Heat recovery ventilation
by Chris Woodford. Last updated: April 5, 2017.
Shut that door and keep the heat in—it's a familiar cry in winter; in summertime, you're more likely to see people closing doors and windows to keep the heat out and save on the air-conditioning. How can you have an airtight, energy efficient home that's also healthy and well-ventilated? Heat recovery ventilation (HRV) and energy recovery ventilation (ERV) (sometimes also called mechanical ventilation with heat recovery, or MVHR) offer a solution, bringing fresh air into your home without letting the heat escape. Let's take a closer look at how they work!
Photo: The inside of a typical heat recovery ventilation (HRV) system. You can see the blue and red air ducts on the left, the diamond-shaped heat exchanger in the middle, and the air blowers on the right. HRV systems are made by many different companies, including Broan, Fantech, Honeywell, Vaillant recovAIR, Renewaire, and Venmar. Photo courtesy of IBACOS and US DOE/NREL (Department of Energy/National Renewable Energy Laboratory).
What's the problem?
Modern homes are usually built to far higher technical standards than buildings constructed a few decades ago and are much more energy efficient, largely thanks to better heat insulation. One key area of improvement has been to make buildings more airtight so they hold onto the heat we put into them for longer. But there's a drawback: our homes need regular changes of air to keep them healthy. Baths and showers, doing the dishes, clothes washing machines, drying clothes indoors, and even simple breathing produce astonishing amounts of water inside our homes: according to leading ventilation manufacturer Vaillant, a typical family will produce 10–15 liters (3–4 gallons) of moisture each day! Let that problem go unchecked and you'll get problems like mold and mildew, dust mites and a greater risk of asthma. Opening doors and windows is the obvious way to get rid of moisture and bring in fresh air, but if you do that in winter you might just as well flush your money down the toilet: all the heat you've expensively introduced into your home will blow away in the breeze. An old drafty house solves this problem by being automatically well ventilated, but it's probably also freezing cold because it's useless at holding onto heat; a modern energy-efficient home solves the draft problem but may be stuffy and underventilated. So what to do?
Photo: The basic principle of a heat exchanger: a hot, outgoing fluid (red) flows past a colder, incoming fluid (blue). Without them actually mixing together, the hot fluid gives up most of its heat to the cold fluid.
Let's look to nature, which solved this problem some time ago. Our bodies are a bit like our homes inasmuch as they need regular supplies of fresh air and have constant clouds of damp, "stuffy" air to get rid of. How do they do it? With an ingenious invention called the nose! As a child, you might have learned that it's better to breathe through your nose than through your mouth because your nose warms and filters incoming air. What your nose actually does is called heat exchange: it lets cool incoming air flow very close to warm outgoing air so heat energy is transferred between the two instead of being lost. As a result, the air you breathe in is warmer and the air you breathe out is cooler—and (among other things) that helps your body to retain heat energy.
What is heat recovery ventilation?
Artwork: How an HRV works (simplified): The hot, moist waste air from the home (passing down the yellow duct) gives up virtually all its heat as it passes through the heat exchanger on its way out of the building. The cold, dry incoming air (flowing through the brown duct) picks this heat up as it flows in. Ideally, no heat is lost. Since the incoming and outgoing air flow past in opposite directions, this approach is known as a counterflow.
HRVs are essentially noses on houses: they consist of two ventilation ducts running next to one another passing between the inside and the outside of a house. One carries cool, fresh air in; the other carries moist, stale air out. The clever bit is that the airstreams run through a device called a heat exchanger that allows the outgoing air to pass most of its heat to the incoming air without the two airstreams actually mixing together (read how this works in our article on heat exchangers). Usually there's a fan (blower) in each duct that can be turned up or down either manually or automatically depending on the temperature and humidity levels. The incoming air supply may also have a bypass fitted to it so that on summer days when it's cooler outside than in, cold outside air can be channeled straight into the home without meeting outgoing air (much like opening a sash window).
Artwork: How an HRV works (in more detail): This is the layout of an actual HRV unit showing the two airflow paths and six isolated compartments in a bit more detail. Fresh air enters the building from outside at point 1 and is pumped into the room at point 2, inside the building, passing through the three compartments colored gray, and following the blue arrowed path. On the way, it picks up heat from the diamond-shaped heat exchanger (red), pulled by the pink blower. Stale exhaust air exits from the room at point 3 and leaves the building at point 4, passing through the three blue compartments along the red arrowed path. It also passes through the heat exchanger, giving up heat, and is helped on its way by the second blower, colored cyan. From US Patent 5,632,334: Heat recovery ventilator with room air defrosting feature by Peter K. Grinbergs and Grant W. Miles. Nutech Energy Systems Inc., May 27, 1997, courtesy of US Patent and Trademark Office, with colors added for clarity.
In small homes, an HRV might consist of a single unit on one wall that effectively ventilates the entire building over time as doors open and close between rooms. In larger homes and offices, there may be ventilation grids in each room feeding into ducts that run between the floors or ceilings of the the building to a single ventilator on the outside wall.
What's the difference between HRV and ERV?
Not all HRVs work in exactly the same way. An alternative system called energy recovery ventilation (ERV) works in a similar way but transfers some of the moisture from the outgoing airstream into the incoming air, so it keeps the humidity in your home at a constant level. That's important if you don't want your home too dry. As a general rule, ERV is a better option if you have air conditioning and live in a humid climate, because it will help to keep moisture outside, reducing the load on your air conditioner and saving on the air-con bills. HRV is often better if you don't have air conditioning, or live in a less humid climate, since it will help keep the humidity down by transferring excess indoor moisture outside.
What are the advantages and disadvantages of HRV?
HRVs and ERVs have an obvious appeal: they give you a warm well ventilated home and stop you "emptying your wallet" into the atmosphere every time you open your windows. In winter, they can help save on your heating bills; in summer, they reduce the need for air conditioning. By keeping excess moisture out of your home, they're better for your building, your furnishings, and your health and they help to keep the "climate" inside your home at a more constant level. Typically they retain about two thirds to three quarters of the heat that would normally be lost from your home through ventilation (some manufacturers claim 85–95 percent), so they really do save energy. How much energy? According to British environmental auditor Nicola Terry's calculations, HRV can safely cut the number of air changes per hour in a "leaky house" by about 50 percent, reducing the energy lost through ventilation by about 65 percent. A small amount of this energy is used to power the electric fans in the HRV system, but there's still a considerable energy saving.
Photo: Large HRV systems use ducts like these running between floors and ceilings. Photo by Warren Gretz courtesy of US DOE/NREL (Department of Energy/National Renewable Energy Laboratory).
On the downside, HRVs are expensive to install initially (several thousand dollars is typical) and they're not guaranteed to pay for themselves (typical annual savings might be a few hundred dollars). You'll see most benefit in extreme climates: where the difference between the outdoor and indoor temperatures is greatest in summer, winter, or both. In milder climates, the benefits are much reduced and may, in some cases, be nonexistent. Don't forget that a typical HRV has a couple of small, electric fan blowers in it and costs money to run: you'll only save money overall if you can recoup the installation costs and generate enough savings to cover the running costs as well. If you're environmentally minded and money is less of an issue, saving more energy in heat recovery than you use in the system itself is obviously the thing you need to focus on. If you're using HRV in particularly cold climates, you'll need slightly more sophisticated equipment to stop the system from freezing up. HRVs also need regular maintenance, with filters that typically need cleaning or replacing every 6–12 months.