Backdraft through Flue


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Old 11-01-10, 02:21 AM
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Backdraft through Flue

I have a wood burning fireplace upstairs and a Furnace/AC unit downstairs. The flue pipe is part of the upstairs fireplace chimney. When I have a fire going in the fireplace I seem to be getting smoke coming in to the basement next to the furnace. The only thing I can figure is that its coming from that flue pipe. The outlet for the fireplace the flue are both side by side and about the same height. Please tell me there is something I can do to stop the smoke from coming into the basement. I tried putting a kitchen towel around the flue pipe to keep the smoke out but it doesn't do any good. If there is anything I can do please let me know. Thanks.
 
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Old 11-01-10, 05:36 AM
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The actual term is cross-drafting. Cold air is sinking down your furnace pipe and carrying your wood stove's exhaust. And I would bet when your furnace is running but you have no fire you have the reverse.

There are codes that dictate the height of a chimney in relation to nearby objects such as roof-lines, or in your case another chimney.

The proper solution is to either relocate or raise. The easiest solution will be to raise your wood stove chimney several feet depending on how close the two chimneys are. Your local codes office can likely tell you how much.
 
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Old 11-01-10, 10:19 AM
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Thank you for the response. The flue for the furnace and the exit for the fireplace are in the same chimney. I don't see a way where I can raise one and not the other. Thank you.
 
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Old 11-01-10, 10:22 AM
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Originally Posted by mystang89 View Post
Thank you for the response. The flue for the furnace and the exit for the fireplace are in the same chimney. I don't see a way where I can raise one and not the other. Thank you.
That is definitely a problem, and worse yet may not be to code. Some combustion devices are not allowed to share the same chimney. I would check with your local codes office to see.

If not to code, I would have it corrected at your leisure, instead of in a hurry like when you go to sell.
 
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Old 11-01-10, 10:31 AM
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Do you mean that both flues combine in the chimney and then go out the top together?
 
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Old 11-01-10, 05:49 PM
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"drooplug
Do you mean that both flues combine in the chimney and then go out the top together?"

Yes, both flues combine in the chimney and then go out separate holes in the same chimney. This house was build in the 1950's so I don't know what the code back then was. Seeing as how my room in now in the basement though I would like not to suffocate from smoke inhalation so I'm still looking for something to at least lesson the smoke.
 
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Old 11-01-10, 06:14 PM
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We have a wood stove in living room, wood/coal stove in basement. Early in heating season, when using only lr wood stove, smoke will cross-draft and go down the coal stove flue. I tightly tie a trash bag around the barometric damper on coal stove, make sure all vents are closed, put rope-caulk around grate handle opening and no problem.

When using basement coal stove, of course there is also no problem. Don't know what you would do to seal a furnace; probably can't as thermostat may call for heat.

Sometimes one flue can be raised by installing a stainless steel pipe inside the top of one flue, thereby establishing a different height to the other flue.
 
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Old 11-02-10, 02:30 AM
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I may try to tie a plastic bag over top the furnace flue pipe and see if that helps cut down on the smoke cross draft coming down into the basement.
I never use the furnace for hardly anything except during the summer when its really hot outside anyway.
 
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Old 11-03-10, 04:17 PM
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Originally Posted by mystang89 View Post
"drooplug
Do you mean that both flues combine in the chimney and then go out the top together?"

Yes, both flues combine in the chimney and then go out separate holes in the same chimney. This house was build in the 1950's so I don't know what the code back then was. Seeing as how my room in now in the basement though I would like not to suffocate from smoke inhalation so I'm still looking for something to at least lesson the smoke.
Huh? How do they combine and then come out separate holes?
 
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Old 11-05-10, 03:12 PM
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By combine I mean that the chimney on the outside of the house holds both the flue for the downstairs Furnace/AC unit and the upstairs fireplace. Both flues are separate from each other, (they do not connect,) but the are housed in the same chimney. When they exit the chimney the flue for the downstairs unit is 3 or 4 inches from the outlet for the upstairs fireplace. I have put a kitchen towel around the hole for the downstairs flue pipe along with 2 plastic bags. I will see if that works or if something I don't want to happen....happens.
 
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Old 11-05-10, 03:24 PM
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The logical thing to do is provide a fresh combustion air source for the furnace alone. Unfortunately, you may lave to separate the furnace from the living area, but the benefit (energy/cost-wise) is that the furnace will burn cold outside air instead of the heated indoor air.

One problem you could encounter is if the proximity of the exit of the flues at the top, especially since they are close and at the same height, you could get some cross-flow even when neither is active, but the climate conditions are just right.

Near-by flue discharges of flues that are close is never good.

Dick
 
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Old 11-06-10, 04:44 AM
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Yeah, this whole thing is becoming increasingly difficult to manage I now have a kitchen towel duck taped to the flue, 2 garbage bags ducktaped around the flue and inside at the bottom coming out of the furnace I have the place where the flue comes from the wall to the furnace ducktaped with another towel around it. After that it started coming from where the flue exits the furnace downstairs so I put another towel inside that whole and ducktaped around it....LOTS of ducktape. That seems to be "managing" the smoke even though I think there is still some that leaks down to the basement.
 
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Old 11-06-10, 05:26 AM
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Have you tried opening a window in the room where the fireplace is? If that stops it, then you know your house is too tight and is pulling the make up air through the furnace flue.
 
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Old 11-06-10, 03:58 PM
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How long should I keep the window open for? In the dead of winter I'd like to minimize the amount of outside air coming inside but I don't want to open the window for 5 minutes then close it before giving it a chance to work.
 
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Old 11-08-10, 05:44 AM
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Originally Posted by mystang89 View Post
By combine I mean that the chimney on the outside of the house holds both the flue for the downstairs Furnace/AC unit and the upstairs fireplace. Both flues are separate from each other, (they do not connect,) but the are housed in the same chimney. When they exit the chimney the flue for the downstairs unit is 3 or 4 inches from the outlet for the upstairs fireplace.
I guess I did not understand your setup originally, but this is actually good news that can be fixed with relative ease. The easiest solution will be to raise your wood stove chimney several feet depending on how close the two chimneys are. Your local codes office can likely tell you how much. My guess is raise the wood stove chimney 3 feet above the other.

The other option is you may be able to install what I call a puffer door on your furnace exhaust. It acts similar to your dryer vent door by staying closed when there is no upward draft when the furnace is not in use. An HVAC pro would know for sure if that is allowed, so the easiest thing to do is hire a wood stove guy, who are much cheaper, and raise that wood stove chimney.
 
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Old 11-08-10, 06:33 AM
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Looks to me like it time to have a Level II (video) inspection performed of the entire venting system for both the fireplace and the gas appliances (is there a water heater in the picture as well?)

The reason I suggest this is that there are many types of defects that can affect the operation of these vent systems but which cannot be diagnosed accurately by an unaided view from either the top or bottom of the flue.

Here's a recent example from a home inspection here in Chicago, I knew there was a functional defect in the system, but it's exact nature was only discovered when the chimney sweep came out a performed the level II:



the flue was partially obstructed at an incorrect transition between flue profiles were the chimney had been extended upwards through a second floor addition.

Though each defect is a bit different, when I discover a functional defect the Level II turns up a significant "hidden" defect around a third of the time.
 
 

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