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by Paul Nalewajk

Is there a rule of thumb that I can use that would tell me how many CFM’s an A/C would need per ton of cooling capacity?

Most packaged air conditioning equipment is designed for, and rated at, 400 CFM per ton. Typically, a 7.5 ton unit will be selected with a drive package that causes it to deliver 3,000 CFM against whatever resistance the ductwork and grilles offer. For example, the unit might be specified to deliver 3,000 CFM at 0.8 of an inch of static pressure.

While packaged equipment is designed at the 400 CFM per ton norm, most units are capable of operating at air volumes between 325 CFM per ton and 500 CFM per ton. As we increase the air quantity on a particular unit, three things happen:

  1. The total capacity the unit delivers increases.
  2. The sensible capacity of the unit increases.
  3. The latent capacity of the unit decreases.

For our purposes, latent capacity is the ability of the unit to lower the humidity or to remove moisture from the air and sensible capacity is the ability of a unit to lower the air temperature.

For a 10 ton packaged rooftop unit operating at standard conditions, consider the following:

Supply Air Quality Total Capacity Sensible Capacity Latent Capacity
3000 CFM 116 MBH 79 MBH 37 MBH
4000 CFM 123 MBH 90 MBH 33 MBH
5000 CFM 126 MBH 99 MBH 27 MBH

As you can see, the latent capacity varies more than 25%. This is extremely important today, when due to indoor quality issues we are forced to design for increased quantities of outdoor air. Typically, an engineer will specify the quantity of supply air and the capacity of the equipment based upon the entering conditions of the return and outdoor air, and the sensible and latent loads he is dealing with.

One exception to the 400 CFM per ton rule that you may encounter if your sites are restaurants is the treatment of make-up air. This is the air that is brought into your space to offset the air being exhausted through kitchen exhaust. Make-up air equipment is typically designed at 200 CFM per ton because the heat content of 100% outdoor air can be two times the heat content of a mixture of 80% return air and 20% outdoor air.

I find it curious that multiple contractors are claiming that your equipment has insufficient supply air at multiple locations. Unless there is a prototypical design flaw, insufficient supply air is usually due to dirty indoor coils, plugged filters, restricted duct work, or improper adjustment of the blower drive.

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