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The Intelligent Use of Energy: Outside Air and Ventilation
Richard R. Vaillencourt, PE, Canterbury Engineering Associates LLC

Editor’s Note: This is the second in a series of articles on intelligent energy use in buildings by consulting engineer Richard Vaillencourt. We hope you’ll find them helpful as you work to optimize your building’s energy use.

Outside air is the most expensive air that building owners and managers can use at their facilities. It is expensive because it is rarely the temperature that you want it to be; therefore, you must apply heating or cooling energy to make it the correct temperature. Of course when it is at the right temperature, it is the least expensive air you can use because you do not have to add energy to change its temperature.

Yet no matter the time of year, outside air is very desirable in terms of indoor air quality, so it is important to optimize its use. In fact, there are several regulations dictating the outside air volume that you must bring in to your facility. The trick for using energy intelligently here is to know when to maximize, or minimize, the volume of outside air based on how close it is to the temperature that you need.

So what temperature do you need? The easiest answer: whatever the discharge air temperature (DAT) is from your air-handling unit. For example, if your DAT is 55 degrees F, then whenever the outside air is 55 F or less, you can use it for ‘free’ cooling – the classic ‘economizer cycle.’ Even if the outside air temperature is below 55 F, it still can be used because in most cases you can blend very cold outside air with the warm +/- 78 F return air to achieve the required DAT.

But what about heating? It is very unlikely that outside air temperature can ever provide free heating. So where else can you find free heat? In your exhaust air. The air that is being exhausted from your building is the temperature of your return air – typically the upper 70s F. If outside air is colder than the exhaust, then you have a Delta T that allows you to minimize the energy lost with the exhaust. With a heat exchanger of various possible designs, the cold outside air can be preheated by the energy in the exhaust. Remember, you already paid to heat that air that you are throwing out. It may be cost-effective to recycle some of it to lower your cost to heat the outside air needed to replace the exhaust.

This is an important point: Every cubic foot of exhaust air will be replaced with outside air. You can only exhaust the volume of air that you return to the building. One way or another, the exhaust volume will get back into your building and become a load on your heating or cooling equipment. If you completely seal up a room and start an exhaust fan, the fan will eventually stop exhausting air. The high negative pressure on the room side of the fan will eventually be so large that the fan will not be able to overcome the pressure difference, and air will stop moving.

So pay attention to your exhausts. The only way that you can beat the laws of thermodynamics is by refusing to play. Shut off your exhaust fans when not needed. Reduce your exhaust volumes to the lowest possible volume unless the outside air is the right temperature. Whenever possible, recycle the energy that you paid for in the exhaust stream and use it to precondition the makeup air.  More Articles/Resources

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