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Balancing the Air Flows with a Pitot Tube
STEP1. Drilla 3/16” hole in the duct (ideally 3 feet downstream of
anyelbows or bends and 1 foot upstream of any elbows
orbends) in the Fresh Air and Stale air streams.
StaleAir From Building
Balancing
Damper
BalancingDamper
FreshAir to Building
Drill 3/16” holes in
Stale& Fresh Air
ducts.
STEP2. Insert the Pitot tube withthe tip facing towards the air
streamin the Stale Air From Building air stream. Move
thePitot tube around in the duct (facing towards the
airflow)and take an average reading. Record the reading.
MagnehelicGauge must be level.
Pitottube tip facing
towardsthe air stream.
STEP3. RepeatStep 2 to measure the Fresh Air to Building duct.
MagnehelicGauge
mustbe level.
Pitottube tip facing
towardsthe air stream.
STEP4.
A)Review the readings and damper down the duct with the
highestduct velocity pressure. Repeat Step 2 and Step 3
untilboth ducts show indentical readings. For this example,
theFresh Air to Building air stream has the highest duct
velocitypressure.
B)Upon completion of balancing, seal
theholes (foil tape recommended).
Determining the cfm
Afterbalancing the air flows, calculate the cfm flow rate.
Example
Thisexample shows how to determine the air flow for a 6” diameter duct.
As shown in the illustration, the duct velocitypressur e reads 0.025” w.c.
onthe magnehelic gauge. Use the chart that came with the magnehelic
gaugeto determine a duct velocity of 640 feet per minute for a duct
velocitypressure of0.025” w.c.
Cfm Calculation
cfm= feet per minute x cross section area of duct
=640 x0.196
=125
Cross SectionArea of some common round duct sizes:
0.087for 4”diameter duct
0.136for 5”diameter duct
0.196for 6”diameter duct
0.267for 7”diameter duct
Magnehelic Gauge reading .025” w.c.