Retrotec Residential Pressure & Air Leakage - Manual (Page 35)

Page 35 of 75
4.6.1. Supply plenum pressure matching method
Run the airhandler with a clean filter in place. Measure the static pressure in the supply plenum with a Blue tube
going to Channel A. Insert a static pressure probe in an area where the pressure signal is stable meaning it does
not fluctuate more than 3% on 5 second averaging. Record this pressure.
The Flow Device with the flex duct must then be connected to a return or air handler cabinet using a flange or
box attached to the flex duct. To ensure the Device measures the exact flow that was there before the Device
was connected, the test fan must be run at a speed that will recreate that same flow rate. Set Pressure to the
previously measured supply plenum pressure and the Device will adjust its speed automatically to recreate the
original plenum pressure and yield an accurate flow measurement through the return register. Use the “@”
feature which will extrapolate your readings to the exact pressure previously recorded in the supply plenum.
When the plenum pressure is matched, the pressure at the connection to the return register will be very close to
zero with respect to the room such that small leaks around the connection will be insignificant. When
connecting to the air handler cabinet, gaps will create more errors since the negative pressure in the cabinet can
be high.
If there is one main return and several small returns, the smaller ones should be taped over so all the flow is
being measured through the main return. Total airhandler flow rate will be the sum of the measured value plus
any leakage (hopefully minor) through the return ductwork. If this leakage is major, then obviously it must be
fixed which will then yield a higher air flow measurement.
Figure 13 Supply plenum pressure matching method