# Double flue chimney problem.



## KAT_Ayanami (Jun 10, 2007)

Hi,

I have a customer that have a double flue chimney.

One of them goes to the first floor of the house, the other one is connected to the one on the second floor.

The problem is that when only one fireplace is running, the other one will get full of smoke and ashes as the second flue on top of the chimney will catch the smoke coming out of the one on the side.

Any cap or solution for this? One of the flues is taller than the other, but the same problem happens no matter what.

Thanks.


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## artisanstone (Nov 27, 2007)

Kat, where are you located? Negative house pressure from the HVAC system can cause this, as can "stack effect" which is usually the chimneys being to short in relation to the house. Are both units operating when they have the problem?


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## KAT_Ayanami (Jun 10, 2007)

Thanks for the answer.

The house is in North Virginia. Close to DC.

The problem only happens when one of the fireplaces is off.

I am going to try to block the top of one of the fireplaces to see if it has internal damage that might connect both flues. And I have also read about exhausto fans. But I do not know if there might be some other solution.

I will ask about the HVAC on the house.

The chimney is quite large. So I do not think it might be short or anything like that.

Thanks!


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## bytor (Jan 23, 2010)

You might also consider installing a 'lock top' damper on top of the chimney. We had a similar problem a few years back and it did the trick for us. Totally seals off the chimney that is not in use. Much cheaper than the fan.
Another consideration; as the problem is more than likely caused by a negative air pressure situation inside the house, have you got adequate make-up air for the fireplaces?


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## Tscarborough (Feb 25, 2006)

Are the tops at the same level? One should extend higher than the other. Forget the TOF fan, they are noisy and expensive. TOF damper is a better choice by far.


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## KAT_Ayanami (Jun 10, 2007)

One of them is taller than the other one. By about a foot or so.

I also think that the damper is better.

I would like to know how to check for the negative pressure issue. Any links or info I could read?

Thanks


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## Diamond D. (Nov 12, 2009)

bytor said:


> You might also consider installing a 'lock top' damper on top of the chimney. We had a similar problem a few years back and it did the trick for us. Totally seals off the chimney that is not in use. Much cheaper than the fan.
> Another consideration; as the problem is more than likely caused by a negative air pressure situation inside the house, have you got adequate make-up air for the fireplaces?


Agreed. This is the easiest and most economical solution.

Or,

You could try a left handed smoke shifter. :whistling


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## Tinstaafl (Jan 6, 2008)

KAT_Ayanami said:


> I would like to know how to check for the negative pressure issue.


Don't have any links, but it's easy to check. The idea is that when you light fireplace A, it's going to get fresh air for the fire from the easiest source. If it's a tight house, that source is likely to be fireplace B.

Get a good fire going so that the problem is showing itself, and then run around opening all the windows. Good chance you'll see the problem disappear.


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## 2low4nh (Dec 12, 2010)

A top damper would work very well in this situation.


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## jgray152 (Oct 4, 2009)

Question: If the house is too tight and you cap the other chimney, would this cause poor draft issues while trying to start a fire? I sometimes see smoke rather enter the room then up the chimney.


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## Tinstaafl (Jan 6, 2008)

Yes, but that's not the only possible cause of a poor draft. Most commonly, when lighting a new fire with a cold chimney, there's a column of cold air sitting there which needs some initial encouragement to head up & out. Generally, you get that going with something that will generate quite a bit of heat quickly, such as crumpled newspaper.

But on some days when the barometric conditions are lousy, it can be tough to get much of a draft going at all. Fortunately, that virtually never happens on a bitter cold day.


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## 2low4nh (Dec 12, 2010)

All NEW fireplaces have a cld air intake hooked directly the the firebx to pull a draft with. We work in homes that are airtight and almost impossible to pull air from. The cold air intake is piped out through the basement. this allows a sufficient flow of air in to the the fire with out sacrifice of heated air leaving the room. on a side note some older units can be adapted by pulling air out of the ash dump since basements are usually not insulated as well. so top damper to stop the cross draft if any further draft issues come up pull cold air from somewhere. Make sure there are no air exchangers or anything of that sort on well fireplace is going.


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## stuart45 (Oct 7, 2009)

I have found that a draught inducing cowl on the pot can help give a better draw when the fire is first lit.


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