Woodburner Gurus

2nd Year of running the woodburner. Didn't think it was drawing very well the last few days, wasn't sure if it was because of the cold, still, weather or something else. So removed the baffle to look at the base of the 7" rear exit flue. Maybe more than 50% blocked by light crispy/brittle tar bits, removed all that (a bucket full) and it's now drawing and burning far better. B-)

The flue was swept before we started this season in the last week of October, similar amount of burnt tar then after the entire winter (5 months?) as I've just removed after a week under three months.

There has been a fire in it most days since the 23rd Oct from about 1500 with the last logs going in about 2100. The stove is quite large 10kW output but with a boiler rated aroud 7kW so I guess the flue gases are generally quite cool. We are burning seasoned hard wood (mostly beech but some birch and a little possibly oak), with the bark falling off and shakes opening up in the end grain. Generally it'll get through 4 x 9" long 4" sized logs every hour, so something over 15 logs/day.

Is that level of light crispy tar build up sort of expected? I don't mind digging it out every 6 weeks or so, more concerned about buildup in the flue and risk of a fire up there...

Reply to
Dave Liquorice
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That sounds a rather fast soot build-up to me. When I was heating this place by woodburner alone (Jotul, similar size, similar fuel, similar rate of consumption, no backboiler) I swept it myself twice a year and got out out half a bucket of soot from the flue, and only light sweepings from within the stove itself.

Reply to
Dom Ostrowski

define "seasoned" - have you done it or are you buying it in?

at what stove air vent level? IOW is the fire "smouldering", "bit of flame & smoky when door opened", "plenty of flame & no smoke", "roaring blaze"

& which vents are you using (if there is a choice)?

Jim K

Reply to
Jim K

Bought in but as I said bark falling of and shakes in the end grain. The supplier stores them in 400kg dumpy bags in a well ventilated barn. We have 'em stacked at the side of the carport. Bought in to sit beside the stove a couple of days before they end up inside it.

3/4 on the top "air wash" vent, under grate vents closed. Never riddle the grate but the fire bars do seem a little wide apart so maintaining a decent ash bed isn't easy. It could be that it just needs a harder burn occasionally.

No smoke, lazyish bluey yellow tinted flames that are frequently not attached to the logs. ie they are burning the gases emmited by the logs. Once a log has been in there for 30 to 45 minutes if you bash it with tongs it will fall apart and be glowing anything from dull red to red all the way through depending on how long it's been in.

Of course a fresh log burns with bright yellow attached flames initially.

Reply to
Dave Liquorice

It's not soot, at least not soot as in fine powdery possibly slightly oily stuff that I remember as soot from the open coal fire when I was a lad. This is hard brittle black lumps and some very light and crispy browner bits.

I think the boiler makes a heck of difference to flue gas temperature, thus flue temperature and how much tar will condense out. When it was swept back in Oct I didn't think that much came down, it certainly didn't sound like much came rattling down. But as I didn't do it I don't really know.

Reply to
Dave Liquorice

mmm can you describe the baffle? presume your descriptions of flame are with this in place?

Jim K

Reply to
Jim K

My wood burner is broadly comparable with yours in power and a back boiler. My observations are that I have had the same thing happen as you but it was because I was burning up the last bits of coal on it.

Mine is capable of running dual fuel. The most likely cause of your build up recently is that the top of the chimney where the flue gasses meet the cold external air is starting to neck a bit with condensing tar. My sweep recommends burning a small amount of smokeless fuel in the mix to make the burning wood soot less inclined to plate out as tar and easier to dislodge. I don't yet know if his advice is sound.

I don't know about yours but mine said the baffle plate should be checked weekly for any obstructions - probably CYA H&S regulations. I do it about monthly or if I have trouble lighting it.

The air draw is decreased so more tar, soot and smoke formed. You can probably keep it running by checking the baffle plate a bit more often and then getting the sweep in to check over the chimney. My money would be on a constriction right up at the top from what you have described.

Just seen your description of the flame patterns which sounds more like the situation I get when I damp mine right down for the night. Relights nicely from the embers. Do you have a carbon monoxide alarm in the room?

The worry is that you have a build up right at the top and plating out of tar in the pipe and at some stage the whole lot decides to make a chimney fire which tends to devalue the steel lining somewhat.

Reply to
Martin Brown

In my experience you need to have a really good roaring blaze every now and again to keep the flue clear of such debris.

AWEM

Reply to
Andrew Mawson

It appears to be a little *underaired* as you sound to be gasifying the wood. I'm no expert though. AJH might have some better knowledge. Our Clearview gets through a heaped garden barrow of dry logs in a full day. I haven't yet fitted a boiler or swept the flue:-(

There is lots of stuff on the manufacturers sites about combustion. ISTR the Clearview one warns about boilers dramatically reducing the combustion temperature and leading to sooty windows.

Do you run the fire closed down overnight? This quickly blackens the glass on ours.

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Reply to
Tim Lamb

Yes, Ascii art is easier than words:

+--+-------+ Where: = | a +---- d = door d v a flue v = air wash vent fed from = d b +---- g = grate d b | b = boiler d b| a = baffle d b| dggggggggg-+ d | +----------+

The baffle is really just an extension to the top of the boiler to force the gases up and round the boiler it's a stubby T in shape. The central "leg" allows access to the rear of the boiler and flue exit.

Reply to
Dave Liquorice

presumably the boiler part is in use? does it produce a "hot" output or just help another heat source?

between 3 and 9 for how long would the boiler be producing "hot"?

generally sounds to me as though the boiler may be cooling the gases etc so much that the tarry nasties are condensing on the first/next coolest surface - your flue exit .....is there a flue liner of some sort above all this? or just an open masonry flue ;>( ???

Jim K

Reply to
Jim K

In message , Andrew Mawson writes

Also, is the flue insulated?

Reply to
Tim Lamb

Neither am I, that's why I'm asking. B-) I was under the impression that to get the most out of the wood you where supposed to gasify it.

No, the last logs go in about 2100 so that by the time the CH thermostat drops back to 15C at 2200 it's not roaring away heating the thermal store for no real benefit. It'll still have a full bed approx 12 x 18" of burning charcoal that doesn't half chuck out the heat...

Reply to
Dave Liquorice

The boiler feeds a 300l thermal store via a load controller that should ensure that, apart from initial start up, the circulating water is always above 60C. Clip on thermometers at the boiler show the flow from the boiler in the range of 65 to 75C and return around 60 to 65C.

I guess all the time it can, the thermal store is depleted by the CH and HW. The CH will come on and off as required and the woodburner generally keeps up with that demand, plodding along heating the store when the CH is not demanding heat.

Could well be right, the flue pipe immediatly out the back of the stove up to the cIosure plate is single skin metal tube, so quite "cold" and looses heat well. I guess the best solution is to give it a good burn every now and again. What do you reckon once a week, fortnight or month?

Above the closure plate flexible twin wall flue liner not quite sure how much insulation is around that in the old stone chimney. I didn't see enough empty vermiculite bags to have filled the entire void. I wasn't overly happy about the installers...

Reply to
Dave Liquorice

You can get thermometers which clamp to the single wall flue above the stove. This lets you know if you have the flue gasses warm enough.

Reply to
David WE Roberts

That is a really dangerous circumstance. You could be gassed or start a chimney fire. I hope you have a CO detector.

It can be got around by having a good hot blaze once a week to burn it off in most stoves before too much accumulates. But don't do it when there is a big accumulation, a chimney fire can destroy the chimney/burn the house down.

It is an inherent thing in non-pyrolysing stoves when reducing zones occur in the fire. Unburnt hydrocarbons condense out in the chimney. (Tar and creosote). It's made worse if the wood is not fully dry. If you have white smoke, this indicates you have a cold chimney/wet wood. and tars will be forming. Most of the time there should be no visible smoke or at worst a slight bluish haze.

Most tarring happens in the top foot or so of the chimney (Where combustion gases meet the cold air). If you get it elsewhere, it is an indication that the chimney is running cold. This can cause damage due to acid attack, especially on masonry chimneys

Sweeping often won't dislodge this stuff BTW. It usually has to be scraped off.

Reply to
harry

Or an infrared thermometer? That would have more uses. CPC have what looks like quite a decent and cheap one on offer ATM. Dual laser, adjustable emissivity (0.1 to 1.0), 12:1 distance:spot. =A332.95 + VAT IN0545703 normally =A338.47 + VAT.

How warm is warm enough?

Reply to
Dave Liquorice

Or an infrared thermometer? That would have more uses. CPC have what looks like quite a decent and cheap one on offer ATM. Dual laser, adjustable emissivity (0.1 to 1.0), 12:1 distance:spot. £32.95 + VAT IN0545703 normally £38.47 + VAT.

How warm is warm enough?

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review of a Stovax product says that the thermometer is graded in three zones: "The display is very simple, just three zones of operation; Creosote, Best operation and Too hot. The temperatures are displayed in both Celsius and Fahrenheit."

OTOH some sites claim you don't need one.

In your case I would guess that a check on the flue temperature would probably be a good thing.

Cheers

Dave R

Reply to
David WE Roberts

that looks neat! =A312 odd on amazon

Jim K

Reply to
Jim K

Still think an IR thermometer has more uses (and I want one) but the thi= s hook has been severly weakened with that price. B-(

Looking at images of stovepipe thermometers it seems that the "ideal" flue temperature is between 120 to 250C.

Reply to
Dave Liquorice

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