Why did my fireplace fill my house with smoke?
September 14, 2010 6:05 AM   Subscribe

Why did my fireplace spew smoke out into the living room when I burned some stuff on a hot day this summer? I've checked the chimney and it looks clean...

Is it possible that on a hot humid day the air pressure from outside pushed the smoke down into the air conditioned house? Most importantly, if I start a fire right now on a cooler Fall day, will it happen again?

I climbed on the roof and checked the chimney with a flashlight. It looked as clean as one could reasonably expect a chimney to be.

Also, yes, the flue was open.

I'd like to have an idea about why this happened over the summer before starting another fire because it's a real bear to get tons of smoke out of the air.
posted by crapples to Home & Garden (25 answers total)
 
Down draught ?
posted by adamvasco at 6:22 AM on September 14, 2010


Was your air conditioner intake running and drawing air down the flue at the time?
posted by The 10th Regiment of Foot at 6:22 AM on September 14, 2010


The chimney could be clean because the squirrel/bird nest or carbon deposit has now burned away.

I'd call a chimney sweep. They still exist.
posted by valkyryn at 6:23 AM on September 14, 2010


If the house is air conditioned, and the air conditioning was running, most likely the air exchange in the Ac was causing negative pressure that pulled air down the chimney. Normally in summer, if you keep the flue closed, that air is being pulled in through cracks and crevices elsewhere. But when you opened the flue, the downdraft ensued, and the heated air from the fire was not enough to overcome it.

It wasn't the hot conditions outside. Normally if it's hotter outside than in, without another factor like the AC, that should set up a natural air flow up the chimney. In winter, you can have the opposite problem of a cold chimney with a natural downdraft.

Try this again on a fall day, without running any AC. As a precaution, before lighting the fire, crumple up some newsprint, light it and hold it up by the flue to warm up the chimney and get the air rising.
posted by beagle at 6:25 AM on September 14, 2010 [1 favorite]


Doy you have central air or window units? I wonder if you had a slight vacuum in the house, relative to outside. Or perhaps it was just so goddamn hot that the flue wouldn't draft.
posted by notsnot at 6:31 AM on September 14, 2010


Was the flue closed? If not, negative pressure seems the likely culprit. AC could be the cause as could a funky draft created by open windows and just the right amount/direction of wind.
posted by that's candlepin at 6:37 AM on September 14, 2010


We have central air - and I don't know for sure whether or not it was running, but odds are that it was since it was July and the AC is on more than it's off during that month.

I don't think a chimney sweep is necessary, unless I'm really missing something. It looked pretty clean in there.

The downdraught thing seems plausible.

I think I'll go ahead and light it up right now, warming the air first as beagle suggested, and see what happens. Worst case scenario is that we spend the next couple of hours with all of the windows open and fans blowing air out of the house.

Thanks.
posted by crapples at 6:38 AM on September 14, 2010


Keep us posted!
posted by beagle at 6:43 AM on September 14, 2010


Air conditioning: irrelevant. colder air is heavier, hot air wants to go up. If your HVAC system is causing enough negative pressure to cause this to happen, it is broken. You actually want slightly positive pressure so the environment doesn't feel drafty.

Either you failed to pre-warm the flue, or there was a wind gust. Now that I think about it, did someone open a door right before this happened? Because opening the door would let a big "gulp" of the cooler air pour out the door and create a temporary negative pressure situation.

What kind of stuff were you burning?
posted by gjc at 6:44 AM on September 14, 2010 [1 favorite]


Turn that AC off first!
posted by The 10th Regiment of Foot at 6:44 AM on September 14, 2010


Agree w/gjc that AC should have nothing to do with this. AC systems cool air that's already in the house; they don't exhaust it outside and create negative pressure.
posted by jon1270 at 6:58 AM on September 14, 2010


Nthing the AC issue.

On another note, if this fireplace is new to you, I'd suggest strongly getting it cleaned, unless you have proof the previous owners did it within the last 3-5 years. Not only will it ensure the safety of your fireplace, the sweep (assuming he's a good one) will show you the best way to light a fire for your particular fireplace. My husband grew up with fireplaces and wood burning stoves but we couldn't figure out why the fires in our fireplace were never successful. The chimney sweep came out, spent a lot of time cleaning the chimney and firebox, and recommended some things to us. Our next fire (and subsequent ones) went beautifully; no smoke in the house, burned a long time, lit fairly easily. Some fireboxes are smaller than others, which requires a little bit of finesse. I know it sounds stupid - fire is fire, right? - but I swear it made a difference.
posted by cooker girl at 7:01 AM on September 14, 2010


How far away from the return register is the fireplace? If you had the central AC on the draft of air returning to the register could have been enough to pull smoke out.
posted by Rhomboid at 7:10 AM on September 14, 2010


Well - I lit some paper and the smoke went right up, so I lit some more paper, and some wood and the smoke is still going right up the chimney. (I have never pre-warmed it before, so it's good to know that I'm supposed to).

The fire has been burning for several minutes now and there are no apparent problems. AC is off (even though some are saying this may not be a factor) and it's cool (65 degrees) outside.

As for the other questions: Entirely possible that someone opened a door. And, I have no reason to believe that this fireplace has been cleaned in the last 15 years. It was originally a wood burning box, then the previous owners converted it to a gas fireplace, then we converted it back. I don't think it saw a lot of action before we moved in because, as I say, the chimney is pretty clean.

However, I like the idea of a chimney sweep explaining all of the nuances of our box to us. That might help us avoid something like this in the future.

For now things seem to be working. I'm going to chalk it up to random air pressures / winds / open doors and other such things and consider inviting a professional to come inspect the place.

Thanks everyone!
posted by crapples at 7:12 AM on September 14, 2010


Agree w/gjc that AC should have nothing to do with this. AC systems cool air that's already in the house; they don't exhaust it outside and create negative pressure.

When the warm air return has the choice of drawing air from the that which is coming through the labirynth of ductwork, around corners, under closed doors, etc. or through a straight shot down the chimney?
posted by The 10th Regiment of Foot at 7:14 AM on September 14, 2010


When the warm air return has the choice of drawing air from the that which is coming through the labirynth of ductwork, around corners, under closed doors, etc. or through a straight shot down the chimney?

Whenever air is being drawn into warm air returns, other air is simultaneously being pumped out into the same spaces through the registers; the pressures shouldn't change significantly when the AC is on. The design of the system won't be perfect, of course, so there will be some imbalances that will resolve themselves via the path-of-least-resistance, but those effects should be very small. As gjc wrote, If your HVAC system is causing enough negative pressure to cause this to happen, it is broken.
posted by jon1270 at 7:28 AM on September 14, 2010


It wasn't the hot conditions outside. Normally if it's hotter outside than in, without another factor like the AC, that should set up a natural air flow up the chimney. In winter, you can have the opposite problem of a cold chimney with a natural downdraft.

It could very well be the hot conditions outside and the cool air inside. The above is incorrect. Chimneys work because the hot air at the bottom is at higher pressure than the cold air at the top. If you have hot air at the top and cold air at the bottom, you are going to have problems. So you could have had a downdraft in the chimney before you started the fire and it could have been strong enough to push smoke into your house. In the winter, when you open the flue, the hot air in your house will cause a natural updraft. So, don't use your fireplace with the A/C on and don't use it when it is hot out and you should be fine.

It could also have been wind or it could have been some other cause of negative pressure in the house (a bathroom or kitchen fan running, for example).

Finally, if your house is well sealed, you may not have enough combustion air for the fire. All the air that goes up the chimney needs to come from somewhere. Normally, it comes in through cracks and leaks in the house, but if you have a very well sealed house, this many not happen (and you'll need an outside air supply for the fireplace).
posted by ssg at 7:48 AM on September 14, 2010


Whenever air is being drawn into warm air returns, other air is simultaneously being pumped out into the same spaces through the registers

This is not about pressure, it's about flow. That's why I asked whether the fireplace was near the return register. The return is going to suck in X volume of air per time period and it doesn't matter where it comes from. If the chimney is closer to the return than a register off in a far away room with the door closed, the flow is going to come from the chimney. Again, this has nothing to do with pressure, and nobody is saying that the AC is going to cause a net imbalance or anything like that.
posted by Rhomboid at 8:03 AM on September 14, 2010


The above is incorrect.

Agreed.

Chimneys work because the hot air at the bottom is at higher pressure than the cold air at the top.

I don't think this is quite right either. The hot air at the bottom rises because molecules are further apart and therefore it's lighter / less dense, not because it's at a 'higher pressure.' After all, these masses of air are not constrained by anything but the weight of the atmosphere; they're all at virtually the same pressure.

IANAPhysicist; I'm strictly armchairing it here.
posted by jon1270 at 8:06 AM on September 14, 2010


The mass of air is constrained by the chimney. In sort of handwavey terms, the hot air pushes against the sides of the chimney, forcing it out the top. If there is no pressure differential in the chimney, then air in the chimney does not move.
posted by ssg at 8:25 AM on September 14, 2010


jon1270: "The hot air at the bottom rises because molecules are further apart and therefore it's lighter / less dense, not because it's at a 'higher pressure.'"

Ideal gas law says otherwise. PV/nRT= constant. Temp goes up, same volume, pressure's going up.
posted by notsnot at 8:40 AM on September 14, 2010


Temp goes up, same volume, pressure's going up.

Yeah, but the volume isn't the same here; the air expands as it is heated, so the ideal gas law seems pretty much irrelevant.

That said, I'm bumping up against the limits of my understanding of chimneys. I'm going to go off and do some reading in hopes of wrapping my head around this a little more cleanly.
posted by jon1270 at 8:46 AM on September 14, 2010


I heat my home exclusively with a wood stove, which here in the Pacific Northwest means that I'm building a fire at least once a day for nine months out of the year. I am so, SO familiar with this problem.

Sometimes there's a downdraft. Sometimes there just isn't enough temperature differential to carry the smoke upward. The solution in both cases is counter-intuitive: burn more, and faster.

What you need to do is get heat pumping upward, as quickly as you can. I accomplish this by wadding up 3-4 sheets of newspaper, tossing them in there as directly beneath the stovepipe as possible, then slamming the door and holding my breath.

Whenever you start a fire, be sure to have a few extra sheets of newspaper hanging around just in case. Use your tongs to hold the lit newspaper directly under the chimney to get the updraft started. Once your kindling catches, you should be set.
posted by ErikaB at 10:26 AM on September 14, 2010


I think the roles air conditioning, and the fact that it was a hot day, played in this drama were probably in making the house fairly airtight when the fire was lit.

In order for any gas to leave the house regardless of the temperature of the gas, either the gas already inside must expand (by heating in the fire, say), or new gas must be generated (as combustion would do in this case), or gas outside must rush into the house to replace the gas that leaves (very limited in this case because of the air conditioning and the fact it was a hot day).

When crapples opened the flue and started the fire, I'd guess that the volume of smoky air generated was soon greater than the volume that could be contributed by the three processes mentioned above, and therefore smoky air ended up staying inside the house.

At some point, hot combustion gases escaping up the chimney might have generated enough of a pressure differential to make the chimney pull air in as well as exhaust it, either by establishing two way flow in the chimney, or by making the chimney alternately breathe gas out and suck it in, each of which would have made for masses of smoky air in the house.
posted by jamjam at 1:06 PM on September 14, 2010


When crapples opened the flue and started the fire, I'd guess that the volume of smoky air generated was soon greater than the volume that could be contributed by the three processes mentioned above, and therefore smoky air ended up staying inside the house.

I think jamjam has it right there, as far as diagnosing what actually happened, and I'll retract. Crapples (the OP) probably has a pretty tight house, as do I. There have been times in winter where I've had to open a window a crack in order to let enough air into the house to make up for the smoke going up the chimney.
posted by beagle at 3:22 PM on September 14, 2010


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