Pitot Tube Air Flow Balancing
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 house
•The unit may not defrost properly
•Failure to balance the HRV may void warranty
Prior to 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.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 dampered back to match the lower reading.
6.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.
Choose the straightest section of duct between the HRV and the weatherhoods. This will be used for both the supply and return ducts. 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. (0 - 62 Pa) of water, preferably to 3 digits of resolution. The tube com- ing 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 duct. The flows should now be balanced.
Actual airflow can be determined from the gauge reading. The value read on the gauge is called 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 either feet per minute or metres per second. To determine the actual airflow, the velocity is multiplied by the cross sec- tional area of the duct being measured.
25
