I am in the midst of an insert install in my downstairs fireplace. I have the 6" lining in place and had to remove the damper, no block off in place but I'm working on it, but that's another post. My current problem is now that it has turned cold I've had a fire or two in the upstairs fireplace which is a normal masonary flue fireplace that I am intending to leave as is. Since sealing the top of the downstairs flu well with a top plate and stuffing the pipe full of towels at the bottom, I am getting significant amounts of smoke coming in the downstairs flue when I burn the fireplace upstairs. I thought sealing the top of that flu would prevent any smoke from reverse flowing down the flu. So, oh wise forum members whom I owe so much to, say it ain't so but could I have cracks between the flues allowing smoke to pass from one to the other? Please advise.
Smoke downstairs
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Assuming that there are two separate tile pipes in the flue, it does sound like leakage is possible. Have both pipes inspected for integrity.
How can smoke get through the side of the lining?Twofireplacestofill said:
You probably need to raise the height of the upstairs flue.
I'm guessing for sure, but it sounds like the upstairs fireplace may be without a stainless liner? If the upstairs stove tiles are cracked or mortar is missing and there is no block off plate in the lower stove, smoke could drop alongside of the new liner, especially if the lower fireplace is a negative pressure zone. Ironically, if this is the case, it seems like the new top cap could make the problem appear worse as the old venting path got plugged up.
Welcome to the world of stack effect. Your house is trying to repressurize itself by drawing in air thru it's best available source, in this case the basement flue. Extending the upstairs flue will help keep out the smoke, but not the cold air. The cracks may be in the tiles that service the upstairs fireplace, or may be in the cap area for the new liner. I recall mine having a 3/16th gap around the liner and the flashing that the liner fits thru, and if that isn't sealed, the smoke and cold air could be drawn in around that. There are lots of things you can work on to seal the flue, like a lower block off plate and looking into the other possible cracks, but that won't stop the fact that your house is losing air that needs to be replaced.
My guess is that most of that air is being exhausted out your open masonary fireplace on the main floor. These things are notoriusly air hungry, and when you take into account the fact that they draw in cold air thru the cracks in your house to replace what goes up the flue when you have a fire, they end up running at near zero, even negative, efficiency.
My recommendation - seal up that fireplace on the main floor or convert to an insert. It is good for ambiance, but poor for heating. The smoke air it is exhausting is the casue of your house wanting to pull outside air into the house, and the source of the smoke as well.
My guess is that most of that air is being exhausted out your open masonary fireplace on the main floor. These things are notoriusly air hungry, and when you take into account the fact that they draw in cold air thru the cracks in your house to replace what goes up the flue when you have a fire, they end up running at near zero, even negative, efficiency.
My recommendation - seal up that fireplace on the main floor or convert to an insert. It is good for ambiance, but poor for heating. The smoke air it is exhausting is the casue of your house wanting to pull outside air into the house, and the source of the smoke as well.
thanks guys. Yes I had extended the upstairs masonary flu 12" with an extenda-flu. It became worse after I sealed the downstairs fireplace with the 6" liner and top plate which I had taken great pains to seal with high temp silicone thinking the smoke was coming in from the top. When I looked into the downstairs fireplace the smoke looked like it was coming out exterior to the liner (I had it pretty well sealed). So, I guess the ultimate answer is inserts or stoves in both fireplaces. Any stack effect issues with metal liners in both? thanks again
You will still have stack effect working within the house, it will just have different effects. In the end, the main issue will be that your basement is a relative negative pressure area, and will have a tendency to cause the stove to draft poorly when the flue is cold. Insulating the two flues will help keep them from reversing flow and improve their function (I assume they are on an outside wall - this is a worse case scenario, as the masonry is cooled by external exposure).
I recommend the Guide to Residential Wood Heating (link in y signature block) a read. It covers a lot of the possible issues around stack effect. Best way to prevent it is to work on air sealing the upper level of your house to prevent the air leakage that is the principle cause. Seal up your attic hatch, caulk your upstairs windows, etc, and you will keep the warm air from leaving, and you will reduce the stack effect tendency.
I recommend the Guide to Residential Wood Heating (link in y signature block) a read. It covers a lot of the possible issues around stack effect. Best way to prevent it is to work on air sealing the upper level of your house to prevent the air leakage that is the principle cause. Seal up your attic hatch, caulk your upstairs windows, etc, and you will keep the warm air from leaving, and you will reduce the stack effect tendency.
Ok so you have a liner... and you sealed it off with towels at the bottom? So... no air should be flowing from outside down the liner, correct? You said you get smoke in the basement, but its coming from AROUND the liner? Not from IN the liner?
Either there is a little smoke getting sucked in up top where the liner meets the plate or the plate meets the masonry, OR your clay tiles are compromised enough smoke can seep from one flue to the other.
Its possible your chimney was previously being used a mark-up air duct for the basement and now that you sealed it off the basement is depressurizing, which could cause more smoke to be pulled through because the outside air would be pushing harder trying to get in.
Either there is a little smoke getting sucked in up top where the liner meets the plate or the plate meets the masonry, OR your clay tiles are compromised enough smoke can seep from one flue to the other.
Its possible your chimney was previously being used a mark-up air duct for the basement and now that you sealed it off the basement is depressurizing, which could cause more smoke to be pulled through because the outside air would be pushing harder trying to get in.
Thanks everyone, I'm understanding this much better. Stack effect is a bizarre phenomenon that's for sure. Cleanburnin I am working through your residential guide to wood heat, excellent resource, thanks for your help.
Just as an aside, smoke in the house = Co which in large enough quantity is a killer. Please make sure that you have a working Co detector. Be safe.
Ed
Ed
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