User Guide

22
Pitot Tube Air Flow Balancing - Commercial
It is necessary to have balanced air flows 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 flows are not
properly balanced, then;
 The HRV may not operate at its maximum efficiency
 A negative or positive air pressure may occur
in the building
 The unit may not defrost properly
 Failure to balance HRV properly may void warranty
Excessive positive pressure may drive moist indoor air
into the external walls of the building where it may con-
dense (in cold weather) and degrade structural components.
May also cause key holes to freeze up.
Excessive negative pressure may have several undesira-
ble 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 back-drafting of vented com-
bustion equipment.
Read the Application Warning on the front of this
manual!
Prior to balancing, ensure that:
 All sealing of the ductwork system has been completed.
 All of the HRV's components are in place and function-
ing properly.
 Balancing dampers are fully open.
 Unit is on HIGH speed.
 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.
 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 adjust-
ed back to match the lower reading. See Adjusting the
Airflow.
 Return unit to appropriate fan speed for normal opera-
tion
BALANCING PROCEDURE
The following is a method of field balancing an HRV using a
Pitot tube, advantageous in situations when flow stations
are not installed in the ductwork. Procedure should be per-
formed 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 forced air furnace or 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 down-
stream 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 manometer capa-
ble of reading 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 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 (supply or return) duct.
Determine which duct has the highest airflow (highest
reading on the manometer). Adjust the higher airflow by
reducing the fan speed (see “Adjusting the Airflow”). 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 indicated by the gauge. This velocity will
be in either feet per minute or meters per second. To de-
termine 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. .)
5 (127 mm) 0.14
6 (152 mm) 0.20
7 (178 mm) 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.
Digital
Manometer
Duct
Airflow
Pitot
tube
Pitot Tube Air
Flow Balancing Kit
c/w digital manome-
ter, Pitot tube, hose
and tool bag.
PART NO.
99-BAL-KIT
Pitot tube and gauge
Outdoors
Note: Duct connections may
vary, depending on model.
Place pitot tube a minimum
of 18" from blower or elbows
Digital Manometer
Pitot Tube
Pitot Tube