American Aldes Heat Recovery Ventilator HRV 120SRD Operation And Installation Manual Download Page 27

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It is necessary to have balanced air flows in an HRV/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 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
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 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

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/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 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

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 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 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 meters 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)

]

÷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.)

0.14

0.20

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.

MAGNEHELIC

DUCT

AIR

FLOW

Pitot tube

High Pressure Side

Magnehelic gauge

Pitot tube and gauge

TI-74-2

0108

Pitot Tube Air

Flow Balancing