AIR FLOW BALANCING
CAUTION
•If the unit’s airflows are not properly balanced...
-The unit may not operate at it’s maximum efficiency.
-Heat recovery core damage may occur.
-The unit’s use could cause negative or positive pressure in your home causing cold air to enter or other combustible equipment to backdraft.
-The unit may not defrost properly.
PITOT TUBE BALANCING PROCEDURE
PITOT TUBE
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 or other manometer capable of reading from 0 to 0.25 in.
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 air- flow (highest reading on the gauge). Reduce this airflow using either the electronic balancing system (if applicable) or damper. The flows should now be balanced. Actual airflow can be determined from the gauge read-
ing. 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 areas 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" diameter (D) duct has cross sectional area (A) of
A = 3.14 x (D/24) 2
A = 3.14 x (6/24) 2
A = 0.196 or about 0.2 ft2
The airflow is then: 640 ft/min x 0.2 ft2 = 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.)
50.14
60.20
70.27
80.35
The accuracy of the airflow 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.
Place pitot tube a minimum of 18" from blower elbows.
Note:
Duct 
connections may vary, depending on model.
Magnehelic
*Pitot tube should be kept at least 12” away from fans elbows and dampers to ensure accurate reading.
*A calibration decal is included to place over electronic balanc- ing system adjustments after it has been balanced.
•The balancing procedure consists of measuring the exhaust air leaving the system and the supply air entering the system and ensuring that these two are equal. A deviation of 10% or less is acceptable. In such cases, it is recommended to have a greater amount of exhaust air than supply