BY: Ilya Benesch, Cold Climate Housing Research Center
Energy Focus: Fairbanks Daily News-Miner September 11th, 2008, Section A3
Heat Recovery Ventilation (HRV) systems are a relative newcomer to the cold climate construction scene, yet have become almost indispensible in today’s super-insulated, air tight homes. They are also becoming an increasingly common element in the current weatherization and insulation retrofitting trend. As older homes are undergoing energy facelifts, and becoming tighter and better insulated, they are also facing the same indoor air quality challenges one would see in new construction. In this article I am hoping to provide a basic understanding of how HRV’s work, their applications, and their advantages.
The HRV is principally designed to supply a regulated exchange of fresh air to the house, while simultaneously expelling stale indoor air. This is of particular importance in a home that is too tight to do so on its own, through passive means. At the core of the HRV unit is a heat exchanger where the airways exhausting the warm, moisture laden indoor air, flow next to the air passages bringing in outside air. At this junction, the cooler incoming air is warmed by the outgoing exhaust air, recovering a substantial amount of heat that would otherwise simply be lost. Typical heat recovery percentages can range from 70 to over 90 percent, depending on the unit and the controls. This is where the HRV shines in comparison to a simple exhaust fan that blows warm air directly outside.
A typical HRV system is designed such that its ducts supply fresh air to bedrooms and living areas while exhausting stale, humid air from bathrooms, kitchens, laundry rooms, and crawlspaces. The HRV should not be substituted in place of a cooking fan, so a range hood should still remain the primary vehicle to remove grease and smoke from above the cook stove. The HRV is designed to be balanced so that it takes in as much air as it exhausts, maintaining close to neutral pressure. Consequently, it is important to note that it is not meant to be a means of supplying air to combustion appliances. Because the system is neutral, it does not present potential back drafting hazards such as one might find in conventional unregulated exhaust fans, which can possibly create excessive negative pressures. Most units run efficiently, using about as much power as one 60 watt bulb when operating, and the industry is constantly improving energy use and performance. As with any appliance, the HRV requires some minor maintenance. Every fall would be a good time to check the built in filters and clean or replace them as necessary.
In addition to regulating air flow in a home, HRV systems can also perform several other important functions. After talking with Rich Musick and Bill Reynolds at Solutions to Healthy Breathing, I learned that it is now possible to install an in-line filter system directly after the inbound fresh air leaves the HRV unit. This allows the home owner to filter the incoming air for particulates and odors. One application in this regard might present itself this winter to those people who may find themselves in neighborhoods with poor air quality due to excessive wood or coal smoke. The potential for pollution levels to increase in Fairbanks this winter is very real, and people with asthma or other breathing issues could benefit from this option.
One other feature worth mentioning is that with the right controls, an HRV can be made to operate in a recirculation mode. A humidity sensor in the unit insures that it meets the house’s air exchange needs, after that it can be programmed to default into recirculation. This would be an excellent feature for people who are using a woodstove as a major source of heat and need a means to distribute that heat to hard to reach areas in the house.
Up front an HRV system may seem like a significant expense, but it is also important to look at it as an investment. By improving indoor air quality, the HRV can help to insure peace of mind by providing a healthy living environment year after year. It also helps contribute to the longevity of the structure by removing moisture before it has the opportunity to do real damage. Keep in mind that in this climate, indoor moisture problems are a big concern not only to the house, but also the occupants if mold is allowed to proliferate. If you own an older unit, a control upgrade may be an option, and prove quite cost-effective and beneficial. If you are thinking about purchasing a system, as always, it pays to learn about specifics, and find an installer who is willing to educate and stand behind their product.
Ilya Benesch is the Building Educator at the Cold Climate Housing Research Center (CCHRC). For questions or comments please contact CCHRC at (907) 457-3454
I like the overview, and you reminded me of some thoughts I’ve had.
If the goal of a HRV is to maintain pressure around neutral for the home, how does one integrate a range/oven fan in keeping balance in the home? Is a cold air intake for the combustion units of the house sufficient to provide supply air for those type of exhaust-only units?
Good question. A good illustration of this scenario would be a 1600sf house I built two years ago that blower door tested at .07 natural air exchanges per hour. In this instance the boiler was part of the house, but had it’s own room with make up air and was meticulously sealed to isolate it. This is not always the case. More and more homes are coming up with really low numbers like this – the REMOTE wall construction systems being a good case in point, so it should be addressed. One popular solution in the past has been to cut a hole in the wall in an area where it best meets demand, and use a cold trap arrangement as you would for a boiler. Not the prettiest fix, but effective. Typically a pipe would come in at about waist level and then turn up and travel 4 feet or so to within 6 inches of the ceiling. Theoretically, the cold air would stay down low until enough negative pressure created a demand to draw it up the pipe and into the room. One drawback here was that any slight pressure differential could induce cold air to come in..ie opening an exterior door, or an improperly balanced forced air furnace.
I talked with Bill Reynolds again at Solutions to Healthy Breathing and he suggested a better option. Say, as you mentioned, you have a high cfm range fan in the kitchen. Somewhere on an exterior wall, preferably in the vicinity of of the fan, you would cut a 6 inch hole and insert a duct from inside to outside. You can place your grilles of choice on both sides of the wall. Inside the duct you would install an inline backdraft damper facing inwards. If the negative pressure gets too big, then the damper is sensitive enough that it can open and supply the needed air, otherwise it stays closed. I have bought these dampers before at Holaday Parks….I remember they ran about $20. I haven’t checked, but Ferguson may carry them as well. Incidentally, those dampers work really well for sealing outgoing exhaust ducts such as dryer vents, bath fans, and range hoods. Just make sure they are facing the proper direction when you install them.
I would like to know how one would go about installing a hrv unit in a house with a boiler system. Thank you.
How come my HRV unit only blows cold air?
I was wondering if you can control the hunidity in a house with an HRV system?
how to design an effective hrv system
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