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Question DetailsAsked on 8/18/2014

How big a furnace should I get for a 1800 sq. foot house?

I have a 20 year old air conditioner that should be replaced. The furnace is 12 years old, so I will consider replacing that, too. I don't want to pay more than I have to, so I wonder what the norm is for the size of a furnace/air conditioner for an 1800 sq. ft. house.

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Voted Best Answer

20 years on the AC - you have either been lucky or been babying it quite well. Can I guess that it is a Carrier ?

A 12 year old furnace would normally not be replaced unless you are going for a major energy efficiency increase or it has a hostory of giving trouble, because most go 25-30 years before they start gasping.

To determine your A/C (or furnace) sizing, you need an evaluation of your heating load based on type and size of construction and insulation, expanse of windows, sun exposure, etc. There are quite detailed computer programs out there now that can be combined with energy audit results and do quite a good job or right-sizing your system, or you can do it with the ACCA Manual J, which is the "bible" on AC and furnace sizing, along with the other ACCA manuals that figure individual room demand and duct sizing, etc. The manual method rarely undersizes a system unless the home has a terrible sun or cold wind exposure, or has a vastly unusuall numberof windows, but commonly does result in a somewhat oversized system. However,that is not necessarily a bad thing - I know I have been thankful at temps below 20-30 below for a week at a time for my 100,000 Btu boiler when 75,000 was the design given by detailed analysis for my area.

You should get a system sizing analysis done by an HVAC contractor - my ballpark guess, depending on if you live in the northeast or in the Mojave or Arizona desert, is somewhere between 3 and 5 tons of AC cooling capacity - with 1800 SF likely more in the 3-4 ton size for most cases. Furnace sizing depends a tremendous amount on your minimum winter ambient temperature so requires knowlege of your specific locale to even ballpark that.

Answered 6 years ago by LCD

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