Another Chimney Ponderance...

[a twig]

So, in time for Christmas, I opened up one of the original fireplaces, ripping out the rusted to bits mini range that had been installed in the 1920s and which was beyond repair. 20 rubble sacks later I had a large open heath, into which I whacked in a large dog grate.

Now, the draw on it is ok, not great, but it did for Christmas, if it got too smoky, a periodic opening of the back door helped to increase the draw to get it all going the right way again. Now I have been looking at remedying the situation further, I shoved the grate as far back into the fireplace as possible, and put fire bricks around the back and sides, the theory being that this will help to force more air to be drawn in from the front. A smoky test fire later, and this have improved but are still not perfect.

This is where the plot thickens. The flue is a shared one, doing the living room fireplace downstairs, and what used to be one in the bedroom upstairs (currently closed off and behind a built in wardrobe. I stepped outside however and noticed smoke coming out of both chimney pots. What I suspect therefore is that when the mason rebuilt my stack and made repairs to the flue (at considerable expense I may add) - he neglected to rebuild the central "divider". Initial torch assisted peering seems to confirm this. Thus I suspect I now have the following:

The smoke goes up the first section, and then hits the now effectively double width flue, loses most of its velocity, cools and sinks into the dead zone in the unventilated closed up fireplace, the rest just about ambles on up and out the two flues.

The ideal solution would be to take the entire lot apart again and rebuild the central divide, but that ain't gonna happen. So... My initial plan is just to replace the double squat pots on top, with a larger diameter, taller, single centrally mounted pot. The extra height should take it further above the neighbours roofline, which should help things. Going to a single central wider pot should increase the final exhaust velocity, which should also help improve draw. It will also reduce the likelihood of smoke going out one pot and back in the other when the wind starts messing around.

 

[a twig]

Where I am a bit more leery is what to do with the closed up fireplace, as it is in the master bedroom.

In theory, putting in some ventilation will stop that being a dead zone, thus improving the overall draw.

However...

Putting in the required amount of ventilation will make the master bedroom somewhat draughty. Reduce the ventilation however, and smoke will inevitably come out into the bedroom, which somewhat negative consequences for the health of my wife and I. At present therefore I am not intending to do this.

The only other option would be to try and "seal off" the upstairs bedroom fireplace as far up the flue as possible, so that the flue does not "see" the dead space so to speak, and just has the decreased velocity of the double width to contend with, as indicated by the red line on the drawing (I have also drawn in the proposed new pot in red as well).

I am more happy about doing this, however, it will not be simple :twisted:

So, that's the predicament and current plan. Thoughts anyone?

 

[a twig]

The ultimate "safe" solution of course would be to install a SS flexible liner and a gather - comme ca:

But that would be somewhat spendy, and given the height of the downstairs fireplace, still might not solve the draw problem anyway.

 

[Flyfisher]

Notwithstanding the less than ideal chimney structure, if the chimney draw is good with the back door open, are you confident there is adequate ventilation for the fire itself?

 

[MikeG]

Well, before anything else I would try to get an independent air supply to the downstairs fire. If that didn't solve it I would take the top off the chimney and repair the dividing wall somehow. Finally, if none of that did the trick, I'd consider an electric extractor for the top of the chimney.

 

[a twig]

Notwithstanding the less than ideal chimney structure, if the chimney draw is good with the back door open, are you confident there is adequate ventilation for the fire itself?

I'm reasonably confident that the ventilation is good. There is a woodburner at the other end of the same room (long and narrow room) which doesn't have any problem at all, indeed it almost draws too much and is difficult to damp down at times. Obviously I'm not trying to have both going at once, and I make sure it is all closed up before lighting the open fire, so there is no "pull" towards that end...

Well, before anything else I would try to get an independent air supply to the downstairs fire.

Could you elaborate on what you mean by independent air supply?

Repairing the dividing wall of the chimney is a non starter. I would need to take the stack down, plus pull out a fair amount of the gable wall to get access. Electric extractor is also a bit extreme for what is in essence a "show fire" to use when we are entertaining etc.


Unfortunately I'm not working in extraction anymore, thus I no longer have access to an anemometer. Otherwise I could measure the face velocity at the hearth, and also get a picture of airflows in the room - which would give me a better indication of where the issues lay. Might see if I can find one on ebay...

 

[paulc]

Based on my experience of installing a wood burner and conversations with the installer & chimney sweep, you really do need to get the feathers repaired. Having holes that allow fumes to go from one flue to another is just asking for trouble with respect to carbon monoxide poisoning in the bedroom. Whist a s/s liner would go someway to mitigating the problem, no respectable installer would fit one once he knows about the problems inside the stack.

 

[a twig]

Based on my experience of installing a wood burner and conversations with the installer & chimney sweep, you really do need to get the feathers repaired. Having holes that allow fumes to go from one flue to another is just asking for trouble with respect to carbon monoxide poisoning in the bedroom. Whist a s/s liner would go someway to mitigating the problem, no respectable installer would fit one once he knows about the problems inside the stack.

There are no plans to open up the bedroom fireplace - for those exact reasons as outlined previously.

Also as above, a dual skin S/S liner would mitigate the problem completely in terms of the "two flue" issue as I could just go to one pot at the top with a new slate, but this would still leave me with draw problems as the flue would then likely be too narrow with respect to the aperture and shallowness of the hearth.

With all due respect, I think you may have the wrong end of the stick regarding installation - feathers are not structural, and installing a S/S liner is the default option when dealing with absent feathers and decayed linings - so not sure why you think anyone wouldn't fit it??

On a separate rant, in my experience no HETA*SE installer would knowingly install things the easy way when there's a more complex way that he or she can charge the earth for and milk the most out of their mickey mouse 3 day cert - hence why I'm not talking to any. Last time I spoke to one he was trying to convince me to rip out my entire hearth and put a new one in as it wasn't "thick enough for reggerlayshuns". Never mind it had done the job for 200 years...

EDITED: Lots of times for typos and clarity...

 

[a twig]

Ah-ha - a late Christmas present from the inlaws arrived the other day, finally got around to opening it and it is a USB borescope - like one of these but significantly longer!

http://www.lightinthebox.com/720p-h...-inspection-camera-borescope-2m_p1840007.html

Any odd suggestions anyone may have about why my inlaws bought me such a device they can keep to themselves :lol: :lol: :lol:

So I shall be off surveying the flue in detail tomorrow - also found a decent anemometer on eBay for tuppence ha'penny so that is now wending its way towards me as well - hurrah!

 

[MikeG]

Well, before anything else I would try to get an independent air supply to the downstairs fire.

Could you elaborate on what you mean by independent air supply?...

Sure. I mean a vent or duct bring air directly from outside to the fireplace, such that the fire doesn't draw air from the room. This is a requirement in all new-build houses with a fireplace. It will clearly be more difficult to retro-fit.

 

[88v8]

Because you had to open the back door, you already demonstrated that there is insufficient airflow for that big fireplace.
To feed the fire, the room needs more air.

This is exacerbated because the opening/flue ratio no doubt exceeds the 6:1 which is the reliable cut-off for not smoking. Your firebricks have helped reduce the ratio, but not enough. You now begin to see why there was a trend towards smaller fire openings. As you already found, one can get away with a larger opening if the room has a good air supply.

In our previous house which had suspended floors, I cut air holes adjacent to the downstairs fires and covered them with sliding hit & miss vents that I opened when we lit the fire. There, two 9x3" for each fire did the job, I suspect you will require rather more but the vent need only be open when you light the fire.

If you have solid floors then the air will probably have to come through the wall. We had the back of one fireplace core-drilled to bring air in.
The wall is 18" thick.

As regards the feathers and carbon monoxide in the bedroom, there is the makings of an Agatha Christie plot here. But you wouldn't see the end of it.
If you can seal up the fireplace, or rather the chimney, you'll be OK, but...
The first But is that with no ventilation the flue will get damp, so the single pot on top must be an H pot to keep all the rain out.
The second But is that I'd have a CO alarm in the bedroom.

Dog grate... a cast fireback might look better than the firebricks to protect the stonework from cracking.

Alternatively you could do what we eventually did with our inglenook and dog grate, which is to fit another woodburner. If it's only occasionally required, efficiency would be unimportant, so you could fit a genuine antique. Good from bad, so to speak.

Ivor

 

[Flyfisher]

Well, before anything else I would try to get an independent air supply to the downstairs fire.

Could you elaborate on what you mean by independent air supply?...

Sure. I mean a vent or duct bring air directly from outside to the fireplace, such that the fire doesn't draw air from the room. This is a requirement in all new-build houses with a fireplace. It will clearly be more difficult to retro-fit.

Holes cut into a suspended floor close to the fireplace and covered with a decorative grille can work well (and improve underfloor ventilation) or if the fire is on an outside wall a nearby hole through the wall plus suitable grilles can also work. The point being to have the holes as close to the as possible to minimise cold draughts.

We had a Baxi open fire at a previous house and it had outside air piped into the base of the fire. Such an arrangement not only provides adequate air for the fire, it is outside cold air rather than inside warm air going up the chimney, thus improving efficiency. As MikeG has pointed out, this ideal arrangement is difficult to retrofit but it's indicative of the principle.

 

[a twig]

Right, here are photos of the offending fireplace - as it is - big cast iron back plate and firebricks in place, shoved as far back as possible:

N.B. The paint, poor state of stonework, blowing plaster and hideous carpet are nothing to do with me, the restoration of the room hasn't been started yet! You should see the cement ribbon pointing at the other end... :evil: :evil:

And with its temporary fire guard / draught blocker :lol: :lol: A bonus point for make, model and year...

Aaanyhoo, thanks, some very useful ideas there. It's a solid floor unfortunately - 2 inches of 1980s concrete over earth... I do need to reinstate the original stone hearth at some point (currently out in the garden where I found it had been dumped), that will reduce the aperture height by another 4 inches or so. I am also conscious that our joiner will be starting on the new DG windows and doors soon, which will reduce "air in" even more once fitted. Behind some of the plaster in the corner there are the remains of a mysterious hatchway heading into the shed (a shed which contains what used to be the outdoor privy at the far end of it). When I get around to hacking off and replastering this room I may be able to put some sort of ventilation hatch/shutter in there. Our walls are a similar thickness to yours Ivor so while I do have the gear to do it myself, it's certainly not something I fancy doing!

One slight puzzler for me regarding the air supply. The cellar door goes off this room as well. When all other doors are closed and that door is opened the smoke pours into the room - yet the cellar is full of cold damp air and it is lower down - so instead all the cold air should come rushing into the living room surely? (Don't call me Shirley...) Instead it seemed to pull the hot air down?! Hey ho... More air always seems to solve things...

Ha, feathers and mysterious poisonings in our sleep, I should imagine Poirot would certainly come running!

Very sensible re: alarm, I'll whack a battery one in for now, and get a proper one wired in eventually. Once I have finished limewashing the land of spar 'oom and got some carpets fitted, the wife and I shall decamp there. Then I can get to work on the master bedroom, rip out the hideous fitted wardrobe and find out what denizens lie behind it and ascertain the state of the second fireplace/second flue.

We have a huge woodburner at the other end of the room in an inglenook, so I think two stoves in the same room might be pushing it a bit. The open fire is entirely frivolous and more so I can sit in front of it in my prized wingback, drinking single malt and reading a good book on the rare evenings I get to myself, rather than any sensible desire to heat the house etc.

Incidentally, are chimney dampers for open fires now against reggerlayshuns as well? Only I notice that they are easy to procure in Europe and in the USA, and do not seem to exist at all in the UK. Companies that used to do them have disappeared. I just was idly looking for something a bit less tacky than a chimney balloon to reduce the draughts to a trickle when it isn't in use.

Maybe I'll just paint the bonnet with the family coat of arms and install a couple of sturdy brackets to hang it from, and use the bonnet strut as the "damping control" :lol:

 

[Flyfisher]

What's that wall like behind the cast iron fireback? Would it be possible to core-drill a, say, 110mm hole to the outside air? If so, that would likely be more than enough to supply the fire with fresh air without causing an undue draught in the room.

Regarding two fires in one room, I don't believe that's an intrinsic problem providing there is adequate ventilation to supply both. When we had our wood burners fitted I recall seeing some calcs for the required ventilation opening area per kW to meet building regs. I can't remember where I saw these, probably the relevant 'part' of the regs I guess, so might be worth checking there. I recall that no additional ventilation was required for stoves rated 5kW or below but that a certain amount was required for larger stoves based on their kW rating.

As for the open fire only being required so you can sit in front of it in your prized wingback, drinking single malt and reading a good book, that sounds far from frivolous to me!

 

[paulc]

When we had our wood burners fitted I recall seeing some calcs for the required ventilation opening area per kW to meet building regs. I can't remember where I saw these, probably the relevant 'part' of the regs I guess, so might be worth checking there. I recall that no additional ventilation was required for stoves rated 5kW or below but that a certain amount was required for larger stoves based on their kW rating.

550mm^2 per Kw above 5Kw assuming no draught stabiliser is fitted. For modern sealed properties, the calculations are based on the full Kw rating of the stove fitted, but as most of our houses are draughty and built way before 2008, we needn't go there.

For two stoves fitted in a single room, I would suggest that the calculations are based on the total rated output of both stoves combined.

 

[a twig]

What's that wall like behind the cast iron fireback? Would it be possible to core-drill a, say, 110mm hole to the outside air? If so, that would likely be more than enough to supply the fire with fresh air without causing an undue draught in the room.

Good thinking. Yes it would be possible, there's still a fair chunk of rubble wall to get through (at least a foot) but all doable. I'd basically just chisel out one or two of the smaller stones. However...

1. The other side of that wall is an access road, used by me and my three neighbours. The hole would also be only a couple of metres away from where said access meets the main road. So potentially noisy and splashy.
   
2. Due to the side slope, the outside road height is fairly high - the ventilation hole could end up halfway up the back of the fireplace unless I come up with some sort of right angle/periscope type solution.
   
3. The neighbours boiler flue and kitchen extract exit on the other side of the access way. While they have now extended their boiler flue a few metres up so I don't get blasted with that anymore, their meals can already be smelt if I leave the nearest window open.

In short - it can be done, but I would rather not, yet It may well come to that in the end though! I might experiment with some ducting offcuts I have lying around - see if I can "pipe" some air in from elsewhere and see what effect that has.

Regarding two fires in one room, I don't believe that's an intrinsic problem providing there is adequate ventilation to supply both...

I agree with you on that - however I think the temperature might get a bit uncomfortable though if I had both going! The room hits low 20s centigrade on just the woodburner alone as it is! I've even had to turn on the electric pump for the radiators, so I can dump some of the excess heat out via the radiator in the conservatory - at times I end up using my back boiler to cool the stove rather than heat the house!

I have some Bath stone kicking around from a previous project, I may get the chisels out and come up with a semi attractive way of reducing the height of the fireplace, maybe effectively fitting another "lintel" across underneath the first. That combined with the raised height hearth may knock the best part of a foot off the aperture height, which should start to bring us back into sensible flue ratios.

 

[MikeG]

.......... The room hits low 20s centigrade.........

[Pedant mode] Eesh, don't do that! Centigrade was a doomed measure of angle with 100 degrees in a right angle. Temperature is measured in degrees Celcius [/Pedant mode]

 

[Flyfisher]

[Pedant mode] Eesh, don't do that! Centigrade was a doomed measure of angle with 100 degrees in a right angle. Temperature is measured in degrees Celcius [/Pedant mode]

[cough] Celsius [/cough]

:wink:

 

[88v8]

Farenheit, thankyou. We fought two wars to keep Napoleon and his system of measurements out of this country.

If it's hitting the 70s, perhaps your woodburner, is, ummm, too big? Although I appreciate it has to be larger where there is a back boiler. We heat the house to the low 60s and wear a wooly. Or two. As I write this it's 63F and will fall as the storage heater run out of puff. Then at 1700hish we'll light up in the dining room.

Fireplace... was imagining something huge. One way of reducing the ratio is to fit a blower across the top, set back slightly inside the opening. Maybe 6" deep? Have seen them in armoured glass. Brass would also work.
What size is the flue?

Mainly though, I guess it just needs more air. The cold cellar creates a downdraught, like a cold chimney.

At the moment the fire is pulling its air from other rooms, if you install (modern) dg, then it will need local air. And so will your existing woodburner. In an old house, draughts are not draughts, they are dispersed and necessary ventilation.

For that fireplace an external snorkel** would work to raise the external air intake. You could use cast iron soil pipe, it would look OK. If you dig down outside, you could make the hole lower and just bury the lower part of the external intake.

However, If you shut off all the other ventilation in the house, It will be like having the doors to the room permanently shut, so one core hole may not be sufficient. Your woodburner may need one as well. And then you have the potential problem of cross-room draughts as the stronger flue pulls air from both intakes. We could have that problem as there is no door between the 8kw stove in the dining room and the 8kw stove in the adjacent sitting room, but we don't because both rooms have appropriate draughts.
In an old house, draughts are not draughts, they are dispersed and necessary ventilation.

Ivor

*^Perhaps your Series III (1976?) has one you could 'borrow'.

 

[MikeG]

[Pedant mode] Eesh, don't do that! Centigrade was a doomed measure of angle with 100 degrees in a right angle. Temperature is measured in degrees Celcius [/Pedant mode]

[cough] Celsius [/cough]

:wink:

Ooops!