Twin wall flues and BSS

[OllyO]

Stoves are rated at a working Kw output, in the manufacturers literature they also state the highest and lowest working outputs so a 5Kw stove could be capable of 6.5Kw for example.

Some stoves are also rated with different fuels. For example chanrnwood multifuel stoves like their 'Cove' are tested with wood and athrasite.  The Anthrasite gives approx 1.5-9 more Kw output than wood.

[system 4-50]

Damn. That's another job on the list. Construct a flue cladding and compare performance with and without.

[MtB]

9 hours ago, nicknorman said:

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. 

 

The air (and oxygen) entering the stove is carefully throttled back to limit combustion and reduce stove output to regulate the heat in the boat, and a side effect of this is all the oxygen gets burnt. Airflow into the room obviously matches the flue gas flow (less an allowance for reduced temperature of incoming air compared to flue gas), which when regulated down is all passing through a gap about 1mm x 200mm on the stove in my boat. I dunno about yours.

I once put a gas analyser probe into the flue of my stove. The CO reading raced up to thousands of PPM before the analyser shut itself down to protect the sensors, suggesting to me there was zero oxygen so again, this suggests a tiny airflow through the boat.

Your point about the boat remaining cold while the stove is initially roaring is more to do with the radiant heat of the interior of the boat being low to start with, along with the lower half of the stove remaining cool while the top is red hot. My stove takes an hour or so to reach max output simply because it takes a while for the heat to conduct downwards to the bottom half of the stove, by which point its output will have roughly doubled compared to when only the top is hot.

[nicknorman]

2 hours ago, Mike the Boilerman said:

 

The air (and oxygen) entering the stove is carefully throttled back to limit combustion and reduce stove output to regulate the heat in the boat, and a side effect of this is all the oxygen gets burnt. Airflow into the room obviously matches the flue gas flow (less an allowance for reduced temperature of incoming air compared to flue gas), which when regulated down is all passing through a gap about 1mm x 200mm on the stove in my boat. I dunno about yours.

I once put a gas analyser probe into the flue of my stove. The CO reading raced up to thousands of PPM before the analyser shut itself down to protect the sensors, suggesting to me there was zero oxygen so again, this suggests a tiny airflow through the boat.

Your point about the boat remaining cold while the stove is initially roaring is more to do with the radiant heat of the interior of the boat being low to start with, along with the lower half of the stove remaining cool while the top is red hot. My stove takes an hour or so to reach max output simply because it takes a while for the heat to conduct downwards to the bottom half of the stove, by which point its output will have roughly doubled compared to when only the top is hot.

Obviously there is a lot of CO, but that doesn't mean there is zero oxygen. And obviously there isn't - it would be unreasonable to expect every molecule of oxygen to be burnt. As I said, the amount of unburnt oxygen as a %, depends on how long the oxygen is in the vicinity of the combustion which depends on the air/gas flow rate. This is why an open fire is so inefficient, a lot of air (including oxygen) is drawn up the flue, having no part in the combustion. A stove is much more efficient because far more of the air as a % is used for combustion, so less heat is carried away. If the stove has a twin walled flue then it is that little bit better because combustion can remain supported with a lower flue mass flow rate.

Back to perceived vs actual, at the gliding club, the toonsers (townies, in English) who weren't used to stoves, always wanted to open the stove door since when the did that, they felt more radiant heat (if they were sitting right in front of it) and it was a constant source of argument because, as you and I know, a stove is roughly 75% efficient with the door shut, and perhaps 30% efficient with the door wide open. Although they felt the radiant heat on their faces, they were of course pulling in a lot of icy air from outside so the effect on the room temperature and thus any bit of themselves not receiving the radiant heat, was negative. But it was hard to convince them of their folly!

Oh and one final point about twin wall stoves - although as we said the draw with a single walled flue on a boat is generally adequate, many stoves do smoke when you open the door, even when you do it slowly. With a twin wall flue, the draw is stronger and so the propensity to smoke when the door is opened to refuel, is less.

Edited June 20, 2017 by nicknorman

[Andyaero]

So to revisit the single or double skin flue debate......have there been any legal changes in the last 6 years, is this a decision I nolonger need to make. Do all new builds with solid fuel stoves have to be double skinned?

[Tony Brooks]

Just now, Andyaero said:

So to revisit the single or double skin flue debate......have there been any legal changes in the last 6 years, is this a decision I nolonger need to make. Do all new builds with solid fuel stoves have to be double skinned?

 

As the ISO/BS 1185 is only advisory, there can not be a challenge. If you are building/fitting to the RCR/RCD then there might be another ISO/BS that applies, but I don't think there is. The BSS requires inspectors to ensure there is no visible heat damage in the vicinity of the stove and flue.

If Alan or someone comes up with such an BS/ISO I am happy to be corrected.

[Alan de Enfield]

29 minutes ago, Tony Brooks said:

If Alan or someone comes up with such an BS/ISO I am happy to be corrected.

 

 

I have looked, many times, for RCD / RCR comments and specifications related to the installation of Solid Fuel Stoves and I have yet to find any mention of them. I am simply guessing, but I can only think that as the RCR / RCD is written to encompass the 'millions' of lumpy water boats in European waters, (which are unlikely to ever have been considered suitable for having a 'coal fire' aboard) someone in the EU looked at the UKs inland waterways and thought - "they look a bit like boats, we'd better include them"

 

This could well be why there is only a "British Standard" (BS8511:2010)  and no (that I can find) "EN" (European Norm) standard.

 

British Standard BS 8511:2010 Code of practice for the installation of solid fuel heating and cooking appliances in small craft

 

 

39 minutes ago, Tony Brooks said:

ISO/BS 1185 is only advisory

 

I think you may have made a typo here as  this is the standard for 7-pin electrical connectors for connecting trailers to the towing vehicle

[Tony Brooks]

4 minutes ago, Alan de Enfield said:

 

 

I have looked, many times, for RCD / RCR comments and specifications related to the installation of Solid Fuel Stoves and I have yet to find any mention of them. I am simply guessing, but I can only think that as the RCR / RCD is written to encompass the 'millions' of lumpy water boats in European waters, (which are unlikely to ever have been considered suitable for having a 'coal fire' aboard) someone in the EU looked at the UKs inland waterways and thought - "they look a bit like boats, we'd better include them"

 

This could well be why there is only a "British Standard" (BS8511:2010)  and no (that I can find) "EN" (European Norm) standard.

 

British Standard BS 8511:2010 Code of practice for the installation of solid fuel heating and cooking appliances in small craft

 

 

 

I think you may have made a typo here as  this is the standard for 7-pin electrical connectors for connecting trailers to the towing vehicle

 

Tru, all the right numbers but not necessarily in the right order,

[pete.i]

I fitted a twin wall flue in my narrowboat. Not because of any regulations, basically there aren't any, although it is wise to kepp any hot bits away from any burny bits. Anyway the reason I fitted a twin wall was because my grandchildren were often on the boat and my stove was next to the steps that led to the cratch. It was very easy to grab the flue when ascending the steps. I could not keep my hand on the flue when It was a single wall cast iron frue and would probably have got burnt had I tried. The twin wall was very cool in comparison and a lot of radiated heat from the single wall cast iron flue was lost when I changed it.

[Tony Brooks]

It is always possible to fabricate a protective cover out of punched or expanded metal to guard against accidental contact with the flue, while allowing heat into the boat.

[Alan de Enfield]

The BSS guidance to examiners :

 

BSS Examiners Solid Fuel Installation.pdf

 

Basically carry on as previously - just look for scorch marks.

[Andyaero]

In that case I am minded to install a single flue to add heat to the inside of the boat, then double flue about 6" from the roof with tall exterior chimney to aid draw. It's a Hungry Penguin Tall.

Opinions?

[Tracy D'arth]

9 minutes ago, Andyaero said:

In that case I am minded to install a single flue to add heat to the inside of the boat, then double flue about 6" from the roof with tall exterior chimney to aid draw. It's a Hungry Penguin Tall.

Opinions?

How are you going to connect twist lock dual flue to the single stove pipe?

Are you going to get a flue collar for the roof big enough to fit around twin wall flue?

Twin wall up there is totally unnecessary if the proper clearance is allowed, its the coolest part of the flue.

Better to fit a single flue with a nice stainless perforated heat shield.

Edited April 29, 2023 by Tracy D'arth

[MtB]

5 hours ago, Andyaero said:

So to revisit the single or double skin flue debate......have there been any legal changes in the last 6 years, is this a decision I nolonger need to make. Do all new builds with solid fuel stoves have to be double skinned?

 

 

As always, the underlying question here is what regulations you want or need to comply with?

 

Candidates would be BSS, RCR and "BS 8511:2010 Code of practice".

 

I'd suggest BSS only wants single skin flue, RCR is probably silent on the matter and BS8511;2010 is only advisory, not mandatory. 

 

So... what do you reckon?! 

 

 

[Andyaero]

1 hour ago, MtB said:

 

So... what do you reckon?! 

 

I'll have the solution that heats the boat in the most efficient legal manner please 😁

[MtB]

23 minutes ago, Andyaero said:

 

I'll have the solution that heats the boat in the most efficient legal manner please 😁

 

So, what do you mean by 'legal'? 

 

You can heat yer bote any way you like without being arrested. 

 

 

And to swerve being prosecuted in smokeless zones, I think all you need to do is burn smokeless fuel.

 

 

Nobody will care how many walls ya flue has! 

[Andyaero]

OK, most efficient to bss standard then, specifically single or double skinned flue.

[MtB]

2 minutes ago, Andyaero said:

OK, most efficient to bss standard then, specifically single or double skinned flue.

 

 

So, what do you mean by efficie..... oh forget it!

 

Just asking the questions to guide you to seeing the underlying issues.

 

For maximum heat energy from your coal into the boat, I'd say single skin. The BSS doesn't care about efficiency, only that you won't set fire to your boat and injure a pssser-by.

 

 

[Andyaero]

So would I, thanks.

[nicknorman]

As usual I disagree with the majority view. A stove is more efficient with a twin wall flue. I agree this is counter intuitive, but then so is quantum mechanics that underpins our existence and lots of other clever things.

 

People witter on about the heat lost by not having a hot flue pipe. Those same people probably relish having the stove door open to feel all that lovely radiant heat, not noticing that the efficiency of the stove has gone from about 80% to about 30% (same as an open fire).

 

it is true that a stove + hot flue pipe presents a bigger hot surface area than a stove + cool (double skinned) flue pipe, and so the maximum possible heat output can be greater. However most people’s stoves, unless they are grossly undersized, spend a lot of their time ticking over with minimal air.

 

A stove with a single skinned flue has to have more air going through it in order that the flue gasses remain vaguely warm and upward bound near the top of the flue. A double skin flued stove needs less air because the flue gasses remain hot and buoyant right to the top. So probably the temperature of the flue gas at the top is higher, but the flow rate and hence the heat loss is much less. Many people don’t understand the difference between heat and temperature.

 

The double skin flued stove maintains a higher combustion temperature inside the stove, = cleaner glass and more complete combustion. Don’t forget that all the air that goes up the flue has to be replaced by cold air coming in from outside, and this is the primary reason why a double skinned flue is more efficient, there is less flow rate up the chimney.

[Andyaero]

Ah, OK, I agree with you now.

See, it's not bloody simple is it.

Cleaner glass is a bonus I hadn't considered.

And, double flue means less worrying about the grandkids hands, tho if they can touch the flue, they can touch the fire.

Why can't someone just give a definitive answer.

The Yorkshireman in me is edging towards single, cos I reckon its waarmer and cheaper?

[nicknorman]

17 minutes ago, Andyaero said:

Ah, OK, I agree with you now.

See, it's not bloody simple is it.

Cleaner glass is a bonus I hadn't considered.

And, double flue means less worrying about the grandkids hands, tho if they can touch the flue, they can touch the fire.

Why can't someone just give a definitive answer.

The Yorkshireman in me is edging towards single, cos I reckon its waarmer and cheaper?

Cheaper to install, not cheaper to run. Warmer yes, but at the expense of more fuel burnt and lower efficiency.

[blackrose]

A twin wall flue isn't required and isn't a particularly good idea for the reasons already stated. Then in terms of draw, a twin wall chimney outside will make no difference over the very short chimney lengths typical on canal boats. I've tried it on a relatively long 80cm chimney and in comparison to my single wall chimney of the same length it made no difference to stove behaviour at all.

 

Where a twin wall flue/chimney would make a difference to efficiency is where you have a longer overall length like a stove on the ground floor of a two storey house or a flue running up the side of a building for example. For boats they're a waste of time and money.

Edited April 30, 2023 by blackrose