My family is now living in an airtight home in Maine with a passive house design. We have no exhaust fans, triple-pane windows and doors, a solar orientation, and thankfully, we have a heat recovery ventilation system (also known as an HRV or air-to-air heat exchanger) that brings in outside air and captures heat from stale indoor air before it leaves our house. Airtight homes need fresh air to ensure high indoor air quality and avoid moisture and mold issues. HRV systems are an energy efficient solution in extremely cold climates.
HRV systems, which can be used in both residential and commercial buildings, are most common in Germany, where they grew in popularity by 17.6 percent in 2012. In Germany, Austria and Switzerland, nearly 215,000 residences were equipped with systems last year. Just over half are for new construction, the majority being centralized systems. For retrofits, single-room systems are common.
The market is expected to grow by 8.6 percent in 2014 after many years of double-digit growth. This market growth is part of an energy efficient building trend towards airtight buildings in Germany, ventilated with HRV systems. These units are gaining traction in the American market, too, both for retrofits and new construction in extremely warm or cold climates. Both Energy Star and LEED-certified homes have requirements for fresh air ventilation, and HRV systems can meet the challenge.
Some of the hurdles for the American HRV market include a shortage of contractors trained in designing, installing, and maintaining HRV systems and lack of consumer awareness surrounding the benefits. If the systems are not understood, they are not likely to be installed. It is best to consider an HRV system early in the project-planning phase for new construction, as it may integrate with the HVAC system.
Although system design and efficiency vary, our Zehnder HRV system supplies fresh, filtered air to the bedrooms and draws air out of the kitchen and bathroom. Our home doesn’t have exhaust fans because these systems vent warm air out of the home in the winter without recovering the energy, thus being very inefficient. Our HRV system takes in outside air, filters and heats it with a heat exchanger before it is supplied to the bedrooms. The system is designed this way because people spend much of their time in bedrooms, and they do not contain doors to the outside (where fresh air enters when people come and go), thus there is the greatest need for fresh air.
Our system contains a booster, so we can turn it up if we want to increase the ventilation speed. We tend to do this if we burned food while cooking or notice condensation on the windows. We also turn down the system to the lowest setting when we are out of town. Our system is up to 90 percent efficient, thus the majority of the heat is captured before the air leaves the home.
Like any mechanical system, the HRV system requires some maintenance. It is recommended to vacuum the dust filters in the system every six months and replace filters every year. The systems are relatively quiet, but it does make a noticeable sound similar to a fan. Our unit is housed in the bedroom closet, which could be disruptive for a very light sleeper. The systems are least efficient when the indoor and outdoor air temperatures are similar, thus it may be more efficient to open windows and turn off the system during these times.
HRV systems allow buildings to become more and more efficient without compromising indoor air quality. The average American spends 90 percent of their time indoors, and it is common for indoor air to contain up to five times more pollutants than outdoor air. Indoor air pollution is caused by a variety of sources, such as air fresheners, offgasing from pressed wood products and solvents in cleaning products.
The market demand for high indoor air quality is rising and bringing in outside air dilutes pollutants. For more information on how these systems work, check out the video below.
Image Credit: Zehnder America