Energy Recovery Ventillators

Jan27 2010 // By: RESCUE GREEN // Categories: Sustainability, Green Building No Comments

The last couple of years have left me observing what an interesting time we are at when comparing the business climate, the social climate, and well, the climate climate. In spite of the business climate having about bottomed out, we actually see a desire by building owners to implement steps to be more environmentally friendly. Sometimes the bottom line dissuades these owners, but everyone is certainly talking about it and at least investigating what steps they can incorporate in their project.

With this interest brings technology, new ideas, studies of these new ideas, and hopefully some incentive monies to help promote them. We have magazine articles promoting technology, government funded studies to verify the capabilities of said technology, and with all of this, a dizzying amount of information we design professionals consider in current or future work. Much of the technology is very good and valid based on the claims made, but this technology can come at a very high cost (read no foreseeable payback), or is best suited for certain climates or building types. I say this because many of you read and hear about products, and then come to us only to be let down when we mention, for example, that the chilled beams you were interested in for your low desert project will take up, oh, about ¾ of the ceiling space and double the mechanical budget. Please know that I am not knocking the technology. I have actually been very interested in it, but I also know that they are currently better suited for a less extreme climate.

There is one technology, however, that I find to be underrated despite often making good financial sense, and certainly meeting up with energy and environmental sensibilities and this is Energy Recovery Ventilator technology. These units take the incoming outside air that we are required to provide to maintain proper IAQ levels, and they temper it with the outgoing exhaust air that is displaced with the outside air. With rooftop package units they are put on the outside of rooftop package units where the outside air intake and economizer would be. When dealing with split systems, often they are located in the attic space along with the air handler equipment. On larger units such as VAV’s, they can be made integral into the units themselves.

This type of equipment is designed to exchange the heat energy from the outside air with the lower temperature exhaust air. Huh? Simply stated, assuming a 105 degree day, the 105 degree outside air comes into the unit where there is an air-to-air heat exchanger, and on the other side of the exchanger is the 75-78 degree exhaust air that would otherwise be tossed outside like yesterday’s trash. The exhaust air will recover between 70 and 77 percent of the difference in heat energy, or 19 to 21 degrees. Now the outside air hits the air conditioning unit at 84-86 degrees instead of 105 degrees. If we have a building with a high outside air requirement such as auditoriums or churches, the savings can be substantial. Even classrooms have high enough occupancies to make a difference. Also, extreme climate conditions at the project site help sway the argument for energy recovery.

To illustrate this point, we have almost completed design on a high school/middle school in the low desert region. When we accounted for the outdoor ambient temperatures and the amount of outside air required for the classrooms, we were able to drop the tonnage of the units 1 to 2 tons each, typically a six ton unit dropping to a four ton unit. Multiply this by approximately 275 units, and we have saved over 400 tons of rooftop package units.

What is the trade off? Obviously, some first cost. Frankly, at about $3.00 per cfm, they appear quite pricey – almost too pricey. But you can see in the above example, the size of the A/C units are reduced by two tons which goes as an immediate credit towards the first cost of the ventilators. Also, there are the reduced duct and grille sizes corresponding to the smaller tonnage and potentially smaller electrical service. What is left of the higher first cost is offset by the fact that there is very little operational cost. These units have a small outside air fan and exhaust fan to help with moving the required amount of air, but the energy use is very small compared to eliminating the corresponding amount of compressor tonnage. In fact, in most applications that I have designed with this technology, we have achieved paybacks in the range of three years and under.

Like every bit of technology in this category, it is not a solution for everything, but as long as we have the two contradictory forces of IAQ and energy savings fighting each other, it certainly is something we analyze early on when we examine the possibilities in our projects.