I'm in the option period on a house that seems to have an a/c that is not effective. 2500 sq ft house that currently has a 4 ton a/c unit.

It is possible that the unit simply needs to be recharged, or serviced in order to fix it, but we have asked the seller to install a replacement. We hav requested a 5 ton unit rather than another 4 ton unit.

Anyone know for sure how many tons you need per sq foot of house. I've googled it alredy for the a-holes out there, and get very conflicting info. My option period ends Friday, I need to act quickly.

It really depends on how well sealed your house is, nearby trees (or anything shading the building), radiant barrier, windows and sun exposure, how much insulation... really it depends.

My AC guy tells me 1 ton per 500 sq. ft.

General rule of thumb is 500 sqft/ton.

BUT, any A/C company that sells and installs a unit using that rule of thumb is cutting corners and making the quick buck. A properly sized unit for the house will be the most comfortable and most efficient. Putting in too much tonnage will cause the unit to go on and off too often, increasing humidity. Ideally, you want a smaller unit that runs more rather than a larger unit that cycles more.

I have a 3700 sqft house with a single 5 ton unit, but it has 2 compressor speeds, and a variable speed fan, and 4 zones. Cost the same as 2 separate high seer units but gets me 4 zones instead of 2, and is more efficient. I'm using less kWh than I was before I added 850 sqft and the new A/C.

The proper thing to do is to have a manual J calculation done on the house. This will calculate volume rather than sqft, takes into account total windows, west facing windows, the size of the west facing profile of the house, shade, insulation, lowE glass, number of doors, etc.

Second, and also just as important to tonnage is the sizing of the supply and return duct work. Static pressure can kill the efficiency of a system.

Finally, go into the house today or tomorrow since it seems to be 95+ now and see if it is cool...

Here is a good writeup:
http://dnr.louisiana.gov/sec/execdiv/techasmt/ecep/hvac/b/b.htm

Oh, and if you need a quote on your mortgage, give me a call or email. My information is in my profile.

-bill - University Mortgage