Does anyone know about chimneys?

[SJPONeill]

Posting this on the off-chance that we might have an expert on wood-burner flues among our number? This is driving me absolutely nuts...

Although it is notionally summer here now…notionally…and we have just had a week of beautiful sun (probably more than we saw all last “summer”), we still have bouts of quite cold weather when our drill is to put the fire on when I get up at 6 to warm the house up and again in the evening if necessary…

Our problem is that the large wood-burner in the lounge that heats the house has taken to smoking continuously and, just like with teenagers, you just can’t tell it not to. We had it refurbished at the beginning of the year, including replacing the flue, removing the damper on the flue and replacing all the seals around the doors. We did this at the same time we re-roofed the house but a few months before deciding to re-roof we asked if ‘hats’ could be placed over the tops of the upper arms of the ‘H’ cap to stop rain water washing soot and much from the inside of the cap onto the roof where it had, over time, created an ugly stain running from the base of the flue down to the edge of the roof. At the time I was surprised that the hats were so small and so close to the opening of the H-cap but figured that our installers knew what they were doing.

You can’t really see if in the picture but there is also a layer of wire netting stretched across each external opening to prevent birds either dying in the cap or building their own little home sweet homes in it – for the first four or five years that we were here, we never had a problem with birds then all of a sudden the chimneys were like avian condos…the gauge of the netting is half-three-quarter inch so is unlikely to be THE problem but may be a contributing factor to the possible lack of draw across the top of the H…

We didn’t start using the wood-burner over winter til around May because, although we had a non-summer, it wasn’t that cold, just wet. But we were surprised that after only a couple of months the wood-burner started smoking worse and worse and so we got the flue swept – twice because the first time they did go hard enough and left a thick layer of creosote scaling on the inside of the flue. The folk who swept the chimney took the hit for not doing the job properly the first time around but were also critical of our wood supply as too wet. Looking back, I think that this is a stock answer as the same wood supply also feeds out wood-burners in the laundry and the guest house, neither of which has any problems with smoking. We always has a wet and dry side of the wood shed and it is possible that they only looked at the wet side i.e. the stuff drying for next winter.

I have been doing a ton of research into this issue on the net and have read up about positive and negative drafts and pressures inside the house. I have tried the fire with windows and doors open to see if it is a draft issue and doors/windows open or closed makes no difference other than to help clear the smoke that pours out the door every time we open it to add more wood.

I keep coming back to the two things that have changed: replacing all the seals on the wood-burner and adding the hats onto the H-cap. From my research it seems that the better sealed a wood-burner is, the more efficiently it operates which brings me back each time to the hats. What I’m wondering is if they need to be lifted higher above the mouth of the H for better wind flow across the top and to offer the least amount of resistance to smoke coming through the H-cap – the caps are, after all only there to stop rain washing soot etc onto the roof…

So I’m hoping that perhaps someone in our global community might actually be a subject matter expert on this topic and be able to offer up some definitive advice that doesn’t include:

pre-warming the flue by stuffing newspaper up it and lighting it,
getting better wood (there is nothing wrong with our wood supply)
replacing the wood-burner or
just hardening the heck up…

[ericinoz]

Hi, I have a slow combustion burner and it sometimes gets the same symptoms you describe of suddenly starting to smoke for no good reason. I don't know the design of yours so I will describe how it works on mine. It may be nothing to do with the flu. In the top of the fire box there are two large pieces of metal, a baffle plate and a length of box section tube with a series of holes in it that runs across the fire box roof. Both are hung from the sides of the fire box in small castings. The box tube carries air from a hole on each side of the chamber that vents out through the small holes.

If you get carried away putting wood and tinder into the fire box before lighting it you can accidentally lift the box tube up, blocking the inlet holes at the sides. The tube then gets jammed out of place causing lots of smoke to enter the room.

Take the tube out and clean the vent holes then reseat it. If that was the problem it should work fine.

[nebeltex]

You are not getting enough "draw" because you sealed the rest of the house too well. as air is warmed, is rises, only to be replaced by other air from below. crack a window or two and stoke it up. removing the damper may have been a mistake. they act in an aerodynamic (and thermo) manner, much like the control surfaces of an aircraft, and should remain present and adjustable...

[mbauer]

Used to make wood stoves.

Sounds like you have done the proper maintenance to your flue (chimney here in Alaska). Also that you burn "seasoned" wood, not wet.

One thing I'm curious about is the twin exhaust up top. I noticed that the diameter is smaller than your main diameter flue.

It looks like the larger diameter at the bottom of the H is designed to take in some additional air to mix with your exhaust.

What gets me is the smaller diameter above this air intake. Is this supposed to work like a bornulli and accelerate the exhasut as it mixes with the exhaust from your stove?
Is it an air intake?

Why not just one straight exhasut? Removing the H assembly, or is this required for air-compliance regulations?

Do you have an air tight stove, that doesn't need a dampner in the flue?

Air tight stoves will have more smoke because the baffle causes the unburned gas to rotate inside the stove to burned again before allowing only the gases that won't burn to exhaust up the flue. (90% or more efficancy ratings)

Yes, the combustion needs an air supply, leaky windows and doors usually supply this, if you don't have a way to supply fresh air for this purpose, when you open the stove's door, flames and smoke will try to hit you in the face. The fire is starved for oxygen and opening the door will supply it. = Flames leap out!

If it is just smoke and no flames, you have a restriction in your system. Possibly the air intake at the bottom of the H assembly has creosote trapped above the wire netting that you mention. This could have happened when they cleaned the flue.

When they cleaned it, did they go from the top down, or did they push a brush up the flue from the botttom? From the bottom could have pushed the soot and debri up and then plugged the opening around the bottom of the H. Make sense?

I've got a book somewhere that explains how a wood stove works, I'll reference it to see if it has any clues.

Mike

[Gil]

I agree with Mike that the flue reduction into the "H" distribution stack seems problematic. Below is a render, taken from the above picture, with the suspect areas noted. Diameters are shown from tracing the original posted photograph [they're for ratiometric comparison only]. Note that the area coming into the cone shaped section reduces down by over a factor of 3!

Another suspect element is the damper replacement. Smoke is an indication that insufficient draw from the firebox is being experienced - something is blocking the venting of the firebox.

Remove the top part of the vent and see if this solves the problem. If it does then call up the folks who installed it. Otherwise the problem is down in the firebox area - come to think of it call the people who performed the overhaul anyway, they're responsible for messing up your stove in the first place. That's your story - don't waver from it...,

+Gil

[SJPONeill]

Thanks, guys, all this is really helpful...

Somewhere in my previous research I came across the formula for building a H-cap for a flue and will try to dig it out when I get home (have to hit the road for a couple of days). They are quite popular down here and this was has given no problems since we had it put on about seven years ago - before that we had a straight cap with the smoke exiting directly out the top - from memory, this was a little too short and the top was just below the peak of the roof and so it wasn't drawing properly. When we moved here it took us ages to find a decent plumber and we found that all three of our wood burners had issues with the flues. All three have the same H cap and but all went well until we refurbished the lounge one which is the one we are having problems with.

By a process of elimination, I wonder too if taking out the damper has affected the dynamics of combustion as it is the only major change we made this year. Replacing the door seals and fire bricks only takes it back to an as new configuration. Ericinoz, I will check the baffle plate and any vents when I get back later in the week - I know that we sometimes give it a knock when we are putting wood into it so maybe we have knocked something loose.

All your comments have been really useful and much appreciated.


Simon

PS. I posted the same request over at UAMF and someone sent me this link which was really helpful in explaining the dynamics of wood burning fires, especially the PDF guide that you can download from the site

[SJPONeill]

PS...I will talk to the plumbers (plumbers do most fireplace installation here) re the dimensions of the hats...

[mbauer]

{H Cowl Description}
The Traditional 'OH' Cowl is designed to ensure that no matter what the wind direction or strength the flue or chimney is protected from downdraught. As the wind passes down the top of the external parts, it creates a wind flow that exhausts the smoke and fumes out of the lower exits.

Having read this, is the large opening of the funnel shaped flue (mounts below the H -shaped flue) where the exhaust gas is supposed to exit? Or is it just a reducer to fit the h-shaped flue?
Kind of interesting way to fix a downdraft issue.

How the Traditional OH Chimney Cowl Works



Any wind blowing up or down the two open-ended air shafts ‘A’ or ‘B’ is intensified where it passes the projections of the horizontal flue ‘C’, and so draws the smoke and fumes out from the chimney.

The main flue ‘D’ projects into the horizontal flue ‘C’, thus there are three ejectors acting simultaneously and drawing out the smoke and fumes, whether the wind blows up or down the air shafts. So powerful is this action that gusts of wind blowing through the air shafts produce an up-draught equal to about 30% of the velocity of the gusts. In normal weather, the ‘O.H.’ Cowl possessing four outlets to the atmosphere improves any sluggish updraught.

Mike

[SJPONeill]

Thanks, Mike...extrapolating from that then, it would be a fair assumption that adding the hats to stop rain washing soot and rust on to the roof has probably affected the critical vertical air flow...maybe they need to be bigger and higher above the upper lips of A and B? Or I just need to figure out a way to wash the roof underneath more easily...

[mbauer]

I'm thinking the diameter is to small for the main flue, it really is reduced in the photo you show.

If it was working ok before the caps were installed, you might be better off just raising them or getting bigger caps. Combination of screens and caps could slow the wind down that enters the assembly.

All in all I learned something and looking forward to what eventually works. Very interesting concept that h shape! Never seen it before.

Went to the links you supplied earlier to see what they say. Interesting about how to stack wood in the firebox and the different ways to get best heat.

The stoves I used to make were 1/4" [6.3mm] plate steel airless, not a big roaring fire, but one that would keep the steel hot to raidate heat out. One load of wood would last 8-12 hours depending on how much air you let in for the combustion. Finnished stoves weighed 400 lbs or more (181kg).

BTW-The fire brick is only used to keep the steel from cracking due to heat-cool down cycles. Without the brick, steel & welds cracks fast!

Mike