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Static Pressure In PSC & ECM Blowers

There are specific things green technicians consider when accomplishing performance evaluations on systems with different types of motors. In our first example in Figure One, we’re showing a simplified air flow chart for a PSC blower motor.

Figure One

In this case, our chart shows that this is a three-speed, PSC motor that could be typically found in an indoor air handler. For our purposes of explanation, we’ll identify this equipment as a gas furnace with the unit operating in the heating mode. Another factor we’ll apply here is that the motor is wired to operate in the medium speed for the heating mode in this specific situation. And, when we consult the chart in accordance with the factors mentioned, and consider that it’s common for the TESP to be in the range of 0.5 inches of water column in a residential system when the blower is operating normally, we note that the air volume according to the chart should be 715 CFM for this particular equipment.

Now, with our normal operating characteristics understood, we’ll consider a different TESP reading recorded, that of 0.7 inches of water column. When we consult our chart with this new information and apply simple arithmetic, we find that the air flow has dropped by 110 CFM. In the event that a technician notes the air flow is lower than that specified by the manufacturer’s performance data chart to this degree, further evaluation of the operation of this equipment is necessary.

One step to consider here would be checking the current draw of the motor. In the event that we found that reading to be lower than normal, we would know that there is a restriction in the duct system that is reducing air flow. The simple reason we would arrive at this conclusion is that the current draw is lower than it should be because the motor has less work to do with a reduced amount of air entering the squirrel cage blower.
When considering an ECM blower rather than a PSC, we would also need to consider a duct system issue if we found the TESP to be excessive. However, with this type of blower, the reduction in air flow would not be as severe as it is with a PSC motor. Because of the different method of operation of an ECM, the actual airflow variable is lower in the event of a duct restriction. In this type of air handler the control system recognizes that the static pressure it is working against is increased, so it adjusts the motor speed in order to maintain the CFM necessary to ensure efficient operation of the equipment.

In considering the chart shown in Figure Two, we’ll determine that the blower is operating on Program B, and that our TESP reading is 0.7 water column inches rather than the 0.5 water column inch reading we would expect. With this information applied, we can see that our air flow loss is only 30 CFM.

Figure Two

An important point to keep in mind here is that although the air flow reduction isn’t as severe as it was in our PSC example, it doesn’t mean we don’t have some work to do. The fact that the ECM ramped up in an effort to deliver and return the proper CFM means that it’s costing more to operate the equipment, so we would still need to look for issues in the air handling system as the cause of the increased static pressure.

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