Furnace sizing for new construction


Old 05-08-03, 07:29 AM
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Furnace sizing for new construction

I just purchased a new house that is 2100 sf, with another 1000 sf of basement that can be finished. I had my good friend, who is an AC contractor out to install an AC unit to the furnace that came with the house. THe existing new furnace (which I have not even turned on) is 70,000 BTU at 80%, so 56,000 effective. The house is 2 stories, and then the basement. He says that the furnace is way undersized for the house and it will be hard to install the proper size AC unit, becuase of plenum dimensions being too small for 3 tons of cooling. He says that someone probably made amistake, and that they usually install 70k units in 1000-1500 sf homes. The contractor and builder who built the development said that they put the same size in all homes, and they have had no complaints. There is no doubt in my mind that they probably skimped. But, my question is- do you think they will do anything for me? My friend says he can get a 100,000 btu unti for $500 and put it in with the AC and probably save me some headache down the road. Do you think the builder or contractor would take back the existing 70,000 btu unit? What if I left it in, what would be the potential problems down teh road for me? Any advice would be greatly appreciated. Also- does anyone know of a chart that is used to figure furnace sizing based on square footage of home? Or a website?
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Old 05-08-03, 02:55 PM
KField's Avatar
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Do a heat gain if you haven't already. 3 tons sounds like alot of cooling for a house your size. Do you have a lot of glass? What part of the country are you in? I wouldn't tear out a brand new furnace unless you have found that it won't carry the load. It is usually surprising how the fudge factors lead people to oversize equipment.

Old 05-08-03, 03:51 PM
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There is no such thing as sizing by square footage and what people "usually" install doesn't cut it either.

A heat load calculation must be done to be sure. If the builder installs 70k furnaces they could work ok.

The way to do it is calculate the heat/cool load and then select the ductwork for the capacity you need.

Some have purchased a single use calc here and were happy but I havn't heard how the free download works:http://www.hvaccomputer.com/main.asp

Considering how long you will have to live with this it would be worth the fifty bucks to get the full version.
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