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Twin wall flues and BSS


OllyO

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Hi folks,

if this topic has been covered already please point me in the right direction.  I search the forum but couldn't find the right answers.

as I understand it the bss now required twin wall insulated flues rather than single skin.

Firstly does this apply to boats after a certain year?

If it applies to all boats does the flue system only require twin wall going through the roof and within x distance of cabin lining?  Or full length from stove to atmosphere?

As we know short chimneys don't draw as well as long ones so keeping exhaust gas at high temp is good for gas velocity so many choose to use twin wall flue outside the boat but that's down to personal preference unless this is noted in the BSS

lastly if boat now needs twin wall flue can the normal cast iron collars do used as the a 6" nominal bore flue pipe is infact over 7" OD?

cheers in advance

olly

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7 minutes ago, OllyO said:

as I understand it the bss now required twin wall insulated flues rather than single skin.

You understand wrong. There's nothing whatsoever about that in the regs. 

Part 8: https://www.boatsafetyscheme.org/media/268789/ecp-private-boats-ed3_rev2_apr2015_public_final.pdf

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In these days of 'internet' it is very easy to do some research yourself - most everything is available and normally a quick google will give you the answers.

The internet also makes it very easy for a 'comment' to be repeated so often that it becomes 'law'.

When told 'stuff' (such as BSS rules, navigation rules, licencing rules etc) I will go to the source and confirm, or deny, the 'new rule' for myself.

 

A lie told often enough becomes the truth. - Vladimir Lenin

Edited by Alan de Enfield
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Whilst an insulated flue might improve the draw of a stove, particularly n the short flues on a narrowboat, I reckon a significant amount of heat is radiated from the matt-black flue, which would be lost in an insulated flue.

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4 minutes ago, dor said:

Whilst an insulated flue might improve the draw of a stove, particularly n the short flues on a narrowboat, I reckon a significant amount of heat is radiated from the matt-black flue, which would be lost in an insulated flue.

I agree - but whenever I have said this the 'experts' here use all sorts of science to prove otherwise - I am not convinced and reckon the surface area of my flue is almost as big as the surface area of my stove - its got to provide a fair bit of heat.

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23 minutes ago, Alan de Enfield said:

I agree - but whenever I have said this the 'experts' here use all sorts of science to prove otherwise - I am not convinced and reckon the surface area of my flue is almost as big as the surface area of my stove - its got to provide a fair bit of heat.

Quite. A twin wall flue allegedly results in more efficient combustion, so less fuel needed to produce a given amount of heat. But since the area of hot metal radiating heat into you boat is substantially reduced, then the end result can only be a colder boat and much reduced fuel consumption. How does that help someone who wants to burn fuel to heat the boat?

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There is British Standard now in respect of  solid fuel flues on boats but the easy to find online version is an early draft.

At present it is not mandatory although  with the RCD referring to standards I am not so sure on new boats being built to the RCD.

Probably more not properly understanding the situation that deliberately giving wrong info. I confirm it is not in the BSS at present.

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1 hour ago, Alan de Enfield said:

I agree - but whenever I have said this the 'experts' here use all sorts of science to prove otherwise - I am not convinced and reckon the surface area of my flue is almost as big as the surface area of my stove - its got to provide a fair bit of heat.

The problem is of course the phrase " Expert" I having lived through very many winters on boats have and always will have unless it becomes illegal a single skinned flue. Simply the best solution for the job in hand.

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51 minutes ago, David Mack said:

Quite. A twin wall flue allegedly results in more efficient combustion, so less fuel needed to produce a given amount of heat. But since the area of hot metal radiating heat into you boat is substantially reduced, then the end result can only be a colder boat and much reduced fuel consumption. How does that help someone who wants to burn fuel to heat the boat?

I may be fortunate but I'm not yet at the stage of 'heat or eat' - I am happy to pay whatever it takes to be at a comfortable temperature. Rather than have to have either a bigger stove, or have it with a turned up 'bigger fire' to maintain the temperature I want, I am happy to have a 'smaller' stove running less efficiently but producing heat from 2x its surface area (fire + flue)

  • Greenie 1
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So how efficent a stove is could be a BSS reg did they not have to do a U turn on it ? BSS regs should all have written justification for why they are in place too much interpretation and where some examiners have failed because of an advisory or what they think should be done .

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I think the claim that using a twin wall flue allows you to burn less fuel whilst keeping the flue gasses at a safe temperature - ie less cooling of the flue gasses - is based upon the belief that most boats get too hot and start opening windows etc. so a cooler stove/flue whilst still keeping the boat at a comfortable temperature will save you fuel (money).  However if you have a big boat and a small stove you will probably end up with a chilly boat when you need it most.  That said I only have twin wall from about 8" below the roof to the top of the chimney, and it suits me just fine.

  • Greenie 1
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1 hour ago, dor said:

Whilst an insulated flue might improve the draw of a stove, particularly n the short flues on a narrowboat, I reckon a significant amount of heat is radiated from the matt-black flue, which would be lost in an insulated flue.

The "Sol.." something or other document that advises on these things is obsessed with getting sufficient draw up the chimney. I have no problems whatsoever with draw in my short chimney, so sacrificing heat transfer from the internal flue to promote more draw by having a double skin inside is inappropriate for me. I get a lot of heat from my flue. Single skin flue (inside cabin) every time for me. But stainless not matt black in my case.

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I used to work as a stove fitter and I vaguely remember a figure of 1kw per 1m of flue pipe so yes twin wall flue does reduce this effect.  There is however not really any reason to run twin wall inside the cabin. From perhaps 8" from the ceiling up to the top could be beneficial thougb

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I'd also add that the maintaining the temperature of combustion gases to guarantee an efficient burn is more applicable to wood burning than coal burning so prob not relevant to the majority of live aboard boaters. 

Stick with the single skin, certainly  makes life a lot easier (and warmer!)

 

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11 hours ago, Alan de Enfield said:

 

A lie told often enough becomes the truth. - Vladimir Lenin

 

9 hours ago, Alan de Enfield said:

I agree - but whenever I have said this the 'experts' here use all sorts of science to prove otherwise - I am not convinced and reckon the surface area of my flue is almost as big as the surface area of my stove - its got to provide a fair bit of heat.

Oh the irony! You have never tried a double skin flue. Lots of people, including several in this thread, have told you that a single skin flue is best. So you believe it, and repeat the "fact" based on zero evidence.

The fact is that a double skinned flue makes a stove more efficient, by reducing the mass flow rate of hot air up the chimney, and hence heat lost up the chimney. Yes I think it would be true that the maximum possible heat output from the stove/flue combination would be reduced with a double skinned flue, but it seems unlikely that many narrowboats suffers from inadequately-sized stoves that can't get a comfortable temperature when running flat out. Normally it is the opposite, the stove spends 99% of its time shut right down.

Anyway I think the situation is that the standard requires a double skinned flue, but compliance with the standard is only required by a new-build RCD-compliant boat. For everyone else it is advisory. It is certainly not required by BSS.

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7 minutes ago, nicknorman said:

So you believe it, and repeat the "fact" based on zero evidence.

You make assumptions based on zero knowledge.

We have a caravan park - some of which are fitted with solid fuel stoves and the latest ones have twin wall flues.

The ones with TW Flues need to have the fire run much higher to get the Home warm.

More relevant to a boat -

3 or 4 boats ago we installed a small 'Pipsqueak' stove in a space that had been a dressing table. This has an 8" 'cube' firebox and is rated at around 3Kw - the flue gave approximately 2x the surface area of the fire - without the 'flue heat' it would not have been big enough.

 

 

IMG_20130912_123236.jpg

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3 minutes ago, Alan de Enfield said:

You make assumptions based on zero knowledge.

We have a caravan park - some of which are fitted with solid fuel stoves and the latest ones have twin wall flues.

The ones with TW Flues need to have the fire run much higher to get the Home warm.

More relevant to a boat -

3 or 4 boats ago we installed a small 'Pipsqueak' stove in a space that had been a dressing table. This has an 8" 'cube' firebox and is rated at around 3Kw - the flue gave approximately 2x the surface area of the fire - without the 'flue heat' it would not have been big enough.

 

 

IMG_20130912_123236.jpg

Can see what you mean,if the Flue were Twin wall the gases would be hotter and draw more heat to exterior,rather than dispersing heat to the Boat/Room.

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3 hours ago, OllyO said:

I used to work as a stove fitter and I vaguely remember a figure of 1kw per 1m of flue pipe so yes twin wall flue does reduce this effect.  There is however not really any reason to run twin wall inside the cabin. From perhaps 8" from the ceiling up to the top could be beneficial thougb

 

Not really.

Although it is correct a twin wall flue will 'improve draw', in my experience it is not necessary. The draw with a solid steel or cast iron flue is more than satisfactory, always.

It is a case of men in suits steering desks trying to improve something that already works. 

 

 

 

(Speeling edit.)

Edited by Mike the Boilerman
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45 minutes ago, Alan de Enfield said:

You make assumptions based on zero knowledge.

We have a caravan park - some of which are fitted with solid fuel stoves and the latest ones have twin wall flues.

The ones with TW Flues need to have the fire run much higher to get the Home warm.

More relevant to a boat -

3 or 4 boats ago we installed a small 'Pipsqueak' stove in a space that had been a dressing table. This has an 8" 'cube' firebox and is rated at around 3Kw - the flue gave approximately 2x the surface area of the fire - without the 'flue heat' it would not have been big enough.

 

 

IMG_20130912_123236.jpg

Well ok, I make an assumption about your knowledge - but based on what you say, it's a reasonable one. If you have identical caravans with identical stoves save but twin wall vs single wall flues, then the twin wall flue will require a bit less fuel to heat the van to the same temperature. If you don't agree, it because you either have differences between the caravans / stoves, or you are using anecdotal and inaccurate evidence rather than fact.

You and I both agree that with a tiny stove, max heat output will be reduced with a twin wall flue, but this thread is not about your specific and unusual circumstance, it is about the vast majority of boat stoves that spend most of their time, even in the depths of winter, operating well below their rated output.

Edited by nicknorman
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3 minutes ago, nicknorman said:

Well ok, I make an assumption about your knowledge - but based on what you say, it's a reasonable one. If you have identical caravans with identical stoves save but twin wall vs single wall flues, then the twin wall flue will require a bit less fuel to heat the van to the same temperature. If you don't agree, it because you either have differences between the caravans / stoves, or you are using anecdotal and inaccurate evidence rather than fact.

You and I both agree that with a tiny stove, max heat output will be reduced with a twin wall flue, but this thread is not about your specific and unusual circumstance, it is about the vast majority of boat stoves that spend most of their time, even in the depths of winter, operating well below their rated output.

 

I often suspect stove heat output specs are measured with the stove roaring furiously, almost jumping off the hearth, in order to maximise the claimed heat output.

So yes, operated normally, a stove is probably only running st 50% rated capacity.

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12 minutes ago, Mike the Boilerman said:

 

Not really.

Although it is correct a twin wall flue will 'improve draw', in my experience it is not necessary. The draw with a solid steel or cast iron flue is more than satisfactory, always.

It is a case of men in suits steering desks trying to improve something that already works. 

 

 

 

(Speeling edit.)

The draw with single wall flue is quite satisfactory, but that is not the issue. Twin wall flues reduce the necessary gas flow rate up the flue, and that means less icy air drawn in from outside - perhaps blowing past the occupants. In other words, improving occupant comfort. But of course the main reason why it's in the reg is the reduction in temperature of hot surfaces (the flue) that could set fire to the structure of the boat, and burn someone touching it by accident. It will also keep the pressure within the stove a bit lower, thus reducing the probability of CO leaks into the boat.

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1 minute ago, Mike the Boilerman said:

 

I often suspect stove heat output specs are measured with the stove roaring furiously, almost jumping off the hearth, in order to maximise the claimed heat output.

So yes, operated normally, a stove is probably only running st 50% rated capacity.

I'd have to check, but I'm fairly sure there is a maximum temperature a stove can operate at, as part of the output spec. Stove thermometers supplied with some stoves have a relatively low max temperature and the instructions advise against operating it  any hotter. Of course one could fill it with hot-burning coal and get the casing glowing red (or even white!) hot to get many, many KWs, but the stove wouldn't last long. Nor the boat!

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33 minutes ago, nicknorman said:

The draw with single wall flue is quite satisfactory, but that is not the issue. Twin wall flues reduce the necessary gas flow rate up the flue, and that means less icy air drawn in from outside - 

 

How does that work then? Let us assume the heat requirement is the same for two identical installations, save for the flues. The twin wall flue will cool the flue gas less because it radiates and convects less heat into the boat than the single wall flue. Therefore the twin wall flue stove itself will need to be heated to a higher temperature to maintain heat output of the whole system. To heat the fuel bed to a higher temperature, I'm inclined to think the airflow into it needs to be higher.

Intuitively, on the twinwall system the flue gas temp will be higher therefore more energy is being wasted to outside, unless the flue flow rate is lower perhaps. But it needs to be higher, to make the stove itself hotter, to compensate for the cooler flue surface inside the bote.

Therefore single wall flue systems draw less icy air from outside...

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36 minutes ago, Mike the Boilerman said:

 

How does that work then? Let us assume the heat requirement is the same for two identical installations, save for the flues. The twin wall flue will cool the flue gas less because it radiates and convects less heat into the boat than the single wall flue. Therefore the twin wall flue stove itself will need to be heated to a higher temperature to maintain heat output of the whole system. To heat the fuel bed to a higher temperature, I'm inclined to think the airflow into it needs to be higher.

Intuitively, on the twinwall system the flue gas temp will be higher therefore more energy is being wasted to outside, unless the flue flow rate is lower perhaps. But it needs to be higher, to make the stove itself hotter, to compensate for the cooler flue surface inside the bote.

Therefore single wall flue systems draw less icy air from outside...

I think you will have seen the real world demonstration of this often, but not analysed it:

You light the stove on a cold day (OK you're going to tell me you lit it in September and it didn't go out until April - but bear with me!). To get it going you have the air vents wide open. It's freezing in the boat. The fire gets going but it's still freezing. The fire is really burning up but it's still freezing. Then you shut the air vents down to tickover and suddenly it's really hot in the boat. So the point is that having a lot of draft really pulls the heat away from the stove and up the chimney. And don't imagine that all the oxygen in the air coming in the stove vent participates in combustion - most of it just goes up the chimney, pulling the heat away. So the aim is to maximise the usage of % of oxygen in the incoming air, so that for a given rate of burn the air flow (that pulls the heat out of the boat) can be minimised. 

With a twin walled flue, the flue gas temperature can be kept hot enough to give adequate draw without much of a flow rate. With the slower flow, more of the oxygen % is burnt and thus the mass flow of hot gas up the chimney is less as a proportion of fuel burnt - the stove is more efficient.

Well I'm not sure if I have explained that adequately but the point is that neither you nor I are experts in the matter. But the experts say that a twin walled flue is more efficient and that is certainly my experience when we replaced the gliding club's old stove with single walled flue (around 4 metre high ceiling, ie long flue) with a same-sized stove with twin wall flue. Very noticeably less draughty, more heat, less fuel used. A number of club members were grumping that we'd lose room heat from the flue (just like on here!) but they all had to eat their words.

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