Chimney flue backdraft question

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Nofossil

Nofossil

Moderator Emeritus
Oct 4, 2007
3,618
Lexington Virginia
www.nofossil.org
I have a strange situation, and I need help figuring out what's going on. Here's the situation:

I have an interior masonry chimney (freestanding - not on an outside wall) with three independent flues - oil boiler, fireplace, and wood boiler. The two boiler flues are on either side of the fireplace flue and extend to the basement floor, with cleanouts about a foot above the floor. The boiler flues are 8" square ceramic tile, and the wood boiler connects into its flue about 6' above the basement floor.

With a good fire going, I opened the cleanout for the wood boiler flue. Sticking my hand in, I feel turbulent cool air. It's hard to get a definitive temperature reading because there's a lot of draft pulling basement air in if the cleanout is open.

Blocking as much of that air as possible, I get about a 60 degree reading inside the cleanout chamber. The basement air and the basement side of the chimney block are at 68 to 70 degrees. The basement and chimney have never been colder than about 64 degrees.

Is it reasonable to think that I'm getting outside air flowing down the flue at the same time that the EKO exhaust is flowing up? Is there some other explanation? Is this a bad thing?

I've not done the barometric damper thing because my expectation is that it would result in enormous amounts of interior air being sucked up the chimney, making all the windows bow inwards ;-)
 
Your basement floor is really 68 to 70 degrees?
 
The best I can come up with is if you have a cracked chimney flue and the cold air is comming down the outside of the flue but inside the block or brick, is it possible you have a sealent issue at the top of your chimney where the flue meets the block that has allowed cold air and or water to infiltrate and cause problems??Dave
 
My basement floor it currently at 68.4 degrees. I inspected the exterior of the chimney this fall and discovered that I need to repoint the mortar between the bricks, but the cap is perfect. I don't see any path for cold air other than the flue itself, unless there's a passage of some sort from the oil flue cleanout to the wood flue cleanout. I'm quite sure that there's no supposed to be any such connection, and I can't see any structural issues of any kind.
 
Assuming the footing for a chimney this size isn't insulated on the bottom, and you are down close to the bottom at the cleanout door, can it be that the heat is being drawn down under your basement floor and that is cooling the air in the bottom of the flue faster than that leaky cleanout door can reheat it? Doesn't seem likely.

You could put a thermometer on the bottom of the flue and pile a bunch of insulation on top of it and see next day what the floor temp in there is when it's not being heated.

I hate those cheap cast iron cleanout doors. I spray some release on the frame, put a bead of silicone RTV on the door edge and close the door back about where it wants to settle. The incense test says it's much less leaky afterward.
I won't even think about barometric dampers. I see those pivoting flaps swinging back and forth when a furnace isn't even running and it looks like it's laughing as warm air goes up and out of the owner's wallet.

Funny, I remember a guy years ago pitching real hard for round ceramic flue tiles. One of his arguments was that cold air could creep down the corners and cause creosote deposits and that's why round tiles stayed cleaner, because they didn't have any corners.
 
I know very little about chimneys but my understanding that with clay liner they are just layed in and not mortared in. All you would need is a small crack and cold air could travel between the liner and brick, Sounds like maybe a crack between the two flues allowing air flow ( slight ) between the outside of the liners.??????????????????
leaddog
 
Can you open the oil boiler side and check for heat that does not belong in that flue while the EKO is running to see if their connected in some way??Dave
 
My guess is that the flues are somehow connected, so that when your EKO exhaust is exiting the top of its flue, cold air is somehow being pulled down into one of the other ones and then entering the EKO's. near the bottom. I doubt that cold air could move down inside a warm/hot series of flues by convection.
 
Feel the flue of the oil boiler where it goes in to the chimney if it is really cold you may have a cross draft connection. A temporary closing cap on the top of the oil chimney should stop that flow. It's not quite the same but I have a negative flow going into my oil furnace when the wind is right and you can feel the cold ooze when you take the back panel off.

Any how...With the wood boiler clean out closed open the clean out for the oil boiler and see if you are getting a negative flow. If you are maybe you can see where the open channel is if you shine a flashlight from one clean out to the other. This time of year you might want to try the light trick first though.
 
It would be really tough to see how the flues could be connected - the fireplace flue is between them. It's closed at the top with a top-of-flue damper. I did check the oil flue and it didn't seem to be cold.

The whole corners thing seems implausible, but I can't figure out a better explanation.
 
I guess I misunderstood you as I thought the fireplace stopped above the boilers and didn't realize it also went to the bottom. As I read a little closer I see you must be right (LOL). Do you think it is just the negative draft caused by the hot air rising up the chimney? (Creating a vacuum on the flue tile so that it draws air in from the un-mortared joints like a venturi nozzle?) You did say you had a good fire going and the heat of the tile above the clean out would pull outside air down to the bottom of the tile if there is even a slight place for it to leak through???
 
Hi All,
It's been a while, hope all is well. First question would be which way is the air moving? Once the cleanout is opened, room air should be drawn in through the opening. The movement of hot flue gases up the chimney should create a low pressure below the flue pipe connection. Probably the best way to determine the direction of air flow would be to tie a small "streamer", or piece of cloth to the end of a stick and hold it near the opening, then into the opening to indicate flow. Just my 2 cents.
 
nofo, I just ran down cellar to test mine even though it is an outside chimney- when I open the cleanout door and stick my hand in it is like someone has the ac on. My guess is the poured base for your chimney is or should be deeper than your cement floor giving you a large cool mass at the bottom of your chimney. By convection this creates a cool area with a strong draft when the cleanout is opened and feeling cooler than it really is. Just be thankful it all goes up and doesnt pour out your cleanout door into your basement. All the chimneys I ever built had flue liners buttered together with refractory cement, don't want to know the guy who lays them in dry.
 
Bartman said:
Hi All,
Once the cleanout is opened, room air should be drawn in through the opening. The movement of hot flue gases up the chimney should create a low pressure below the flue pipe connection. Just my 2 cents.
Click to expand...

We were just trying to determine the source of the colder than room temperature air. The exhaust air is moving up the center of the flue as intended but because it is I think there is negative pressure air (just like you pointed out) coming down between the flue tile and the chimney and is finding any crack or crevice it can and since the clean-out is below the chimney cold air is being felt and measured there.
 
OK, try this. Hop into your time machine, late 60's ought to do it. Go to your local head shop and buy some of your favorite incense sticks. Set one alight and standing up on the bottom of your flue where you can see it from the cleanout door. Cover the cleanout opening with the door open with the vinyl shrink wrap used to cover old windows for the winter. Then you could watch it with a flashlight and perhaps see what's going on without the air currents caused by the door being open.
 
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