Woodstove leaking flue gasses, but no smoke

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anthony_d

New Member
Dec 26, 2020
7
Scotland
Good Evening,

I've got a Contura 520, with soapstone surround. It's been operational for 9 years and up until mid November it's been completely reliable.

But now I've got a problem, it's smelling of flue gasses coming from the top of the stove, near the front.

There is no visible smoke (I've checked all over many times using a laser for assistance), and so far the source of the leak has been elusive.

I had a chimney sweep come in to try and trace the sourse. He found a gap at the interface between the flue collar and the wood stove. He applied fire cement, being unable to replace the gasket rope (that would require the entire chimney above to be removed in order to remove the flue collar).

After applying fire cement, he smoke bombed the stove, using the door to pump the stove. This yielded no further emissions of smoke. I've since done my own smoke bomb tests a few times.

He didn't charge money for this. He indicated that a proper job would need to be done as the fire cement would eventually crack. However he indicated that it should be enough to keep me going until that time.,

Unfortunately the flue gasses are still coming off the stove and into the room.

Now I've checked all over my stove - I've checked the welds, the back and there are two box tubes that pass diagonally front to back through the stove. I've blocked those off with some aluminium foil, just to try a process of elimination. I've also applied fire grout to any potential candidates for smoke leakage (gaps in the welds, screw tappings etc...

The door seal fails the paper test, but there's no visible evidence of smoke leaking through the door seal. In any case, I 've used clamps to press the door even tighter against the seal, just to rule it out.

My chimney sweep says that there's a negative gauge pressure inside the firebox and chimney, so any leak should pull air in rather than push flue gasses out.

However I applied his logic and checked the flue with a camera - it was completely clear and the chimney sweep already confirmed that it was drawing air in.

So I'm now no longer clutching at straws. I've run out of straws to clutch onto. It's almost as though the smell is being emitted directly from the top steel plating, daft as that sounds.

So does anyone have experience of leaking flue gasses, without there being leaking smoke? How do you trace it?

I would hate to have to condem my stove. It was expensive for one thing, and the source of the failure has not been confirmed.
 

moresnow

Minister of Fire
Jan 13, 2015
1,853
Iowa
Replace the loading door gasket with new original gasket. You may be surprised. Sounds like it's failing anyway.
 

EatenByLimestone

Minister of Fire
The stove does run at negative pressure. So it should draw in. You can check for leaks by moving something like insense around the seams.

Have you changed anything else about the house? New bathroom fan, new stove fan, air sealing or anything that might change the way air moves throughout the house?
 

anthony_d

New Member
Dec 26, 2020
7
Scotland
The stove has a dedicated air supply duct directly from the outside. It doesn't get receive any air from the house itself. I have checked that this supply duct is clear. I've also double checked by doing a test run with a window open. I was still getting leaking flue gasses.

The door seal - I've repositioned the door hinges so that it now closes more tightly. It made no difference and as others have alluded to, it shouldn't be emitting gasses into the room. My one caveat is that this is a Swedish stove, and as such the flue exit from the firebox is at the front of the firebox, not the rear. One documented consequence of this is that when the door is opened (including slow openings) then smoke often does come out into the room.
 

EatenByLimestone

Minister of Fire
Outside connections, at least here, aren't tight. Check to see if it pulls smoke in anywhere with insense.

Otherwise, you may be getting smoke leaking in from outside. Or physically pulling the smoke out of the stove if your house is depressurized by a furnace, bathroom vent, dryer vent, too short of a chimney, etc.
 

anthony_d

New Member
Dec 26, 2020
7
Scotland
Otherwise, you may be getting smoke leaking in from outside.

I'm not too sure what you mean by that. The smell is definitely most concentrated right above the top of the stove. If it's coming in from the outside, i.e. via a window vent, then that is where I'd expect the smoke smell to be concentrated.

So negative pressures inside the house are not a feature, and I can confirm that by a process of elimination:
1. I can leave the window wide open and I still get the smell of flue gasses.
2. If I crack open the stove door, the fire gets considerably more vigorous. That confirms to me that there is suction inside the stove.

That leaves the flue itself. If it's blocked then positive pressure in the stove is a certainty, and I'd completely understand the problem.

However, It was swept last august. It's been in daily operation since mid September right up until mid November when the flue gasses in my lounge were first noticed. There's still a draw of air into the stove, and the chimney sweep confirmed this himself. I've also confirmed using a camera that the flue is completely clear. Furthermore, the smell gets stronger when the fire gets hotter, so I don't think this is down to a cold flue.

My chimney sweep reckons I'm chasing shadows, but he didn't do a test burn, and my girlfriend confirmed also the smell of flue gasses. So I'm not imagining it.

So here theory and observation diverge:
1. Why the smell of flue gas, but no visible sign of smoke?

The mind boggles.
 

begreen

Mooderator
Staff member
Nov 18, 2005
90,662
South Puget Sound, WA
Got kids?
 
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jetsam

Minister of Fire
Dec 12, 2015
5,283
Long Island, NY
youtu.be
Doesn't sound like you should be having a problem.

Do you only smell this at the end of the burn when the fire has died down a lot? Only at the beginning while the wood is offgassing? All the time?
 

anthony_d

New Member
Dec 26, 2020
7
Scotland
Do you only smell this at the end of the burn when the fire has died down a lot? Only at the beginning while the wood is offgassing? All the time?

The smell of flue gasses are a function of temperature, and combustion strength.

When the firebox is cold, and I've got a smokey mix of kindling, then there's no smell. The smell only starts to be noticed above firebox temperatures of 100 degrees C. By 300 degrees C the smell is really quite strong.

If the combustion is largely complete (embers only), and the firebox temperature is still high, then the smell starts to subside.

So this suggests to me that the smell is indeed a product of combustion gasses, and not any kind of other offgassing (paint etc...)
 

jetsam

Minister of Fire
Dec 12, 2015
5,283
Long Island, NY
youtu.be
Well, the draft would have been low under the smoke bomb test, and higher with a fire in the stove.

If you are smelling firebox air, there will be CO in it. Maybe try a handheld CO leak detector (amazon has one for £35) to pin down where it is coming from?

I would also question why it is happening. If you drill a hole in a burning stove, you shouldn't get a lot of odors, just a hotter fire. The new hole will be an intake while a fire is burning. You're sure the flue is drafting properly?

Have you tightened up the house this year- new windows, new doors, weather seal? Have you added a new exhaust fan or dryer? Has your HRV fan died without you noticing? Anything else causing a new pressurization situation in your house?




 
Last edited:

begreen

Mooderator
Staff member
Nov 18, 2005
90,662
South Puget Sound, WA
I was serious when I asked if you have kids. We had an odd situation many years ago where the phantom smell when the stove got hot turned out to be a toy soldier that their child had lost between the rear heat shield and the stove body when playing on the stove in the summer. It took a while to locate that one.
Has anything at all changed with the stove since last season? Was a special cleaning product used on it or were any joints sealed?
 
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anthony_d

New Member
Dec 26, 2020
7
Scotland
I think I'm onto something. I did a new test tonight, this time leaving the stove door cracked open.

The flue gas smell was still there, but much more subdued. Eventually I closed the door, because the firebox temperature was low. Interestingly the smell remained low, even once the firebox temperature increased.

I've seen the flue pipe insides and that's clear. So I'm wandering whether the air supply ducting is constricted, consequently leading to low flow in the flue pipe, and thus low suction.

There's 2 metres of 10cm diameter pipe under the floor, which becomes 40cm of 5cm insulated diameter pipe between the floor and the bottom of the woodstove. Both these pipes were visually checked, and apart from a few spiders webs at the main inlet, they were clear.

So I'm just wondering if a spider crawled up into the stove air supply and got stuck there.

Once the firebox cools down again dies down. I'll remove the 5cm pipe, and try vacuuming it and checking for flow in the firebox inlet. Then I'll retest. It's the only straw I've got left to clutch onto.

To answer other questions:
1. I've got no kids, but I can relate the the story being told (plastic bag melted onto the bottom of my car engine once!)
2. I haven't carried out draught proofing in the house this autumn, or fitted new exhaust fans / boilers or other air extraction devices.
3. Door window cleaning is with water/ kitchen towels. Any stubborn stains are cleared with an extra bit of ash from the firebox. Chimney is swept annually, and stove nooks and crannies are cleared out with a pipe brush. No chemical cleaners are used.
4. While checking for airflow for the above problem I did once use a candle, and yes a little wax dripped onto the hot steel. That's now all evaporated and I certainly knew all about it with the new smoke that it generated. That was certainly a much more acrid affair than what I'm dealing with here.
 

begreen

Mooderator
Staff member
Nov 18, 2005
90,662
South Puget Sound, WA
5cm pipe sounds quite restrictive. Most stoves require 7.5 to 10cm. What does the manual call for?
 

anthony_d

New Member
Dec 26, 2020
7
Scotland
5cm pipe sounds quite restrictive. Most stoves require 7.5 to 10cm. What does the manual call for?

The manual quotes 67mm for the inlet diameter. I haven't measured the supply pipe, just eyeballed it. 67mm seems plausible.

The air supply box tube ducting that is welded into the stove however has a 30mm x 30mm cross section, and that's got a much greater length than the 40cm for the external supply duct.
 

anthony_d

New Member
Dec 26, 2020
7
Scotland
I've made a youtube video that illustrates my problem a bit better, including some camera footage that indicates that the flue is clear of any partial blockage.


I still have a problem