What size AC unit should I use for a 1,986 sq. ft. home in Kingwood, Texas?
The house is less than six years old. It currently has a 2-1/2 ton Carrier unit. I like the temperature around 70 F, but can tolerate up to 73. In the summer the current unit can only get to about 77 on hot days. I recently had the unit checked out by a Carrier dealer. He said the unit was in good condition and reported no problems. When I asked about its inability to get to 73 in the summer, he checked the size of the unit and my square footage and came to the conclusion that the house needed a minimum of a 3-1/2 ton unit, but recommended a 4 ton.
Sizing an AC unit strictly by square footage is not correct practice.
The size of the unit will vary greatly depending on the variables from one to the next. You must take into consideration things like the type, size and number of windows, orientation of the house, tightness of the house, insulation levels, etc. The only correct way to size a unit is to run a Manual J. This can be performed by a qualified HVAC contractor or a third-party energy auditor.
Since your house is six years old, I assume the contractor who installed your system will not come replace it if it is truly undersized. Rather than going to the expense of replacing/upgrading your unit, you can get an energy audit and increase the performance of your home.
Hire a certified Building Analyst through the Building Performance Institute and have them do an inspection of your home including a blower door and duct blaster test. Odds are if your home is six years old and not an Energy Star home, you have a very leaky home and possibly leaky ductwork. If your ductwork is in your attic or crawl space and it is leaky, you do not have a chance; your cold air is not even making it inside your home and you are wasting energy.
The blower door can point out leaks in your home and you can seal these up -- seal around all lights in the ceiling, around all top plates and all wire and plumbing penetrations in the attic. Do the same exact thing in the crawl space and exterior walls. Increase insulation where you can.
The duct blaster will tell you how leaky your duct system is -- seal all the joints at the system and the boots with mastic. If your unit is in the attic, you could also consider installing a radiant barrier on the bottom side of your roof rafters to help keep the attic cool.
Performing these low-cost measures will improve the performance of your home and will help "rightsize" an undersized unit.
As a side note, avoid people that recommend power attic ventilators or solar attic fans as well as people that would recommend installing the radiant barrier on the ceiling in lieu of the rafters -- these salesman do not understand building science and you will be wasting your money and potentially making your problem worse.
For more information:
Read "I keep hearing that I should get a home energy audit. What should I expect to pay and expect to get?" for pricing and other best practices of an energy audit.
Also, check our energy efficient home Q&A to see what questions other homeowners are asking.