Q: 2 Flues in same chimney can exhaust from 1 be drawn into other?

[Dave A.]

The 2nd flue (fireplace) is about 1' above the central heat flue but the clay liners are touching at the top of the brick chimney.

Am installing wood stove insert in one. Meanwhile the central heat (propane boiler) is running in the other. 6" flex liner in 12x12 clay masonry flue is not connected to stove (trying to connect and having difficulty, but that's a different problem) is open in fireplace. When boiler is firing can smell exhaust in fireplace -- not strong, but definite.

CO detector is nearby and not going off. Not feeling light headed or anything, so doubt there's CO.

Had an insert in there previously connected to ss liner and never noticed this before.

 

[HotCoals]

You can't have a open fireplace on the same flue as a furnace I don't think!
Only one appliance in each flue ..if I'm wrong someone will let me know..lol.
But I did do it years ago..but that's a story for another day..worked good for a month..you will not believe what I did..lol.

 

[Dave A.]

You can't have a open fireplace on the same flue as a furnace I don't think!
Only one appliance in each flue ..if I'm wrong someone will let me know..lol.
But I did do it years ago..but that's a story for another day..worked good for a month..you will not believe what I did..lol.

There are two separate flues exiting from one chimney -- two holes in the chimney at the top, they are not joined -- or not supposed to be anyway.

 

[brian89gp]

Pictures help.

You normally have a vertical separation between more then one flue to prevent cross-drafting.

 

[Dave A.]

I don't think that's what's happening. I'm wondering if the smoke coming out the top of the one can get sucked down the one next to it from the top, possibly because of a downdraft situation.

 

[chimneylinerjames]

What is happening is the furnace needs air for the combustion. The easiest spot for the furnace to draw air in our case appears to be down the fireplace chimney. You can simply test this theory first by opening window/door a little bit near the furnace. This way it will draw air in from the window instead of the fireplace flue. If this works, you can install an outside air kit for your furnace. Basically make a way near the furnace where it can draw in outside air easily. You can also try raising the furnace flue higher than the fireplace flue.

If you try to raise the flue for the furnace, it may stop the exhaust from being drawn down the fireplace but you may still have air being drawn down the chimney. This air may then smell like creosote.

 

[Dave A.]

What is happening is the furnace needs air for the combustion. The easiest spot for the furnace to draw air in our case appears to be down the fireplace chimney. You can simply test this theory first by opening window/door a little bit near the furnace. This way it will draw air in from the window instead of the fireplace flue. If this works, you can install an outside air kit for your furnace. Basically make a way near the furnace where it can draw in outside air easily. You can also try raising the furnace flue higher than the fireplace flue.

If you try to raise the flue for the furnace, it may stop the exhaust from being drawn down the fireplace but you may still have air being drawn down the chimney. This air may then smell like creosote.

No window, but increased outside air, (was waiting to get the boiler on but still can't really tell whether that did anything). Can't say it's completely gone. Not sure.

If the Boiler is pulling air in for combustion through the exit of the ss liner at the top of the chimney which is the higher flue, why am I smelling exhaust out of the bottom of the ss liner (which at the moment is temporarily not connected to anything.)

And if it needs air why would it pull from the flue exit above and to the side of it, rather than take all the air from directly above it?

BTW thanks for the replies.

Also, wondering if this is this just a temporary situation because right now the fireplace flue (ss liner) is open at the bottom. Because I never noticed this problem before. When there was just a fireplace, it usually had the damper closed. And when there's a stove insert in the fireplace usually the stove door is closed.

I don't know if this is effecting anything but the boiler has an exhaust blower, it could have been direct vented (and plan to do that this summer) and have a feeling the present boiler flue setup is too short for the way it's currently vented. Plus the boiler is in the attic above the room with the fireplace.