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Testing and Unbalancing Variable Air Flow System Performance

Dec 5, 2012 5:00:00 AM

A recent Test and Balance (T&B) report for a central air handler made me really question the value of some of our current T&B practices. The system was a conventional variable air volume (VAV) air handler with a return air fan and single duct reheat terminal VAV boxes. The purpose of the T&B process is to adjust and record air handler performance to achieve the specified maximum flow rates.

VAV Air Handler Schematic VAV Air Handler Schematic


The report stated the return fan was unable to achieve design flow rates because the fan had an undersized motor. The air flow was below spec and the motor amps were at the maximum rating.

There were two items in the final T&B report which caught my eye.

Carrying Its Load

First, the return fan discharge pressure, with the fan operating under full load conditions, was a negative value, which means the return fan was operating in series with the supply fan. This forced the supply fan to do some of the return fan’s work, so the test did not represent real-world full load conditions.

The correct method is to test maximum return air flow by putting the system in 100% outside air mode. This inherently creates a positive return fan discharge pressure (the pressure required to overcome resistance of the relief air path, relief damper and relief louver).

Not All Amps are Created Equal

The report listed the return fan variable frequency drive (VFD) operating at 50 Hz (below max speed of 60 Hz) and the VFD display indicated 8 amps, the motor’s max rating. Instead of adjusting the fans’ sheaves (pulleys) to achieve design flow at full speed, the T&B tech simply told the control technician to limit the fans’ max speed to 50 Hz.

In the field a few weeks later, the T&B tech and I looked in to this deficiency together. When I mentioned my check calcs indicated the return fan should have plenty of motor capacity, the response was, “No way, see… the 8 amps on VFD display are already at max rating.”

An amp at 460 line voltage is not the same as an amp measured downstream of a VFD operating at 50 Hz (383 volts output). I was surprised to find that the technician was unaware of this fact. The tech adjusted the sheaves, tested the fan in 100% outside air mode (with positive discharge pressure) and recorded amps at line voltage entering the VFD. The fan, now having to work harder than before (more discharge pressure to overcome), achieved design flow, and even had a little room (amperage) to spare.

A VAV box during installation A VAV box during installation


Why Are We Doing This?

The purpose of T&B is to ensure a system has the capacity to operate at max design flow. It seems to me current T&B procedures are not providing the basic information we need to evaluate a system’s performance. Nor do accepted T&B procedures address modern, variable flow system performance.

In the VFD amperage example, fundamental questions remain unanswered. For instance, how much power is the motor using? Fan power consumption is an important part of code compliance and operating efficiency. Can air flow be increased if needed for future building use changes?

Our industry uses outdated, pre-variable flow testing procedures. For example, tests typically assess individual VAV box design flow rates by setting all terminal units to max flow concurrently. This approach does not address system diversity, nor will it ensure an individual VAV box will be able to achieve design flow while the air system operates in the normal part load mode.

We still test outside ventilation air volume with a VAV central air handler at max flow. But air flow pressure gradient profiles tell us 20% outside air at max fan flow actually results in less than 20% outside air when the fan operates at its normal 60% to 70% part load capacity.

Our industry needs to review our T&B procedures relative to our real objectives with modern variable flow air and hydronic systems. We certainly need full load testing procedures. However, with VAV systems part load testing is at least as important as full load testing since the system is at part load for the vast majority of its operating life. There should be more focus on system power measurement, especially at part load because part load is where the real operating cost reductions are.

What are you seeing in T&B reports for your projects? How do you think we can improve our methods and procedures?

Written by Thomas Anderson