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I am having problems out of one of my units at my house. We can't get the house below 76-78 during the day. I know it's been hot but I think I should still be able to get it cooler.
It's a 5 1/2 ton unit on about 2500 feet, older mid 80s ranch style house, no shade. The guy that came out the other day tried to tell me that no matter how good the unit was that you could only hope for a 20-22 temp difference between outside and inside. I know this is bull because I have never had this problem before at other hosues I have owned.

I was told when I bought the house that the inside unit was not replaced when the outside one was. Could this be a problem?
Any ideas?

Do you know what the air temp is, coming out of the vents? You should get about a 20-degree difference between this temp and the temperature INSIDE the house. If your thermostat reads 80, the air temp at the vent should be about 60.

Look and see how much insulation you have in the attic? if not much that could be the problem. You could always put in some more insulation. You can rent a insulation blowing machine and do it yourself. You shoud have aleast about 9 1/2 inches of bat insulation. When i built my houst i put in about 18 inches of blown in insulation.

You probably actually have a 5 ton unit. That is the largest size available for homes, since most don't have three phase power. Without getting into alot of calculations on thermal load, you can estimate 400- 500 square feet per ton. So, you are on the marginal end. I would say on the 97- 100 degree days we have been having lately, you are doing as well as to be expected.

Had to get some freon put in my unit at the office. It wouldn't get below 78. Mine is an old unit that needs replacing though.

we have rigged a hose up to my office, double wide with a shingle roof, during working hours. cut holes in it and just open up the valve slightly. 7 degree difference.