I bought a house a few months back. I received several bids on a new furnace, and went with a guy who was referred to me by my realtor. The furnace he installed is a Trane XV95. It's 80,000 BTUs, and it's a 3 or 3.5 ton unit. Model number: Tuh2b080a9v3vab

My house is 2400 sqft above grade.

When the summer started we found that the AC isn't keeping up with the heat. I had my thermostat set to 72 but regularly the house would be up at 76-78 degrees in the afternoon.

I called the guy who installed the furnace to see what he thought. He says that it's a duct work problem, that the air returns are too low on the wall, and he wants to move them higher.

I had another guy come by and take a look, and he looked at the furnace and said it's undersized for the house, and it's probably causing the problem. He says it's a 3 ton and the house needs at least a 4 ton drive to push the air from the basement up (the house is a tri-level, four levels if you count the basement).

So I call back the installer and he claims that the furnace is actually able to push 1400 cfm, making it a 3.5 ton, which is sufficient for the job, and that isn't the problem.

So I'm stuck between disagreeing people, and I have no idea who is telling me the truth. I am trying to do some research to determine if the furnace is really a 3 ton or a 3.5 ton. They both want to sell me a new AC, though neither of them has said that would actually fix the problem, just be more economical for me.

Can anyone lend some advice? Is my furnace likely to be the problem? We haven't had any problems with the line freezing up. I blocked all the vents in the basement and lower level of the house so that the air would be pushed into and pulled out of the higher levels.