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The "right" way to run a woodburner?

Posted: Fri Nov 12, 2010 8:22 am
by mikepepler
Rather than add this on to the long thread about woodburners/stoves, as it's mostly about models and installations, I thought a new thread might be good to discuss what the best way to run a woodburner is.

My personal interest is that we've had ours in for about 6 weeks now, and are still trying different ways of using it. What we have is:
- Woodwarm Wildwood 9kW with side/back/roof boilers (no firebricks)
- rear flue exit to a 90 degree elbow with hatch for sweeping
- about 1m of uninsulated stovepipe to register plate
- about 6m of flexible flue liner inside a chimney, insulated around it with vermiculite.

(there's more info here ... hp?t=15203)

Anyway, we started off reading the instructions that came with the stove, and with a thermometer on the side of the stove (we were told it was more use here than on the stovepipe). We were lighting the fire using both air supplies, closing the bottom air once the stove got to about 150C, and mostly closing the top air supply once the stove got to 300C (the instructions said to let it get this hot - max is 400C). This seemed to work well, but the window was getting a bit black.

I'd also read that it was good to have the fire hot when you light it each day to dry off or burn off any tar deposits in the stovepipe and chimney from the stove slumbering overnight. However, after a few weeks of use, I realised that when I did this, the tar deposits in the stovepipe (NOT the chimney, thankfully) were igniting. I found this a bit alarming, as I know chimney fires are dangerous, so I went out and bought my own equipment for sweeping the chimney, and proceeded to investigate.

It turned out that while the insulated flue inside the chimney just had a bit of ash in it, the stovepipe had a layer of burned tar. Not very thick, just 1mm perhaps, but I thought it was a lot for a few weeks use. So, panic over - I wasn't about to set the chimney on fire, but I still wasn't happy about the tar in the stovepipe.

We've now changed the way we're using the stove:
- putting in fewer logs and leaving the air supply open wider
- added a second thermometer onto the stove pipe, and control the air to try and keep it above 150C, to avoid tar condensation.
- don't shut the air supply in the stove down as long as there are yellow flames visible - only if it's died away to a bed of hot charcoal.
- sweep the chimney ourselves once a month to keep an eye on it, and get a professional sweep in once a year.

This seems to be working better so far, given that the window has stayed clean, but I've not re-checked the stove pipe yet, as we've only been doing it differently for a week.

There seems to be a lot of conflicting advice on the web, so my questions to all you seasoned stove users (bearing in mind that a stove with boilers will always run a bit cooler) are:

- How dry does the wood need to be? (Ours is seasoned 2 years, and typically 20-22% moisture)

- Is having a hot fire to "burn off" the tar a good idea or not? If so, how hot do you need to get the stovepipe? I've read warnings that letting a lot of hot oxygen go up the stovepipe is likely to ignite the tar, but surely that's inevitable if you're trying to get a really hot fire to burn off the tar? Or is igniting the tar while it's in a thin layer what you want to do anyway? I've seen some people say yes, others no...

- Is it OK to have a fire, let it die down to embers for a while (causing the stovepipe temperature to drop down to 100C or even lower), and then get it going again with some kindling followed by logs? Or does doing this repeatedly create more tar (the stovepipe takes 5mins to get back above 150C)? Would it be better to just keep the stovepipe hot, and when you let the fire die down, let it go out completely until the next day?

- Are "chemical chimney cleaners", which claim to dry out the tar so it can be swept out easily, any good? I gather some are corrosive to flexible stainless steel flue liners, but haven't seen a definitive list of what chemicals to avoid (other than salt).

- Any other specific thoughts on the best way to run a woodburner?

Thanks for any advice, even though I'm sure we will have a lot of different opinions between us!

Posted: Fri Nov 12, 2010 10:25 am
by re
Just my opinion, but it sounds to me like you were shutting down the air too much. Your wood sounds fine.
This is what I do (with a stockton5):
- both air vent open to light it
- when it gets to about 200 shut down the bottom vent but leave the top vent open
- chuck on some small logs
- when it gets to 400ish shut the top vent but only about half way
- when I chuck on extra logs, open the top vent fully for a few minutes until it really gets going then shutting it down to half again (I never shut it down lower than this.)

I find the key is to make sure it's never just smoldering. I notice with my stove you should always be able to hear a slight roar, otherwise there's not enough air. The glass rarely gets dirty this way. But if it does, then letting it get a bit "too hot" (500C) for a few minutes clears it.

We had it swept after using it for a full winter and the sweep said there was hardly any soot to clean out.

Hope this helps.


Posted: Fri Nov 12, 2010 11:50 am
by emordnilap
What re said. Our system is happiest running quite hot fires and letting them burn right down before adding new wood. I think most of the potential grunge is thus either burnt or carried away and has the added bonus of keeping the window clean-ish. A scrub with a screwed-up sheet of newspaper cleans it.

Posted: Fri Nov 12, 2010 12:15 pm
by mikepepler
Thanks for the comments, I agree I was shutting down the air too much. Seems like the way I'm running the stove now is like you, re, though the temperatures are slightly lower as my stove manufacturer gives an upper limit of 400C.

One question though - are you monitoring the stove temperature, or the stovepipe, or both?

Any thoughts from people with boilers in the stoves?

Posted: Fri Nov 12, 2010 1:16 pm
by re
mikepepler wrote:One question though - are you monitoring the stove temperature, or the stovepipe, or both?
I measure it on the stovepipe, which I assume is hotter than the actual stove.

Posted: Fri Nov 12, 2010 1:21 pm
by JohnB
I had a Hunter Herald 16kw with a wrap around boiler, and could never get it to work properly. Someone who showed me how to use it said that each installation had it's unique characteristics that you had to learn. I think some of the problem was due to the location of the bungalow, but I never managed to make sense of it. This experience has put me off the idea of a wrap around boiler, but it may be a one-off, due to combination of a big stove and a shorter flue than I would have had in a two storey building.

Posted: Fri Nov 12, 2010 1:23 pm
by mikepepler
re wrote:
mikepepler wrote:One question though - are you monitoring the stove temperature, or the stovepipe, or both?
I measure it on the stovepipe, which I assume is hotter than the actual stove.
Well, now I have a thermometer on the stove and the stovepipe, I've found that while the stove is warming up, the stovepipe temperature can be a lot higher than the stove itself. However, once it's been going for a while, the stove catches up, and ends up being slightly hotter.

If you shut the air supply down too much, the stove gets hotter still (less air to cool it), but the stovepipe cools down (losing more heat to the room than is coming from the small amount of flue gases) - this is the mistake I was making before, when I was monitoring only the stove temperature, as I thought everything was nice and hot...

Posted: Fri Nov 19, 2010 10:10 pm
by adam2
If much smoke is emitted then that indicates incomplete combustion, which apart from wasting fuel, is apt to deposit tar and soot in the flue.
Adjust the fuel supply to give the desired heat output, and then alter the draught to the minimum that will result in a clean burn with least smoke.

This is soon achieved by instinct, but initialy it can be helpful to observe the chimney frequently.

Posted: Sat Nov 20, 2010 4:53 pm
by RenewableCandy
Fils shut our stove right down when it was burning, crashed out on the sofa and woke up with a splitting headache.

Everyone has now had the CO lecture from yours truly. They'd better bloody listen this time.

Posted: Sun Nov 21, 2010 11:36 pm
by snow hope
No boiler round my Stockton 8KW.
When lighting the fire, top air control fully open, bottom air control half open. Five minutes after lighting fire, close bottom air control. Ten or fifteen minutes later, close top air control to half way or sometimes leave 3/4 open to get a really good fire going with maximum heat output.

If room gets too hot, almost completely close top air control. Window then turns black, so after it cools down a bit, turn top air control back to about half way - can just hear the air being sucked up.

Have never used thermometers anywhere near the fire.

Is there really a CO danger if the fire is turned down low?

Posted: Mon Nov 22, 2010 9:40 am
by adam2
If the stove and flue are in good condition, correctly installed and used sensibly, then there should be no risk of CO poisioning.
However any small imperfection could allow flue gases to escape into the living space, as could freak wind conditions.
I would therefore urge that a CO alarm be installed in any room with any fuel burning appliance. CO can kill silently and without warning.

Although CO is slightly heavier than air, it may in practice rise to upper floors as it will be mixed with hot air which naturally rises.
A CO detector might therefore be advisable in bedrooms as well as rooms containing a stove.

A very low level of CO can kill those in poor health, and a slightly higher level is fatal to the healthy.

Posted: Mon Nov 22, 2010 2:45 pm
by RenewableCandy
TBH it was probably just poor air quality rather than CO as such.

We never shut the door in that room (it'd get too hot) we just let the warm air flow up te stairs.