What is an HRV or ERV system?
If your home is reasonably airtight, it may need an energy-efficient ventilation system to supply fresh air. These systems, known as Heat Recovery Ventilators (HRV) and Energy Recovery Ventilators (ERV), save energy and improve indoor air quality. In this article we will touch on maintenace, installation and operation of your HRV or ERV system.
Do you need an HRV or ERV system?
For new construction, yes. A new home that meets current building codes or high-efficiency standards like Passive House, should be quite airtight, and needs to have an air ventilation system.
If you have an older house that is drafty now, it depends how airtight you are planning to make it through air sealing renovations. If you are doing a major upgrade that makes the house much more airtight, for example by wrapping the house in a new layer of insulation and air barrier, or having closed-cell spray foam applied around the whole exterior, you might make the house airtight enough to need a ventilation system. That’s good news because you will save boatloads of energy (and money!) and improve the air in your home.
The best way to know whether your renovated house needs a ventilation system is to have a home energy assessment that includes a blower door test before and after your renovations. In a blower-door test, a Certified Energy Advisor temporarily installs a large fan in an exterior door and uses the fan to blow air out of the house to test how airtight the house is.
“ Airtightness is measured in Air Changes per Hour (ACH), which indicates how many times per hour the volume of air in your house exchanges with the outside air through existing holes and cracks in the building envelope.”
Why is air ventilation important?
We all need fresh air to breathe. Beyond that, the emissions from activities inside the house need to be removed to the outside, so they don’t build up to damaging concentrations. These include carbon dioxide (from breathing), and water vapour (also from breathing, but mostly from showers and cooking). If your house is not getting enough air exchange, one sign can be water condensing and dripping down windowpanes when it’s cold out, especially in rooms other than the bathroom and kitchen.
Venting emissions with a simple exhaust fan, like a bathroom fan or range hood fan certainly helps. But whole-house ventilation systems like an HRV or ERV have significant advantages – they cover the whole house, and they recover the thermal energy in the exhaust air and use it to preheat the incoming fresh air in winter, or precool it in summer, saving you money on your energy bills.
Which should I choose – an HRV or an ERV?
The short answer is you can use either one. They have different advantages depending on your climate and how you operate your house.
Heat Recovery Ventilator (HRV) – Recovers heat!
Winter season: HRV recovers heat energy from the exhaust and uses it to preheat the fresh air, saving you energy. Heat is transferred across a heat exchange core, so your fresh air is not contaminated by your exhaust air.
Summer season: If you have air conditioning, you can use an HRV to precool the fresh air coming into your house. If it’s not too hot out and you don’t have or aren’t using air conditioning, you can also open windows for fresh air and shut off the HRV.
Energy Recovery Ventilator (ERV) – Recovers heat and moisture!
Winter season: ERV preheats the fresh air with energy from the exhaust air, and transfers moisture from the exhaust air to the dry fresh air coming in, making the fresh air supply more humid.
Summer season: If you are using air conditioning, the ERV precools your fresh air supply and transfers moisture out of the humid fresh air into the exhaust air, making your fresh air supply to the house less humid.
“The key difference is that an ERV will make your home more humid in winter, and less humid in summer, compared with an HRV. In winter this is not often an advantage, because one of the reasons to ventilate in winter is to get rid of excess humidity.”
Where an ERV really shines is in summer in a hot, humid climate. On those hot, muggy summer days the ERV will remove some of the humidity from the outside air before distributing it through your house, making your house less humid and more comfortable. This advantage only applies if you are using air conditioning, because if not, then the indoor and outdoor humidity will be relatively similar anyway and no humidity transfer will happen.
An ERV can also help in a place with an extremely cold and dry winter climate, because there it can help humidify a house that might otherwise be too dry. If you would otherwise need a humidifier in your house in winter, an ERV could be advantageous.
In the end, the difference between an HRV and an ERV is not that large. In a reasonably airtight house, either one will be a significant improvement over not having ventilation. An HRV is a good default choice, with an ERV being helpful in those climates with seasons of extreme humidity – extremely cold and dry or hot and humid.
Things to consider when choosing a model of HRV or ERV
Efficiency: Called “Sensible Recovery Efficiency”, rated by the Home Ventilating Institute (HVI), ranges from 55% to over 90% heat recovery. Higher saves you more.
Good controls: Look for units with multiple or variable speeds, and/or smarter controls based on humidity, so they can ramp up and down as needed.
Quiet: There are two fans inside an HRV or ERV, and they are going to run much of the time, so you want a quiet system. Unfortunately, it’s hard to find sound ratings for these units – HVI doesn’t give sound ratings for HRVs because it depends so much on where and how they are installed. Talk with a professional installer about which ones they’ve found are quieter, and how to keep the whole ventilation system quiet, with such considerations as unit placement, duct design, sound isolation, and large enough air diffusers. You could even ask to go listen to one in operation.
Air filtering options: Consider what kinds of advanced air filtration are available, such as HEPA filters that remove smaller particles, if you are especially sensitive to dust or pollen in the air. All HRVs and ERVs come with a basic air filter, so they all decrease the amount of dust particles in your home.
Correct design of ductwork and installation of a whole-home heat recovery ventilation system can be tricky and involves a lot of details. We recommend working with an experienced home ventilation professional on this.
Care, Maintenance, and Operation
All HRVs and ERVs have an air filter inside them that needs to be cleaned regularly, every two months. You can do this job yourself. Check the manual of your unit, and here is a great guide to HRV maintenance.
Winter operation (cold climate): Run the ventilation system continuously at a low speed, and ramp it up when there is extra moisture in the house, such as when showering or cooking. This is where multiple speed settings and easy-to-use controls are helpful.
Spring / Summer / Fall (moderate climate): In the non-heating seasons, if you are not running air conditioning you could just open some windows for fresh air and turn off the ventilation system.
Summer (hot climate): If you are running air conditioning, it’s best to keep the windows closed and use the ventilation system.
What if the power goes out?
An HRV or ERV requires a continuous supply of electricity. If the power is out, it will stop working unless you have a backup power supply. This is no big deal, but if your house is extremely airtight you may need to open a few windows a bit to let in fresh air for the duration of the power outage. This will lose you some heat in winter, but may be needed if the air is getting stuffy or water is condensing on windows.
If your house is, or will become, reasonably airtight (less than 3 air changes per hour), a heat recovery or energy recovery ventilation system is a good investment in energy savings and air quality. Choose a quiet model with high efficiency. It doesn’t matter a lot whether it’s an HRV or and ERV. An HRV is a reasonable choice unless the outdoor climate makes your house get uncomfortably dry in winter or humid in summer.