The “Ask a Builder” series is dedicated to answering some of the many questions Fairbanks residents have about building, energy and the many other parts of home life.
FAIRBANKS — To begin 2015, we are running a series on heat recovery ventilators (HRV) in this column. Indoor air quality is a perpetual concern in cold climate housing, and has become more so in recent years as homes are more airtight and better insulated than ever before. This means they use less fuel for space heating, but also that they require mechanical ventilation, such as an HRV, to keep the interior atmosphere healthy.
Heat recovery ventilators, or HRVs, provide fresh air to homes while recovering heat lost by exhausting stale, warm air to the outdoors. Balanced HRV systems cost several thousand dollars to install, though the price depends on the size of the building and the amount of ductwork that is needed. For a smaller home, the balanced ventilation system might run from $6,000 to $8,000 for the HRV, ductwork, control system and labor. Larger homes will require larger HRV or ERV models and more ductwork. On the other hand, homes that already have ductwork (for a furnace, for example), will save money if the ductwork can be used for the HRV as well. While the upfront cost may sound high, the energy savings for homes in cold climates are also significant.
Operating costs for HRV systems include the electricity to run the fan and any frost protection devices. To get an idea of the cost of running an HRV system, let’s consider a few models available in Alaska. The table above shows the electrical use — at low and high speed — of three different HRVs. (The electric use and speed of the fan also depends on the external static air pressure, available from product spec sheets.) This table gives examples of the electrical use but should not be used to compare the efficiency of the three models. (See Table B)
The operating cost of an HRV depends on how often it runs in each speed. The table above lists the approximate operating costs in three Alaska cities if the HRV runs 22 hours per day on low speed and 2 hours per day on high speed. (See Table A)
In each case, it costs less than $100 per year to operate the HRV and continually provide fresh, warm air to a home.
In Alaska’s climate, HRVs need a defrost mechanism to keep the exchange core from freezing on cold days. Some models, such as the Venmar EKO and the Lifebreath 100 ECM, are programmed to switch to recirculation mode (discussed early in this series), and recirculate warm indoor air through the core to prevent freeze-up for a portion of each hour. The Zehnder 350, on the other hand, uses an electric resistance heater or a ground loop to preheat incoming cold air. The electric pre-heater draws 1,000 watts. The ground loop strategy has a small 7-watt circulator that routes incoming cold air to a heat exchanger, where it is warmed by energy gathered by a glycol loop buried in the ground outside a home (so an electric heater is not necessary). Both the electric heater and the ground loop are controlled by temperature sensors, which detect when the appliance is approaching freezing. These defrost mechanisms will add some operating cost to the system during winter months. For instance, running the ground loop circulator of the Zehnder on cold days will add less than a nickel per day in Fairbanks. The electric pre-heater will add more, around $1.50 per day, depending on the outdoor temperature. It would be cheaper in Juneau and Anchorage, because of lower electric rates.
Besides electricity for fans and any defrost systems, there are no additional maintenance costs for an HRV. Filters should be checked and cleaned once a year but can be vacuumed and re-used. To lower operating costs with any ventilation system, look for features like variable speed, electrically efficient fans, and high heat recovery efficiency. Also, an HRV with programmable controls can reduce ventilation when buildings are unoccupied and save money.
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Ask a Builder articles promote home awareness for the Cold Climate Housing Research Center (CCHRC). If you have a question, contact us at email@example.com or 457-3454.