Retrotec DucTester 300, Operation Manual

Page: 62 / 89

Page 62 of 89
©Retrotec 2017
12.5 Measure large exhaust and intake flow rates
The previous methods can be utilized to measure large rooftop exhaust or air intake flow rates. The
essential idea behind the test method is to construct a tent over the source of the flow and to use a
blower door to bring the pressure inside the measurement tent to zero and in so doing to not affect the
flow rate that was there originally. There are two methods to do this:
1. Measure the pressure difference across the tent with at least two pressure hoses T’d together
going to the Blue port of the gauge and to connect the red port of the gauge to at least two
pressure hoses T’d together going to the Red port of the gauge. This works well in calm weather
but when the wind is blowing it will be difficult to determine zero pressure.
2. Measure the pressure difference between inside the duct referenced to indoors. This pressure
signal c
an be WiFi’d to the operator on the roof who can measure the flow rate into or out of
the tent required to re-establish the pressure in the duct prior to the installation of the tent.
Construction of the tent is from stock PRV pipe parts. They can be press fit together to create a
framework and covered in plastic using Red Tuck Tape to hold 3 mil Poly together. The blower door fan
may rest on the ground and be
taped to the poly our mounted in
a panel that is taped to the poly.
Super tight fit is not essential
because the tent will be very close
to zero pressure during the
measurements.
Use the “Set Pressure” function on
the gauge to set the pressure to
zero for method 1 and to the duct
pressure in Method 2. Do NOT
use the “@” function for Method 1
but you should use it for Method
2.
Rough guidelines for the minimum
size of the tent are?
Tent size on each side Maximum flow into tent Maximum flow out of tent
2 feet 750 CFM 1500 CFM
3 feet 1500 CFM 3000 CFM
4 feet 3000 CFM 6000 CFM
6 feet 6000 CFM 12000 CFM
Larger tents generally work better because velocities are lower.
Figure 43 Tent made of poly and PVC pipe courtesy of Terry Brennan of Camroden
Associates