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Pitot Tube Air Flow Balancing
It is necessary to have balanced air flows in an
HRV. The volume of air brought in from the outside
must equal the volume of air exhausted by the
unit. If the air flows are not properly balanced then:
• The HRV may not operate at its
maximum efficiency
• A negative or positive air pressure
may occur in the house
• The unit may not defrost properly
• Failure to balance the HRV may
void warranty
Prior to balancing, ensure that:
1. All sealing of the ductwork system has been
completed.
2. All of the HRV's components are in place
and functioning properly.
3. Balancing dampers are fully open.
4. Unit is on high speed.
5. After taking readings of both the stale air to
the HRV duct and fresh air to the house duct,
the duct with the lower CFM ([L/s] velocity)
reading should be left alone, while the duct
with the higher reading should be dampered
back to match the lower reading.
6. Return unit to appropriate fan speed for
normal operation.
Balancing Procedure
The following is a method of field balancing an
HRV using a Pitot tube, advantageous in
situations when flow stations are not installed in
the ductwork. Procedure should be performed
with the HRV on high speed.
Choose the straightest section of duct between
the HRV and the weatherhoods. This will be
used for both the supply and return ducts.
Drill a small hole in the duct (about 3/16"), three
feet downstream of any elbows or bends, and
one foot upstream of any elbows or bends.
These are recommended distances but the
actual installation may limit the amount
of straight duct.
The Pitot tube should be connected to a
magnehelic gauge or other manometer capable
of reading from 0 to 0.25 in. (0 - 62 Pa) of water,
preferably to 3 digits of resolution. The tube
coming out of the top of the Pitot is connected to
the high pressure side of the gauge. The tube
coming out of the side of the Pitot is connected
to the low pressure or reference side of
the gauge.
Insert the Pitot tube into the duct, pointing the tip
into the airflow.
For general balancing it is sufficient to move the
Pitot tube around in the duct and take an
average or typical reading. Repeat this
procedure in the other (supply or return) duct.
Determine which duct has the highest airflow
(highest reading on the gauge). Then damper
that airflow back to match the lower reading from
the duct. The flows should now be balanced.
Actual airflow can be determined from the gauge
reading. The value read on the gauge is called
velocity pressure. The Pitot tube comes with a
chart that will give the air flow velocity based on
the velocity pressure indicated by the gauge.
This velocity will be either feet per minute or
metres per second. To determine the actual
airflow, the velocity is multiplied by the cross
sectional area of the duct being measured.
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