Heat recovery ventilation
by Chris Woodford. Last updated: August 14, 2020.
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 air inlet and
outlet ducts on the left and right, the diamond-shaped heat exchanger in the middle, and the air blower on the right.
HRV systems are made by many different companies, including Broan, Fantech, Honeywell,
Vaillant recovAIR, Renewaire, and Venmar. Photo by Dennis Schroeder
courtesy of NREL (US Department of Energy/National Renewable
Why do we need to recover heat?
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?
Animation: Your nose works as a kind of heat-recovery ventilation system. When you breathe out (1),
your nasal passages are warmed by heat from the outgoing air (2). When you breathe in, the cold incoming air
picks up some of this heat (3), which would otherwise have been wasted.
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 (or, more technically, regeneration): outgoing air warms your nasal passages
as it leaves; cool incoming air picks up some of that same
heat energy on its way into your lungs. As a result, the air you breathe in is warmer
than it would otherwise be, while 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
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.
Photo: A typical heat recovery ventilation (HRV) system seen from the end. You can see the air inlet and
outlet ducts much more clearly from this angle. Photo by Dennis Schroeder
courtesy of NREL (US Department of Energy/National Renewable
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
(properly ventilated homes that are neither too hot nor too damp are less likely to harbor dust mites, a very common trigger of asthma.)
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 (typically about 50–100 watts, and as high as 300 watts in some cases), 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 NREL (US 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). On the other hand, money
isn't the only relevant measure; what price good indoor air quality?
You'll see most benefit from HRV 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. Finally, if your home really struggles
with damp (or you generate a lot of moisture in kitchens and bathrooms),
you might need more than one HRV unit or a more sophisticated setup.