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Field Test Best Practices: A Resource for Practical Residential Building Science

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Orifice Plate

An orifice plate, also known as a flow plate, is a calibrated airflow measurement device, typically used to measure return airflow at an air handler unit. It is placed in the filter slot, and sealed to the sides of the air handler case. Round holes are located in a flat plate, and the differential pressure across the plate is measured while the air handler blower is operating. This pressure is correlated to the volumetric flow through the orifices.

Flow plates are typically provided with the necessary calculations and calibration factors to convert the pressure measurement to a flow in the field. Unless the orifice plate is damaged, it typically does not need recalibration. However, the pressure transducer used for this measurement should be handled carefully and calibrated on a regular schedule.

Application

Installation of the orifice plate will change the airflow and pressures in the air handler cabinet. The best practice method for using an orifice plate is as follows:

  1. Install pressure taps in the supply ductwork after the air handler and in the air handler just after the filter.
  2. Measure the differential static pressure across the blower and coils while the blower is running in the intended stage/mode.
  3. Remove the filter and install the orifice plate.
  4. Adjust the fan jumpers to achieve approximately the same differential pressure as above.
  5. Measure the differential pressure across the orifice plate and calculate the airflow.
  6. Remove the orifice plate, reinstall the filter (or a clean one), reset the fan jumpers, and verify the same differential pressure as before.
  7. Cap the pressure taps, or seal them with mastic.

Note that different size and quantity of orifices may be needed, depending on the overall flow rate, so that underlying assumptions are valid. At extremely low and extremely high air speeds, those assumptions will become invalid. Consult the manufacturer's user manual for the range of use for your orifice plate.

Orifice plate in use in a field test. Plate has 12 large openings in the back, one of which is attached to a wire.

Orifice plate in use
Ed Hancock