Just about to start overhauling the chimney stacks (I know should have thought about it in the summer but a recent leak prompted a trip onto the roof to seal it and of course a close-up look and it's always much worse than it seems from ground level).
It's a Victorian house originally with big open hearths (now reduced by putting fire-boxes in) and wide, open flues.
I've always had a sneaky suspicion that despite regular'ish brushing the tar from the logs we burn will stick to the flue and could take fire at some point.
Whist doing the stacks I reckon I'm best putting a flue-liner in.
My question is what to do immediately behind and above the flre-box.
Should I make up an inverted cone with a spigot on top ( job for a man and his mig welder) to fill out the flue and so I can join the flue liner to it?
If so how far above the fire-box should I locate it?
How thick should the metal for the cone be?
How important is it to achieve a good seal all around the cone where it meets the flue and liner?
Any problems reducing the flue size by using a liner and the amount of draw through the flue?
Any suggestions/links appreciated.
Thanks, Pewe

Hmm, not exactly an expert, more an idiot with an urge...

Anyway, I have a liner above my log burner having the same doubts about an open chimney.. I thought about a metal plate for the liner but the installer shook his finger and told me some horror stories of heat+metal+warping and an ineffective seal.

SO, there a piece of board there that looks like cement board, but isn't.. go to your Hetas equipped shop and ask them.

If your anywhere near Malvern, I believe there is a sheet of that fire resistant board stuff there you can have.... Would need to check its still there first... Its an offcut from when we had a liner put in for the aga

Thanks guys.
I was thinking of making the cone out of some thick checker-plate I have kicking about but it's the seal between that and the flue I'm concerned about - maybe fire-clay?
Think I'll do some googling about the fire-board.
Funnily enough I imagine it's the same as the soffit board I've attached to the F27 headers to stop the GRP bonnet from over-heating/melting.
Teg thanks for the offer but we no longer visit Malvern regularly - age caught up with her last year.
Cheers, Pewe.

Be careful, as there are a fair number of building regulations regarding chimneys. Might be worth doing some research on this before making too many plans...

Many years ago I had an old, poor condition, chimney lined for a wood burner, they dropped a long inflatable tube down the chimney then having blocked the bottom filled it up with a lightweight quick drying cement. Then they deflate the tube remove it and you are left with a very smooth easy to clean flue. Don’t know if they still do this sort of thing but it worked well for me.

Had a couple of old farmhouses over the past 30 years, both of which necessitated flue work.

I'm no engineer, but I understand the basics, (venturi effect, Bernouli's theorem etc as I used to be an aviator).

There's a 'basic law' called the one-in-nine, which says that the opening at the hearth (area) should not exceed 9 times the minimum cross-sectional area of the flue at any point of its travel. Flue geometry affects this, as does the proximity of other tall buildings etc. If you stick with one-in-six or so, then you stand a better chance avoiding back-draught problems. Bends in flues are commonplace to prevent snow and rain wazzing down onto the hearth. (Another technical term).

Old huge fire-places seldom 'draw' properly and suffer blow-back because of not sticking to the above. If they do work, check the flue size, they're always enormous and Father Christmas could get down in all his gear. Evidence of blowback? Just look over the hearth lintel and you'll see if there's a problem. I fell foul of this when I had a big fireplace built in a farmhouse we had refurbed. Luckily, I lit a 'trial fire' (old crisp packets and leaves) before the final work on the hearth was completed.---Wrong geometry and blowback. Had to reduce the size of the open hearth.

It is possible to 'beat the system' and get better than one in nine with a chimney-mounted extractor fan. Electrically operated ones with variable draw, controlled from a switch by the hearth are available. (No experience of these as we solved problems by sticking to the rules of physics).

Second house (another farmhouse), we uncovered a huge inglenook fireplace. For lots of reasons, we decided to go with a 9kw 'Jetmaster' open logburner. Fabulous bit of kit. The flue was colossal just above the hearth with superb brickwork tapering the flue down to 'normal' proportions. (Actually, small proportions with the open inglenook 1:15 or so).

I did the sums with the area of the Jetmaster and decided a 10" flue gave us the 1:9 we needed to work with. A fire-proof board was fabricated to surround the flue and battoned to the brickwork horizontally above the level of the oak lintel. This was painted black in some Stovax product which was fireproof. The 10" blackened steel flue protruded about 3 ft above the board, ie, into the brick flue, thus we had a partial flue-liner.

Some folk recommend that a flue liner should go the full length of the chimney, for ease of sweeping, and soot adherence to a steel liner is less than it is to old brickwork. We cut a small hose-friendly hatch into the plate so the sweep could vac' out whatever soot fell either through daily use of the fireplace or from sweeping. We were 10 years in the house and had it swept three times with no issues.

We burned chiefly a mix of anthracite cobs and hardwood (oak and elm). The heat helps burn off tar residues and vapourises them. Damp, unseasoned pine is the worst. Heavy in tar and burns cool, perfect for gumming up the flue/chimney.

Fine tuning can be done by the selection of a good, tall, well veturied chimney-pot. A directional vane is a good idea, since a static one can be prone to problems with an adverse wind direction/speed.

Hope you find the above useful. ( I learned the 'hard way.' Bob.

[Edited on 12/10/11 by bobinspain]

Pewe

Just re-read your post.
What you propose would be fine, provided you stick to 1:9 (or better).
Plenty of fire-cement to 'gob up' the cone base where it meets the brick flue.
The solution I employed with the fire-board was suggested by the heating engineer who insatalled the Jetmaster. The brick flue, behind the oak lintel which was 5' above hearth/floor level was approx 12 sq ft. (6ft wide by 2 ft deep).
If a simple inverted cone will do the job, then good oh !

Bob

when i was a lad, we had done what dingz suggests above. No idea what it cost though cos i was 8.