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determing to cfm

Does anyone know what is the best way to determine total airflow on a variable speed furnace when a duct traverse is out of the question? i am trying to find out how much air the furnace is moving in the 1st. stage cool, 2nd. stage cool, and cont. fan modes. i can not do a duct traverse due to the furnace being installed in a finished basement, and i can not get to all of the ducts that come off the plenum. since at this point i am more concerned with how the furnace is doing in the cooling months, and the furnace has an ecm, i am not sure the sensible heat formula is going to help. thanks to all.


  • Techman
    Techman Member Posts: 2,144

    Does this unit have the "cfm blinking light"? Hooking up a set of gauges will tell you how well the system is running, KindaSorta.!
  • Spence
    Spence Member Posts: 316

    Place one manometer probe at the entrance to the blower compartment downstream of the filter. Place the other at the outlet side of the furnace downstream from the cooling coil. The result will be the external static pressure that your blower "sees" which you will compare to your performance table.
  • don_9
    don_9 Member Posts: 395

    Yep sometimes you have to drill hole in the cabinet to get the reading.
  • RJ_4
    RJ_4 Member Posts: 484

    Hot wire anemometer.   you can get one for $175.00     when reading air flow at diffusers always factor the free air through the grill or diffuser.  also what Spence talked about, using fan curve for unit
  • Spence
    Spence Member Posts: 316
    ESP Readings

    Some manufacturers put a hole for you in the cabinet to save the drilling. Others give you a peep hole so you can see the IFC LEDs which you can pop out for the return pressure readings. On the supply side you can take out the high limit card.