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. That's it.
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 soon created.
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.