I recently had my home HVAC system replaced with a 4-ton communicating Armstrong system (variable-speed AC and blower, 2-stage furnace) with a ComfortSync A3 thermostat (a rebrand of the Lennox iComfort S30).

California's Home Energy Rating System (HERS) program requires a certified rater to perform certain tests depending on the type of HVAC work you have performed. For my case, they'll be performing a duct leakage test and an airflow test.

For the airflow test, I want to make sure the system is running at full capacity, so they can get an accurate measurement. However, the Comfort Sync A3's normal user interface only indicates if the system is currently on or off, not what rate it's currently running at.

I asked the installer how to check the current rate the system is running at, but he didn't know how to do this, so I'm looking through the system's Dealer Menu.

In the Dealer Menu, under the Tests section, there is a "Cooling - Maximum Rate" test, which sounds like it would be the best way to test the system at maximum capacity. If I start this test, it shows that the "Cooling Rate" is at 100%. The fan RPM and "blower CFM demand" start off low, but ramp up for a bit and then stop at:

  • Blower RPM: 979
  • Blower CFM Demand: 1287

However, the Installation Report section of the Dealer Menu says the system is configured with "High Normal Cooling Airflow" to be 1570 CFM.

  1. Am I correct that the "Cooling - Maximum Rate" test should be the correct setting to use when measuring the maximum airflow? (This test is not described in the thermostat's technical manual)
  2. Does "Blower CFM Demand" indicate the amount of air that the blower is trying to push, or the amount of air that it's actually pushing? (this term is not defined in the thermostat's technical manual)
  3. If "Blower CFM Demand" is how much air the system is trying to push, why would it stop at 1287 when the system is configured for 1570?
  • BTW: my Trane communicating air handler has both a CFM Demand and a CFM Calculated number, which represents how much air the unit thinks is actually going through it. (and in my case, which confirms that I don't have enough return area for genuine 100% operation)
    – KMJ
    Commented Jun 2, 2023 at 21:28

1 Answer 1


It turns out I just wasn't patient enough! It took 10 - 15 minutes for the blower to spin up to full speed.

  1. Yes, "Cooling - Maximum Rate" ramps the unit up to full
  2. "Blower CFM Demand" is the amount of air the blower is trying to push (the number eventually reached 1570)
  3. Even during the test, the blower settles a medium speed (in my case, 1287) for a while (in my case, 10 - 15 minutes) before ramping up to maximum (in my case, 1570)

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