UKC

Read more: https://www.diynot.com/diy/threads/terrible-sudden-creosote-problem-in-flue...

Is the flu clean, it's not lined with tar from the larch and when it warms up liquefies? How much larch were you burning? Tar can set as a thin glaze and softish bristles will just run over it. 

This sounds like a collection of small problems giving a bad result.

Firstly if liquid is dribbling/dripping out of the flue/register plate that means the seal is leaking. This allows cold air to leak into the flue and reduce the draw and cause condensation.

If the chimney has been left open for 4 days (why not use a fire cement which can be fired with hours?) it will have allowed cold air to completely chill the chimney, causing poor draw and condensation.

Using damp fuel of whatever kind allows moisture to enter the chimney from the fuel which condenses (moisture from smokeless will give brown water, evaporated sap from timber will give a variety of tar like substances, pine wood of any kind, even seasoned well outside for a couple of years. is very bad for this!) The moisture absorbs heat from the liners as it condenses, cooling the flue and causing more condensation.

Smokeless fuel has a very short flame and tends to rely on radiant heat, so it's flue gases are somewhat cooler than those from wood or bituminous coals, again causing a cool flue, poor draw and condensation.

A Dovre has secondary air, which should be closed off until the flue is up to temperature and running correctly, when it can then be opened a little to help burn off some of the incomplete-combustion gases!

All wood/multifuel stoves need to be run hard at least once a day to keep flues cleaner and warmer, effectively hot enough to evaporate and/or burn out any form of condensates. Usually people unconsciously do this when the arrive home to a cold house and give the stove some wellie to warm the house up. Also keeping the stove running non-stop, with this daily boost will also help to keep the flue warm and condensate free!

This may or may not be well informed, but are your flue section joints the right way round? Shouldn't they be assembled so that condensate runs back into the stove and doesn't escape?

As a point of physics, condensation causes energy release to the surface on which the condensate forms; the latent heat of vaporisation. It does not absorb heat from the surface on which it condenses.

Some useful insights there thanks, especially from Johncook. I knew smokeless nuggets give very little flame but hadn't seen it put into words - they form a great base of glowing embers on which to burn the logs but are poor for warming up the flue and take an awful lot of energy to light, especially if slightly damp. We have always started our burn with the door ajar for ten minutes but we have now tried lighting with only wood and leaving that door ajar for up to an hour until the grate is a roaring mass of burning wood. The result? No more condensation. I'm beginning to understand that the problem is of warm damp fumes rising too sluggishly in a cool flue - certainly when you go outside the smoke is only emerging very slowly from the cowl, not with any vigour so it's pretty cool.

It doesn't explain why the problem has suddenly started but at least we're getting there. The house still stinks though - even RR Junior when he got in our car with us last night when we collected him from Uni said "something smells smokey or peaty in this car" - it's our coats, which we hang on the newel post just outside the lounger door!

Has it always done this or has something changed effecting the draw? Correct flu diameter, cowling? Extra draft proofing on house doors and Windows, extractor fans running more in kitchen or bathroom restricting airflow... 

The weather can effect draw; but I don't think the UK has lacked wind this winter. 

Nothing has changed. And the weird thing is that, as I wrote, we have an identical stove, flue and setup in the front room, which actually draws less well because it's on the lee side of the roof yet we have  never had condensation in that flue.  I think we have got into the habit of piling smokeless nuggets onto the fire and not giving it sufficient draft, meaning we have been burning cold damp fuel, sluggishly.

Sounds like you've solved it, or at least identified a problem to test more. Apart from storing wood a shed, once in the house it's in a little store near the stove for perhaps 24hrs before it goes on. Which helps take the edge off it. Very High pressure and zero wind is the only thing that kills the draw on ours. 

If you have the two stoves they could be competing for air which will slow down one stove or even make it smoke back into the room. Any slowing of the draw will cause condensation in the slower chimney, or soot build up, and a smell in the rooms, and often a dark mark on the carpet at the bottom of the doors.

Another trick if you are burning just wood, is to not clean out all the ash. Wood produces its best performance when burning in a bed of ash. You get a very white glow from the ash. If you are burning pure wood and the grate is clean, stick two or three glossy magazines on the grate and light the wood on top of these. The titania in the glossy paper will have the same effect as wood ash.

Good luck. and try to keep the flue warm. (A chimney is the structure, a flue is the hole through it!) Check register plate and seals on pipes to make sure that only gasses through the stove can enter. Any leak will lower the draw and make the flue harder to get warm.

Thanks Johncook. The two stoves are in two separate rooms separated by two doors and when we installed the stoves I hired a core drill and made holes though the walls of both rooms near the fireplaces to ensure a good air supply, otherwise, as I know from experience, air finds its way under doors and through windows causing drafts - and I'm married to a draft detector!

Over the years we have learned that wood alone doesn't burn too well and needs a bed of embers. We use the smokeless nuggets for that and one load will last all evening. The register plate seems pretty well sealed; our sweep put masses of heat-resistant silicone where the plate is attached to the top of the fireplace and the stove draws and burns like a small nuclear reactor when there's a good flame creating lots of heat. The diameter of the flue is definitely bigger than the stove pipe though and it's this, which has been allowing the warm damp fumes to rise slowly and condense. For three evenings now we have lit with lots of draft and suffered no condensation so yesterday I felt confident to get a scraper and wire wool and clean up all the crap that had dribbled down from the rodding port and congealed all over the stove pipe. 

The flue passes though the outside wall in the bedroom and there's a noticeable difference in the temperature of the wall immediately where the flue passes through. Daily use of the stove will keep the flue warm in winter when the structure of the house is at ambient, i.e. probably around 4 - 6c.  

Our house has been smelling like a Scottish bothy or a big old house where there's a huge cold chimney with loads of stuff dribbling down. Slowly the smell is dying away though as we are washing things and airing the house.

I hope things stay good for you.

I would ensure that the rodding eye is also well sealed. Any leakage of air into the flue will reduce draw/cool flue gasses. 

Try the magazine trick if you want to burn wood and then only clear out the ash when it really gets too much and starts to fall out of the front of the stove. (I used to use the old littlewoods catalogues, but they have been superceded by the internet!)

Good suggestion; we get the Ribble Valley magazine shoved through our letterbox, an expensive infomercial mag printed on high-clay glossy paper. It's too stiff and glossy to use in the loo so your idea might work.