External Static Duct Pressure

Probably one of the most frequently ignored factors in setting up a duct system is the external static duct pressure (ESP).

In years past the temperature rise or drop across the coil or heat exchanger could verify that the air flow was in the 'ballpark', but with the higher efficiencies, particularly of air conditioning systems and heat pumps, air flow becomes absolutely critical.  In fact, if the air flow is not per manufacturers recommendations, it is impossible to even get the refrigerant charge correct.

Ironically, measuring ESP is a fairly simple procedure and, in all likelihood, will result in increasing customer satisfaction with the system's performance, comfort and efficiency as well as find more work on a jobsite.  Several studies have concluded that most heat pump and air conditioning systems have duct systems that are inadequate to supply sufficient air flow.

To correctly measure ESP, a static pressure tip is used in conjunction with a pressure gauge capable of reading in the hundredths of a water column inch.  Bacharach's electronic instruments which measure differential pressure (typically used for draft measurements), can measure ESP.

The two Bacharach combustion test instruments commonly used to measure ESP are the PCA 25 (and higher series) or the Fyrite Pro 125.

In the photo's above, the differential pressure taps are the two on the right hand side, the port on the left hand side is for sampling combustion gases.

We suggest purchasing a static pressure tip (below) which has a magnet mounted on the base, this helps hold the sensor in the proper position (with the tip pointing into the air flow).

Take your measurements on both the return and supply plenums of the furnace, as it was shipped from the manufacturer (including the filter).  

This means that if it was a gas or oil fired furnace, the measurement would not include the AC coil.  If a heat pump is being tested, the coil would be included.

Drill two holes large enough to insert the static pressure tip, one on the supply side and one on the return.  Pressure measurements are then taken at each location.  The measurement on the return side will be negative with a positive reading on the supply but you disregard the positive/negative and just add the two numbers together.

For example, if the return reading is negative .2WC" and the supply side is positive .2WC", the total static pressure is .4WC".

Once the ESP has been determined, look at the fan curve for that particular blower and determine the CFM from that chart.

In this example, a .4 WC" static pressure would translate to approximately 1700 CFM.