7
PITOT TUBE BALANCING PROCEDURE — The follow-
ing is a method of field balancing an HRV using a Pitot tube.
This is advantageous in situations when flow stations are not
installed in the ductwork. The procedure should be performed
with the HRV on high speed. See Fig. 24.
1. Operate all mechanical systems on high speed which
have an influence on the ventilation system (i.e., HRV,
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 bal-
ance of the unit.
2. Drill a small hole in the return duct (about
3
/
16
-in.), 3 ft
downstream of any elbows or bends, and 1 ft upstream
of any elbows or bends. These are recommended
distances but the actual installation may limit the
amount of straight duct.
3. The Pitot tube should be connected to a Magnehelic
gage or other manometer capable of reading from 0 to
0.25 in. wg, 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 gage. The tube coming
out of the side of the pitot is connected to the low pres-
sure or reference side of the gage.
4. 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.
5. Repeat this procedure in the supply duct. Determine
which duct has the highest airflow (highest reading on
the gage).
6. Use the HRV dampers to correct the airflow to match
the lower reading from the other duct. The flows
should now be balanced.
7. Actual airflow can be determined from the gage
reading. The value read on the gage is called the veloc-
ity pressure. The pitot tube comes with a chart that will
give the airflow velocity based on the velocity pressure
indicated by the gage. 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.
FORCED AIR
FURNACE
COOL AIR
RETURN
RETURN AIR
1 M
(3'-3") MIN
RECOMMENDED
* UNIT IS NORMALLY BALANCED ON HIGH SPEED
WITH FURNACE BLOWER ON.
OUTDOORS
* DUCTS CONNECTION MAY VARY
DEPENDING ON MODEL
40" MIN
NOTES:
1. Furnace blower may be required to operate when ventilation from HRV is required. The furnace should be set to run continuously or inter-
locked with HRV.
2. A minimum separation of 39 inches (1 m) is required between the two direct connections.
3. The exhaust air connection should be upstream of the supply air connection to prevent exhausting any fresh air.
4. Weather hood arrangement is for drawing purposes only. 6
′
(2 m) minimum separation recommended. 18
″
(460 mm) above grade minimum.
5. Due to the differences in pressure between the HRV and the equipment it is being connected to, the HRV’s airflow must be confirmed on site,
using the balancing procedure found in the installation manual.
Fig. 18 — Direct Connection to Furnace (Return/Return) Airflow Diagram
Fig. 19 — Direct Connection to Furnace
(Supply/Return)
