16
PITOT TUBE BALANCING PROCEDURE
PITOT TUBE
BALANCING PROCEDURE
The following is a method of field balancing an HRV/ERV
using a Pitot tube, advantageous in situations when flow
stations are not installed in the ductwork. Procedure
should be performed with the HRV/ERV on high speed.
The first step is to operate all mechanical systems on high
speed, which have an influence on the ventilation system,
i.e. the HRV/ERV itself and the forced air HVAC system or
air handler if applicable. This will provide the maximum
pressure that the HRV/ERV will need to overcome, and
allow for a more accurate balance of the unit.
Drill a small hole in the duct (about 3/16"), four feet down-
stream of any elbows or bends, and two feet upstream of
any elbows or bends.
These are recommended distances but the actual installa-
tion 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 ref-
erence 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 typ-
ical 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
other duct. The flows should now be balanced. Actual
airflow can be determined from the gauge reading. The
value read on the gauge is called the 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 in either feet per minute
or meters per second. To determine the actual airflow,
the velocity is multiplied by the cross sectional areas of
the duct being measured.
The accuracy of the air flow reading will be affected by
how close to any elbows or bends the readings are
taken. Accuracy can be increased by taking an average
of multiple readings as outlined in the literature supplied
with the Pitot tube.
Magnehelic
Magnehelic
Gauge
Duct
Air
Flow
Pitot
Tube
A
The duct’s airflow
velocity is measured
with a magnehelic
gauge and a pitot
tube. See “Pitot Tube
Balancing
Procedure” next
page.
• To avoid airflow
turbulence and
incorrect readings,
the airflow velocity
should be measured
on steel ducting a
minimum of 3 duct
cross-section from
the unit or elbow and
before any transition.
• The balancing procedure consists of measuring the exhaust air leaving the system
and the supply air entering the system and ensuring that these two are equal. A
deviation of 10% or less is acceptable. In such cases, it is recommended to have a
greater amount of exhaust air than supply air as so to increase the supply air’s
temperature.
AIR FLOW BALANCING
PRACTICAL
TIPS
• If the unit’s airflows are
not properly balanced...
- The unit may not operate
at it’s maximum
efficiency.
- Heat & Energy recovery
core damage may occur.
- The unit’s use could
cause negative or
positive pressure in the
building causing cold air
to enter or other
combustible equipment
to backdraft.
- The unit may not
defrost properly.
A
*
Pitot tube and gauge
