Author Topic: Sizing a furnace.  (Read 2853 times)

Offline nash668

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Sizing a furnace.
« on: February 16, 2015, 07:54:32 AM »
Currently have a 50000 BTU furnace for a 1500 square foot home town home. 80% mid efficient Rheem. Its 11 years old and slowly dying… Anyway, just wondering you guys suggest. Thank you.

Offline Admin

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Re: Sizing a furnace.
« Reply #1 on: February 16, 2015, 08:16:47 AM »
I use a very simple and crude method that has never failed me.

If the house is an older standard build use a factor of 37.5.

If the house is a new tight build use a factor of 25.5.

1500 sq ft x 37.5 = 56,250

1500 sq ft x 25.5 = 38,250

The old house @ 1500 sq ft should have an output of at least 56,250 BTU's.

The new house @ 1500 sq ft should have an output of at least 38,250 BTU's.

If you're installing a 2 stage furnace then don't be afraid to go with the bigger option.

Offline Porcupinepuffer

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Re: Sizing a furnace.
« Reply #2 on: February 16, 2015, 09:07:58 AM »
Is it a town house with houses on both sides of it? We installed an adorable little 40,000 btu carrier last week in a town house about the same size. The salesman was saying they were having some issues with oversizing with the 50's in some of the town homes.

Offline nash668

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Re: Sizing a furnace.
« Reply #3 on: February 16, 2015, 09:42:05 AM »
What kind of issues?

Offline Admin

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Re: Sizing a furnace.
« Reply #4 on: February 16, 2015, 09:47:16 AM »
If need be, most manufacturers sell de-rate kits so that the outputs can be lowered on the smaller units.  You will see once the builders start using the F280 guidelines furnaces will have smaller outputs to create longer runtimes.

For example a 1457 sq ft home built to SB12 would have a heat loss of 44,499 BTU's and require an output of 58,000 BTU's.  A 60,000 BTU furnace would be installed.

The same home built to F280 would have a heat loss of 33,723 BTU's and require an output of 38,400 BTU's.  A 40,000 BTU furnace would be installed.

Offline Porcupinepuffer

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Re: Sizing a furnace.
« Reply #5 on: February 16, 2015, 10:12:52 AM »
What kind of issues?

Not sure. I wasn't really paying attention since so many of them are full of bologna. Most of them don't know to size a furnace at all. They just look at what was existing. Which is rather scary when we're going from some 120k btu oil furnace to a 98% eff gas furnace.

Offline nash668

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Re: Sizing a furnace.
« Reply #6 on: February 16, 2015, 04:53:47 PM »
That 1500 sqft includes the basement. Does that make a difference?

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Re: Sizing a furnace.
« Reply #7 on: February 16, 2015, 05:05:37 PM »
I don't include the basement, but in your case at 1000 sq ft you're probably installing a 45,000 BTU furnace either way.