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Back drafting fireplace

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I'll start with the question and then give the details. Can a radon mitigation system cause a fireplace to back draft?

Here's the situation. I got a call from a friend last week telling me about smoke in his basement. They moved into their house about 6 months ago-a typical two-story home (approx. 2500sf) about 8yrs old, not a tract home, but not custom built either.

The way it was explained to me was that a fire was started at about 1:00pm. with no apparent problems in the main level. At around 6:00pm my friend goes into the basement to find it filled with smoke.

The fire deptartment comes out and couldn't find out where it was coming from. They had a Chimney Sweep check it out and could find no problems. Then they called me.

Here's what they have; a masonry fireplace with an enclosed ash dump; in the basement, a 132,000btu mid-efficient furnace, a 50gallon warm air draft water heater and a recently installed mitigation system. Apparently this is the 1st fire that they made using real wood. Previous to that, they used Dura-flame logs.

Can the reduction system depressurize the building enough to cause the fireplace to back draft? My one thought is that the mitigation installer sealed the entire basement, all the cracks and sealed down the sump lid, but not inside the ash dump, the only direct connection from the fireplace to the basement.

I know there are so many different factors that can contribute to this, but I thought that I'd start with the radon system. Thanks for your help.

Tony

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I guess it is possible, but I would not say that the mitigation system is the cause. Does the basement have a window in it?

One way to tell would be to turn the fan off on the radon system. Light a fire in the fireplace and see what happens. Then if it still does it you know that the radon system is not the problem.

Is this a new construction home?

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Hi,

I don't know much about radon systems but I think your theory is reasonable. A fireplace, exhaust fans and clothes dryers in tight houses can make a furnace and water heater backdraft and vice versa, so I should think that a radon mitigation system might be able to do that. The only thing is; if it were the radon mitigation system why didn't it just dump all that smoke outside?

Where are the furnace and water heater getting their combustion air from? Is there a clothes dryer in that basement too? Sounds like it's time for a little investigation using a smoke bomb.

ONE TEAM - ONE FIGHT!!!

Mike

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If the basement is sealed and the radon system is pulling air from under the slab, how does it affect the pressure in the house? Mitigation fans are variable, 15-80 CFM, at the low end not enough to turn the volume of the house once a day.

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Thanks for your answers so far. To answer some of the questions. This is an 8 year old home. R-30 in the attic and R-13 in the walls w/ mid-efficeint mechanicals. Not particularly tight construction.

The basement where the mechanicals are located is unfinished and the dryer is on the main level.

The basement floor is sealed, but not the ash dump floor. That's what made me think about the ash dump acting like a vacuum. I didn't think radon reduction fans were that strong, but I can't think of anything else causing smoke in the basement, but not on the main level.

My only other thought was that something in the ash dump caught fire, but when I opened the door to look inside it was fairly clean.

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If the basement is sealed and the radon system is pulling air from under the slab, how does it affect the pressure in the house? Mitigation fans are variable, 15-80 CFM, at the low end not enough to turn the volume of the house once a day.

Hi,

That's what I thought. What about a clothes dryer running in a tight basement where the fireplace flue from the basement is right next to the fireplace flue from the living room fireplace and if the flue at the basement fireplace is left open? Seems like then you'd have a big vacuum cleaner nozzle right next to where the smoke is leaving the flue for the living room fireplace.

ONE TEAM - ONE FIGHT!!!

Mike

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We've discussed adjacent unused flues before, and the mid efficiency mechanicals could move enough air out of the building to create a negative draft. Makes lots more sense than the fan.

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Simple test.

Turn off the radon fan.

Put a small dish in the ash dump. Place a rag in the dish and pour a small amount of peppermint oil or eucalyptus oil onto the rag. Close up the ash dump.

The basment should not smell of the oil. Let it sit like that for a few hours.

Refresh the oil.

Turn on the radon fan.

If you get a strong smell of oil in the basement, it's the radon system. If you don't, you've eliminated the radon system from the equation. Go look for something else.

- Jim Katen, Oregon

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Jim's on it... make sure that there is no combustible material used for 'forming' the firebox slab too.. Ash dump doors can get cherry-red and re-radiate heat down into that space and burn the forms.. I've seen it.. But it usually takes a long period of pyrolysis to do that..

Radon fan could be the problem.. and that ash dump floor.. If that's the case, mud-in that floor :)

(I have the world's oldest pack of butts in my toolbox to troubleshoot this stuff with radon vents... I use em perhaps 2x a year.. )

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(I have the world's oldest pack of butts in my toolbox to troubleshoot this stuff with radon vents... I use em perhaps 2x a year.. )

Someone ever lights up one of those nasty things in my house to test my HVAC system and they'll be taking a trip to the hospital to get their lip sewn up.

OT - OF!!!

M.

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I like Jim's idea, and I would also encourage you to look for leaky supply ducting in the HVAC system. If there is leakage to the exterior, the de-pressurization of the living space could lead to smoke being drawn into the home as pressures try to equalize. You say "mid efficiency" furnace. It's not one with a combo side intake/exhaust that could be drawing smoke into it, is it?

They may have not noticed the problem before since they used relatively clean burning fire logs and not real wood.

Any other changes recently that might have "tightened up" the home?

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Seems like an easy "worst case depressurization" test. Get a manometer and some hose, turn on all the various air-exhaust devices in the house (radon fan, bath fans, range hood, dryer, possibly the air handler, etc.), and see if you can pull a vacuum in the basement relative to wherever you think the smoke might have come from. It can take a little experimenting to figure out which devices have an effect, and what it is, and you may also need to check which interior doors are open or shut to establish the worst case. You also need to consider whether wind outside is contributing.

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A good fire in the fireplace is the most likely cause of "depressurization" in the home. If the furnace isn't running, smoke from the fireplace flue is being drawn into the home through the furnace flue.

Every time we lit up our open-hearth fireplace (you could park a VW in it), smoke would immediately be sucked down the boiler flue (along with some birds and an occasional plane). This is in a drafty 300 year old house - we always had to open a door and a window.

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Lot's of ifs!!! Turn off the fan and do the peppermint oil test or cinnamon oil for the holidays! If all else fails call in a fireplace specialist to figure it out.

If this is the first time that they have had a fire in the fireplace, it might be that that the fireplace itself has a problem and does not draw properly. Chimney might be too low, wind might be deflected off another roof or wall near the chimney just a bunch of stuff that can mess with a fireplace drawing properly.

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(I have the world's oldest pack of butts in my toolbox to troubleshoot this stuff with radon vents... I use em perhaps 2x a year.. )

Someone ever lights up one of those nasty things in my house to test my HVAC system and they'll be taking a trip to the hospital to get their lip sewn up.

OT - OF!!!

M.

shEESH!

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There is another rather remote possibility: if this is a tall home (several stories), with a very tall solid masonry chimney, it can be difficult to establish a good proper draw - especially if all that mass it particularly cold.

Cold air falls. Have you ever noticed the very strong smell of creosote in a home? It is usually due to this very problem - a creosote laden flue and cooler air descending down the cold chimney and into the home. That principle is also why over-sized masonry flues collect creosote so quickly: the moisture in the smoke condensates on the cool flue surfaces before it ever clears the top of the chimney.

I recall old schoolers speaking of holding burning newspaper well up into the throat of the fireplace to get the flue to begin to draw properly. I used to think of it (humorously) as a bit of a "good luck charm", but more often than not, the old folks prove us to be the dummies. ;-)

With all this in mind, you might have your friend try just that - preheat the flue a bit and don't try to start with Dante's inferno. Kick up the fire in stages and see what happens. You have nothing to lose.

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I'll start with the question and then give the details. Can a radon mitigation system cause a fireplace to back draft?

Here's the situation. I got a call from a friend last week telling me about smoke in his basement. They moved into their house about 6 months ago-a typical two-story home (approx. 2500sf) about 8yrs old, not a tract home, but not custom built either.

The way it was explained to me was that a fire was started at about 1:00pm. with no apparent problems in the main level. At around 6:00pm my friend goes into the basement to find it filled with smoke.

The fire deptartment comes out and couldn't find out where it was coming from. They had a Chimney Sweep check it out and could find no problems. Then they called me.

Here's what they have; a masonry fireplace with an enclosed ash dump; in the basement, a 132,000btu mid-efficient furnace, a 50gallon warm air draft water heater and a recently installed mitigation system. Apparently this is the 1st fire that they made using real wood. Previous to that, they used Dura-flame logs.

Can the reduction system depressurize the building enough to cause the fireplace to back draft? My one thought is that the mitigation installer sealed the entire basement, all the cracks and sealed down the sump lid, but not inside the ash dump, the only direct connection from the fireplace to the basement.

I know there are so many different factors that can contribute to this, but I thought that I'd start with the radon system. Thanks for your help.

Tony

When there are fossil fuel appliances in the home, the radon remediator should perform a test to ensure that the suction from the radon fan doesn't affect the operation of the appliances.

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Question - Where does the water heater vent terminate? If it's next to or near the chimney flue, then consider this - the fireplace and the furnace are both using air from within the home. There has to be make-up air getting sucked in from somewhere. The draft vent for the water heater is wide open, and if the water heater isn't heating, just on the pilot, then make-up air could be getting sucked down the vent and into the basement, and if the vent terminates right by where the smoke is exiting, then you got smoke in the basement. Even if the vent isn't right next to the chimney flue, on a cold, still night, the smoke could be drawn to it. Make-up air sources have been reduced by the sealing of air leaks in the basement. Try cracking a window about 1/4" in the same room as the fireplace when using the fireplace, problem will go away. Or make sure the furnace is turned down and not running so you don't need so much make up air at the same time.

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In order to do your scented oil test, you'd have to make sure that no other appliances needing combustion air from inside the home kick on during your test. I'd be less likely to suspect the radon mitigation system because it's pulling air from under the slab, and more likely to suspect the HWT or furnace either creating negative pressure in the basement and sucking from the ash dump, or pulling smoke down their flues.

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