I have 1 1,200 square foot house. I live in Florida. I have new windows and good insulation on the sides of the house. I am not sure about the insulation in the roof. It might not be great. I currently have a 2 1/2 ton unit in my house which is 22 years old. It is a 10 SEER unit that has been estimated to be operating more like a 7 SEER unit by this time and it leaks Freon. So, I am breaking down and getting a new unit.

My problem is that I am just not sure if I should upgrade to a 3 ton unit. I had 3 quotes. One guy told me I needed a 3 ton, that he has been in many similar houses to mine in the neighborhood, and that is what I need. The second guy did a J sheet and said I am right in the middle of where it says I need a 2 1/2 ton, no more no less. He measured the windows, estimated insulation etc. The third guy asked if I already had an estimate and decided to rely on the quote from another guy.

So, now I am actually having someone else put the unit in after I found that all my quotes were overpriced. I am using someone that is coming from a far distance since he is a friend of a friend so it is hard for him to do something like a J sheet before he puts in the unit since he would have to make 2 separate trips.

My question is, what are the most important factors in deciding what the proper tonnage is on the unit? I am worried that if I put a 2 1/2 ton in again that the upstairs will still be too hot.

Thank you so much if anyone can help. I don't want to make a mistake on such an expensive purchase.

Go with the best informed contractor.

How has your old 2 1/2 ton unit performed? If it kept the house comfortable on the hottest days, then do not get the bigger one. It is better to run a smaller unit longer to take out more humidity. The temperature changes will also be more gradual. I tend to trust the J sheet guy since he did the calculation.

The reason the upstairs may be too hot may not be related to unit size. If the thermostat is downstairs, then it will get the downstairs to the right temp and shut off. The upstairs may not be cool yet. If that is your problem, the best way to solve that problem is to put in zoned cooling. That way, if only the upstairs needs cooling, cold air is only delivered to the up stairs.

If your A/C is running constantly and not keeping the house cool, then you need a bigger unit. You may need to adjust the results for loss of cooling capacity of the old unit (how did it perform a few years ago?).