Experienced stove users, what’s your take on this?

We have used multi-fuel stoves for over 20 years and consider ourselves pretty experienced users but this Autumn an annoying problem has arisen, which we’ve never seen before. We got a load of wood from our supplier and it included about one-third larch. I now know that larch has a bad reputation for producing a nasty runny tar but quite soon after we started on the load, we began to get dribbles of evil-smelling brown liquid coming out of the register plate and running down the back of the fireplace. This is easily removed with a damp cloth but it stinks and is marking the paint.

We had the chimney swept about a month ago and the bloke commented that a lot of tarry clinker-like crap came down and he reckoned we ought to sweep more often. Following the sweep even more tar dribbled out; the entire area around the register plate is a nasty suppurating mass like an oil well and it’s even coming out of the sweeping port cover. We thought the sweeping had liberated a big burst of tar but it has continued to come down since then. We burn hot and dry, we don’t let the stove idle and we buy seasoned wood, which we further season in a sunny, breezy store and even stack on the hearth for a few days before burning. Most of this year’s wood is ash with some other stuff thrown in; it has a moisture content of 8% to 12% when I measure it. Nonetheless the odd damp bit must be getting in because last week I could hear hissing while I was lighting the stove. I think I know what wood is doing it but can’t identify it from the bark. It’s not larch, we have finished all that.

Been thinking about this and I guess that if you burn a 250gm chunk of wood with, say, 10% moisture content, you will be sending 25gms of water up the flue, which is quite a lot. Last night we even heard a loud hiss as something came down and hit the flue pipe or the register plate.

Our flue is unlined, clay rings, but draws very well and the pot has a SS cowl, which looks OK from the garden but when it’s next dry I plan to nip up on the roof and check the cowl and the flaunching aren’t letting in rain water, of which there has been rather a lot this Autumn.

Our sweep (who installed the stove) is coming back soon to clean it all out and try to work out what’s happening but my guess is that we have two problems:

1 – The gross contamination of the clay flue with larch tar.

2 – Despite our efforts we are still getting the occasional piece of damp wood.

Any hints, comments, experience of this kind of problem?

You are unwittingly burning wood that isn’t dry enough would be my guess.
How are you testing moisture content? It should be on a freshly split face ie, split it and measure.

You could try and burn just coal DON’T MIX WITH WOOD for a while to see if that helps clear it up/dry it out.

You must be right. I forgot to mention that we create our bed of embers using smokeless nuggets, which are generally good for an evening of burning logs on top. Maybe we should do a weekly hot burn with smokeless only.

I test moisture content on the cut faces of the dried logs. I guess that might be giving me an over-dry reading.

Why do you need a bed of hot embers, this suggests that your wood is not dry enough.

I had to use smokeless fuel when we had the log burner installed in our latest house, the seasoned wood we bought was not dry enough to burn and would only burn on a bed of hot embers.

We now get all our wood green and keep it for 3 years drying so we know what we have and where we stand. No need to ever measure moisture as you know its all dry.

I think you are measuring the moisture of the wood wrongly. You need to split the wood at the time of measuring and check in the middle of the freshly split surface to get a correct reading.

My suspicion is that most of your problems are down to the moisture content of the wood. I have never burnt Larch so can’t comment on that but I would have thought it should still burn ok if it was properly dry

The key to running a wood burner well and easily is a good source of wood that is consistent and correctly seasoned.

THe type of wood is irrelevant, if it is damp it will create this tarry stinky mess.
The moisture content is high, guaranteed, regardless of what the supplier says.
Best plan would be to forget that load of wood and burn something else meantime.

It can just take a few logs among a good load to create an alarmingly large amount of tar.

It’s horrible stuff to get rid of…

This is why most installers reline chimneys, it’s not a ploy to sell more stuff, it’s because the flue is sealed top to bottom and any tar produced will drop back into the stove.
It also makes a nice smooth flue, whereas yours (I’m guessing) probably had a large void above the register plate where the smoke can slow down and condense.

I forgot to mention that we create our bed of embers using smokeless nuggets, which are generally good for an evening of burning logs on top.

Sulphur + condensation = chimney destruction….There’s no better way to wreck the chimney than mixing solid fuel with wet wood.
Best with one or the other.

Useful points thanks. I share the concern about damage to the flue and will be sure to take the day off when the sweep comes so as to have a good look and a chat.

I agree that wood must be wetter than you think. Air dried won’t get to less than 18% ish so I think perhaps your meter is duff. The odd hisser isn’t unusual though, how it happens I have no idea but you seem to get the occasional one.

Clay flue will be big and allow the gas to cool, is it an external chimney too? That will make it colder. Perhaps the pointing is damaged too allowing water in. Liner will be a big help but you need to clean the flue first.

Sweeping far more often would help. Seems odd to me but apparently creosote good through 3 stages, 1 is dry and powdery, 2 is the tar you have, 3 is hard dry tar and really really hard to clean out. Sweeping more often before you get a build up of type 2.

I’d clean, line and if needed point the flue, and think about your wood too.

I had a thought, I’m guessing as a long term stove user you don’t use a flue thermometer. I suggest you try one, they are less than a fiver. I know you can see when the stove is burning well but you can’t see how hot it is and a tiny bit of water does seem to make a large difference in temp. For example, I’m currently burning a lot of 80 year old 2×4, so it’s well dry except the tarp on my stack slipped and a teenie bit of rain got on a few. They are only ever so slightly wet on the top side but the difference it makes to flue temp is dramatic. I’m running with vents almost wide open and watching a ‘fire of Hadees’ scene to get a hot but far from insane temp, if the wood was bone dry I’d have everything shut and the scene much calmer but a hotter flue. A flue thermometer can help you run hot enough with those few slightly damp logs.

The annoying issue is you bought logs a few weeks before burning.

Log suppliers logs unless your buying kiln dried or very lucky are never as ready.

They have high throughput.

12months in the shed before I’ll burn them.

It sounds very much to me that you may have water ingress into your chimney. I have experienced an identical situation where cracks in the chimney/pot haunching allowed rainwater in and which then entered the masonry flue. This then ran down to the register plate as a sticky creosote like tar. The smell was incredibly pungent and pervasive and would linger on your clothing.

The cracking was not visible from the ground, only from close up inspection from the roof.

I have also seen a similar scenario where driven rain was defected upwards over a skew and into those downward facing openings on those cowls with several small vents. The net result was the same – water running down and collecting on the reg plate with tar deposit.

Edit I have reread your post and I am almost certain this is your cause. If you haven’t had the issue before and you have been running stoves for 20 years I doubt wood moisture is your problem. You also mentioned lots of rain this autumn and the problem arising in the same period – when you check the haunching and pots go over everything thoroughly – we see water ingress in almost unnoticeable cracks. I’d also be checking everything for possible cracking – is it a gable chimney/is the gable rendered?

Larch burns quite hot, quickly. May well have melted any residual tar and started a chain reaction of breaking it up. If you haven’t smouldered the larch, it won’t have created additional tar.

Surprised you need the coal as well as the wood, usually just open fires people do that with unless yours is a big multi fuel home range type thing that burns continuously.

My parents have a range and clay chimney that burns continuously through the cold months on coal and wood. Only tarring in 30 years was chimney stack needing some pointing/flashing attention.

I used to get a big fire going and deliberately set fire to the resin build-up in the stovepipe. The first time it looked like Haley Bop had tried to embed itself in the top of my chimney blue flames like a gas burner, the resin burned so hot that the black enamel part from the stove top to the ceiling would glow red. As it burned down to carbon it would flake off as the pipe cooled down and could be shoveled off the top of the baffle. T’was lovely.

I think you’ve got a couple of issues here, firstly you’re getting moisture in the chimney which may be coming from bad flaunching but I doubt it’s getting in down the side as you have a clay liner that would stop the moisture entering even if the brickworck was damaged.

The second issue is your wood supply. You need to be buying right now (at least) for next winters supply. I have a log supplier 500m from me and they have a massive stack of pine trunks that they chop up, store for a few weeks and then sell – no way are these dry enough.
As has already been said, you’ll never get firewood down to 8% moisture and a realistic good figure is anything <20%.
To measure the moisture split a log and then measure at the outer edge of the split face (i.e. near the bark) and not near the middle as the moisture moves from the middle to the outside – so the middle can be dry but the outside is still too damp.

The next problem is your flue. One of our stoves also has a clay lined chimney and when the stove was being fitted I asked the installer whether he could just fit an adapter to go from the liner to the stove.
His reply was that if he did that we’d be getting tar running down the flue and into the stove…. exactly as you are doing now.

So, although it seems a waste, you should put a steel liner down the clay flue. When this is being fitted they’ll redo the flaunching as part of the installation.

Buy some kiln dried logs to use now (or just go grade 1 coal…. not smokless as there’s proper nasties in there that will eat a steel liner) and keep your current supply for next winter.

Oh and Mucker, if you were deliberately lighting a chimney fire then you’re nuts. Lucky you didn’t set fire to the floor joists!

+1 for water in flue
+1 for fitting a flue inside your flue
+1 for mucker being a complete and utter pyro nutcase
+1 for wood supply being (somewhat) worthless this year
+1 for never mix smokeless and wood, as it makes lovely sulfuric acid to eat holes in your flue
+1 for switching to smokeless only for this year

I’d recommend going smokeless only, unless you insist on the middle class bling of only running that woodburning machine on wood because “it looks nice”, it’s far less hassle as a source of heat, although it carries the minor drawback of being about as green as Tortoise tourism in the Galapagos.