The air exchange system: The system in our homes that perhaps remains the biggest mystery to new homeowners. We throw the acronyms ERV and HRV around as though they're as universally understood as PB&J or LOL, but to many, the notion of active, pre-filtered fresh air exchange in your home is at odds with most homeowners' experience with the drafty New England farmhouse (which our homes, of course, are not).
To help shed some light on what these systems are and what they do, we tapped local expert, Kurt Johnson, of Fresh Air Ventilation in Lewiston, Maine. He offers some great insights, tips, and clear explanations on these critical systems.
Can you quickly define “ERV” and “HRV” - and explain the differences between them?
Energy Recovery and Heat Recovery Ventilators. Both transfer heat energy from exhaust air to fresh inbound air. ERV also transfers moisture. (Winter).
When would you recommend going with an ERV over an HRV, or vice versa?
ERVs are a good choice for most climates with normal occupancy, according to the National Research Council of Canada. In our climate here in Maine, unless there is an excess inside source of moisture (very wet basement, 5 people plus, or other moisture source) ERVs are the best choice.
Are there any concerns around cold climates, when it comes to ERVs?
Extremely cold climates present a potential core freezing issue for both HRVs and ERVs. Thankfully, we don't live in an extreme climate here in Maine. ERVs have less potential problem with this than HRVs.
How do you determine what size unit is appropriate for a given home?
Scientifically, this is the most challenging question. Opinions tend to drive the answer and I think that is a big mistake. ASHRAE has a minimum standard "Ventilation for Acceptable Indoor Air Quality" known as 62.1 or 62.2. This consensus standard has changed little since 1925 even though our buildings have become much tighter and many new chemicals have been added to the indoor environment. The Standard basically considers changing the full volume of the space once every 3 hours as the minimum. Since there are times that additional ventilation would be beneficial to the inhabitants, I think that higher rates should be available to the homeowner. How high? According to research at Lawrence Berkeley Labs, https://iaqscience.lbl.gov/ventilation-rates-and-health-homes, benefits in health and performance have been shown in multiple studies up to about 1 air change per hour or 1 ACH. Therefore, I suggest that a system should be sized so that the normal rate is near the ASHRAE minimum or higher and the system capable of up to 1 ACH. Interesting to note, in Europe, many of the guidelines are from 1 to 2 ACH rates, higher than the ASHRAE.
Are there certain brands you would recommend over others? And if so, why?
Caveat Emptor - Let the buyer beware - You get what you pay for. If you spend more on a unit, you likely get more. More airflow, more energy efficiency and maybe more serviceability. I suggest choosing based on how you pick a car. I drive a Chevy Bolt. Chevy is a big company, been around for a long time, good quality vehicles and when I need it fixed, someone local knows how.
Once a unit is installed, what do you need to do to maintain it?
Clean filters every 3 months and the core every fall. Also cleaning the intake screen on the outside of the house is very important! After that, maybe every 3 to 5 years have a professional come in and clean the fan blades and check to make sure air flow is still good. Clean exhaust diffusers/vents in rooms as they get dusty.
How much energy does a typical unit use, over the course of a year?
Energy consumption is rather small. Depending on the unit and ventilation rate, common units use as little as 20 watts and upwards of about 150. That means about $3 - $20 a month. On the heat retention side of it, about 50 to 90% savings compared to running an exhaust fan for the same volume. You can figure between $9 to $20 a month energy loss.
How frequently should filters be changed - and can they be cleaned, or do they need to be replaced?
Most filters are washable. At some point they should be changed. Maybe every 5 to 10 years. HEPA filters should be changed every 6 to 12 months or as needed.
If you have your windows open, should you shut down your ERV/HRV?
Studies have been done to see if people opening their windows is enough. What they found was that many people are just not consistent enough with keeping them open to be reliable from a ventilation standpoint. It’s important to note, off gassing happens at a higher rate with increased temperature and humidity. I encourage people to open windows as much as possible. But with rain and other factors, I find it difficult for the other people in my house to be as consistent as I would like. Look at it this way, Since there is no heat loss, the only cost is the electricity. For my basements benefit, and the rooms that don't get windows opened, I leave my system running 365 days a year.
If a homeowner wanted to learn more about their air exchange unit, are there resources you’d recommend?
Most, if not all, manufacturers have websites with information. A word of caution, sometimes a manufacturers rep. will talk from an equipment standpoint and not from an air quality standpoint. Your health is most important to you! Equipment does not reduce your risks but how you use it can.
Are there seasonal considerations settings for a unit - for humidity, or otherwise. Or are there things to be mindful of around snow accumulation at intake or exhaust ports?
Dehumidistats should be set at 35% for winter. It's not a perfect set point, but I think the most reasonable. Like any other mechanical, maintaining equipment is essential. Both to unit longevity and functioning. Blocked air pathways happen and making sure air is flowing is essential. I recently made a service call and found blockage in a duct from a bathroom. Moisture was dripping from the vent where previously there was not a problem. The user of a system needs to understand it, and keep an eye on it. Think of it as the lungs of your house. If the lungs go bad, so does the house and the people inside it.
Thank you to Kurt Johnson for contributing valuable insight on ERVs and HRVs!
