Measuring Airflow through a Return Side Tap


Averaging Flow Sensor

I’m putting together an advanced on-line residential ventilation course for GreenTrainingUSA, and in the process a couple of issues came up.  One of these is testing airflow on the supply pipes to air handlers.  This popular approach includes a pipe from the outside of the building attached to the return side of the air handler.  The opening can be controlled by a damper that opens when the air handler turns on and air is drawn into the air handler along with the return air from the house.  It gets circulated around the house, blended with the other air.  Some of these systems include a control that monitors the run time of the air handler.  If the air handler does not run long enough to satisfy the ventilation requirement, the control restarts the air handler just for ventilation.  Some of these controls also monitor the outside air humidity and temperature, overriding the ventilation operation if it is too cold or too humid outside.

This approach effectively puts the house under positive pressure.  The pipe to the outside is like a hole in the return side of the HVAC system and since more air is being sent to the house than is being removed from the house by the air handler, the house is effectively under positive pressure.  The question is: How much air is being drawn through that pipe into the house?

It is often difficult to measure the flow into the system from the outside of the house either because of the location of the exterior hood or because of the irregular surface of shingles or clapboard or brick.  A measuring instrument that is sensitive to air motion is likely to be impacted by air movement on the outside of the house.

Drilling a hole in the ducting and measuring the pressure when the air handler is running, will provide the static pressure in the ducting.  Let’s say that produces – 5 Pascals of pressure.  The duct can then be disconnected from the exterior hood and a duct tester can be attached to the pipe.  Turn the air handler on again and then turn on the duct tester fan and increase the flow until there is – 5 Pascals of pressure.  At that point, the air moving through the duct test fan will be equal to the air moving through the ducting when the system is operating under normal conditions.

A simpler way to approach this is to get a small flow station and insert it into the ducting.  I recently found some remarkably inexpensive ones from Dwyer (Series PAFS-1000).  The one for 4” ducting is $7.25 and the one for 6” ducting is $8.50.  These work like simple pitot tubes.  You need to drill a 7/8” diameter hole for the sensor.  Insert the sensor into the duct and attach Channel A of your manometer to the two taps and read the pressure.  Use this process:


Air movement through a duct measured with a pitot tube (or averaging flow sensor):

VP = TP – SP (ΔP in Pascals)

FPM = SqRt(VP) x 253.29



VP = velocity pressure in Pascals

TP = total pressure in Pascals

SP = static pressure in Pascals

FPM = feet per minute

CFM = cubic feet per minute

A = area of the duct in square feet

So, for example, if you get a pressure reading of 1.3 Pascals and it is a 4” diameter duct, because you are looking at the pressure difference between the two ports, that is the velocity pressure or VP.  The FPM velocity is the square root of 1.3 times 253.29 or 329 feet per minute.  The area of the duct is 0.087 square feet, so the CFM equals 29.  This process is not perfect, but it’s a lot easier than trying to measure the flow from the outside or the duct tester approach.

We need to get better data on the performance of this ventilation approach.   29 cfm wouldn’t meet many of the ASHRAE 62.2-2010 requirements even if the air handler was running 24/7.

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