# What is the formula used to determine blower fan size given total air volume and system BTU capacity?

I've completed a Manual J calculation for a new house and determined the BTU requirements for heating and cooling (35,352 and 15,772 respectively). In attempting to determine the turnover rate and blower size, I'm finding two unsatisfactory answers. The first is, "just use 400 CFM per ton". This is a rule of thumb and many sources say it's not super accurate. The other answer is "buy several hundred dollars worth of books and become a rocket scientist because there are too many variables to understand".

I don't buy either of these, since the optimal air flow rate over a heating or cooling coil of a certain size should be easily determined by a simple formula. You probably just need the BTU value of the heating/cooling element, the volume of the air being heated/cooled and perhaps a constant or two.

What is the proper formula used to determine blower fan size given total air volume and system BTU capacity?