What’s Controlling Your Ventilation?

Indoor Air Quality is certainly a hot topic in the HVAC industry. Practically every trade journal includes at least one Indoor Air Quality article, and sometimes more. This interest in Indoor Air Quality has boosted sales of all types of specialty sensors, including refrigerant leak detectors, carbon monoxide sensors, and carbon dioxide sensors. These are understandable, but what is this new VOC sensor we are reading about that senses not a specific gas, but actually monitors “indoor air quality”? How does it work? Does it work? What does it sense? How is it applied?

The VOC sensors monitor the level of “Volatile Organic Compounds” in the air and output an analog signal. However, can the air quality really be determined by measuring this signal? The problem with these sensors lies in the broad spectrum of gases they monitor. The VOC sensors detect a wide variety of gases including carbon dioxide, carbon monoxide, hydrogen, methane, and R-123, and output a signal when these gases are sensed. However, you cannot determine from this signal which gas is present or the level of gas in the air. Did the signal rise because a truck exhaust pipe is against the outdoor air intake louver with the engine running, because a chiller has a severe leak of R-123, because chemicals in the storage room are leaking, or because an employee near the sensor is wearing too much cologne? Any of these can cause the VOC sensor output to rise, but obviously, the action you would take would be very different in each case.

Another problem in applying VOC sensors is determining what is “good air quality” and what is not. Since the output does not represent a specific level of a known gas, you must somehow establish a baseline for what is acceptable air quality. With some VOC sensors, you take a reading when the air quality is deemed to be good and use this as a baseline. How do you know when the air quality is good? Other VOC sensors automatically establish a baseline for air quality. However, if the output rises and stays at an elevated level for a period of time, this new level is automatically established as the baseline for acceptable air quality. It was not acceptable an hour ago, but since it has not improved, it is acceptable now!
One common argument for VOC sensors is that they require no calibration. Obviously a sensor that is not gas specific and has an output that cannot be statistically defined cannot be determined to be out of (or in) calibration. In an industry which tends to over-specify accuracy (0.25% pressure transmitters for duct static, 1% humidity transmitters, etc), it is difficult to imagine a sensor with an undefined output being specified to control building ventilation.

A common sense approach to indoor air quality is best. Demand controlled ventilation based on ASHRAE 62-1989 using a carbon dioxide sensor will yield acceptable indoor air quality, providing there are no extenuating circumstances. (See “What Are You Breathing?” in the December 1994 20/20 Insights for more information on this strategy. ) You will also have an industry accepted standard to reference in case your ventilation design is challenged. Gas specific sensors are available for monitoring other potential pollution sources. Carbon monoxide sensors may be used to monitor outside air intakes, refrigerant leak detectors at the chillers, etc.

If you feel that indoor air pollution due to VOCs is a potential problem, a VOC sensor could be used as a troubleshooting tool. It could effectively monitor rapid increases in the level of chemicals in the air from such sources as copy machines, printers, off-gassing of construction materials, etc. If the source of pollution is located, a common sense approach to solving the problem may then be taken. This might include purging the building with outside air during unoccupied periods, relocating problem causing equipment, installing exhaust fans, or improved air circulation. Any of these solutions are better than allowing a sensor, which is not gas specific and has an undefined output, to control building ventilation. ?