I don't know how all chimneys are built, but I have a 2-flue chimney and this wouldn't be possible for me. My flues are completely separate. There would be "zero" chance of smoke from one flue getting into the other one.

I really don't know if they would have tied both the flues together inside the chimney. On top of the chimney...are there (2) flues coming out the top or just one?

What is happening is that when you light a fire upstairs you are creating a negative pressure inside the house, if the house is fairly airtight then the easiest way for the fire to get air is through the open flue of the downstairs fireplace in other words the smoke goes up the chimney and gets sucked back down the other flue along with the necessary fresh air. Theres a few things you can do.
1 extend the height of the upstairs flue tile enough this makes one flue higher than the other so that the smoke escapes and doesnt get pulled down with the fresh air . You can try this by temporarily perching a new piece of flue tile of the same size on top of the woodburning flue tile on top of the chimney
2 install a fresh air kit in the upstairs fireplace .
3 Try & get the damper working properly downstairs. this may not be possible if the gas fireplace has a ventpipe going through it
4 IF & thats a big IF the downstairs flue is not being used in any way at all either now or in the future, you could block it off at the top. ( personally I don't recommend doing this )
5 install a locktop damper on top of the downstairs flue
6 simply crack open a window near the fireplace upstairs when using it.

When gas logs are installed in a fireplace, the damper is typically fixed in an open or partly open position. I believe this is required by building codes. It is here in central North Carolina, and I suspect in most localities. It would be unsafe to close the damper altogether, that is, to use the gas logs with the damper completely closed. If you decide to close the damper, to correct the downdraft smoking problem, you shoudl probably disable the gas logs so that no one accidentally uses them.

Dear wlai,
I'd love to know if you figured this out. I'm having similar issues and I see it's been a few years since this post. Thanks.

btw, it is required that the damper be held permanently open on a retrofit of gas logs to a masonry fp. This way, if you forget to open the damper with the new gas logs (which produce little smell or smoke as indication of a damper left closed) you won't kill yourself with oderless CO.

Sorry to resuscitate this post, but I too have a similar problem - smoke down-drafting into an adjacent furnace flue. Chimney is well above highest point of the house. Berlin, if you're still around, you seem to know what you're talking about, and several of your posts on this topic recommend raising the fireplace flue higher than the adjacent furnace flue. I had a well-respected chimney sweep recommend a "flue stretcher" (see link below) - have you (or has anyone) seen those, and have any thoughts as to their efficacy? Seems like it could effectively raise the flue in a cost efficient manner. Thanks all!

Here is a link that might be useful: Flue stretcher

the height of the chimney would NOT have anything to do with smoke coming down a different flue than the one the smoke is exiting!
the house is tight and the chimney flue in use is looking for air to breath
it is getting that supplement air from where ever it can find a source which is the opening to the second flue
here is a test you can try...run the fire as to cause you problem when the smoke is entering the house thru the second flue opening try cracking open a window a few inches in the room where the fire is running
have another person watching the smoke entering as you do this
give it 5 minutes the smoke will stop coming in

Better just to buy the heater with the isolation system. Metal doors with vacuum gasket or something like that.

Here is a link that might be useful: crownstoves.co.uk

Guys, I have read all these posts, and have tried the "flue stretcher" solution. I have a sheet metal business so I can fabricate just about any type or length required to extend my chimney flues---I have found this does not work in my case as I installed a 2' & 4' metal extension flues and still got smoke coming in---what does work is opening a window or cracking a door, after about 5 minutes, the smoke dissipates and the house is back to normal. I recommend installing an air intake of some kind into the fire box if possible. This will help with the negative pressure and solve the smoke issue. This may require a licensed professional.

We have the same problem except our wood burning stove is on the opposite wall of fireplace in the sunroom. If we have a fire in living room fp then the sunroom is smoked out. We don't know why they have a stove in their so our plan is just to get rid of it. Kind of difficult if its two fireplaces though. After reading it seems many people have smoke issues so not sure why fp makers do double flues?

It sounds like you don't have a fresh air kit in the firebox or you aren't using it when you light the fire, or it may be plugged up with a birds nest or something else . Google fresh air kit for fireplace.

I never had this problem until I had my chimney cleaned. All I did was open the chimney.

i know this is an old post but wanted to revisit the issue. We've recently moved into a house that was build in the 60's and had to have the flue relined. We have a large wood burning fireplace. 3'x5' with a double flue. The second smaller flue is used by our water heather that vents through the old boiler which vents through the flue. Our house smells like a camp fire after every fire. I've read through this chain and the top of the chimney is much higher than the highest point of the roof. We've tried cracking a window and still the smell is there. It starts in the basement and works it way up. Any thoughts on this would be great. Thanks in advance.

If the fireplace doesn't already have a fresh air intake kit in it then you may have to get one installed, this happens mostly due to the fireplace needing air for combustion and the house is too air tight, it therefor pulls air down from the second flue and brings the woodsmoke along with it. Another better option, (if it is an open fireplace), is to have glass doors installed and use them along with the fresh air kit, this has the added advantage of closing the fireplace up when not in use and prevent room heat escaping up the flu. A fresh air kit and glass doors also increase the efficiency by not pulling cold air into the house to feed the fire, therefore not having to heat that cold air, as its contained within the firebox. https://www.nrcan.gc.ca/sites/www.nrcan.gc.ca/files/images/enerpedia/fig4-6_e_2.jpg

If you click on the link it will show you the combustion air (fresh air kit) below the firebox but instead of the fireplace plug you would have glass doors.

thank you Brickman

Easiest answer is to use both fireplaces this takes away the drafting effect

Same situation here, smoke in my basement after starting a fire upstairs. I've been able to mitigate the issue by starting up the gas log in the basement several minutes before I light the wood fire upstairs. After a while of burning both I can shut down the gas log with no further issues. I always thought it was a hot/cold air issue.

Although this is a really old thread, I will suggest that the solution offered by Brickman would be the best: adding an outside air intake and Fireplace Glass Doors. Keep the glass doors closed when you are not using either fireplace. If you cannot install a fresh air intake inside the fireplace, crack a window in the room where you are burning the fireplace to prevent negative pressure in your home and make sure the glass doors are closed on the other fireplace.

Been selling fireplace doors for over 30 years and this has worked well in almost every case where this issue has been brought up. In cases where it did not work there were other contributing factors such as a leaky chimney or chimney that needed to be cleaned.

Here's a link find fireplace doors: Custom Fireplace Doors

I appreciated the comments regarding this issue. Which I currently have as well.

I have a question on a Flue Extender. Which flue would you extend if they are both at the same height?

My conditions:

Single chimney with 2 flues.

The height of the chimney appears to be correct for the house.

one 12” for woodburning fire 1stfloor

one 8” for pellet stove in basement.

House is very air tight

1st Fl wood burning does have a fresh air intake built into the fire box when it was built.

And the fireplace has newer glass doors installed.

When you run the wood burning 1st fl fireplace you get smoked out in the basement.

Unless you are running the pellet stove in the basement at the same time.

I don’t want to have to run the pellet stove every time we want a fire.

When running the pellet stove in the basement without a fire on 1st fl you can barely notice a smell.

So if I am able to (supply seems hard to find) get flue extender or stretcher which flue do I extend? First Fl larger flue or basement smaller flue

Also does anyone know what height difference is needed? Most I found so far seem to be 1 foot.

Thanks.

I hope I don’t add to the confusion, but we have three flues in one chimney. One flue is for an outside fireplace, the other two are for gas logs inside the house. When we burn a fire in the outside fireplace, smoke comes in through the two inside fireplaces. This would seem to negate the fresh air kit idea suggested by some, since there is plenty of fresh air outside. All of our flues are at the same level and extend above the roof line. Any suggestions?