Air Flow Balancing - Robertshaw SER Serie Manual De Instalación, Operación Y Mantenimiento

AIR FLOW BALANCING

CAUTION
• If the unit's airflows are not properly balanced...
- The unit may not operate at its maximum efficiency.
- Energy 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 ERV using a Pitot
tube, advantageous in situations when flow stations are not
installed in the ductwork. Procedure should be performed with the
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 ERV
itself and the forced air furnace or air handler if applicable. This
will provide the maximum pressure that the ERV will need to over-
come, 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 instal-
lation 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 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.
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).
Reduce this airflow using damper. The flows should now be bal-
anced. Actual airflow can be determined from the gauge reading.
Note: Duct connections may
vary, depending on model.
• 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.
16
Place pitot tube a minimum of 18" from blower elbows.
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 ft 2
The airflow is then: 640 ft/min x 0.2 ft2 = 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
8 0.35
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.
* Pitot tube should be kept at
least 12" away from fans
elbows and dampers to ensure
accurate reading.