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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 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

the

front

this

manual!

Prior

balancing,

ensure

that:

1. All sealing of the ductwork 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 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 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 adjusted back to match the lower read-

ing. See

Adjusting

the

Airflow

7. Return unit to appropriate fan speed for normal operation

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 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 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

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

manometer capable 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.

nsert 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  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)

5 (127 mm)
6 (152 mm)
7 (178 mm)

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.

DUCT

AIR

FLOW

Pitot tube

Digital manometer

Pitot

tube

and

gauge

Place pitot tube a minimum

of 18"

from