How Many BTU's For A Unit Heater For My Garage - DIY Chatroom

story · 10-26-2009, 02:19 PM

Hi All!

I have a garage/ex-living structure that's about 30' x 30', with about a 12' ceiling. It's well-insulated, and I practically live out there during the winter (for my job).

I had an HVAC guy come over to give an estimate, and asked him what size unit heater to get. He said 75,000 to 100,000 BTU, which was right in line with another estimate I had from someone else.

BUT . . . I have a chance to pick up a 200,000 BTU heater for *great* bargain. I know the benefit will be quicker heat and less time with the heater running, but I'm specifically wondering about this: Is it true that this larger heater will require such a huge amount of gas at start-up that it would be terribly inefficient?

Does anyone here know how unit heaters work in general--and if a unit that was (supposedly) over-large would be a relatively minor inefficiency, a significant inefficiency, or a major inefficiency?

Also, are there other factors to think about in going with a 200,000 BTU model, as opposed to a 100,000 BTU model?

Thanks for any replies!

John

hvac122 · 10-26-2009, 03:02 PM

Where the heck do you live that you need a 75-100,000 unit heater? I live in Nebraska and have a 30x30 garage with a 30,000 btu unit heater (smallest I could find) and can heat the space in a matter of minuites to 70°.

A 200,000btu unit I would think woule use so much of the air in the garage to burn it would either suck in in through the cracks or back down the flue. Not a good idea.

story · 10-26-2009, 03:27 PM

Quote:

Originally Posted by hvac122

Where the heck do you live that you need a 75-100,000 unit heater? I live in Nebraska and have a 30x30 garage with a 30,000 btu unit heater (smallest I could find) and can heat the space in a matter of minuites to 70°.

A 200,000btu unit I would think woule use so much of the air in the garage to burn it would either suck in in through the cracks or back down the flue. Not a good idea.

I should have explained more: it's has two 10'x10' rooms off to one side, another 15' x 15' on the other side, and a loft/storage that's about 20' x 20'. It's practically a house.

So, I'm still wondering what problems might arise from a unit heater that's actually too large for the space. Is it an actual problem that the combustion would take a lot of oxygen from the space?

Thanks in advance for any more info!

john

Viper16 · 10-26-2009, 03:32 PM

Quote:

Originally Posted by story

I should have explained more: it's has two 10'x10' rooms off to one side, another 15' x 15' on the other side, and a loft/storage that's about 20' x 20'. It's practically a house.

So, I'm still wondering what problems might arise from a unit heater that's actually too large for the space. Is it an actual problem that the combustion would take a lot of oxygen from the space?

Thanks in advance for any more info!

john

Alot of the larger burners can have external intakes (taking up outside oxygen) You could maybe use a barometric damper if there was a demand for more external air.

hvac122 · 10-26-2009, 09:48 PM

I do think you may have problems with combustion air. Don't know complete layout. You would also need to make sure the gas service to the house is large enough.
My other thought is that with multiple rooms you would want something sized pretty close to what you need so the unit has a long enough run time to get some heat to those. A unit like your talking would come on and go right back off again and could leave cold spots. I suppose it depends on what the rooms are used for.
You never answered, where do you live?

Home Air Direct · 10-26-2009, 11:14 PM

I had a building a couple years ago that was 50x60 with 16ft ceiling, well insulated. We had a 145,000 btu unit heater and it was too much. (Illinois)

I called on an HVAC shop that was 30x40 with terrible insulation that had a 200,000 btu sterling unit heater. The contractor knew better, but like yourself, he picked it up for a bargain. What he got was a very LOUD unit, just by the nature of it's size, that would explode on with heat, satisfy the stat and then shut off. The room would cool down quickly and it would come back on "Barrrrrooooom! Run an shut off. His gas bill was nuts. It took more just reach temp to set the fan off then it would have taken with a correctly sized unit to completely heat his shop.

What you have is a terrible idea and not worth the "Good Deal".

rschwanke · 10-27-2009, 03:11 AM

I have to agree with the guys above - 200,000 btu might blow you out and use a lot more fuel than you would like. I just built a 30x40 garage, heated slab, with radiant for the 2nd floor as well. I live in the cold a** north and I'm putting a 60,000 btu boiler in. I too wanted to go bigger, but I didn't want the fuel bill. If you can't pass up the great deal, put the thermostat on the far end of the building

story · 10-27-2009, 02:41 PM

Okay, I'm getting the picture! 200,000 BTU would be ridiculously too much. Ahhh . . . the benefits of forums like this! Saved me from a big mistake.

Thanks!

John
(Michigan)

sktn77a · 10-27-2009, 08:00 PM

I would also be leery of going as high as 75000-100000 BTU unless you have no insulation, single pane windows and a lot of leakage. Get a proper heat loss calculation done, not a quick "rule of thumb" estimate as they are invariably wrong (too large).