Log burner flue behaviour

[reb78]

The log burner in the kitchen is on 24/7 this time of year. Its a small Villager C model - so cheap and no frills but works fine and keeps the whole house warm.

It draws well. Doors closed and you can get it roaring in seconds with the damper open and vents at the front open. This last week though, if you open the doors with the flue damper open smoke will waft out of the opening. Last week it would be pulled up the chimney as long as the flue damper was open fully. Close the doors and its fine and it draws well (will pull tissue and hold it against the front vents).

I swept it at the end of last season. The stainless liner seems good visually looking through the inspection hole on the register plate. The brushes run up fine.

What causes this? It was fine till a week ago. We haven't sealed the room more or anything. Is it true it could be the weather? Wind has changed direction multiple times since it started doing it but it did coincide with the high pressure that seems to be hovering over the UK.

 

[jerrytug]

Re atmospheric pressure: yes it has been 1030 millibars recently, unusually high for sure. Unusually low might be 50 millibars lower. So the variation which affects our weather so much is only a small percentage, 50 out of a thousand. Probably unlikely to affect your burner. And the atmospheric pressure will be the same at both ends of the flue..
Different fuel? Cold flue?

 

[reb78]

Thats interesting re the pressure. You see the figures on the weather but they never mean much to me.

We are on the aode of a steep north facing hill if that makes a difference. Weather can vary significantly on the south side of the hill and wind direction high up (clouds) is not always the same as in the valley at the ground level.

Fuel wise, its predominantly (99%) wood. Occasionally a handful of coal. I have switched from mostly Ash to Elm this season. The Elm fell in late 2019, was logged early 2020 and sat in the sun for two summers so is nicely seasoned.

I cant really work out whats going on. We will be putting a solid fuel Rayburn in this space at some point so any big works to the flue could be done then but I am not sure there is a problem with it.

 

[stevethegrease]

We are also on the side of a north facing slope. Seems to us that in some conditions cold damp air rolls down the slope and sort of inhibits the smoke. ( OK then pushes it back down the chimney). It wouldn't do any harm to run the brush up again though, just to make sure nothing is dislodged/blocking the chimney. Maybe buy a CO monitor just to be on the safe side.

 

[Gavlar]

I'd check the chimney is clear. If it's not drawing an obstruction is likely. If you've burned a lot of unseasoned timber recently they can block quite quickly. If you haven't got one, get a CO monitor. If it's not drawing, CO may build up.

I'm too slow! Steve types quicker than me!

Just a thought, is it foggy or misty? I can't see the air pressure affecting the draw as it's the same inside and out but damp weather can hold smoke down. Also if the chimney is very cold it will affect the draw until it warms up.

 

[zeppelin builder]

ive seen jackdaws building nests in chimneys sods drop twigs in till they hold . came back from holidays to find this one time

 

[gaz1]

The log burner in the kitchen is on 24/7 this time of year. Its a small Villager C model - so cheap and no frills but works fine and keeps the whole house warm.

It draws well. Doors closed and you can get it roaring in seconds with the damper open and vents at the front open. This last week though, if you open the doors with the flue damper open smoke will waft out of the opening. Last week it would be pulled up the chimney as long as the flue damper was open fully. Close the doors and its fine and it draws well (will pull tissue and hold it against the front vents).

I swept it at the end of last season. The stainless liner seems good visually looking through the inspection hole on the register plate. The brushes run up fine.

What causes this? It was fine till a week ago. We haven't sealed the room more or anything. Is it true it could be the weather? Wind has changed direction multiple times since it started doing it but it did coincide with the high pressure that seems to be hovering over the UK.

please get your chimney reswept it reads as you have something in the flue or at certain points thats blocking

you could have a partial blockage

if you have good draw not much affects it high winds and it should pull well

 

[waddycall]

Definitely sweep it, and get a CO alarm (and fit it as per the instructions). Check that the damper is still correctly aligned with the handle! Check the baffle plate is correctly positioned and that there’s nothing obstructing the throat in front of it. Don’t take any risks though this stuff does actually kill. I certainly wouldn’t want to use it overnight until it’s resolved.
It’s probably worth playing around with opening doors and windows to check it’s not a ventilation issue. With the doors open the flue will be trying to pull more air in than when the doors are shut.
If you don’t find anything then a cctv flue inspection would be a good idea. I was involved in a job where the flue wasn’t performing properly. The customer had it swept and inspected and was assured it was fine. It didn’t seem right so I got a cctv inspection done and found some major issues including the liner not reaching the pot and piles of twigs blocking the flue. None of this was visible from the bottom.

 

[Blobber]

On this damp/murky/misty/still weather our wood burner is harder to light, it is a big one though! I could see if yours is just a small burner how easy it would be to get a bit of smoke in the room until the flue starts to draw

 

[dobbslc]

We've just had our chimney swept and our burner is harder to light and keep going lately, the sweep also completely emptied the ash pan though so it might just be that as our burner likes at least an inch or two of ash.

 

[julianthegypsy]

I'd echo what everyone else has said about checking/sweeping the flue, but add that if you're on the side of a hill, depending on which way the wind is blowing you can pressure pushing smoke down. The solution is an anti downdraft cowl. Not hugely expensive and stops the problem dead. There are various types, the old fashioned H type work perfectly and the newer ones are a simpler ring type but they do the same job that even if the wind pushes down the smoke will go out and down or out and up but not back down the chimney. They really do work very well indeed.

 

[Gareth J]

The log burner in the kitchen is on 24/7 this time of year. Its a small Villager C model - so cheap and no frills but works fine and keeps the whole house warm.

It draws well. Doors closed and you can get it roaring in seconds with the damper open and vents at the front open. This last week though, if you open the doors with the flue damper open smoke will waft out of the opening. Last week it would be pulled up the chimney as long as the flue damper was open fully. Close the doors and its fine and it draws well (will pull tissue and hold it against the front vents).

I swept it at the end of last season. The stainless liner seems good visually looking through the inspection hole on the register plate. The brushes run up fine.

What causes this? It was fine till a week ago. We haven't sealed the room more or anything. Is it true it could be the weather? Wind has changed direction multiple times since it started doing it but it did coincide with the high pressure that seems to be hovering over the UK.

No harm getting it swept but could well be wind /air pressure. It might be simply that your house negatively pressurised slightly with a certain wind direction/speed. Very occasionally this happens to me. The reversed flue flow makes it tempting to open a window in the room (argh there's smoke in the house!) but that makes things worse. Solution is to open a door/window/s into the wind to help positively pressurise the house. Once the fire is lit and drawing well, all is well. Certainly shouldn't get down draughts if the fire is going and the flue is hot. If you do, and the flue is clear, maybe it's a design issue; flue gases cooling too much? Run the fire hotter? insulate the flue? Chimney stack not high enough?

Aspect and location can really exacerbated issues, I know a few who live in dank, dull wooded valleys where you get weird settled mists and frosts that have way more draw troubles than those in exposed locations.

 

[julianf]

We have a solid fuel Rayburn.

It has more controls on it than your average fire anyway, but...

If you open the door to refuel without closing the lower vents (ie have two openings in line) then some smoke will escape from the fueling opening.

It has an opening and dampener on the flue also. In the wrong situations you can get smoke to leak from that too - only if you've got all the rest set very very wrong on, but it's possible.

I'd suggest that, if it draws fine normally, it's nothing to do with the flue needing sweeping (sweeping isn't for draw anyway, it's to remove stuff that could cause a chimney fire) but then, who knows I guess... Fires science but so complex they're almost more art.

 

[waddycall]

Sweeping is for both draw and fire prevention! Can’t understand why you’d say that.

 

[pedrobedro]

Ours does leak smoke on first lighting sometimes in really cold or damp weather and I have to open a window till it blazes up.

 

[reb78]

Thanks All.

I will run the brushes up it today, even just to satisfy myself it isnt blocked, but I agree with Julian - its hard to fathom a 6" flue blocking enough to cause poor draw when I have only been burning since mid November this year on a clean flue.

It has a cap on the top so birds (large ones for sure) cant get in there and deposit nest material in the flue.

When I swept it at the end of last season I didn't actually get much out and it was swept thoroughly and the brush (which is a snug fit) ran freely.

I have a CO alarm in there already. I will check its install height etc is as per the instructions though.

I took the baffle plate out and cleaned and reinstalled it so thats all fine and the damper is still attached and moving as it should in relation to the handle as I had that thought as well.

Interesting comment on opening a window to pressurise the house - I hadnt thought of that so will try it. Its an old cottage, although the POs had installed 'lovely' double glazing, I would say it still isnt sealed (one day i will replace the plastic frames with something far more attractive...)

 

[daleyd]

Ours very occasionally will be a pain to light until it’s going and the flu gets a bit of heat in it, once you get heat in the flu that’s what causes the draw. I can usually tell when it’s going to happen so try to get a chunk of rolled up newspaper going first to heat the flu slightly then light the fire.

I think when installing fires/log burners and they do a spill test they get the flu warm beforehand.

 

[Gareth J]

Thanks All.

Interesting comment on opening a window to pressurise the house - I hadnt thought of that so will try it. Its an old cottage, although the POs had installed 'lovely' double glazing, I would say it still isnt sealed (one day i will replace the plastic frames with something far more attractive...)

Has to be a window facing the wind (with other windows facing away from the wind closed) to have a hope of working.

 

[julianthegypsy]

Double glazing will reduce draughts, but at the same time, it's reducing ventilation. The stove is trying to pull air out of the room, use it to burn the fuel then send it up the chimney. If the room is too well sealed, it can't pull air in so it can't draw. Any stove installation over 4kw these days must have ventilation fitted somewhere in the room for this reason. The same applies to smaller stoves, even though it's not in the regs. So open a window somewhere.

 

[SyncroMatt]

Good advice above, I would agree if it is very still and there is little Venturi effect from the wind then it takes time to get a good draw going. Essentially the upcoming hot air can take a while to displace cold air in the flue.

One thing no one has mentioned. This stove is in your kitchen, do you have an extractor fan? If that is running it can reduce the air pressure in your kitchen pulling air from other parts of the house or a window.

I find on a still day with the extractor running it can pull air back down our stove flue witnessed by smell and some times a touch of smoke. Open a window a crack and all is fine.