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It is necessary to have balanced air flows in an HRV/ERV. The vol-
ume 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/ERV 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 HRV/ERV properly may void warranty
Excessive
positive pressure
may drive moist indoor air into the
external walls of the building where it may condense (in cold weath-
er) and degrade structural components. May also cause key holes
to freeze up.
Excessive
negative pressure may have several undesirable effects.
In some geographic locations, soil gases such as methane and
radon gas may be drawn into the home through basement/ground
contact areas. Excessive negative pressure may also cause the
backdrafting of vented combustion equipment.
Read the Application Warning at the beginning of this manual!
Prior to balancing, ensure that:
1. All sealing of the ductwork system has been completed.
2. All of the HRV/ERV's components are in place and functioning
properly.
3. Balancing dampers are fully open.
4. Unit is on HIGH speed.
5. Air flows in branch lines to specific areas of the house should
beadjusted first prior to balancing the unit. A smoke pencil used
at the grilles is a good indicator of each branch line's relative air
flow.
6. After taking readings of both the stale air to the HRV/ERV 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.
7. Return unit to appropriate fan speed for normal operation
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 furnace or air handler if applicable. This will
provide the maximum pressure that the HRV/ERV will need to over-
come, and allow for a more accurate
balance of the unit.
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 refer-
ence 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 proce-
dure 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 deter-
mined 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 indicat-
ed by the gauge. This velocity will be in 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.
This is an example for determining the airflow in a 6" duct.
The Pitot tube reading was 0.025 inches of water.
From the chart, this is 640 feet per minute.
The 6" duct has a cross sectional area of =
[3.14 x (6"÷12)
2
]÷4
= 0.2 square feet
The airflow is then:
640 ft./min. X 0.2 square feet = 128 cfm
For your convenience, the cross sectional area of some common
round duct is listed below:
DUCT DIAM. (inches) CROSS SECTION AREA (sq. ft.)
5 0.14
6 0.20
7 0.27
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.
Pitot tube and gauge
TI-74-2
1203
Pitot Tube Air
Flow Balancing
Kit
c/w magnehelic
gauge, Pitot tube,
hose and carry
case.
PART NO. 99-167
Place pitot tube a minimum of 18" from blower or elbows
Note: Duct connections may vary,
depending on model.
Outdoors
M
A
GN
EH
ELIC
Pitot
tube
Magnehelic
gauge
MAGNEHELIC
Magnehelic
gauge
Pitot
tube
Additional dampers
may be required
Pitot Tube Air Flow Balancing
29