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Gauge Attachments
When sampling an airflow, various attachments are available
for use on a digital manometer. Consult with your Lifebreath
distributor for available options such as a pitot tube, flow
measuring station, and an airflow measuring probe.
Figure B illustrates a digital manometer with a pitot tube
attachment. This combination will measure the system air
velocity pressure accurately, regardless of the duct size or
shape (either round or rectangular).
Determining the CFM
After balancing the airflows, calculate the CFM flow rate.
Example
This example shows how to determine the airflow for a 6 in
diameter duct. If the duct velocity pressure reads 0.025 in w.g.
on the digital manometer, use the chart that came with the pitot
tube to determine a duct velocity of 640 ft/min. for a duct velocity
pressure of 0.025 in w.g.
CFM Calculation
CFM
= feet per minute x cross section area of duct
= 640 x 0.196
= 125
Attention
Installations where the HRV/ERV is ducted directly to the return of a furnace may require additional dampening
on the fresh air to building duct. This is due to the high return static pressures found in some furnace
installations.
AIR
FLOW
Pitot tube
Digital
manometer
Cross section area of some common duct
sizes:
0.087 for 4 in duct
0.196 for 6 in duct
DUCT
Figure B
Digital manometer with a
pitot tube attachment
0.139 for 5 in duct
0.267 for 7 in duct
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