advice: higher btu furnace than I need

[71Datsun510Wagon]

Good Evening All,

My garage is approx. 26' x 28' with 10' walls. I want to install a hanging natural gas, forced air furnace. I live in Western Canada where winter temperatures can be -40 degrees Celcius (often much colder with wind chill factor).

Local furnace contractors advise that a 70-80,000 btu furnace will meet my requirements. The cost is about $730 CDN funds (plus the cost of installation by a licensed gas fitter)

A guy I work with has a 120,000 btu hanging furnace that he says has been recently inspected and in excellent working condition. He is asking $250 but I suspect I could likely buy it for about $200 (plus installation cost)

Is the larger 120,000 btu furnace massive overkill for my garage? Would I be using more natural gas to run the larger unit?

Would it be better to wait and buy the smaller furnace? Any advice ??

My personal preference is to buy the new smaller furnace but budget-wise I think the larger, cheaper unit would work fine. However it has also been six years since I built the garage and my "garage enhancement budget" always seems to take a back seat to other family expenses (kids, wife, mortgage, etc)

My intention is to run the furnace for about 6 hours or so before I would do work in the garage. Otherwise it would be turned off. At this point I don't have a need to heat the garage all winter long.

Thanks!

Rob

[bmwpower]

Garages are funny when it comes to heat. Throw open a large garage door in -20 below and you'll know what I'm talking about. You'll want something with more capacity than a normal home heater since you have these things to consider. The higher the Btu's, the quicker the unit will react to drastic drops in temp. This will translate into less time for the heater to run before you enter the garage as you say.

The only true way to tell what you need is to take many factors into consideration: Insulation, windows, doors, ducts (how many and what placement), etc. All this can be calculated by a HVAC guy/gal using a manual J calculation. I have no idea how to do this calculation (sorry), but your local contractor may have done this.

That being said, I have a 100K Btu (80%) gas furnace forced air furnace in my garage, 30x30x14. I have yet to try it in the winter since it's recently installed.

Worst comes to worst, buy the $200-250 unit and if it doesn't work out, I'm sure you could sell it to someone for close to what you paid for it.

[rickairmedic]

Rob I would say you will be much happier with the bigger furnace . I helped a buddy of mine install a 100,000 btu furnace in his 1 and 1/2 car wood framed garage we pulled the unit out of a large 3 bedroom home so yes it is overkill for his aplication ,however even on the coldest days here which can get down to 0 degrees F with wind chills in the below 0 area it takes only a few minutes to bring his garage up from NOOOOOOOOOOOway am I going out there too dang I am sweating like a mad man temps :D. I do heating and AC for a living every day and can tell you in a garage more is better .


Rick

[astroracer]

I have to jump on the bigger is better bandwagon also. I am heating my 30 x 26 shop with an 80K BTU house unit and it does a fine job.
One thing I will suggest/recommend is leave the unit running at all times. Set it as low as the thermostat will go when you aren't out there (50-55*). The problem you will have with spot heating is condensation. Heating up a cold shop will cause a lot of condensation which really wreaks havoc on machined tool surfaces. Before I put in the furnace I was doing as you are planning, heating the shop only when I went out to work. All I had was a salamander though so the "heat all the time" option wasn't available. I spent more time cleaning surface rust off the lathes and machine tools then I did doing actual work... You can try it but you will quickly see what I am saying...
Mark

[trovato]

The problem you will have with spot heating is condensation. Heating up a cold shop will cause a lot of condensation which really wreaks havoc on machined tool surfaces. Before I put in the furnace I was doing as you are planning, heating the shop only when I went out to work. All I had was a salamander though so the "heat all the time" option wasn't available. Mark


I think you may find that the problem was not that the heat was only used when needed. With a salamander all of the products of combustion are blasted into the heated space. With a furnace, they are exhausted outside. I think this may be the cause of the problem when using the salamander.

[danski0224]

Bigger may be better, but sometimes smaller is good, too.

If you want quick recovery after opening the door, then go big. You will pay for that capacity.

If you will maintain a temp around 40*F all the time, then a much smaller unit will do the trick, especially if you are ok with a slightly delayed warm up after opening the door.

Make sure the garage is insulated well.

A buddy heats his 2 car with a 45K btu unit heater, and has no problems here in the Chicago area.

Energy costs are going up. Choose wisely.

[mikeyr]

Dumb question but costs wise isn't it about the same ? smaller BTU units runs for lets says 15 minutes to heat the garage and higher BTU unit runs lets say 10 minutes, isn't the gas cost going to be about the same in the end since you ran less time ?

[bmwpower]

Maybe. Maybe not. Depends on the efficiency as well as other factors. For example, how far into the heating cycle does the heater reach optimum efficiency?

[ZRWON]

Good Evening All,

A guy I work with has a 120,000 btu hanging furnace that he says has been recently inspected and in excellent working condition. He is asking $250 but I suspect I could likely buy it for about $200 (plus installation cost)
Is the larger 120,000 btu furnace massive overkill for my garage? Would I be using more natural gas to run the larger unit?
Would it be better to wait and buy the smaller furnace? Any advice ??
My personal preference is to buy the new smaller furnace but budget-wise I think the larger, cheaper unit would work fine. However it has also been six years since I built the garage and my "garage enhancement budget" always seems to take a back seat to other family expenses (kids, wife, mortgage, etc)
My intention is to run the furnace for about 6 hours or so before I would do work in the garage. Otherwise it would be turned off. At this point I don't have a need to heat the garage all winter long.Thanks!Rob
I AGREE with you re:BUYING THE CHEAPER UNIT...should work fine!
I suspect it'd cost less or about the same though if you ran your furnace continously with the thermostat set at lowest temp and then turn it up when you start working in there. I keep mine set @ 50 F year around, and turn it up to 62F when I spend much time out there. It takes less time (and fuel) to get temp to working level because I don't need to warm the garage ceiling, walls, and all the other stuff that's in there as I did when I only ran it when I was working out there. My temp is much milder than yours which might be a serious consideration. My shop is 32X26 with a 10Ft ceiling. I have a 45000BTU ceiling mount propane mounted in the corner pointed at the opposite corner. Works great/
http://img.photobucket.com/albums/v346/zrwon/the%20shop/DOGHOUSE/shop8.jpg