Size of new AC unit?

Old 07-30-08, 11:59 AM
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Question Size of new AC unit?

I am getting quotes on a new furnace and AC unit for the 50 year old house we just bought. It is an 1100 sq ft. Cape Cod, with the upstairs addition being newer than the rest of the house. Not sure if this matters, but the first floor has all new windows, as well, and the second floor windows are older and not as well insulated, making it much warmer. There is also only one return vent on the second floor.

In the quotes I have been receiving for AC units, one company said a 1.5 ton unit would be fine, because when you get a unit that is too big for the house it actually is counterproductive. Another company stated we would need at least a 2 ton unit, but quoted a 2.5 saying there is no way a 1.5 ton unit would be big enough to cool the house and a 2.5 ton unit would be best.

Which is right?
Old 07-30-08, 01:57 PM
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Both contractors need to do what is called a Manual J which is a detailed calculation to determine exactly how large of a airconditioning system (or furnace) you need for your particular house. Make sure they provide that calculation before you sign a contract.

Not knowing where you live, its hard to say if 1.5 tons is too small or not, but it does sound rather small for a house of that size and age. By comparison, I have a 2.5 ton AC in a 1250 sq. foot house, but I live in the midwest where 100+ temps can occur during the summer.

For what's its worth, 1.5 ton central AC units are about as small as most manufacturers build, and are most commonly sold for apartment use. Again, have your contractors do the Manual J calculation. If neither will, ask them if they will guarantee in writing that they will change out your new AC unit for free if it proves out to be too big or small within the first two years. Bet that prompts them to do the calculations and show you how they arrived at their bids!

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