Air Exchange - ERVs and HRVs

If you've been reading the comments, you'll see that my last posting brought an excellent question from reader Terry Smiley.  She asked whether we were going to use an air-to-air heat exchanger in our house as they had done in their SIPs house in Montana.  The short answer is yes, but it was not easy to decide exactly what to use.  Building a very tight house is great for energy efficiency.  Unfortunately, it also introduces some potential problems that need to be dealt with.  Probably the most important of these issues is making sure you have enough fresh air in the house.

One of the biggest benefits of using Structural Insulated Panels (SIPs) is that air infiltration is kept to a minimum.  Thus, in the middle of winter when the wind is blowing hard, very little air can penetrate the envelope of the house and make its way inside.  Unfortunately, this creates a potential problem.  You see, in traditionally constructed houses, the fact that there are significant air leaks in a house actually has a benefit; it regularly brings fresh air into the house.  To see the potential problems with a SIPs house, imagine for a moment that the house is perfectly tight and has no leaks whatsoever.  As time passes, the occupants will use up the available oxygen in the house and the carbon dioxide levels will rise.  If no doors or windows are open, such as in the middle of winter, there is no natural way for fresh air to get inside the house.  This can be a potentially dangerous situation, particularly if there are volatile organic compounds (VOCs) present in the glues and paints that are used on the interior of the house.

Since a tight house is designed not to let any air infiltrate the envelope, some kind of mechanical means of bringing fresh air into the house is critical.  It is recommended to have approximately 0.35 air exchanges per hour, or about 8 air exchanges per day to maintain good indoor air quality. The way this is normally handled is to provide some means of air exchange.  Fresh air from outside is exchanged for (stale) interior air on a regular basis.  The simplest possible way of achieving this would be to simply keep a door or window open.  While this would certainly do the trick, it is not an ideal solution except perhaps in very mild climates (in which case it wouldn't be prudent to construct such a tight house to begin with).  A far better option is to provide some type of mechanical means of exchanging the interior and exterior air, such as by using some arrangement of fans and timers.

Of course, if you simply exchange interior and exterior air, as is, in the middle of winter, then you are exchanging warm air for cold air.  In terms of efficiency, this is not much better than simply leaving a window open all winter.  (In terms of distribution throughout the house, it is actually quite a bit better.)  A far better solution is to run the incoming air and the outgoing air through a heat exchanger.  Essentially this just brings the incoming air tube in contact with the outgoing air tube so that the incoming cold air warms up at the expense of the outgoing warm air (which cools down).  Heat exchanger technology can be quite advanced and it is common for these units to extract 80% (or more) of the available energy from the outgoing air.  (Incidentally, these systems work just as well in the summer when you want to cool the incoming air at the expense of warming up the outgoing air.)

There are two basic types of air-to-air heat exchangers available: Heat Recovery Ventilators (HRVs) and Energy Recovery Ventilators (ERVs).  They both transfer energy between the incoming and outgoing air streams.  The main difference is that ERVs also transfer moisture between the two air streams.  In a place where the summers are hot and humid, it definitely seems advantageous to transfer moisture from the incoming air to the outgoing air.  If the winters are cold and dry, it likewise seems advantageous to transfer some of the moisture from the outgoing to the incoming air.  Thus, at first glance, it appears as though an ERV would be the better choice.

Unfortunately, this are never quite so simple.  It turns out that in addition to the local climate, the way you live can have a large impact on whether an HRV or an ERV will perform better.  For example, a family that spends a lot of time at home and cooks frequently will have a much higher natural moisture level in the house compared to a family that eats out often and is home less frequently.  Thus, an HRV could be the better choice in the winter.  We have opted to purchase an ERV unit and also purchase a separate HRV core that can be easily swapped with the ERV core.  While this might seem like overkill, it turns out that the separate HRV core only cost about two hundred dollars and this will give us complete flexibility to try out both units to determine which one performs best for each season.