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©Retrotec 2017
5.1.1.
Range Configurations
Open
74
47
29
Figure 3: Open Range is for the leakiest ducts and houses while Ranges 74, 47, 29 are for progressively tighter ducts.
All Retrotec calibrated fans have multiple Range Configurations in order to measure a wide range of air
leakage flow rates. Select the correct Range Configuration by running the fan speed up to ensure it is
running at over 50% speed and the gauge is giving a flow result. If not change to a lower range and
repeat.
A calibrated fan measures flow by measuring the pressure developed inside the fan, which is often
called Fan Pressure. As the fan speeds up, a suction pressure develops at the inlet of the fan that causes
air to flow. By measuring this Fan Pressure, airflow is calculated using flow equations that are inside the
Retrotec gauge and FanTestic software.
When the fan slows down, the Fan Pressure can become too small to accurately measure flow. To
increase the Fan Pressure, a restriction plate is placed in front of the fan. The fan, consequently, has to
turn faster to maintain the same room or duct pressure, which creates a larger, more accurate Fan
Pressure. By providing a set of flow restricting plates with calibrated holes, Retrotec DucTester fans can
measure flows from 0.005CFM to 800 CFM.
The Model 300 has 4 standard Range Configurations: Open, 74, 47and 29. Open range is accessed by
removing the cover over the fan inlet called End Plate 300. The numbered Ranges are 4 inch Plugs that
snap into the End Plate and will restrict the flow to allow you to measure lower flow rates. The numbers
refer to the approximate hole diameter in millimeters. Most residential ducts in new homes can be
tested using Range 47. Older existing houses can be tested using Range 74 and super tight ducts on
Range 29.
When depressurizing, the Flex Duct must be removed in order to change the Ranges.
Refer to the DM32 Flow Calculator on retrote.com for Range Plug flow capacities.