Fix for chimney downdraft?


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.
    Dave
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Hot air rises unless it is opposed by a positive pressure coming from above. The problem you are having is that there is a negative pressure in your house. This may be cause by furnace combustion in the house that uses inside air instead of out. It may be caused by a bathroom vents. It may be caused by your clothes dryer ect.
If you recently put better siding up or installed more efficient windows your house may be tighter and facilitate a negative pressure more easily.
The solution is to either use outside air for your fireplace or install some sort of vent in the wall of the house to eliminate your negative pressure. Maybe both?
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On Feb 16, 4:38�pm, snipped-for-privacy@mucks.net wrote:

I AGREE WITH ALL OF THIS!
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open a door or window when you light the fire until the draft gets going Mark
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snipped-for-privacy@mucks.net writes:

There isn't an overall negative pressure in the house, because the same model of gas fire is also installed in the living room above the den, vented through the same chimney (but a different flue), and it always has an updraft. In addition, the furnace and water heater have a direct vent to the outside for combustion air. I can even open the front door of the house, which is only a few feet away from the den, and there is still a downdraft in that flue of the chimmney on some days. It happens with no vent fans running, with the clothes dryer off, and whether or not the furnace is running.
Having said that, the pressure in the basement room almost certainly is slightly lower than that upstairs. The basement is cooler in the winter, and the house itself acts as a sort of chimney. The real chimney is outside the house, so the air in the vent eventually drops to outside temperature and provides no draft of its own. There's an explanation of this effect at
    http://www.woodheat.org/chimneys/trichim.htm
in the "Put the chimney inside the house" section.
Now, *most* the time when I try to use this fire, there is already an updraft, or the downdraft is weak enough that the hot gases from the gase fire overpower it and the chimney develops a proper draft within a few minutes. Sometimes a draft isn't established on the first try, but running the fire for a few minutes, shutting it off, leaving it for 10 minutes, and starting it again will give a proper draft on the second try. But sometimes (maybe 10-20%) the downdraft is strong enough that the hot gases from the fire can't overcome it.

We moved in less than a year ago and this was our first winter, so we don't know whether the fire has always done this.

There is already an outside air vent into the ceiling of the room. There's another large vent directly through the basement wall to supply combustion air for furnace and water heater. Even opening the main door of the house, 10 feet away from this room and half a floor higher, does not eliminate the downdraft on cold days. Even with plenty of access to cold air from other outside sources, there's still a downdraft in the chimney sometimes.
    Dave
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http://healthandenergy.com/backdrafting_of_chimneys.htm
Then your solution should also include ventilation to equalize the temperature/pressure between upstairs and downstairs. This should minimize the "stack" effect.
There is also the possibility that your flue opening is on the windward side of your house which can cause pressurized are to flow down the flue.
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snipped-for-privacy@mucks.net wrote:

I saw a house on TV a while back that had this problem and I think the solution was to install a small fan made for this purpose on the top of the chimney.
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On 16 Feb 2007 17:10:57 -0800, snipped-for-privacy@optonline.net wrote:

Restaurants sometimes use this type of fan for safety reasons, but it will definitely create a upward draft.
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On Fri, 16 Feb 2007 19:59:44 -0500, snipped-for-privacy@mucks.net wrote:

If all else fails...Jimmy Rig......If possible rig a fan to draw air into the house as close to the offending flue as you can. Just pressurize the house in this manner for five minutes before lighting the fire.
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snipped-for-privacy@mucks.net writes:

The main entrance to the house has a half-flight of stairs up to the living room, and a half-flight down to the den. This stairway is always open. About the only way to improve ventilation further between living room and den is to cut a large hole in the floor between the two rooms.

Could be, but both flues are in the same chimney.
    Dave
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http://www.fluesystems.com/fans /
http://www.chimneyfans.com /
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I once had as wood stove in the basement and needed to exhaust some of the heat out of immediate area. I cut a hole in the ceiling close to the front of the stove. Upstairs this hole was at the end of a bedroom hallway. This vent had a fan included. You need a vent like this that will allow you to draw air into the basement upon starting the fire. Hopefully this will be enough to equalize the pressure/temp difference between upstairs and downstairs.
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http://www.fluesystems.com/fans/chimney_top_fans.htm
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snipped-for-privacy@cs.ubc.ca (Dave Martindale) writes:

An update: I've used this gadget twice more when there was a downdraft in the chimney. It seems to work really well. A 10-second shot of compressed air seems to be sufficient to draw some of the warm exhaust from the fire into the chimney vent. Once the warm air reaches the vent, it starts warming it up and a natural updraft takes over.
It takes so little volume compressed air to reverse the downdraft that if I was doing this over again, I might try to use a "dust gun" can as the gas source. And the quarter-inch copper tubing is vastly oversized for this use; a length of copper capillary tubing from a dead refrigerator or dehumidifier might be sufficient.
    Dave
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replying to Dave Martindale, Roberto Zavattiero wrote:
highly recommended solution
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