Twin Wall Chimney

[scb61]

Good Evening and a Merry Christmas to you all - I am installing a new log burner in my narrowboat and I am a novice in many respects, I would like to install the new twin wall system to comply with new regulations, however I am finding it extremely difficult to source a new chimney collar.

My (limited) understanding would indicate that the existing 6" hole in the steel roof will need to be increased to 8" to allow the twin wall flue to pass through and into the chimney collar, I am unable to locate the new type of chimney collar required to accept the twin wall flue anywhere and would be very grateful for any assistance offered

Edited December 23, 2020 by scb61
typo

[Alan de Enfield]

23 minutes ago, scb61 said:

Good Evening and a Merry Christmas to you all - I am installing a new log burner in my narrowboat and I am a novice in many respects, I would like to install the new twin wall system to comply with new regulations, however I am finding it extremely difficult to source a new chimney collar.

My (limited) understanding would indicate that the existing 6" hole in the steel roof will need to be increased to 8" to allow the twin wall flue to pass through and into the chimney collar, I am unable to locate the new type of chimney collar required to accept the twin wall flue anywhere and would be very grateful for any assistance offered

 

Just be aware that there are actually no regulations with regard to fitting a 'wood burner' in a boat. There is 'guidance'.

 

The British Standard for domestic installations cannot be achieved in a boat, so its just a case of do what you can to make it safe - you do not need a twin-wall flue (and anyway - why would you want to burn wood / coal to send 50%+ of the heat up the chimney instead of allowing it to radiate into the boat).

[jetzi]

If you want to make the flue safe to touch an alternative to a double skin could be to lag it. But I agree with Alan that it's a bad idea - you want that heat in your boat not out the chimney, and besides, the stove is the main source of burns, not the flue.

 

I just re-reinstalled my stove last week as it wasn't bolted to the hearth. I'm not sure where/whether bolting it down is regulated, but it's a really good idea - boating bumps had caused the stove to shift slightly and crack the fire cement around the flue. It wasn't leaking, but it still made me a bit uncomfortable. 

 

The title mentions a twin wall chimney which is a good idea unless you like scrubbing muck off the side of your boat - the twin wall in this case is for rainwatater. If it runs down the inside of your chimney and on the outside of the collar, then it will carry soot and muck out onto your roof and down the side of the boat - the aim is to keep the water on the inside of the flue, so make sure it's sealed well at the top so that water running down the inside of the chimney ends up in the flue. 

[dmr]

We have a single wall flue, with the stove just ticking over the top of the flue is nicely warm but not too hot. My wife wraps her clothes round it each morning to get them nicely warm.

 

I still find it hard to understand how a twin walled flue is better, except for reduced danger if the stove is accidentally over-fired. If there is no heat radiated from the flue and the heat output into the boat is not reduced then the stove itself MUST be hotter which is an increased risk.

 

............Dave

[Tony Brooks]

You can get punched stainless/aluminium sheets from which you could fabricate a flue guard maybe 4" larger than the flue OD. that way heat could still get out but the guard should be a few degrees cooler than the flue itself.

[jetzi]

21 minutes ago, dmr said:

I still find it hard to understand how a twin walled flue is better, except for reduced danger if the stove is accidentally over-fired. If there is no heat radiated from the flue and the heat output into the boat is not reduced then the stove itself MUST be hotter which is an increased risk.

Wouldn't a twin walled flue increase the temperature of the exhaust gases, rather than the temperature of the stove? My guess is that the same amount of smoke/exhaust MUST be produced, but said gases would cool slower as they are in contact with a hotter inner skin that radiates less heat to the outer skin than a single walled flue would radiate to the air. Slower cooling gases would rise faster and draw more heat out of the top of the chimney - so therefore losing more heat to the outside. The other safety advantage I see here is that a better drawing flue would suck the toxic gas out faster past any possible cracks and leaks than otherwise, maybe the double skin is more about this than about potentially burning oneself on the flue?

 

Or are you saying that in order to maintain the same cabin temperature you'd have to fire the stove hotter than otherwise would be necessary? That makes complete sense if so because it's a bit silly to say "a colder overall stove/flue combination is safer than a warmer one". 

[BWM]

52 minutes ago, ivan&alice said:

If you want to make the flue safe to touch an alternative to a double skin could be to lag it. But I agree with Alan that it's a bad idea - you want that heat in your boat not out the chimney, and besides, the stove is the main source of burns, not the flue.

 

I just re-reinstalled my stove last week as it wasn't bolted to the hearth. I'm not sure where/whether bolting it down is regulated, but it's a really good idea - boating bumps had caused the stove to shift slightly and crack the fire cement around the flue. It wasn't leaking, but it still made me a bit uncomfortable. 

 

The title mentions a twin wall chimney which is a good idea unless you like scrubbing muck off the side of your boat - the twin wall in this case is for rainwatater. If it runs down the inside of your chimney and on the outside of the collar, then it will carry soot and muck out onto your roof and down the side of the boat - the aim is to keep the water on the inside of the flue, so make sure it's sealed well at the top so that water running down the inside of the chimney ends up in the flue. 

That isn't how a lined chimney works, they have nothing to do with rain protection. 

 Their function is to provide a surface for stove by products to condensate on which allows it to stay within the flue, as an ordinary chimney fits over the collar, which directs the condensed water/ooze mixture to the gap between. 

 A coolie hat attachment will prevent rain but i have never found it necessary, but they can provide entertainment as they change shape in a tight bridge. 

[dmr]

10 minutes ago, ivan&alice said:

Wouldn't a twin walled flue increase the temperature of the exhaust gases, rather than the temperature of the stove? My guess is that the same amount of smoke/exhaust MUST be produced, but said gases would cool slower as they are in contact with a hotter inner skin that radiates less heat to the outer skin than a single walled flue would radiate to the air. Slower cooling gases would rise faster and draw more heat out of the top of the chimney - so therefore losing more heat to the outside. The other safety advantage I see here is that a better drawing flue would suck the toxic gas out faster past any possible cracks and leaks than otherwise, maybe the double skin is more about this than about potentially burning oneself on the flue?

 

Or are you saying that in order to maintain the same cabin temperature you'd have to fire the stove hotter than otherwise would be necessary? That makes complete sense if so because it's a bit silly to say "a colder overall stove/flue combination is safer than a warmer one". 

 

I really don't know!   The twin wall advocates insist its a more efficient system, but like you I do't see how this can be if its putting more hot gas outside the boat. A twin walled flue will certainly draw better, this is why I use a longer twin wall chimney (when not cruising) with insulation between the walls. A good draw is good but the twin wall does not radiate heat into the boat which is a significant loss and I suspect outweighs any gain in combustion efficiency.  A good welded steel stove minimises the problem of leakage from gaps, our lovely old cast iron range was really bad.

And much more important, the twin walled flue and chimney with a red rubber seal look just terrible on a boat ?

 

.............Dave

[jetzi]

5 minutes ago, BWM said:

That isn't how a lined chimney works, they have nothing to do with rain protection. 

 Their function is to provide a surface for stove by products to condensate on which allows it to stay within the flue, as an ordinary chimney fits over the collar, which directs the condensed water/ooze mixture to the gap between. 

 A coolie hat attachment will prevent rain but i have never found it necessary, but they can provide entertainment as they change shape in a tight bridge. 

Whether rain or condensate, it's still water and muck, and the lined chimney was an improvement in our boat which prevented it. The lining fits into the inside of the collar so anything that runs down the inside won't end up on the outside of the collar. I noticed a lot more muck in the rain with our single skinned chimney, so whether it's an intended function or not, the lined chimney helped in our case.

[Alan de Enfield]

3 minutes ago, dmr said:

The twin wall advocates insist its a more efficient system, but like you I do't see how this can be if its putting more hot gas outside the boat. A twin walled flue will certainly draw better,

Fully agree.

I actually worked out the surface area of my flue and the surface area of the stove and they were very similar.

 

By not allowing the radiated heat from the flue it would appear to me that virtually half of the heat produced is dissapearing 'up the chimney'.

 

The fire may be more efficient, but if I have to use twice as much 'coal' to heat the boat then it becomes a 'no-brainer' - single skin rules the day.

[Tony Brooks]

21 minutes ago, BWM said:

That isn't how a lined chimney works, they have nothing to do with rain protection. 

 Their function is to provide a surface for stove by products to condensate on which allows it to stay within the flue, as an ordinary chimney fits over the collar, which directs the condensed water/ooze mixture to the gap between. 

 A coolie hat attachment will prevent rain but i have never found it necessary, but they can provide entertainment as they change shape in a tight bridge. 

and as the hot gasses condense on the underside they run down the slope and drip black gunge onto the roof. I tried one once and the did without a "pound shop" stainless saucepan sans handle makes a decent heavy weight rain cap in summer and may be cheaper than the chandlers rain cap.  I squashed mine so it would not sit right down onto the roof collar and thus provided some ventilaion.

[BWM]

18 minutes ago, Tony Brooks said:

and as the hot gasses condense on the underside they run down the slope and drip black gunge onto the roof. I tried one once and the did without a "pound shop" stainless saucepan sans handle makes a decent heavy weight rain cap in summer and may be cheaper than the chandlers rain cap.  I squashed mine so it would not sit right down onto the roof collar and thus provided some ventilaion.

..or like the bright spark above you could rejoice in the fact that your lined chimney acts as a funnel for rain! 

[jetzi]

32 minutes ago, BWM said:

..or like the bright spark above you could rejoice in the fact that your lined chimney acts as a funnel for rain! 

No need to be nasty. I realise it isn't that easy to follow. can draw you a diagram if it would help you understand.

[nb Innisfree]

1 hour ago, Tony Brooks said:

and as the hot gasses condense on the underside they run down the slope and drip black gunge onto the roof. I tried one once and the did without a "pound shop" stainless saucepan sans handle makes a decent heavy weight rain cap in summer and may be cheaper than the chandlers rain cap.  I squashed mine so it would not sit right down onto the roof collar and thus provided some ventilaion.

I've sometimes wondered if a coolie hat had an identical cone inverted under the top cone, then any condensate would run down to the tip and drip back down into the flue. 

[Slim]

I rather think that some contributors to this topic are confusing the terms 'flue' with 'chimney'. The flue is within the cabin, the chimney outside. A twin walled flue is not a requirement but is recommended by some, Normally retailers of said item. it's cost is  multiple times that of a single skinned flue with corresponding profit levels. A double skinned chimney prevents most condensed tar type much condensing on the outer skin , down to the collar and onto the roof. A colie hat is a take it or leave it matter.

My set up (and that of most) is a single skinned flue, double skinned chimney and a colie hat until it falls or is knocked off.

As for a collar cap I prefer a £1 B & Q plastic paint kettle.  

[Tony Brooks]

32 minutes ago, nb Innisfree said:

I've sometimes wondered if a coolie hat had an identical cone inverted under the top cone, then any condensate would run down to the tip and drip back down into the flue. 

I toyed with converting one like that but decided an open chimney when the stove is running and rain cap when not was easier and harder to  flick off with a rope.

[nb Innisfree]

59 minutes ago, Tony Brooks said:

I toyed with converting one like that but decided an open chimney when the stove is running and rain cap when not was easier and harder to  flick off with a rope.

Yeah that's what I did, never had any probs with rain entering the chimney even in heavy downpours, when stove was out I had a heavy steel cap directly on the collar

 

[BWM]

2 hours ago, ivan&alice said:

No need to be nasty. I realise it isn't that easy to follow. can draw you a diagram if it would help you understand.

Nothing nasty about the truth, and the last thing i need is a diagram from someone who clearly doesn't understand the subject matter and becomes argumentative when this is pointed out. 

 You are in a small minority of one if you consider directing rainwater into your flue as a positive, and to assume that a lined chimney was designed to do so. 

[jetzi]

3 hours ago, BWM said:

Nothing nasty about the truth, and the last thing i need is a diagram from someone who clearly doesn't understand the subject matter and becomes argumentative when this is pointed out. 

 You are in a small minority of one if you consider directing rainwater into your flue as a positive, and to assume that a lined chimney was designed to do so. 

 

Educating people is a nice thing to do. The nasty part - calling them stupid whilst doing so - is unfortunately a popular pastime on this forum.

 

I accept that it isn't what the lining is designed for. However a lining that fits into the flue from above has the effect that any liquid (whether condensate or rain) is directed into the flue. Water in your flue is not a good thing, but it is better than the alternative which is that the liquid runs down the side of the boat.

 

Happy to be shown that I'm wrong.

[BWM]

I didn't at any stage use the term stupid and it is your sense of defensiveness that causes you to group others into that accusation, your habit of asking questions of the forum whilst having your own fixed conclusion will inevitably lead to clashes with others, but my response earlier was to your assertion as to the purpose of a lined chimney. After pointing out that you were incorrect and describing the actual function so others may benefit from the exchange, instead your attitude was to further the absolute cobblers that directing rainwater into your flue as desirable attribute, purely to protect your own fragile ego. 

 Your penchant for reinventing the wheel isn't helpful to newcomers and will be challenged by those who care about people receiving the correct advice when asked for.

[jetzi]

I appreciate the viewpoint on my conduct, I'm sure there is some truth to it and I'll keep it in mind in future. To be clear I was objecting to the "bright spark" comment. It was probably an overreaction, so I apologise.

 

I already said this but I accept that I was wrong in saying that the purpose of the inner skin is to direct rainwater rather than other runoff to the inside of the flue rather than the outside. Thank you for pointing it out, now I know. I unfortunately can't edit my original incorrect post anymore but hopefully anyone reading will continue and see that I'm wrong about that point. I also already said this, that water in your flue is not a good thing, but it's preferable to having it run down the outside, isn't it?

 

Anyways, merry Christmas and new year to you.

[blackrose]

I once tried insulating a metre long chimney with a thick layer of rockwool over a period of cold dry nights. After about a week I concluded that with such a short chimney (compared say to an outdoor flue pipe running up the side of a building) it made no difference to the draw of my stove. A stove pipe supplier had previously said the same thing to me when I was making enquiries into twin wall insulated chimney and my test confirmed it.

 

My single wall chimney fits inside the collar with a skirt at the bottom just over the collar to prevent water ingress. The stove burns hot enough and the chimney is long enough that I don't have problems with tar on the roof.

 

Edited December 27, 2020 by blackrose

[BWM]

9 hours ago, ivan&alice said:

I appreciate the viewpoint on my conduct, I'm sure there is some truth to it and I'll keep it in mind in future. To be clear I was objecting to the "bright spark" comment. It was probably an overreaction, so I apologise.

 

I already said this but I accept that I was wrong in saying that the purpose of the inner skin is to direct rainwater rather than other runoff to the inside of the flue rather than the outside. Thank you for pointing it out, now I know. I unfortunately can't edit my original incorrect post anymore but hopefully anyone reading will continue and see that I'm wrong about that point. I also already said this, that water in your flue is not a good thing, but it's preferable to having it run down the outside, isn't it?

 

Anyways, merry Christmas and new year to you.

Cheers, i've been corrected on here and it's not always easy to differentiate between pedant and helpful correction - the internet is undoubtedly the bluntest tool of communication!

[scb61]

Many thanks for your comments which are always very helpful for a novice like me - I have decided to go with the normal chimney flue and not the twin wall 

My next question concerns plumbing systems - stand by any experts please!!