# 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?

• Sounds like noise level would also be a factor. Oct 18, 2017 at 21:22

## 2 Answers

Unless your house is very small or extremely energy efficient I think you should read the book again and recalculate the size of the heat loss and heat gain. The 400 CFM/ton of A/C is accurate enough for me. Sizing a furnace is not rocket science and is not an exact science since there are too many variables. Years ago when I was still doing this work (before retirement) furnaces were sized on their input or output and the size of the drive required. Do not try to be too exact unless you know "exactly" how the house was built and can guarantee that it was built to those specifications. As far as the "proper formula" used I have no idea, it never had to be perfect.

Manual J gives you the sensible cooling load. A good rule of thumb is add 30% for the latent load. A little less for really dry climate and a little more for really humid climate. If you need more accuracy ACCA has a whole series of books. 350 CFM per ton will dehumidify better, 450 cfm per ton will be slightly more energy efficient. Anything less than 300 cfm per ton and the equipment may ice up. As far as heating goes that is determined by the furnace manufacturer. Every furnace has a minimum and maximum heat rise. They have charts that list various heat rises at different static pressures that come with the furnace. Because all furnaces are built a little different with different resistance these charts are the most accurate. One furnace may specify a rise of 40-75 degrees while another may specify 30-60 degrees. Both of these furnaces would require different air flows to make them operate properly. Because air conditioning almost always uses more air than heat I size my ducts for air conditioning and they are then too big for heat (that would be Manual D) If you are using manual J I would strongly suggest Manual D. The other manuals are somewhat niche books