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Old 03-22-2008, 10:58 PM   #1
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Furnace sizing


I replaced the furnace but I think I went to big. I would like to get a method of sizing the correct furnace for my home. There has to be a formula of some type.
My home is about 1450 sq.ft. ranch on a slab, relatively new Anderson windows ( about 8 years).
A 95,000 BTU unit with a 3 ton drive was in there, I put in a 100,00 BTU with a 5 ton drive. I had planned to duct some heat to the garage. It is a counter-flow unit.
Is it possible to convert it to an up flow and use the existing floor registers as returns?????

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Old 03-23-2008, 12:27 AM   #2
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Furnace sizing


Try to Google HVAC load calculations. There are a lot of factors that go into it. Measurements of outside walls, which direction do they face, window size and types. The insulation in the walls, square footage, etc. As far as your furnace converting to upflow you need to check with the mfg. The 100,000 btu sounds like it's in the ballpark,but the 5 ton seems like overkill. You want the a/c to run long enough to cool and dehumidify. With a 5 ton you will cool quickley and not dehumidify very well.

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Old 03-23-2008, 01:23 AM   #3
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Furnace sizing


It's a 5 ton drive unit not the a/c. the a/c is a 2 1/2 ton unit.
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Old 03-23-2008, 09:01 AM   #4
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Furnace sizing


Heat Loss is calculated as:
(Square feet of exterior wall or ceiling)/(average r value of the materials that it's built with)*(temperature differential) + btus needed for air infiltration loss.
It's more complicated than that, but that's the gist of it.

There are free calculators online. I like the one that Slantfin has on their website.
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Old 03-25-2008, 07:52 AM   #5
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Furnace sizing


Quote:
Originally Posted by thumbkins View Post
I replaced the furnace but I think I went to big. I would like to get a method of sizing the correct furnace for my home. There has to be a formula of some type.
My home is about 1450 sq.ft. ranch on a slab, relatively new Anderson windows ( about 8 years).
A 95,000 BTU unit with a 3 ton drive was in there, I put in a 100,00 BTU with a 5 ton drive. I had planned to duct some heat to the garage. It is a counter-flow unit.
Is it possible to convert it to an up flow and use the existing floor registers as returns?????
havc talk.com has a easy load calc you can use for 50.00
at 1450 sq ft you screwed up big time. the furnace heat btu is way to big and to go with a 5 ton drive was even more dumb. you will have so much air and noise you can forget about sleeping at night. do a load calculation to prove me wrong.

Last edited by coolmen; 03-25-2008 at 07:53 AM. Reason: add a few words
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Old 03-26-2008, 08:05 AM   #6
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Furnace sizing


Quote:
Originally Posted by thumbkins View Post
I replaced the furnace but I think I went to big. I would like to get a method of sizing the correct furnace for my home. There has to be a formula of some type.
My home is about 1450 sq.ft. ranch on a slab, relatively new Anderson windows ( about 8 years).
A 95,000 BTU unit with a 3 ton drive was in there, I put in a 100,00 BTU with a 5 ton drive. I had planned to duct some heat to the garage. It is a counter-flow unit.
Is it possible to convert it to an up flow and use the existing floor registers as returns?????
If you are now saying it's a 2 1/2 ton a/c and 100,000 btu I think you are ok. Double check on a heat loss/heat gain calculator.

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