Preventing a fireplace filling room with smoke

[Tim Pollard]

Hello

I am new to this group but I'm hoping some fireplace expert can point me in the right direction with a solution to a problem with my new fireplace.

I recently extended my living room which would have resulted in my existing fireplace being off centre. To solve this, my builders blocked up the old fireplace and made a new opening in the new centre of the wall, joining this to the existing chimney about 2 metres up. The chimney is a 30s unlined one. The connection beteween fireplace and chimney is by a clay flue pipe. The displacement (ie. amount the fireplace was moved) is about 1.25m.

Now the old fireplace drew perfectly - except it was a 70s built rough stone horror (just like my granny had in her bungalow built in 1972). The new one has only been used once, but a small proportion of the smoke 'leaked out' into the room, making its use a bit untenable! It wasn't enough to suggest that the chimney was blocked, but rather that the chimney wasn't drawing as well as it had.

I suspect that the problem comes from a number of factors:

- The opening is taller than it was, so there is a bigger gap between grate and flue. The builders made a brick insert in the shape of a gothic arch, 85cm high by 58 wide, 35 deep. The old opening was lower

- around 70cm, I think.

- The clay pipe is roughly 25cm x 25cm, which I suspect is less than the old one (but I have to be honest and admit that I never did stick my head up the old opening).

- There is now a bend in the flue which (maybe) reduces the draw efficiency.

Can anyone tell me what is likely to be the problem and how I should solve it? I had wondered whether buying a hood to reduce the distance between the fire and the effective top of the opening would help? If so, does anyone know where I can get one in the South London (Croydon) area?

When I'm feeling brave I intend to try putting the grate up on bricks, to see if raising it closer to the top of the opening will improve things. Sensible or just a license to smoke some people?!

Any other ideas would be most welcome!

Tim P

[Jb]

I'm no exoert but had one thing has occurred to me. Has the new work made the room more draught proof and the fire can't 'pull' enough air through?

[Tim Pollard]

Good thought - it is a bit more sealed, but it always was quite draft free. We did try opening a window when we filled the room with smoke, but it didn't seem to improve things much!

[Kalico]

How sharp are the bends they have put in?

Also, is it clear that the smoke in not coming from the flue but rather just from the fireplace opening? I wouldn't have thought that a raised height of

15cm or so should make too much difference, although it could until the fire has got nice and toasty.

The correct way to think of a fire and flue is as a machine. The fire is the the engine and needs the necessary input to run correctly. There should be sufficient air available to it and it will not run efficiently until fully hot and the exhaust gases are running fast up the flue.

Another thought - what terminal do you have fitted to the chimney pot, if any?

Rob

[Huge]

We had exactly the same problems as the OP until we fitted (at the chimney sweeps suggestion) a "Colt Cowl". Marvellous bit of kit.

[Rick Dipper]

I had this issue, the opening was too tall, the local fire shop sole me a small "hood" to fit in the top or the opening.

Rick

[The Natural Philosopher]

small "hood" to fit in the top or the opening.

Ok, I hjad this problem in SPADES when the dumb builder built my inglenooks.

There is a page on the web that I discovered that covers this all in detail.

There are a few salient points you need to know.

The most important is that the fire APERTURE - if you like the area bounded by the grate, the sides of the fire and the lintel over teh top of it should be no more than 5, or at most 7 times the cross sectional area of the flue. Any more than this and the velocity of air being drawn into the fire will not be enough to suck stray smoke into the fire etc etc.

The second point is that starting fires with e.g. papaer and damp kindling, does not produce enough heat to overcome the resistance of the cold air trapped in the flue. Many canadian contributors to the sites I found that - especially when they had chimneys running up an outside wall - they needed to get a small blaze going first to 'start' the chimney. This efect is worsened if the flue is longer and not vertical: It takes more to start it.

In my case I am in teh process of installing giant cowls to reduce the effective area of 'draw'...so far with pretty decent results. I couldn't get teh aperture down to ideal, but its now small enough that it takes some severe sideways draught to allow a little somoke to escpae. ...in your case a simple strip along the top of the fire may be enough, or it may be more aesthetically pleasing to simply raise the grate. My in-laws have an amazing fire which is actually in the MIDDLE of the wall, with a log store underneath. Just raising the grate on a couple of blocks should be enough to stop the problem, or you can test by taping a sheet of cardboard over the top of the fire aperture.

To ensue the chimney 'starts' well, you need s good blaze from the word go. That means newspaper and dry kindling, or better a firelighter, or even as I sometimes do, ten minutes with a plumbers blowlamp to get the fuel going. Once the cghimney is going, you need to mae sure the room has adequate ventilation - I have underfloor ducts for the fire to avoid draughts, they work very well. ANY negative pressure in the room will stop the fire drawing, sister-in-law had a dreadful fire that wouldn't draw unless the doors were open. Eventually it was left unattended and burnt the house down. Literally and seriously.

So the key to a good fire is not too much aperture to flue, warm flue, and adequate room ventilation and a decent hearth space to avoid exploding material getting onto flammable stuff. The latter two points are covered by building regulations which are utterly sensible and should be studied and applied (or exceeded) rigorously. The design of flues and apertures is a lost art these days, and should be the subject of intense google searches for what info there is out there.

[Andy Hall]

On that subject, what precautions do you need to take to avoid small burning particles that are going up the chimney from landing on the roof and burning it? Just curious.. .andy

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[The Natural Philosopher]

Fisrt of all, most thatch house that burn, don't burn from outside. The classic cases are electrical shorts or hot flue gasses escaping via a cracked flue with rotting mortar, and setting teh dry thatch underneath aliht.

To this end, modern thatched rooves are subject to the followng regulations

- final exit height of stack and pot MUST be 2.2 meters above thatch ridge etc. this is reckined to be enough to carry sparks away.

- thatched roof must be a minium distance from boundary of property, or (in my case) there is no adjacent buildings, nor planning consent or indeed possibility of such to put up a property closer than, IIRC 5 meters.

- fireproof breathable board (supalux/masterboard/multiboard is used UNDER the thatch to reduce chances of an attic fire setting thatch alight.

- smoke alarms everywhere. This is pretty standard anyway no matter what the roof.

- no timber stricures are allowed closer than I think 20mm to teh stacks. In my case we used supalux board spacers and metal hangers to carry the ridge to stack loads.

- all flues must be lined with ceramic flue liners or steel insulated liners.

- some fairly srong regs on use of e.g. ceiling mounted spots under thatched rooves. Mine are actually under fully boarded attic floors.

That takes care of most of the normal mechanisms. The one remaining poetntial problem is a chimney fire, which can indeed eject huge quantities of burning material. In normal use the flue gases cool down fast as they rise up teh chimney, and no burning material is carried up.

Chimney fires are all about dirt flues and burning too hot. If you get one, you puit teh fire out below immediately with buckest of water, and block teh flue to starve the oxygen. It goes out n a few seconds. Its very unlikely that it would hurt damp wet thatch, but I have a hose always connected outside anyway.

The fire that hapopened was to a timber farmed house but tiled roof. The fire was piled high, surrounded by firewood and newspapers, had no guard, a very small hearth and a history of rolling logs off the front. Which is probably what it did, onto the carpet. In short, it was criminally negligent to leave it burning while we all went down the pub.

Open fires need to be treated with respect. But so does a motor car. We are less used to fires, than cars, these days...

[Andy Hall]

.

That was one thing that I was thinking about, and should be avoidable with proper sweeping of the chimney anyway.

I was thinking more in terms of log fires and during lighting with paper etc. where small pieces of burning material can go up the flue, or do you feel that they should have cooled sufficiently by the time they have gone up the flue and potentially blown across and fallen onto the roof? Presumably all this is OK or there would be many more fires. I can appreciate that the greater danger is from within.

.andy

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[Capitol]

small "hood" to fit in the top or the opening.

inglenooks.

I'm not disagreeing with this comment, but it is possible to work with aperture ratios approaching 14 if the entry to the chimney is funnelled. Most standard fire places use an angled restictor throat at the entry to the chimney, to produce a pressure change which aids gas flow. Sorry, I can't remember all the details, perhaps someone else can enlighten us. When I researched the problem of building fireplaces, I found that all the experienced fireplace experts, said "Try it and see if it works!, but allow for a fanned flue if it doesn't!!" I'd agree however that some form of a reduction in aperture is probably the easiest solution. It's probably worth asking a few questions of an experienced fireplace showroom, they may have an off the shelf solution for a few pounds. Regards Capitol

[Tim Pollard]

Thanks for all the responses - lots to chew on there (and I'm now very pleased I don't have a thatched roof).

Looking in more details at the fireplace, my clay pipe is about 10" by

7" - ie. area roughly 70 sq in. My fireplace area (w x d) is 23" x 14" or 322 sq in. therefore my ratio of areas is 322/70 or 4.6, so should be well within the accepted practice referred to above. It is off centre (about 3" from right side, 10" from left) which may be a factor. The angled piece seems to run up at about 45 degrees.

To answer some of the other questions posed above. At the top of the chimney is a conventional chimney pot - not capped. The smoke is not flooding out, but rather just leaking out slowly into the room from the hearth. I'm going to try a selection of the ideas above later (when i;m feeling brave).

Does anyone know of any sources on line for cowls to go in the fireplace.

Thanks again for all the ideas.

TP

[The Natural Philosopher]

The flues are pretty tall. Mine is, on a storey-and-half house, 9 meters from grate to the pot from memory. Being fairly large fires as well they don't draw to a roar easily. I have set coal flues alight - usually by getting a good roar going when starting using hardboard/cardboard etc. Even these rarely carry burning stuff all the way up tho - only time I have EVER seen the top spouting red hot stuff is in a flue fire. Everyone should start one just for the experience :-)

And if its that hot, it gets carried well clear anyway.

Basically, the regs have got it about rigfht IMHO. You have to work pretty hard to have a problem with open fires that are designed to regs these days, and swept properly.

[The Natural Philosopher]

I would agree with that to the extent that muy smike hoods are running on estimated 10:1 ratio and work fairly well. They don't 'draw' like a coal fire throtled down does though. I suspect my 5-7:1 was a ratio for open hearh coal that needs a better draught.

Those are hopeless. The noise is incredible. They are the open fire equivalent to Saniflo.

Few hundred pounds. They know bugger all as well, I tried. In fact I am probably the nearest thing to an expert that exists. And I know next to bugger all. You can get pre-desined coal fireplaces that will sork whjen slotted into standard flues, but no one build wod burning firepalces any more. After a struggle, mine are beginning to perform well - not just adequately, but well - and the experience is as I summarised.

The key is reducing fireplace aperture by raising the grate, or fitting a smoke hood.

Insofar as smoking is concerned. You can fine tune that by throttling the flue in a smooth choke type design, that accelerates airflow around the aperture top, but too much throttling will reduce flow rates too much.

[The Natural Philosopher]

These are nearly all custome made.

In your case a simple flap of bent stell that screws up into teh top of teh hearth a cpoup[le of inches back from the mantle piece and comes down a few inches at an angle towrds the front of the fire should suffice. I suspect the problem is more that you have (as I did) a 'flat top with a hole in it' rather than a carefully blended reducing fire-to-flue design...ou shold attempt to recreate that.

[Alan Campbell]

Hi,

sorry for the late reply - Google managed to lose my last reply. I found the following site very useful when I did my fireplace -

think one of your biggest problems is possibly the shallow angle of the new piece of flue.

Let us know how you get on - I for one am very interested.

Alan.

[Reader]

Have a think about getting one of these;

am planning on getting one of these very soon. It solves all the problems of sizing etc. I also like them because they have a lifetime garauntee which means i will keep it from one house to the next.

They aint cheap though.. The size i want is around £850.