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External Static Pressure In HVAC Systems Part Two

In Part One of our discussion on total external static pressure testing of HVAC systems, we discussed checking the manufacturer’s installation information to determine what the TESP should be in a given system. As our chart showed, there isn’t just one number that applies to all systems regardless of the tonnage and the amount of air being moved by the indoor air handler.

However, one general application rule we can consider when evaluating any system in regard to the static pressure in the duct system is that most manufacturers of residential and light commercial equipment don’t want a pressure difference of more than .50″ W.C.  if a standard PSC, multiple-speed motor is employed by the air handling system. If the system employs a variable-speed, ECM-type motor, the TESP can be higher that that of PSC systems.

And, when it comes to evaluating an HVAC air flow system and going beyond the aspect of  isolating a given component in the duct system, such as the filter as we showed in the last segment, exact positioning the probes in the proper place varies, depending on the specific design of the system. For example, if you were evaluating a split system that employed a gas furnace as an air handler with an indoor coil positioned in the upflow position, Figure One shows you where you would effectively test the TESP of the blower itself in this particular type of system.

Figure One
Figure One

In this illustration, you’ll note that we have positioned the probe on the return side of the system after the filter so we can be sure we’re getting information directly at the inlet of the squirrel cage assembly of the blower. And when it comes to the probe on the supply side of the system, it’s positioned so that it is ahead of the indoor coil. Testing at these two points will tell us if the air handling system is operating properly in regard to the furnace itself, providing proper air flow through the duct system and the heat exchanger in the furnace. You’ll also note that we are just slightly above the general ‘.50 or less’ rule we mentioned for PSC blower assemblies. As we mentioned in the last segment on this subject, accomplishing static pressure tests provides us with the information we need to pursue a complete evaluation of the equipment operation and correct any potential problems affecting the total air flow through the duct system. If this system was a PSC rather than a variable-speed motor, we would need to expand our testing in order to bring the TESP across this blower down, and ensure the proper operation of this equipment.

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