While waiting for a dream job, I'm doing service work on my own and I notice homes that are only 5 years old have undersized units? Can anyone tell me what size an HVAC unit should be or how to size a unit for homes? I had a service call for a condensing unit not running and I noticed it was an obviously undersized unit for the home. It was a 3100sf home with a 3 ton or 3 1/2 ton condensing unit. Could anyone tell me what size unit that home should have? I'm thinking it should at least have twice what it currently have. The customer said it takes a long time to cool the entire house.