How do HRVs and ERVs work?

Today’s buildings are being built tighter than ever before.  Extreme care is taken to install modern siding, use high-quality insulation, and seal all cracks to prevent air from leaking in or out.  Such tight buildings are designed with the intention of keeping as much heat inside as possible in order to save energy and reduce heating costs.  This technique, though, has some unintended negative consequences.  While drafts are typically thought of as unwanted, indoor air quality can suffer from a lack of air exchange.  Nobody likes to breathe in old, stale air, and without proper ventilation, humidity and pollutants can build up and damage your property and your health.  Luckily, there are several solutions to this problem.


The simplest way to increase ventilation is to create an opening in the walls yourself.  No, don’t go bashing in your walls – just open a window or door.  To facilitate air flow even more, you can put a fan in one window facing outwards and open another window on the other side of the building.  As air is sent out through one window, the negative pressure brings in a fresh breeze through the other window.  However, unless the temperature is just right outside, it is clear that this is not an ideal option.  Not only will you be too warm or cold for your own comfort, but you’ll likely be wasting money heating or cooling your indoor air, which is swiftly being sent outside.  Also, because you have little control over what air is coming in or leaving through these openings, you may not be exhausting the pollutants from inside your home, and might even be introducing new contaminants from outdoors.

A great solution to all of these problems is using a heat recovery ventilator (HRV) or an energy recovery ventilator (ERV).  Both an ERV and a HRV provide the same principal function, but ERV’s can have additional benefits when they are installed in certain climates.  At a basic level, the purpose of either system is to provide fresh outside air while exhausting stale indoor air.   As we mentioned previously, introducing fresh air could affect the comfort of your home and increase your heating and cooling costs.  The main benefit to using an HRV or ERV is that it transfers much of the heat from one air stream to the other.  This means that in the winter, your hot, stale, indoor air is being used to preheat the fresh, cold air from outdoors.  With the incoming air now much warmer than it began, you will use much less energy conditioning it once it gets inside.  During the summer, the cold air from indoors steals heat from the incoming, hot air, cooling it, and reducing the cost of cooling your home to a comfortable level.

So how do ERVs and HRVs provide ventilation without wasting energy?  It’s quite a simple process, actually.  The physics behind it is related to the second law of thermodynamics, which generally states that energy will spontaneously move from areas of high energy to areas of lower energy.  What we know as temperature is actually a measure of the average kinetic energy of molecules of a substance.  Therefore, when two substances with two different temperatures meet, energy (i.e. heat) is transferred from the high temperature substance to the low temperature substance  (see our article on heat pumps for a longer description).  The transfer of energy is never complete, but if given enough time, heat will be transferred to the lower temperature substance until the two have equal temperatures (think about mixing one cup of hot water and one cup of cold water to make two cups of warm water).

In HRVs, two sources of air are brought into the system with different initial temperatures.  Since it’s getting colder out, let’s talk about when the outdoor air is cold and the indoor air is relatively warm (especially when being heated).

Air flow in an HRV or ERV during the winter.  In the summer, temperatures reverse.

The HRV has a heat exchanger inside, usually made of aluminum, which allows indoor and outdoor air to pass through without actually touching each other.  This helps to prevent any cross contamination between the stale indoor air and fresh outdoor air.  The heat exchanger is usually made such that there are many small channels for air to flow through as opposed to just two big ones.  These smaller channels allow for the air to spread across a larger surface area, allowing heat to travel more quickly between the two air streams.  As an example, imagine two pots each boiling a cup of water.  One pot is a small saucepan.  The other is a large soup pot.  The water will boil faster in the soup pot because the same amount of water is spread over a larger area. This allows for more of the water to be in direct contact with the hot pot and absorb heat more quickly.   In the same way, the warm indoor air will be able to transfer its heat to the incoming outdoor air more rapidly if it can heat more air at a time due to increased surface area.  In addition to the heat exchanger, ERVs and HRVs also include a blower to distribute the now warm fresh air throughout the house, a second blower to exhaust the stale air outside, and filters to make sure you are not introducing any unwanted contaminants from outdoors.


As discussed in our previous article about preparing for the heating season, low humidity levels in the winter can cause property damage and even health issues, such as scratchy throats and dry skin.  An added benefit of using an ERV instead of an HRV is that ERVs also transfer moisture.  An ERV heat exchanger is usually made of a different material, often paper, so that moisture can transfer between incoming and outgoing air while still preventing cross-contamination of air streams.  Like temperature, moisture in the air will also move from an area of high concentration to an area of low concentration.  This means that if you use an ERV in the winter, your warm and humid outgoing air will transfer both heat and moisture into the cold and typically dry incoming air.  This is not to say that you should throw away your humidifier, but it can certainly make you more comfortable and decrease the load on the humidifier you are currently using.  In the summer, the ERV will work as a dehumidifier, since the exchanging process is reversed.  This can help prevent your home from having too much moisture in the air, which can cause issues such as mold and mildew growth.

While ERVs and HRVs can be expensive to install, they have incredibly low maintenance and operating costs. Because ERVs and HRVs are mostly passive devices that only use energy to circulate air, they are very efficient and often generate far more in energy savings than what is needed to operate them.  How beneficial either one is, though, will depend on both your climate, the type of building you would like to ventilate, and the method of installation.  Due to the complexity of the decision, it’s best to contact a trustworthy dealer or contractor who can properly size your home and determine the right solution for you.