I have to install a new air conditioning unit. The old one is dead (Heil 60LC-000087CR; a 5 ton machine manufactured in 1991)

Two contractors have bid to replace it (Trane 13 SEER and a Lennox 14 SEER). Both have recommended 5 ton machines.

However on this site and other literature - proper sizing is critical; not too big, not too small. I worked through a heat load estimate on Heatload.com (37,175 BTU, 1043 CFM) and a worksheet in a book called "HVAC Fundamentals" (43,932 BTU). Both analysis took into account room area, volume, windows/type, orientation, insulation, and location - story level/over uninsulated rooms, etc. Both analysis gave a result that would indicate a 4 ton machine.

The contractors will conduct a Manual J analysis for $400.

Knowing a little bit of knowledge is a dangerous thing, I asked the contractors their opinion of going to a smaller machine.

They are concerned that the existing duct work was sized for a 5 ton machine. Without proper duct size and dampers the air flow will be reduced (1,600 cfm for the 4 ton and 2,000 cfm for the 5 ton), hot/cold spots will be felt. However the ducts work fine for the furnace (Trane XR80).

I should note that I have taped up most of the ducts joints to reduce the leakage I found when I purchased the house (built in 1953, do not know the age of the duct work - assumption is it was installed in 1991 but it looks older.) We are also adding insulation to the attic where there is none now.

So the question is - 4 ton or 5 ton?

Thanks very much.