The HVAC system on my home is going to have to be replaced this year. I've been getting estimates for replacing the system but, I have some confusion about some of the things that I'm being told by various contractors. I have a lot of questions/confusions but I'll try to list them as succinctly as possible. My home is a 3 story townhouse with approx. 2080 sq. ft. of living space and currently has a 20 year old 3.5 tonne A/C unit. The lowest level has about 20% of the total sq. footage. Questions: Most contractors have included replacement of the A/C lines as part of the job. Contractor A (with very good Yelp reviews) has told me that it isn't necessary if the lines are good because line leaks are almost always due to corrosion at the coils. He did open the furnance and there is significant corrosion there. He said that he would pressure test the lines and save me about $1500 if they don't need to be replaced. This seems reasonable to me since I could always replace the lines later if they fail. So, is he cutting corners or are the other contractors just not ? Most contractors have just looked at my current 3.5 tonne system and given me estimates for 3.5 or 4.0 tonne units. Contractor A measured the size of my home, did a load calculation and said that I only need 3.0 tonnes on the A/C. I've looked at some calculators online and we seem to be at the top end of the 3.0 range but I know that we also don't want an oversized system. I think that if there were cooling issues later, I could always have an attic fan installed to decrease the loading on the A/C. Does it seem reasonable to put a 3 tonne unit in 2100 sq. ft with an attic fan as a backup plan? RE the corrosion issue on the lines. Contractor A has stated that the main location of corrosion is usually on the coils where two different metals are coming into contact with each other. Seems reasonable given what I've seen. He stated that the Trane system that he wants to install doesn't suffer from that since all parts and components are made of aluminum. Of course there will be a single metal difference where the A/C line connects to it but that seems easy to fix if it corrodes. My question is whether aluminum works well for an A/C coil or if there is anything else I should be wary of with respect to aluminum. Thanks for any answers.