Figure 1. Energy recovery ventilators may be used in conjunction with packaged rooftop equipment. (Courtesy of Greenheck.)
Increased ventilation is often prescribed as a means of improving indoor air quality. Simply bringing in more outside air can dilute odors and other contaminants, reduce CO2 levels, and make building occupants feel healthier and more productive.

But there’s usually an energy penalty to pay: heating in the winter, cooling in the summer. Bringing in more outside air means there is more of a load on the hvac system, producing more of a strain on the existing equipment, raising operating costs, and/or requiring additional equipment.

Energy recovery ventilators (ERV) are emerging more and more, particularly in commercial and institutional applications, as a way to deliver more air while controlling energy costs. One manufacturer, Greenheck (Schofield, WI) has combined its expertise in ventilation products with new technology in air-to-air heat exchangers.

The Heart of the Matter

“The heart of the energy recovery ventilator,” according to Aaron Gotham, product manager for ERVs at Greenheck, “is a light-weight polymer enthalpy wheel. Silica gel desiccant is permanently bonded to the polymer, which provides a long and reliable energy transfer life.”

Silica gel is a highly porous solid adsorbent material that structurally resembles a rigid sponge. It has a very large internal surface area composed of myriad microscopic cavities and a vast system of capillary channels that provide pathways connecting the internal microscopic cavities to the outside surface of the “sponge.”

Gotham stated that the product is still in the educational stages, since not everyone is familiar with the technology, but that ERVs are seeing some applications now that should be expanded on later. In fact, he is hopeful that one day use of an ERV will be pretty much standard operating procedure on most systems where 15% of the supply air is from outside air.

So far, utility subsidies have been nil, although that would seem a logical way to help introduce them into the marketplace. But with deregulation, most utility subsidies have dried up, Gotham said. “We are optimistic that prices will continue to fall on the ERVs and the energy savings will make up for the initial cost anyway. We’d rather make our way in the marketplace without utility subsidies.”

Humidity Fighter

Use of an ERV is even more important in hot, humid climates where bringing in outdoor air can also mean bringing in moisture. Adding moisture can not only make building occupants feel uncomfortable, and require additional cooling, it can actually damage building interiors and porous wall coverings, furnishings, and carpeting. Moisture encourages mold growth and dust mite activity.

Greenheck offers a 50-page application manual, which begins by outlining the need for mechanical ventilation. There are references to ASHRAE Standard 62, with its recommended ventilation rates for various commercial spaces, from supermarkets and classrooms (15 cubic feet per minute) to smoking lounges (60 cfm).

School classrooms and auditoriums, with their varying occupancies, are a good application for ERVs. In Houston, five Greenheck rotary ERVs were specified to provide 20,215 cfm of outdoor air to the school, or approximately 20 cfm per student. Without the ERVs, 68 additional tons of a/c capacity would have been needed to cool the additional outside air. With that outside air, CO2 levels in the school dropped by 50%, from 1,800 ppm to 900 ppm. Following the success at one school, Alexander Elementary, the district decided to add ERVs to three more schools.

Greenheck recently expanded its product line to include a Mini-Vent 450 ERV that provides 300 to 500 cfm of outdoor air, with external static pressures up to 1 in. wg. It is said to be ideal for school classrooms and small offices. It can be mounted on the floor or hung from the ceiling.