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© Retrotec 2017
Appendix G: Measure Supply or Exhaust flow with a gauge and
box
The Retrotec gauge can be used to measure the amount of air flowing through a hole. This feature enables
the gauge to be used as an Exhaust Fan Flow Meter, by simply cutting a couple of holes in a cardboard box.
The open end of the flow box should have rough dimensions which are at least two times the register
dimensions, and the depth of the box should be at least the average of the other two dimensions.
Because flow through a known size hole depends on pressure across the hole, and air flow into an enclosed
space will cause pressure, we can use a box with a hole in it to measure the system air flow. This method
partially blocks the flow from the air handler so readings are not exact, especially if pressures in the box are
above 8 Pa.
To create an Exhaust Fan Flow Meter
1.
Cut a hole in one side of a medium-sized cardboard box where it is only
one layer thick, and leaving about one inch of cardboard around the edge
for stiffness.
2.
Cut a 2" x 2" square hole in the center of the other side of the box, again
where the cardboard is only one layer thick. This is the flow measuring
hole. For accuracy, the small hole should be at least 1.5 inches from the
edge of the box and its area should be less than half the area of the end of
the box.
3.
Tape any cracks in the other sides of the box to prevent air from leaking.
4.
Punch a 0.25 inch diameter hole near a corner of the open end of the box
for the pressure tube. Insert a tube in the hole.
5.
Connect the tube to the “
r
ef B” (yellow) and "
input A" (blue) ports of the gauge using a T
connector.
6.
Fit the box over the exhaust fan grille while it is running, and seal in place around the box edges.
7.
Observe the pressure in the box on “A”. The same pressure will show on “B” if displaying p
ressure
on “B”
.
8.
Increase the size of the flow measuring hole in the box until the pressure is between 2 and 8 Pa.
9.
The gauge will calculate the exhaust fan flow if you
[Change Device]
and choose
“Hole Flow”
from
the “Generic Device” screen of choices
.
10.
Enter the area of the hole into the gauge when prompted by the gauge.
11.
Read the Exhaust Flow in CFM directly from the gauge on
“
B
”
.
The Hole Flow function calculates airflow rates through a hole using the following equation:
𝐹𝑙𝑜𝑤 = 𝐻𝑜𝑙𝑒 𝐴𝑟𝑒𝑎 ∗ √𝑃𝑟𝐵 ∗ 1.0755
Where:
Flow
is in units of CFM
Hole Area
is in units of in
2
PrB
is the Channel B pressure,
is
in units of Pa
Figure 73: Exhaust fan flow
meter
