Air Flow Balancing
It is necessary to have balanced air flow in an HRV. The vol- ume of air brought in from the outside must equal the volume of air exhausted by the unit. If the air flow is not properly bal- anced, 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 HRV properly may void warranty
CAUTION
Ensure balanced air flow in HRV. 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.
Read the Application Warning in Clearances & Requirements sec- tion of this manual!
WARNING
Ensure balanced air flow in HRV. Excessive
negative pressure may also cause the back- drafting of vented combustion equipment.
Prior to balancing, ensure that:
1.All sealing of the duct 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 flow 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 dampered back to match the lower reading.
7.Return unit to appropriate fan speed for normal operation
8.A
Balancing Procedure
The following is a method of field balancing an HRV using a Pitot tube, which is advantageous in situations when flow sta- tions are not installed in the duct system. Procedure should be performed with the HRV on high speed.
The first step is to operate the air handler on high speed. 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 magnehelic gauge or digital manometer capable of reading from 0 to 0.25 in.
Insert the Pitot tube into the duct; pointing the tip into the air flow.
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 read- ing on the gauge). Then damper that air flow back to match the lower reading from the other duct. The flow should now be balanced.
Actual air flow 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 air flow, the velocity is multiplied by the cross sectional area of the duct being mea- sured.
This is an example for determining the air flow 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 air flow is then:
640 ft./min. x 0.2 square feet = 128 cfm
For your convenience, the cross sectional area of some com- mon round duct is listed below:
DUCT DIAM. (inches) | CROSS SECTION AREA (sq. ft.) |
5 | 0.14 |
6 | 0.20 |
7 | 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.
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