A few weeks ago, I described a problem with a gas fire installed in our
basement rec room. The fire is vented through a metal liner installed
in the flue of an original wood-burning fireplace. The chimney is on
the outside of the house, and when it is cold outside the chimney gets
full of cold air and there is no natural draft. In fact, there's a
downdraft in the vent.
The gas fire has a built-in draft diverter between the heat exchanger
flue gas outlet and the inlet of the chimney vent. When there is a
downdraft in the vent, the descending cold air spills out the sides of
the draft diverter. Under these conditions, when you start up the gas
fire, the combustion gases also get spilled out the sides of the draft
diverter, never making it to the vent entrance. Thus the vent never
sees any hot air, never warms up, and the downdraft continues. After a
few minutes, the gas fire shuts down because of its vent failure safety
system (a thermal switch).
What is needed is something to get hot air flowing up the chimney for a
short period, just long enough to start it heating up, and then it will
have a natural updraft that continues as long as the fire is operating.
For people with fireplaces with downdraft problems, I've seen the
suggestion of starting a paper fire at the back of the fireplace to
preheat the chimney. But there's no real way to get a flame back where
the vent starts - it is hidden behind the gas fire and accessible only
from the draft diverter vent slots on the side of the fire unit, which
are pretty narrow.
So I built a compressed air jet to get the chimney air started in the
right direction. I started with about 18 inches of 1/4 inch OD soft
copper tubing. I flattened one end and soldered it shut, then drilled a
small hole in the tubing wall near the closed end. I attached a needle
valve (the sort you'd use to supply water to a furnace humidifier or a
refrigerator icemaker) to the other end of the tube, and then connected
the needle valve inlet to a compressed air quick-connect fitting.
When I need to use it, I bring my portable compressed air tank into the
den and connect it to the valve. Then I thread the copper tubing
through the draft diverter vent on one side of the gas fire, feeding
enough tubing to position the air outlet hole at about the centre of the
entrance to the vent duct. There is a mark on the tubing indicating
the right distance to insert it through the vent.
And then I turn on the needle valve for a little while. The small jet
of high-velocity compressed air from the tubing seems to be enough to
overcome the natural downdraft and take some of the air from the
diverter hood up the chimney with it. If the gas fire is lit, then the
hot exhaust gases start going up the chimney and a natural updraft is
That's the theory anyway. I've only tested it once so far, since it's
only been cold enough to have a downdraft once since I built this
device, but it worked great. I think the compressed air was on
for less than a minute when I tried it. I'll do more testing when we
get more cold weather.
It's not the most convenient of solutions, since there's a portable air
tank and an air hose involved. But if it lets me use the fire on cold
evenings when a downdraft would otherwise prevent that, I'll be happy.