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It is necessary to have balanced air flow 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 flow is 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 HRV properly may void warranty
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.
Read the Application Warning in Clearances & Requirements section
of this manual!
Prior to balancing, ensure that:
1. All sealing of the duct 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. Air flow in branch lines to specific areas of the house should be
adjusted 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 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
8. A field-supplied balancing damper for the stale air side is required
for system balancing.
Balancing Procedure
The following is a method of field balancing an HRV using a Pitot
tube, which is advantageous in situations when flow stations are not
installed in the duct system. Procedure should be performed with the
HRV 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 itself and the
air handler, if applicable. This will provide the maximum pressure that
the HRV will need to overcome, 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 digital
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 end of
the Pitot is connected to the high pressure side of the gauge. The tube
coming out of the branch 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 air flow.
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 air flow (highest reading on the gauge). Then damper that air
flow back to match the lower reading from the other duct. The flow
should now be balanced.
Actual air flow 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 air flow, the
velocity is multiplied by the cross sectional area of the duct being
measured.
This is an example for determining the air flow 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 air flow 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.
Ensure balanced air flow in HRV. Excessive
positive pressure may drive moist indoor air into
the external walls of the building where it may
condense (in cold weather) and degrade
structural components. May also cause key
holes to freeze up.
CAUTION
Ensure balanced air flow in HRV/ERV. Excessive
negative pressure may also cause the back-
drafting of vented combustion equipment.
WARNING
Air Flow Balancing