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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 42 Supply plenum pressure matching method
Summary of Contents for DucTester 300
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