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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 com-
ing 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 multi-
plied 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 sup-
plied 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. Excessive
negative pressure may also cause the back-
drafting of vented combustion equipment.
WARNING
Air Flow Balancing