A load calculation is a detailed study, most frequently done with the aid of computer software.

While the items you mentioned are important in evaluating system replacement, many contractors will ultimately size units based on a 'rule of thumb', such as so many tons per so many square feet. Many will replace with what is currently installed. Others will guess and cite their 'years of experience'.

When doing a load calculation (manual j), such things as dimensions and orientation to the sun, as well as geographical location, insulation, infiltration, etc.

That's why an identical house around the corner can have a different heat gain/loss profile. \

In evaluating heat gain, it needs to be expressed in terms of the sensible (that which you can feel) gain and the latent (humidity) gain and equipment needs to provide both capacities.

The ductwork needs to be evaluated, especially with respect to returns (should have on both levels). If upstairs is a big problem, should consider additional attic insulation and radiant barrier, and this should be reflected in the heat calc.

In discussions I have read, most professionals do not recomment an attic fan.

V

Attic fans are a bad idea. They can depressurize the attic. If that happens, the air in the floor below will be drawn upward through every available crack and crevice in the ceiling and its fixtures. That will depressurize the interior of the house in turn, and make up air will infiltrate the house from outside. Your cooling bills will be higher as a result.

You should have a return register on both floors, not just the second floor. Did the contractors comment on the feasibility of installing a return register on the second floor? If they didn't, ask them why not. They should have least explored the subject with you.

Better yet, find another contractor.

Thanks for the comments. I've actually got a guy here this morning who did a real load calculation with all the measurements on a nifty little palm pilot thing. That's a good thing to know how an identical house elsewhere in the neighborhood can have different profiles.

I only have one return register on the first floor (cheap builders). The guy last night looked into the feasibility for one on the 2nd floor, but it's not feasible at all, i.e. major construction.

Also good to know about the attic fans. I was trying to figure out if the salesman was simply trying to increase his commission. The contractor this morning did not mention one at all, and he's explained much more to me than the other 2. Thanks again!