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Old 05-10-2015, 09:30 PM   #1
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Fireplace Smoke Drawn into Furnace Combustion Air Intake


Hi - I purchased this home 2 years ago and have been quite satisfied with the furnace and fireplace. The furnace has a vent and a combustion air intake that are in one flu and my fireplace is in a separate flu - both in the same chimney. When they are operating at the same time my basement often smells of smoke. I've never been able to see any smoke, or had a smoke alarm go off but the smell is strong.

At first I wondered if it was a back-draft or insufficient make-up air situation but if I do not run the furnace I do not get any smoke smell in the basement. I haven't noticed any correlation with winds or wind direction. I have my HVAC guy coming later this month to do a bi-yearly inspection/cleaning and I'd like to know what options we may have in rectifying this situation. Or is there some other testing I need to do to pinpoint the cause? I also have a chimney sweep coming on the same day so I thought I could ask him too but as I said I'd like to give these pros a heads-up if there is some work that can be done while they are here.

I've attached pics - the difference in height between the various flues does not seem to be the 8-12" (?) that I've read is recommended. I believe that my fireplace is in the far-side flue and not the flue in the middle.

Any thoughts or suggestions? I appreciate any feedback.

Thanks,
David
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Old 05-10-2015, 09:40 PM   #2
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Fireplace Smoke Drawn into Furnace Combustion Air Intake


My question would be why did they not vent the furnace out the side wall

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Old 05-10-2015, 10:18 PM   #3
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Fireplace Smoke Drawn into Furnace Combustion Air Intake


What floor is the fireplace located on?
What floor is the furnace located on?
Where is the furnace located, near the chimney and if so is it in a small(er) room with a door that is closed?

I'm thinking that if the furnace is located near the base of the chimney and when the main fan runs for the furnace it pulls the area around it (including the clean out doors at the base of the chimney flues, all 3) into a vacuum and that negative pressure is pulling the smoke down the chimney and out of one or maybe all three clean out doors. This can be caused by leakage in the return air ductwork.

Just a thought.
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Old 05-11-2015, 06:56 AM   #4
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Fireplace Smoke Drawn into Furnace Combustion Air Intake


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Originally Posted by old_squid View Post
What floor is the fireplace located on?
What floor is the furnace located on?
Where is the furnace located, near the chimney and if so is it in a small(er) room with a door that is closed?
Hi - thanks for the response. This is a ranch with a walk-out basement and your speculation is correct about the locations. The fireplace is on the main level and the furnace is directly underneath in the basement against the same end wall. The furnace, however, is not in a closed room. It is in a room that is the depth of the house and probably 12 feet across with an open door to the rest of the basement.

Regarding the clean-outs ... there is an ash dump in the fireplace that I don't use as I am unable to access any clean-outs which I presume are BEHIND the furnace. So many questions about how/why they installed it as they did.

The furnace has a direct vent - I think that's the right term - meaning that it gets the combustion air from outside (the hooked pipe in the chimney). How can I test to pinpoint the problem and then what can I do to correct it so that I can run the fan/furnace at the same time as the fireplace is going?

Thanks again!
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Old 05-11-2015, 09:00 AM   #5
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Fireplace Smoke Drawn into Furnace Combustion Air Intake


Correct me I am wrong but I think you are implying you have a direct connect vented into the chimney flue? The direct vent should be through the wall
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Old 05-11-2015, 09:09 AM   #6
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Fireplace Smoke Drawn into Furnace Combustion Air Intake


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Originally Posted by jimn01 View Post
Correct me I am wrong but I think you are implying you have a direct connect vented into the chimney flue? The direct vent should be through the wall
Thanks for responding. The intake and exhaust are both via PVC up through the flue and out at the top. If you are able to see the picture you can see the vent, which I think is the one with the cap and the intake vent which is the open-mouthed upside down pipe.

Since the furnace is below grade how would you vent through the wall without going up? Or is that what you are implying ... go up through the flu and then exit the chimney mid-way to the top?
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Old 05-11-2015, 10:19 PM   #7
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Fireplace Smoke Drawn into Furnace Combustion Air Intake


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Originally Posted by theorbo View Post
Hi - thanks for the response. This is a ranch with a walk-out basement and your speculation is correct about the locations. The fireplace is on the main level and the furnace is directly underneath in the basement against the same end wall. The furnace, however, is not in a closed room. It is in a room that is the depth of the house and probably 12 feet across with an open door to the rest of the basement.

Regarding the clean-outs ... there is an ash dump in the fireplace that I don't use as I am unable to access any clean-outs which I presume are BEHIND the furnace. So many questions about how/why they installed it as they did.

The furnace has a direct vent - I think that's the right term - meaning that it gets the combustion air from outside (the hooked pipe in the chimney). How can I test to pinpoint the problem and then what can I do to correct it so that I can run the fan/furnace at the same time as the fireplace is going?

Thanks again!
It's not the venting for the furnace that I'm concerned with, it's the ductwork itself. You could prove this one way or the other by firing up the fireplace and then just run the furnace fan without actually calling for heat. Easy to do if you have AC, just switch the fan switch on your thermostat from AUTO to ON. If the smoke smell returns then it's a ductwork issue.

Without being there the best suggestion I can give is to see if you can find someone in your area that does blower door testing of homes. This test is designed to identify drafts and air leakage in the home which is kind of what you're having an issue with...... but in your case the air leakage is coming into the house and pulling smoke with it.

Where is your ductwork located? Is it all in the basement?
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Old 05-12-2015, 04:20 AM   #8
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Fireplace Smoke Drawn into Furnace Combustion Air Intake


Where does the middle flue go? One of my thoughts is the smoke could be getting pulled into the furnace intake if it heads over in that direction. Smoke is pretty lazy exiting the flues. Two things I would try during the burning season. Crack the basement door to see if that changes anything. Add a temporary extension to the furnace intake.
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Old 05-12-2015, 10:01 AM   #9
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Fireplace Smoke Drawn into Furnace Combustion Air Intake


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Originally Posted by old_squid View Post
You could prove this one way or the other by firing up the fireplace and then just run the furnace fan without actually calling for heat.
Thanks, old_squid ... sounds like a good idea about the blower/door test if I cannot figure it out another way. And good idea to distinguish between the fan being on and the furnace being used. Would having the fan on - only - not pull air down the furnace intake? Does the furnace intake only suck air when the furnace is lit? I shall try the test of just having the fan on ... running that for some time while a fire is going and then if that doesn't do anything, then try turning on the furnace.

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Where is your ductwork located? Is it all in the basement?
Yes, all of my ductwork is in the basement.
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Old 05-12-2015, 10:05 AM   #10
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Fireplace Smoke Drawn into Furnace Combustion Air Intake


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Originally Posted by zappa View Post
Where does the middle flue go? One of my thoughts is the smoke could be getting pulled into the furnace intake if it heads over in that direction. Smoke is pretty lazy exiting the flues. Two things I would try during the burning season. Crack the basement door to see if that changes anything. Add a temporary extension to the furnace intake.
Thanks, zappa. I am not sure about the middle flue but that is something I can find out. The cross-drafting is what my hunch was initially, smoke rising from the fireplace chimney and then being sucked-down the furnace combustion air intake. How would my HVAC guy add a temporary extension to the intake? Extend the tube further downwards?

I am going to try old_squid's test and see if I can rule-out any ductwork issues.
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Old 05-12-2015, 10:52 AM   #11
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Fireplace Smoke Drawn into Furnace Combustion Air Intake


It's hard to tell from the pic but if the pipe exit is far enough over the edge of the chimney, just add a pipe with no glue. It will be a tight fit and shouldn't fall out. I would go pretty far down just to test the theory.

Might also want to check on how well the flue is sealed where the pipes exit. Normally not a big concern but you need to be thorough when trying to solve an odd problem like this.

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