Should I Install An HRV?
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.