may make it feel cooler because of the air moving past skin. That's something that can be added later if you find it to be the case.

Found this, it may be helpful: Heater Size BTU Calculator - Heat Wagon Portable and Construction Heaters

For 42x60x14, assuming the building is "fairly tight" (heat retention, ranging from Very Loose/Loose/Fairly Loose/Fairly Tight/Tight/Very Tight) 60° inside desired temp and 20° outside temp, we get 80K.
If you want 50°/0° with a "fairly loose" building, it says 128K.

You're going to have to decide how tight you think the building is.

 

[5030]

I have ceiling fans (attached to the lower chord on the trusses) in both shops to move the air downward and to provide circulation as heat rises. I just bought 4 blade generic fans, nothing fancy. I believe I got them at Menards for under 50 bucks each and they are 3 speed. They stay on low all the time.

 

[sixdogs]

I'd ask Morton. Sorry.

All the best, Peter

Nice idea but been there and done that. Morton won't offer any advice and that's corporate as in the field. It's their policy not to offer any suggestions or advice because of the variables involved. They say to defer to the HVAC people and I'm working on that now. It seems Morton would offer some commentary?

 

[sixdogs]

Seems likely to me that you're going to get substantial heat stratification - heat rises, you're on the floor - so installing some fans may be helpful as well. Could be that enough cold happens at the roof that the air in the place is similar from man-height to higher; also a fan moving air while keeping the temperature more consistent it may make it feel cooler because of the air moving past skin. That's something that can be added later if you find it to be the case.

Found this, it may be helpful: Heater Size BTU Calculator - Heat Wagon Portable and Construction Heaters

For 42x60x14, assuming the building is "fairly tight" (heat retention, ranging from Very Loose/Loose/Fairly Loose/Fairly Tight/Tight/Very Tight) 60° inside desired temp and 20° outside temp, we get 80K.
If you want 50°/0° with a "fairly loose" building, it says 128K.

You're going to have to decide how tight you think the building is.

That's a good guesstimate site. Thank you. I have two sliding doors 18' wide by 12' high. There is one inch of that lite fibre board insulation in them and rubber casketing around the doors except at the bottom.

There is a gap there but I'll plug that with either 2"x2" diced of foam board and/or fiberglass. One of the doors I'll seal tighter and just won't use in the cold months.

Using your calculator, 125,000 is all that I would need. 150,000 would be too much.

 

[sixdogs]

I have ceiling fans (attached to the lower chord on the trusses) in both shops to move the air downward and to provide circulation as heat rises. I just bought 4 blade generic fans, nothing fancy. I believe I got them at Menards for under 50 bucks each and they are 3 speed. They stay on low all the time.

Fans of some sort do sound like a good idea. I have a few of the barn-type fans so that would work. Anything to move the air. I'm starting to get this figured out and feel like I'm making progress . Thank you all.

 

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That's a good guesstimate site. Thank you. I have two sliding doors 18' wide by 12' high. There is one inch of that lite fibre board insulation in them and rubber casketing around the doors except at the bottom.

There is a gap there but I'll plug that with either 2"x2" diced of foam board and/or fiberglass. One of the doors I'll seal tighter and just won't use in the cold months.

Using your calculator, 125,000 is all that I would need. 150,000 would be too much.

What cold-outside-air temperatures are you expecting this to deal with, and how warm do you want it inside then?

50° is still somewhat chilly if you're just sitting around, though you can easily keep warm by some movement and work. I'd say 40° is too cold unless it's just to keep things from freezing (though I'd be happy to have a 40° shop in those climes as an alternative to entirely unheated!), and I do sweat some on a 60° day if I'm working consistently (well, I'll sweat shoveling snow below freezing, but that's really hard work).

 

[sixdogs]

What cold-outside-air temperatures are you expecting this to deal with, and how warm do you want it inside then?

50° is still somewhat chilly if you're just sitting around, though you can easily keep warm by some movement and work. I'd say 40° is too cold unless it's just to keep things from freezing (though I'd be happy to have a 40° shop in those climes as an alternative to entirely unheated!), and I do sweat some on a 60° day if I'm working consistently (well, I'll sweat shoveling snow below freezing, but that's really hard work).

I figure if I'm not working, 55 is good and if I'm there working, 60 or even 65 wouldn't hurt. That's what I'm thinking.

Outside temps for my part of Ohio in the coldest months could be in the 20's at night and 30's in the day. In Jan-Feb we could occasionally go to a single digit at night.

December is still 30's-40's and March starts to warm up again.

 

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I figure if I'm not working, 55 is good and if I'm there working, 60 or even 65 wouldn't hurt. That's what I'm thinking.

Outside temps for my part of Ohio in the coldest months could be in the 20's at night and 30's in the day. In Jan-Feb we could occasionally go to a single digit at night.

December is still 30's-40's and March starts to warm up again.

My guess is 125 would do you fine then, especially if you tape off that door.
Worst case of "too big" a furnace is that it'll just run for shorter periods of time, and cost more up front.

 

[sixdogs]

My guess is 125 would do you fine then, especially if you tape off that door.
Worst case of "too big" a furnace is that it'll just run for shorter periods of time, and cost more up front.

Are there other downsides to a bigger furnace?

 

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Are there other downsides to a bigger furnace?

Any furnace that's at least "big" enough will run just as often, because they'll be running once the thermostat says "too cold". The bigger the furnace is, the less long it will run for; I suppose if it's severely oversize it could heat the area around it too much before the heat gets to the thermostat, so it may overshoot. But if it's just a bit too big (like 150 vs 100 or 125) I think you'll see it just run for 5 minutes instead of 10, not a big deal.

That's assuming the different size units are all the same efficiency, of course.

I don't think it's like an A/C where you really do want to size it right; ideally an A/C is big enough to actually cool the place, and small enough to just run all the time just getting the job done, but that's because the compressor motor lasts longest that way. The furnace has a blower fan but kicking it on isn't as big a deal as kicking on the A/C compressor.

If it was my shop, I'd probably size it for worst case, like "occasionally it's 0° and I'd like to be at least 50° because damn that's cold but 50 will be tolerable" and probably "fairly tight" from what you've said.

But hey: I'm NOT an HVAC guy and I haven't ever heated that kind of a space. Do you know anyone with a big shop that you can ask what they've got there?

Oh, one more thing, very important.
That calculator is "how many BTUs to heat the space". A 100K BTU unit with 80% efficiency is only giving you 80K BTU of heating; 20K BTU are going out the tailpipe. Definitely keep that in mind.

More on efficiency: a 100K BTU propane furnace will use about 1 gallon per hour (when going, of course). You can figure out how much the efficiency will cost and what the break-even is on a pricier more efficient system... also though, if you have tanks, a more efficient system will burn through your tank more slowly, which is less $$ quantifiable - though a wintertime refill is likely much more expensive than a summertime one, if you can manage to time the refills that way.