BURLINGTON, Vt. - While summertime humidity moderation, especially in southern climates, is a recognized advantage of energy recovery ventilators (ERVs), Mark Stephenson has found ERVs to be the best solution for his customers in northern Vermont.

Stephenson, of Vermont Energy Contracting and Supply Corp., started his full-service HVAC contracting company in 1985. His company serves northern Vermont, including places so far north the customers speak French.

The severe cold climate, high energy prices, and a tradition of Yankee craftsmen quality construction convinced Stephenson that air-to-air heat exchangers (as they are sometimes called) should be an important part of his business. And besides, his offering of ERVs set his company apart from other contractors and demonstrated that he could help his customers with the latest HVAC technology.

Today, Vermont Energy sells and installs high-efficiency American Standard furnaces and Mitsubishi Electric mini-split air conditioning systems in addition to the RenewAire ERVs that he has sold since he founded his business.

"I get questions all the time about why and how I can use ERVs instead of heat recovery ventilators (HRVs) here on the shores of frozen Lake Champlain," said Stephenson. "The answer is, as long as you have the right product and proper installation, ERVs are, from my experience, a better choice."

After connecting the ductwork, Stephenson closes the RenewAire ERV. In his estimation, installing ERVs in cold climates makes sense. (Photo by Carolyn Bates.)

The Differences

Contractors and builders can choose heat recovery ventilators (HRVs), which transfer temperature difference from one airstream to another, or energy recovery ventilators (ERVs), which transfer both temperature difference and a portion of the humidity difference. There are many who believe that an ERV should not be installed anywhere north of Florida, but Stephenson is not one of those believers.

"In Vermont, HRVs are in de-frost mode for over a thousand hours every winter, reducing ventilation capacity and wasting energy," he said.

"The ERVs I use rarely lose exhaust capacity without a defrost system of any kind. Since ERVs do not produce condensation, they can be mounted in any orientation, which makes my technician's job easier and gives my customers a neater installation."

In Stephenson's estimation, an ERV matches the benefits of an HRV - plus offers some of its own. An ERV moderates humidity year-round by transferring a portion of humidity from the exhaust airstream to the fresh airstream. If indoor humidity is too high, it lowers the humidity level. If indoor humidity is low, humidity will be recaptured and will not overdry, he said.

According to Stephenson, an ERV will deliver drier air than an HRV when the temperature is above 40ÞF because the higher humidity in the outdoor air will be rejected to the exhaust air-stream before it gets delivered to the house.

At temperatures below 25 degrees, the net dehumidification rates are virtually identical for HRVs and ERVs, he said, because the reduced ventilation rate of HRVs using recirculation defrost matches quite closely the amount of humidity transferred within ERVs.

ERVs eliminate core condensation and significant frost formation in virtually all applications, said Stephenson. Significant airflow restriction will not occur in ERVs unless the outdoor temperature is below zero, he said. ERVs are designed to operate reliably in extreme winter weather, he said. Since a portion of the humidity in the exhaust airstream is transferred in the gas phase, condensation does not occur and eliminates the need for a condensate pan or drain.

Other Believers

An award-winning Vermont home builder also speaks enthusiastically about the use of ERVs in the North.

"RenewAire ERVs help me achieve the highest possible energy performance, which is why my homes were named the most efficient homes in Vermont by Vermont Energy Star Homes the past two years," said Allan Rossetto of A. Rossetto Construction.

"ERVs provide necessary ventilation to ensure a healthful and comfortable indoor environment for the families that buy my homes, and I like the flexibility of installing the unit in unheated attics and not have to worry about freezing drains."

Rossetto first learned of RenewAire ERVs while attending Efficiency Vermont, the state's annual building conference and exposition. This expo is one of the primary marketing initiatives that Stephenson uses to promote his company and products.

"There is no better way than to display your products and talk to influential builders and energy experts," said Stephenson.

A tool Stephenson likes to use at these shows and other sales settings is the Home Ventilation Institute's (HVI) certified products directory for HRV and ERV equipment.

"The HVI directory is a great resource to determine ventilation capacity of various manufacturers' equipment and to compare the energy efficiency of one manufacturer's product to the next," said Stephenson.

The HVI directory provides third-party certification for over 25 brands of HRV and ERV products. It is available for free download at www.hvi.org.

Today, many building scientists recognize the temperature and humidity moderation benefits of ERVs, even in very cold climates. Stephenson agrees with that assessment.

"When I started selling ERVs, there were only one or two brands," he said. "Today most manufacturers have an ERV core option for their HRV boxes. Unfortunately, few of these products are certified by HVI, so I can't show my customers a reliable comparison."

Publication date: 02/14/2005