Set Static Pressure on Rooftop Unit (Video)

After installation of the new Axion rooftop unit, the proper blower speed must be set for proper operation. Failure to properly set the unit static pressure can cause unit operational problems, including motor failure.

First, measure the total ESP (external static pressure) of your duct system

  • Using a manometer, measure both supply and return ESP
  • ESP should be taken in the duct drop, not thru the RTU doors
  • Add the two together to get total ESP
    • Example: .4” supply and -.5” return = .9” ESP
  • Add the static for the economizer and curb adaptor if used
    • This information can be found on manufacturer paperwork


Select the required CFM for the unit you are working on

  • Figure approximately 400 CFM per ton of unit cooling capacity
  • For example, a 3-ton unit requires 1200 CFM while a 5-ton unit would be 2000 CFM

Locate the air flow chart on the outside of the unit control box cover

  • Using your total ESP and required CFM locate the DC voltage setting
  • Using a DC voltmeter on P+ and N- terminals on terminal board and measure the DC voltage
  • First, select the voltage range A, B or C on the switch as follows:
    • A 4.1 to 7.5 DC volts
    • B 6.9 to 8.7 DC volts
    • C 7.7 to 10.0 DC volts
  • Second, use the dial to fine tune your DC voltage to what is required by the chart
  • Double check your static pressure and readjust as need.

Be aware, failure to properly adjust the air flow static pressure during installation of the unit can cause operational issues and potential motor failures.

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