Back in the day, when most buildings were drafty, a continuous supply of fresh air was a given. A tremendous amount of heat loss was also a given. But energy was cheap! What are you gonna do?

Today, we expect better. Modern homes are sealed much more thoroughly and that brings with it some tradeoffs. For one, we can spend drastically less—and waste drastically less—on heating and cooling. But we can also find that our indoor air quality suffers. Tight, well-sealed homes can lead to rising levels of mold, allergens, particulates, and moisture, all of which can affect human health for the worse. Plus, excess moisture will wreck your house.

Fortunately, there are two machines for your home’s heating/ventilation/air conditioning (HVAC) system that capture the heat or cool from the stale indoor air, transfer it to the fresh incoming air, and also filter out those nasty bits that are floating around. One is the heat recovery ventilator (HRV) and the other is the energy recovery ventilator (ERV). The main difference is that the ERV also transfers moisture, so it tends to be the choice in warmer and more humid areas of the U.S.

How Do HRVs and ERVs Work?

Both HRVs and ERVs must be installed as part of a forced-air system with ductwork, preferably separate from the furnace/heat pump and AC. They pull fresh outdoor air in through ductwork into a heat exchanger inside the HRV/ERV. Meanwhile, stale and polluted indoor air is also coming into the heat exchanger to be exhausted outside, so a filter captures the mold, particulates, and allergens.

Now here’s how a heat exchanger works; it’s very simple. The second law of thermodynamics states that when two things are in proximity to each other, heat always moves from the hotter thing to the colder thing. That’s why your hot food cools in room temperature air, and your hot bathwater cools down to the ambient air temperature over time.

In the case of an HRV/ERV we have two airflows, one warm and one cooler. The heat exchanger separates the two airflows, indoor and outdoor, with thin metal plates. The two airflows do not actually come in contact with each other. As the air flows across the plates, heat transfer occurs across the plates. The hotter airflow gives up some of its heat across the metal plates, while the colder airflow accepts some of that heat.

In this way, the HRV/ERV captures heat from the indoor air in the winter and transfers it to cold but fresh incoming air. The HRV/ERV “prewarms” the incoming air so your furnace or heat pump has less work to do. In the summer, the HRV/ERV can capture the warmth from the incoming air, lowering its temperature and transferring it to the stale outgoing air. That way, your air conditioner has less work to do on the incoming air.

Do I Still Need A Furnace and AC?

Yes, you do. HRVs and ERVs are not heaters or air conditioners, so you will still need a way to heat and cool your home, such as your furnace or, even better, a heat pump, as they’re more efficient. If you live in a very low-humidity area an ERV may control your indoor humidity very nicely, though!

HRVs and ERVs are extremely efficient, also. One manufacturer, Zehnder, claims that their units are up to 90% efficient in transferring energy (heat or cool) from one airflow to another. That may mean that your heating and cooling systems can be downsized, as their load has been lessened somewhat.

The best way to install an HRV or ERV, according to Zehnder, is separate from your forced air furnace and AC system. That’s because using the same ductwork for both systems will often change the system pressure when the furnace and AC switch on, which could lead to your HRV/ERV operating poorly on shared ductwork.

Your forced-air furnace or heat pump, along with forced-air AC, will turn on and switch off when the thermostat calls for it based on conditions so that it will go from zero pressure to its operating pressure and back to zero many times per day. The HRV/ERV, in contrast, should usually run all the time to continuously filter your indoor air, especially when temperatures are cold and your furnace is running. Also, if you use a gas cooktop your HRV/ERV should be running to help eliminate those pollutants. Alternatively, you can set the unit to run for a period of time every hour.

In warm months, however, it may be a different story. If you often have your windows open, then your home is already getting complete air changes, so running your HRV/ERV would accomplish nothing. On the other hand, if your outdoor air quality is not good, you’d be better off closing your windows and running your HRV/ERV to ensure good indoor air quality.

Why Not Just Use A Fan To Exhaust Stale Air?

This is a bad idea. A fan, such as a bathroom exhaust fan that vents outside, can definitely get rid of the extra moisture in the bathroom. It creates a couple of problems, though. This type of fan pulls air from throughout the house, and also tends to pull air in from outside the house through gaps and cracks and anyplace air can sneak in. It’s called “makeup air” and it’s both unfiltered and unconditioned, which leads to spending more on your heating and cooling than you need to. This situation also creates a pressure imbalance that tends to produce drafts that you’ll notice most in winter.

In contrast, an HRV or ERV has a dedicated ducted incoming supply and balances the incoming and outgoing air supplies as it’s running so you never (or rarely) feel as though air is whooshing around in your house. You’ll experience a constant and steady flow of fresh and filtered air.

How Do I Know If I Need An HRV or ERV?

If you have a new and well-sealed home and often keep the windows closed, then an HRV or ERV is definitely a good idea. If you have a drafty older home, you’ll need to address those areas where air can easily get in and out through air sealing before installing an HRV or ERV makes much sense. And if you live in a mild climate with good air quality and usually have the windows open, then an HRV or ERV just won’t do much for you. In that case, though, a stand-alone air purifier may be the thing to pull allergens, mold, and pet gunk out of the air that you breathe.

What Do HRVs and ERVs Cost?

It’s tough to give a good estimate of a system cost because each one is a custom installation. The HRV/ERV unit itself will typically be several hundred dollars to well over $1,000, plus ductwork and installation. Your best bet is to get bids from 2-5 HVAC professionals. Good luck!