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How do I measure total air handler flow?

This test is used to measure total air flow through the air handler. First make sure all supply and return registers are open and untaped (keep filters installed if they are reasonably clean). Set up a pressure gauge to measure the duct pressure WRT the house at the supply plenum or a few feet away from the supply plenum in a main supply trunk. Use a static pressure probe to measure duct system pressure and be sure the static pressure probe is pointing into the air flow. Turn on the air handler and measure the normal operating duct pressure WRT the house. Record the normal operating duct pressure and turn off the air handler. Do not move the static pressure probe used to measure duct pressure because we will need to use it later in this test.

Open the air handler cabinet access panel and seal off the return opening in the cabinet from the air handler fan using tape and cardboard. Now install the Duct Blaster system in the access panel opening of the air handler cabinet. This is typically done by attaching the Duct Blaster's square transition piece to a piece of cardboard that has been cut to fit over the opening and taped in place. In this configuration, all return air flow will be moving through the Duct Blaster fan, with the return ductwork effectively sealed off from the supply system.

Turn the air handler fan back on and re-measure the duct pressure WRT the house. Now turn on the Duct Blaster fan and adjust the fan speed until the duct pressure equals the normal operating duct pressure measured above. Once adjusted in this way, determine the flow through the Duct Blaster fan by measuring the fan pressure and using the flow table. The measured Duct Blaster fan flow is your estimate of the total system air flow including flow through return registers, plus return duct leakage, plus leakage at the air handler access panel. The only component of total system airflow that is not included in this measurement is any leakage on the return side of the air handler cabinet (other than the air handler access panel).

If more fan flow from the Duct Blaster is needed to complete this test, remove the flexible extension duct from the fan and connect the exhaust flange from the fan directly to the access door opening using tape and cardboard.

Note: If you have installed the Duct Blaster in an unconditioned space (attic, garage or crawlspace airhandler), open a door from the house to the outside to prevent pressure changes in the house when the Duct Blaster is operating. Also open any vents or doors between the unconditioned space and outside to prevent that space from experiencing pressure changes from Duct Blaster fan operation.

Single Return Systems:

In single return systems, there is a simplified procedure which can be used under certain circumstances. If you are already hooking up the Duct Blaster to the central return to conduct an airtightness test, and you verify the duct system is tight (following retrofit or as part of a new construction performance test), then the air handler flow test can be performed without moving the Duct Blaster fan to the air handler cabinet. In this case you will need to measure the normal operating duct pressure prior to installing the Duct Blaster system to the return grill. Once your airtightness test has been completed, then remove the temporary register seals, turn on the air handler fan, and adjust the Duct Blaster fan to achieve the same normal operating pressure measured earlier.

The main advantage to this procedure is that you do not have separate the return side of the system from the air handler fan, which can be a laborious and time consuming process.

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    Graham Ralfe

    I read somewhere that when using duct blaster at return grille there is a multiplication factor to be used...I believe it was 1.4 times actual reading equals true airflow...thanks