Even Exchange: Heat Recovery Ventilators

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Unlike windows, heat recovery ventilators (HRVs) can filter out dust and pollens, such as these, before they enter your home.

Only a few years back, a “tight” house still lost 40 percent of every heating dollar to air infiltration. Plenty of fresh air found its way in through small cracks in the foundation and walls and around doors and windows, replacing the warm air that escaped through cracks and gaps in the ceiling and roof. Because of this constant exchange, older homes are notoriously drafty and expensive to heat. But from a clean air standpoint, they’re a healthier place to live.

Opening a window is the simplest solution, although doing so defeats the purpose of a tight house and provides only temporary relief. Local exhaust fans can be used to remove moisture and other pollutants from high-use rooms, such as kitchens and baths, but the negative pressure created by these systems draws in outside air from every crack and crevice. A negatively pressurized home can potentially draw in carbon monoxide from an attached garage or radon gas from the surrounding soil. The biggest problem with both solutions is that they sacrifice the heat your home was designed to save.

A heat recovery ventilator (HRV) is a better option. HRVs work much like an open window, but they can recover up to 85 percent of the outgoing heat. In addition, HRVs (unlike windows) can filter out pollens and dust before they enter your home.

HRVs aren’t new. They’ve been used throughout Europe–where super-efficient homes first encountered air quality and humidity problems during the winter–for more than a decade. Consumers and builders in the United States have quickly adopted the new technology. Today, many local building codes require mechanical ventilation systems in new construction.

How HRVs work

In one of Aesop’s fables, a satyr befriends a man, lost on a winter’s night. As they reach the satyr’s cave, the man starts blowing on his fingers. When asked why, the man answered, “My breath warms them.” The satyr then placed a dish of stew before his guest. When the man raised the spoon to his mouth, he began blowing on it. The satyr again asked him what the man was doing. He answered, “The stew is too hot, my breath will cool it.” The satyr, wanting nothing to do with a creature who can blow both hot and cold with the same breath, promptly threw the man out of his cave.

What the satyr considered magic is actually the principle of heat transference; heat energy was transferred from the man’s breath to his fingers, and later from the stew to his breath. This is basically how HRVs work. At the heart of an HRV is a heat-exchange core. This core is made up of alternating aluminum or plastic passages that allow incoming and outgoing airstreams to flow next to each other but prevent the streams from mixing. In winter, the heat energy derived from the outgoing stale air blowing out preheats the incoming fresh air blowing in; in summer, the opposite occurs.

An energy recovery ventilator, or ERV, is a close cousin to the HRV, except that it transfers moisture in addition to heat. The added humidity control can be a big advantage in climates where dry winters last for months. (By expelling all the moist air, an HRV can make a home too dry in the winter.)

Although wall- and window-mount models are available, most HRVs and ERVs are designed as centralized whole-home units. HRVs can be easily retrofitted into homes that already have forced air heat, simply by feeding the fresh air intake directly into the furnace. Independent HRV systems are also available for homes with hot water, steam, or electric heat; however, the required five-inch to seven-inch diameter ductwork and extra labor usually limits this option only to new construction or major remodeling projects.

The cost of installing an HRV system varies widely. Depending on your existing heating system, existing ductwork, and the size of your home, adding an HRV can cost anywhere from $1,000 to $4,000 or more. When shopping for a system, look for units that have high-efficiency filters to trap pollutants before they enter your home. Some units also offer pre-heater or pre-cooler coils to make incoming air more comfortable during temperature extremes.

Do I need an HRV?

Not all homes can benefit from an HRV. According to the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), the minimum standard for residential ventilation is .35 air changes per hour (ACH), or about eight changes per day. Many older (drafty) homes exceed that minimum naturally, especially on windy days. However, even for older homes, higher circulation rates may also be desirable when more people are home, or if you suffer from home-related allergens.

To test if your home is tight enough to benefit from an HRV, contact your local utility company or heating contractor and ask them to conduct a blower door test. This test checks your home’s tightness by using a fan to blow air into the house and then gauging how well it holds pressure. Your heating contractor should also be able to help you decide on the size of the HRV and the proper locations of the intake and exhaust vents.

Want to compare HRVs?

The Home Ventilating Institute’s product directory compares dozens of HRVs by their recovery efficiencies, airflow capacities, and other features. For more information, call (847) 394-0150.

Home with Standard Vents
Mechanical vents are good at expelling unwanted moisture and pollutants, but if a home’s tight, they can depressurize the inside space. The replacement air that’s drawn in may bring pollens, molds, and in some cases, radon. Air entering through vents and chimneys can also cause fuel-burning appliances to vent carbon monoxide and other pollutants inside (back drafting).

Home with an HRV
Whether run independently or hooked into the existing hot-air system, HRVs provide a balanced clean-air airflow. Note that HRVs and ERVs can replace bathroom exhausts, but you’ll still need a vent hood over your stove–vaporized grease could clog the exchanger and contaminate the ducts. Install supply and return vents, about ten feet apart, in each bedroom, bathroom, and common area.