"Back drafting fireplace question" or "Ack, I can't breathe in here!"

Soooooo… here’s me with yet another heating question…

Not satisfied with getting the oil furnace back to good working order Dylan and I had our fireplace chimney cleaned so that we could have a nice romantic blaze to sit beside.

I get the fire going on the first shot. Absolutely amazing. The chimney is clean. The flue is open. The smoke is going up it.

About a half an hour later, the smoke apparently gets bored with going up the flue. Perhaps it was too cold outside. Anyway, things got progressively less romantic as the house slowly filled with smoke.

Any idea what could cause this? (not the less romantic thing - I got that one figured out)

the chimney effect needs a couple of things to roll on, perhaps most important if which is direction fo flow. Smoke out is the preferred direction. One way (the best?) to accomplish this is to make sure there is low pressure at top of chimney. Cross breeze works. 'Course, you can’t just up and order a breeze when you want a fire. Ssme chimnies have a baffle like piece at the top. This redirects the smoke and exhause, which can create a small low pressure area (from movement).

Perhaps, however, the problem is circulation. As air and smoke goes up the chimney (which, BTW, we tend to pronounce “chimbley” here in southern Maryland) air needs to go into the room with the fireplace. Perhaps your weather stripping is so wonderful, the room is air tight!!

Alternatively, there may be feeding vents around the base of the firebox which might be clogged or closed. (Not in every fireplace).

Regardless, the direction of flow favors “into the house” for some reason. It’s either blockage (not bloody likely since you had it cleaned) or a pressure differential.

Of course, this all assumes the flue didn’t close itself about the time you were bringing in the brandy and Mrs. Dewt was setting up the quilts in front of the hearth. (Had that exact thing happen - shot the mood straight to hell.)

You can also get back draft if the chimney wasn’t built high enough. Is the top of your chimney higher than the highest point on your roof? If not, the roof can block cross winds which help the chimney draft. We once cured a back drafting fireplace by adding a couple of feet to the chimney’s height.

What the other guys said, plus:

  • Crack a window open if your fireplace doesn’t have the vents spritle talked about. I have a wood stove, in a room surrounded by windows, so I just crack the one behind the stove to give it air. Obviously, if you do this, the window should be as close to the fireplace as possible to minimize the feeling of cold drafts.
  • This is pure speculation, but: I had a friend from Minnesota who said they always set a piece of paper on fire and sent it up the flue just before starting a fire to “warm up” the flue. You can get cold air coming down the flue on really cold days, since cold air sinks, so while I’ve never had a necessity for this (north Jersey), maybe it gets real cold where you are and you have to do something like this. Of course, getting a roaring fire going right from the start will also warm that flue up right quick.

When I was a kid, we always did that “priming the flue” thing that pantom mentioned. I don’t think it really warms it, I just think it helps the direction of flow, kinda like siphoning something. Instead of giving a suck, you get the air moving higher up in the chimney by lighting a sheet of newspaper and holding it as high up in the fireplace as you can. Do this right after you light the fire, before it really starts going. This should “prime” the air flow in the right direction.

Good luck, I bet you two looked pretty silly in your knickers with soot all around your noses. :smiley:

dewt: please e-mail me.

The question not yet answered is “Why did the smoke exhaust properly for a half-hour, then reverse?” One possible answer is that the furnace turned on just then. Unless your furnace has a separate air-intake, when it turns on, it is getting its air from inside your house. This lowers the air pressure a little bit, maybe enough to reverse the flow in the chimney if your house is pretty airtight. Cracking the window, as ohers suggested, will help.

Well, the furnace is recieving and blowing air. Unless the intake was very near the fireplace, the furnace should generally have an equilibrium as to air flow. Actually, it’s more likely that a furnace vent near the fireplace would create higher pressure than the other way around.

My assumption is that something external messed with the flow dynamics. Perhaps a swift, ill-directed breeze came and sat upon dewt’s chiminey. :wink:

If your house is really tight (few air leaks) and someone turned on a kitchen stove exhaust fan or a bathroom exhaust fan then they would be fighting the fireplace. I have never seen a home with any outside air intakes on their heating or air conditioning units, but combustion air is frequently pulled from inside the conditioned space, and that has to be made up from somewhere. A home furnace could also pull in outside air if the ducts were leaking heated air into the attic or any other space outside the building envelope.

Basically, the air pressure inside the house is somehow becoming lower than the air pressure at the top of the chimney. Fires cause air to be exhausted from the home, and because homes are not air-tight this can only happen for so long before some outside air finds a way back in. Ideally, this should be outside air introduced from somewhere around the fireplace, so it can be immediately exhausted without cooling the house down. If you crack a window on the other side of the house, cold outside air will flow into the house from there defeating the purpose of having a fire.

A big gust-a wind can blow that smoke right back into your house! I oughta know cause it happens to me too…

I suggest a glass fireplace door, keeps the smoke in the chimney.

Precisely why, if you crack a window, it should be as close to the fireplace/wood stove as possible.

Democritus, you’re thinking of the air that gets moved through the furnace ductwork. In addition to this air movement, the furnace pulls air in from it’s surroundings (the basement, in my case), mixes it with fuel (natural gas for me), burns it, extracts heat from the burnt gasses, then exhausts those gasses up the chimney. The gasses going up the chimney have to be replaced by air pulled in from outdoors, so the house is at lower pressure than outdoors while the furnace is operating.

Engineer Don wrote:

Southern California, huh? No, you probably haven’t seen one. :slight_smile: High-efficiency furnaces do have an air intake tube. I don’t have this kind of furnace, but my mother does. When she had it installed, they ran two PVC tubes out the wall of her house, one for intake, one for exhaust. The furnace doesn’t use the chimney any more.

dewt, what kind of heating do you have?

Doh! My bad. Thanks for clarifying.

I also put in a chimney cap. They are about $29.00 & you can put it on yourself. It keeps wind from blowing down the chimney.


My reply is gone. (or never actually made to the thread, more likely)

My deepest most sincerest apologies for starting a thread and apparently dumping it. :frowning: I got somewhat caught up in the pit, but I did have an update/reply for you guys. Hmmmm.

Anyhoo, it went something like this. (abridged)

(next day)
Hey, I just started it up again, did the flue priming newspaper trick and it’s been going great for an hour now… blah blah blah thanks blah blah quote witty comment blah thanks… [end post]
And then, about an hour after I supposedly hit ‘submit reply’ it started backing up again; coinciding quite interestingly with the imminent arrival of a real estate agent and her potential buyers of this smoke filled house I’m trying to sell.

I didn’t have time to post just then.

I grabbed the burning logs, threw them outside. Opened windows upstairs and downstairs. Then I pulled the spray-top off a bottle of Febreeze and with arms extended, spun briskly in a graceful, if hurried circle. Then figuring if there was going to be smoke it should be at least nice smelling, I lit some incense. Ok, all the incense.

Two hours later, with the windows closed, I answer the door smiling. :smiley: ( <- insert gritted teeth) "Hello, please do come in. This is our living room. :smiley: Note the large fireplace. :smiley: We had it cleaned and checked only recently. :smiley: In fact we had a merry little blaze going in it this morning. :smiley: "
Democritus suggested:

Lol. Somehow, and I’m not sure how it happened or how to explain it, but I got a visual of that.
ZenBeamI heat by oil and the furnace is new. (1 year) It uses recycled air from the house. As standard as it comes.

Comparing thoughts from this thread and my own imagination, I’ve come up with this theory.

The house is old, (built in 40’s) and drafty as an architects filing cabinet; particularly when it comes to the roof (which mother nature will not allow to survive another year). This fall, eager not to repeat the errors of the preceding winter, I went on a seal’n’peel binge and caulked every damn crack/window/door in the house all to hell. Dylan made me unstick the front and back doors and a window or two but aside from that the bulk of the flow from the 1st floor was eliminated. (I think the bulk of flow upstairs is through the cracks in the walls rather than the windows)

It’s about 15 degrees Celcius below freezing outside and about 20 above inside. That’s a significant difference and I imagine that the air flow is quite strong. By sealing up the 1st floor I’ve created this huge pressure difference.

Curious to see that the flow of air heated by fire directly below the chimbly is not strong enough to over come the flow of air through the roof. Hmmmm.

But then again, the pieces of the puzzle start falling into place…

A half an hour is about the time a fire takes to ‘settle in’. That is, it hasn’t got the blazingness of a fire just started, but it’s not about to go out either.

For the second fire, I used wood that I’d bought at the local store. It was very dry and chopped into relatively small pieces. The fire was burning quite merrily for a couple of hours as I tended it and fed it. When I started to prepare for the viewing (kind of a surprise) my attention was turned elsewhere and the fire burned down a bit. Apparently enough for the roof to regain it’s title as ‘supreme evacuator of dewts air’ and pull all that smoke back into the house.

We actually do have a chimney cap up there. One of the few things around here not falling to bits. One side of the fireplace has a glass door too. That’s the side the smoke chose to come out of (also why I didn’t notice the problem until the house was filled with smoke)

I think I’m going to take Engineer Dons advice and drill a hole in the wall…

** sigh **

ya, I know… preview is my friend.


A bump so that those who responded (we know who you are) get my thanks.

The only other thing I think that can cause it is a dirty chimney. Having it cleaned is about $55.00 but you said it was clean? Did you look up it with a flashlight to make sure?

All the suggestions posted so far deserve testing. Here’s another which might be the cause.

Traditional open fireplaces have a “choke” or “throat” just above the open fireplace part. This is easier to draw than describe, but here goes: If you look up the chimney, you see a shelf just above the fireplace opening, like a ceiling to the firebox area. It is attached to the inside of the back wall and slopes upwards towards the front wall, leaving a gap of only about one quarter of the full front-to-back size of the fireplace. You can’t see it, but above this shelf the chimney has a chamber of about the same size as the fireplace below the shelf. The chimney then gradually tapers up towards the chimney pot. This traditional design somehow stops smoke going into the room. The proportions have been developed pragmatically and there is a well-known (to builders but not to me) rule of thumb for it’s design.

The shelf which creates this choke effect is usually made of stone or brick, and is usually permanently built in, but it is possible yours was made of a thick steel plate or a stone slab which you accidentally shifted while cleaning the flue. Or maybe it was removed by someone thinking it was blocking the airflow (common sense gone wrong).

I recommend you search for suitable diagrams and check whether your fireplace and chimney are properly choked.

<<< bringing in the brandy and Mrs. Dewt was setting up the quilts in front of the hearth. (Had that exact thing happen - shot the mood straight to hell.) >>>

uh… exact thing happened to you… and Mrs.Dewt? :wink:

( I’m kidding, I’m kidding! ok? Hey, it was there, it was too good to pass up, I just HAD to use it!)