Not long ago, my wife and I decided to install an Energy Recovery Ventilator in our home to improve the quality of our indoor air in our home in Durham, NC. Prior to this decision we had begun to tighten up the house and reduce energy loss with spray foam insulation (SFI) on the roof deck and a sealed and insulated crawl space.
The ERV would help ensure we were getting an appropriate volume of healthy, fresh air into the house. Air tightness, which is great for energy efficiency, reduces natural air exchange in the home; oxygen that gets used by appliances and people does not get replaced as quickly as in a leaky, albeit less efficient home. This can result in trouble breathing and sleeping, and can present a real problem if there are combustion appliances in the home, unless you make clear and appropriate mechanical provisions for bringing fresh air into the house.
Filtering Air and Saving Energy
Letting random, unfiltered air flow through a house is potentially bad for health as it draws in crawl space air, soil gases, etc., and makes heating and air systems work harder. With this in mind, we considered two options:
1. An occupancy sensor with an adjustable timer for bath exhaust fans, which can be paired with a passive inlet through the exterior wall
2. An Energy or Heat Recovery Ventilator (ERV or HRV)
There are merits to both, and cost implications. While option 1 is less expensive, it still brings in, through a simple dust filter, whatever air is outside. Since this is most critical in months when we don’t have the windows open, that air could be 100º/95% humidity (August), or 20º/30% relative humidity (February). Neither is suitable in terms of comfort and both would cause our heating/cooling system to work harder.
Ultimately, I chose an ERV because it not only filters the air that comes in but captures or recovers some of the energy that is lost when air is exhausted from our home. An ERV takes air out of the house, typically from bathrooms, utility or mudroom, or even a conditioned crawl space, and delivers fresh air from outside, usually to spaces where we spend a lot of time like bedrooms. When it does this, it runs the two volumes of air past each other so that both sensible air temperature (what you feel and what a thermometer registers) and latent heat (heat contained in moisture) are transferred between the volumes of air. These air volumes are not mixed but rather run adjacent to each other and are separated by thin conductive material.
Most ERV and HRV manufacturers also provide options for controlling the systems from adjusting the speed of the fan on the unit itself to a full functioning touchpad thermostat-like module that can be programmed to a schedule. ERVs are about 60-70% efficient, meaning that if I have 68º/50% RH air inside, and it’s bringing in 18º/30% RH outside air (a 50º/20% spread), the 18º/30% air will be warmed to somewhere around 50º/42%RH before it is delivered to my bedrooms. This eases the load on our heating and air conditioning system while introducing the fresh air that helps us breath easier and sleep better.
ERVs or HRVs are usually installed either in line with heating and air systems, or separately with independent controls. We chose a separate installation so that the systems could operate independently of each other. The day it was installed I immediately noticed a difference. The air felt fresher and odors were diminished. We felt like this was a worthwhile investment for the health and comfort of our growing family.
While simplified here for the sake of the homeowner audience, ventilation and energy recovery is actually a fairly complex subject and care should be taken in the selection and application of equipment. If you are serious about purchasing an ERV or HRV, I suggest you first read this article, “Musings of an Energy Nerd” at one of my favorite websites, GreenBuildingAdvisor.com. The information is thorough, yet easy enough for a (nerdy) layperson to understand.
Bradley Yoder is a homeowner in Durham, NC, and works as Building Performance Specialist at BuildSense.