Question About BTU's and Unit Heaters

Old 10-26-09, 10:33 AM
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Question About BTU's and Unit Heaters

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!

Old 10-26-09, 11:20 AM
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Is the 200K bargain new or used? I would think the age and technology of the unit will play more into the efficiency of the unit rather then weather it is 100K or 200K. That said, the short cycling of an over sized unit will waste a bit more gas during start up.
Old 10-26-09, 07:09 PM
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Longer runs = more efficient. Plus longer equipment life.
Old 10-27-09, 05:14 AM
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Hi story. Depending upon how well insulated, I suspect the 75 to 100K estimates were oversized already. The HVAC guys always like to error on the side of TOO BIG. If that is the case, then a 200K unit will be a killer. Unfortunately air does not have a lot of capacity, so it take little time to warm it up and the energy doesn't last long, so your heater will be short cycling. I'm not sure what type of work you are doing, but I know I like my work areas cool. Sweating one minute and being cool the next would not suit me.

Being curious, I just ran an estimated heat loss on a 900 sq. ft. building with 12' walls and a slab. A lot of guessing, but 27,600 btu's per hour. It was just a guess, but it makes you wonder just how much oversized a 75 to 100k unit would be.

Old 10-27-09, 12:09 PM
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Thanks for the info, everyone. That helps a lot. Thank goodness for forums like this! I could've made a big mistake. Looks like 100,000 or less!

Thanks again,

Old 11-05-09, 02:26 AM
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I agree. HVAC guys and every other engineer/inspector that i have met always over estimate.
So when they say 100,000, that's usually the max. But it's always better to check with the one selling the equipment.
In terms of over capacity being inefficient, that would be true. If your heater unit heated the room to quickly, it will just shut on and off sporadically. And it always takes more to start something than keep it going. hope that helps.

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