Let’s start with “What is an HRV?”

A Heat Recovery Ventilator is a system which pulls fresh air into the house and pushes stale air out of the house under fan pressure, that’s the ventilator part. The heat recovery part is accomplished by forcing the incoming cold air (in winter) through a series of shallow and broad chambers which are in contact with another series of shallow and broad chambers containing stale warm air going the other way. In this way the cold air coming in is warmed by the warm stale air going out. This is a simplification, you’d have to see a diagram or the real thing to completely understand it.

Why would we want an HRV? The answer is to increase and regulate the number of air changes per hour in our homes. Houses built before the mid-seventies that haven’t been upgraded have more air changes per hour than are needed to maintain a healthy supply of fresh air in the house. After the mid-seventies, houses became more tightly sealed because of rising fuel prices. The R2000 program was in development: R2000 homes were so tight that the stale air didn’t escape and the air in the houses was unhealthy to breathe.

Enter the HRV. If your house was built after 1975 or has been upgraded, you may want one. Consistently high humidity all over the house during the winter is one sign that you need an HRV. The only way to be absolutely sure is with a rather costly test used by the R2000 program. If you have an HRV, maintain it! Check the filter once a month and do a general cleaning yearly. If you need one, you want it working properly. If you heat your home with electric baseboard heaters or hot water or steam radiators, an HRV isn’t going to work well. It needs circulating air to affect the whole house. HRVs are also sometimes used to eliminate overly humid air from bathrooms or to eliminate smoky air from houses where people smoke. In both cases I would recommend either an open window, or better, a ventilating fan.

A call from a man we’ll call Craig presented this problem: “We had an addition put onto the back of our house and it is much colder than the rest of the house. Do you know why? What can we do about it?” Craig told me the room is on the north side of the house and has a lot of windows – OK there’s the first problem. Windows are poor insulators and on the north side of the house the heat loss through these windows would be high. There was obviously more to this problem, so I drove down to Craig’s place to have a look.

Craig wasn’t kidding about his addition. It was 10C/50F in there. I checked the windows and doors for leaks – none. I checked the connection between the addition and the original house. All the work had been done professionally and since the addition had been put on in the past year, I had no doubt that it had been insulated properly. The township building inspector would have made sure of that. So we went down to the basement where I found once again that the work had been done professionally and again there was no source of outside air. Next I turned to the heating system: three warm air supply ducts and one cold air return, all 5 inch diameter. This is not ideal, but should have been sufficient for the size of the addition. What struck me was the length of these ducts. In a forced air system like Craig’s, the air in the duct cools as it gets further from the furnace. In fact, Craig recorded a 9C/16F difference in the temperature of the air coming from the heat register closest to the furnace and the register in the addition.

The best place therefore, for a furnace is in the centre of your basement, not at one end. The pipe lengths will be shorter and the heat more evenly distributed. The only solution that I have found for this problem is to insulate the ducts. This is best done by a professional since the insulation goes on the inside of the duct and special insulation is used for the purpose. If you have a forced air system and a cold room in the house, try this test. Place an ordinary thermometer on the heat register closest to your furnace. Allow the furnace to come on normally and shut off normally. As soon as it shuts off, read the thermometer and record the temperature. Take the same thermometer and put it on a register in the cold room and repeat the process. It is a very simple and telling test.

Mike Lancop is a certified home inspector and member of the American Society of Home Inspectors, the Canadian Association of Home Inspectors, and Independent Home Inspectors of North America, and owner of Buyer?s Edge Home Inspection Services, High River, Alberta.