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  3. Using the Duct Leakage Screening Test in measureQuick

Using the Duct Leakage Screening Test in measureQuick

Accessing the Quick Tests

Quick tests are accessed in a project in the latest software release (1.1.5 or newer, you may have to go to the app store to update to this version) to access the test, start a cooling project and tap the quick test from within the workflow. This will bring up the test, which will require a temporary remapping the the probes to complete the duct leakage screening.

Duct leakage screening can be used to determine if a duct leakage test is required with a Duct leakage testing tool like a “Duct Blaster(r)”
Probe Placement is Critical for Accurate Testing!

Duct leakage accounts for some of the largest losses in a cooling system, and specifically duct leakage in the return. Using the same formulas that are used to set mixed air when setting up an economizer, mQ can calculate the approximate return duct air leakage as well as the capacity losses due to air leakage and radiant heat gain in the ductwork. mQ then normalizes the duct leakage for 20 pascals of pressure, the typical pressure used for duct leakage testing if the return air static pressure is also measured with a manometer.

For accurate measurement, shut the cooling condenser turned off (remove the outdoor disconnect) and allow the fan to run for 15 to 30 minutes to allow the evaporator coil to dry if it is wet. This is a great test to perform before you start the cooling system.

When setting up for this test, we highly advise the use of 4 psychrometer probes, one in the closest supply to the air-handler (place the probe tip in the register), one at the return air inlet, and one on each side of the blower at the inlet of the fan volute. Attach the probe at the blower inlet with the magnet. As air travels up the return, it will travel up the side of the duct where the leakage occurs. Air often does not mix well before entering the fan so we measure on both sides of the fan (blower) so as the air mixes entering the fan that we can detect any changes in temperature. Using a single probe on one side of the blower you may miss leaks of one side of the system.

If desired, in place of a live reading, the attic air temperature can be measured and captured by pressing the capture button or by manually entering the attic air temperature after measuring. Measure the air temperature of the attic, not that of the decking as deck temperatures is important. Higher than the actual air temperature will significantly lower the duct leakage calculation output. Attic air temperature will not change much over the short time of measurement, so a real time measurement is not required. Ideally, set up the test, then close the attic access to get the most accurate measurement.

The more information that you supply mQ such as the return duct length, the duct diameter or circumference, and the R-Value of the exterior insulation, the more accurately mQ can estimate the duct leakage.

For all practical purposes, if there is no duct leakage or radiant heat gain, the temperature entering at the return air inlet will be the same as that entering the fan and the losses and air leakage would be zero.

Air leakage for attic stairs in a common problem and a huge capacity loss for an air conditioning system.

When performing this testing, make sure that you are not short cycling air from an attic access into the return air grill. If the return air temperature is more than a degree or two above what the thermostat is registering you may want to use painters tape to temporary seal the attic access to see if the duct leakage goes away. While this is not leakage into the duct from the attic, it is an unintentional path of leakage into the return that will significantly reduce the capacity of the cooling system. There are many attic stair solutions available that will eliminate this issue.

You will also notice in the diagram a negative pressure trap. Air leakage up a dry trap is also a common point of undesired air leakage into the duct system. A negative pressure trap will eliminate this issue. If the trap is dry, a ball gets sucked up against the trap outlet to stop the air leakage. Although these are not inexpensive, the payback is high as these types of leaks add both sensible and latent load.

If you are using the measureQuick Premier Testing and reporting, the duct leakage can be added to the report in place of the map. If this is a new installation you can also add the temperature compensated pressure test results after testing the piping system.

Updated on April 10, 2020

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