It is necessary to have balanced air flows in an ERV. 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 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 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 weather) 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 on the front of this manual!
Prior to balancing, ensure that:
1. All sealing of the ductwork system has been completed.
2. All of the ERV’s components are in place and functioning properly.
3. Balancing damper are fully open.
4. Unit is on HIGH speed.
5. Air flows 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 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 and ERV using a Pitot tube, advan-
tageous in situations when flow stations are not installed in the ductwork.
Procedure should be performed with the 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 ERV itself and the forced air fur-
nace or air handler if applicable. This will provide the maximum pressure that
the 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”), three feet downstream 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 con-
nected 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 (sup-
ply or return) duct. (Fig. B)
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 indicated by the gauge.
This velocity will be in either feet per minute or metres per second. To deter-
mine 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 readings 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.
main balancing
dampers
motors
Pitot Tube
Note: for best results, keep
Pitot tube will away from
dampers and motor turbulence
Pitot Tube
MAGNEHELIC
DUCT
AIR
FLOW
Pitot tube
Magnehelic gauge
PITOT TUBE AIR FLOW BALANCING
Figure A:
Pitot Tube Air Flow Balancing Kit
c/w magnehelic gauge, Pitot tube, hose and carry case.
PART NO. 99-167
Figure B:
Pitot tube and gauge
Figure C:
Placement of Pitot tube
TI-74-2-ERV
9811
ERV
19
