Balanced mechanical ventilation is code required in all new construction in my home state of Minnesota. Indoor air quality has become a big topic with today’s pandemic. My market uses HRV’s or heat recovery ventilators almost exclusively. They work great at supplying the home with filtered fresh air and at controlling indoor humidity levels. You can read more about HRV’s here.
I recently had a conversation on social media with Mark LaLiberte from Construction Instruction about moving to ERV’s or energy recovery ventilators in my very cold climate. Something I did not think could be done. I still have some reservations about the switch, I feel homeowners need to have a good understanding of moisture loads inside their home, but there are ways, and some advantages to making the change. First, lets look at the differences between an ERV and HRV.
The biggest, and most important difference between an energy recovery ventilator (ERV) and a heat recovery ventilator (HRV) is the difference the two handle moisture. In an HRV, the outgoing stream of air, taking stale air out of the home, will contain some water vapor. As this air crosses (without mixing with) the incoming cold, fresh air from the outside, there is a possibility there will be some condensation and frost that will accumulate on the heat exchanger core, especially during the heating season. The purpose of them crossing, but not touching is to transfer heat from the outgoing air stream to the incoming air stream, increasing the incoming air temperature before distributing it into the home. HRV’s are designed to handle this condensation issue by using a defrost cycle, which will change any frost that forms back to a liquid and allow it to exit the HRV case through a drain. No moisture is re-introduced back into the home. This removal of indoor humidity and replacing with dryer outdoor air can lead to homes becoming very dry during the winter months.
The top left blue port in the above photo is the incoming fresh air from outside. This air moves across the heat exchanger core and filters to the lower right hand corner of the HRV where it is distributed into the home. The lower left area is where the outgoing stale air stream crosses the heat exchanger, exchanging some of it’s heat with the incoming air stream. It moves to the upper right stale air exhaust port and is forced outside the home. Condensation often occurs when the warm, inside air that has some moisture content comes in contact with the cold heat exchanger.
The above photo shows the drain pan and drain for an HRV. These units can produce large amounts of moisture in cold and very cold climates and need a place to drain all this water.
An ERV will handle moisture in the incoming and outgoing streams in a different way, it allows some of the moisture to move into the incoming fresh air stream, (again, without the two air streams actually touching), which can increase the indoor moisture level. Part of designing comfort into a home is by using moisture, specifically latent heat. Most people will find 70°F and 50% relative humidity more comfortable than 70°F and 20% relative humidity. It has to do with evaporating moisture off our skin, more evaporates at lower humidity levels than higher levels. Having a little more humidity inside a home during cold and very cold temperatures can be more comfortable than very dry air. An HRV will make the air more dry, and ERV’s can add some moisture back into the air.
So, why aren’t we using ERV’s in cold and very cold climate? Because most homes in cold climates, even the new ones, aren’t designed to handle moisture levels over 30% during the heating season.
This photo was taken very early in the heating season at a new home. The interior moisture level was a little more than 40%, outside temperature was around 20°F. The windows became dehumidifiers at these relatively warm temps and moderate indoor humidity levels. Keep this moisture level around 40% and drop the temps to this past heating season’s polar vortex low of -35°F and this window will be solid ice and possibly inoperable.
Another consideration is air leaks, especially high in the building envelope. In my climate, stack effect can push moisture from a home with high indoor humidity into unconditioned spaces where it can come in contact with a condensing surface. Even new homes with code minimum air tightness levels can experience frost forming in these areas. These are two big reasons humidity levels are maintained at 30% or less during the heating season.
What am I saying? In a cold and very cold climate, if you want increased indoor humidity during the heating season, the home needs to be designed for the moisture load. A very tight envelope will help to keep the moisture inside the home and great triple pane windows that most likely won’t become dehumidifiers are two important parts. Additional details such as exterior insulation to keep condensing surfaces above certain temperatures and ways to dry assemblies that become wet, such as vented rainscreens and eliminating low perm products such as polyethylene sheeting will be required. These details will allow for a higher indoor humidity levels, and permit the change from an HRV to ERV. Comfort, health and building durability can all be achieved through good design, ERV’s can be part of the equation in a cold or very cold climate.