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Fitting a multifuel stove.


pete.i

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Morning.

I know this subject has been covered but I cannot find a specific answer to my query.  I want to fit a stove to the boat and whilst there is a mass of information of what is needed for the hearth and the surround and distances etc and I have also downloaded the pdf called stoves in boats which was featured in one of the threads., But my question is about the flue and whether it needs to be a twin wall or just a single walled pipe. I'm told that I will lose a lot of the potential heat output if I use a twin wall flue. I realise that the distances will change for a twin wall compared to a single walled flue. All the boats that I have seen or had any experience of have had single walled flues. The only advantages that I can see of using twin wall is the very marginal one of safety. I say marginal because the stove does not have any preventative measures used to protect people from burning themselves. Also a twin walled flue can be installed much closer to combustible materials. A single walled flue is also usually made of substantially more robust cast iron and is cheaper whilst a twin wall flue is thin thin steel sheet or stainless wrapped in more thin thin sheet.

 

So my question is what is the consenus of opinion? I am aware of the safety aspects of all of this incidentally but, as I say, in all the boats that I have looked at and the ones that I have owned a single walled flue has been the norm. 

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7 minutes ago, pete.i said:

So my question is what is the consenus of opinion?

For me - Single wall flue every time. (For all of the reasons you mention)

My primary reason being that having worked out the surface area of the flue, and the surface area of stove, and finding them similar, it was fairly obvious that if I allowed all of the heat to go up the flue and out of the chimney, instead of radiating out into the boat I would be losing half of my heat.

 

Other folks try to scientifically prove that you use less fuel so is more efficient - I am yet to be convinced.

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The argument for double wall insulated flues is that they help the fire draw better by keeping the flue gas warm. Flue gas has to be warm so that its density is low and will rise in the flue. The rising gas is what draws fresh combustion air in to the stove through the vents. It is claimed that stoves with insulated flues can be stopped down lower and so use less fuel without going out. There is some stuff online, which I am too lazy to go search out of a proper experiment with the same stove and under identical conditions other than the flue, which demostrates this.

Boat stove flues are generally a lot shorter than is recommended in the house installation destructions, so anything to help it draw has to be a good thing.

 

Relative surface area of flue to stove is  not so significant as the temperature of most of the flue will be a lot less than the stove. Radiation of heat is something like the fourth power of absolute temperature. Convection less so, so the loss of surface area is not as important as people think the argument for insulated flue goes.

My stove has a single skin flue. If I was installing new I would seriously consider insulated. Have some vague memory that insulated may be required for a new build to RCD standard, but that would need to be confirmed by someone knowledgable.

 

Jen

Edited by Jen-in-Wellies
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Twin wall flues are called for by the Code of Practice.  They are not mandatory, but a professional ignoring the CoP would be on poor ground in the event of an incident caused by the flue.  Similarly, an amateur might find  themselves  in difficulty with an insurance company.  That said, if there is good reason to fit a single wall flue then you can do it and it is perfectly possible to make a safe and effective single wall flue. 

 

The first port of call must always be the stove manufacturers requirements- for example I believe from their current stove instructions that Morso make a boat installation kit.   I have never seen one, even on  a shelf but if it suits an installation situation it would be a good idea to use it. 

 

I have read the arguments for twin walled flues in terms of efficiency and for me the jury remains out.   If the flue is more efficient then I accept  the stove can be run more slowly to produce the same amount of heat.  What I don't understand is how that heat then gets into the room.  Transfer of the same amount of heat through a smaller area requires that the heating surface gets hotter, but if the stove gets hotter the flue gases get hotter too and so the flue draws even better and a runaway up ("overfiring") will happen much more easily. 

 

I have a single wall steel flue, specially fabricated for the job in 3.5 mm steel tube.   It has a right angle bend, to allow use of the stove back flue entry, and two lesser bends so the flue follows the line of the cabin.  It works fine and has done for nearly thirty years.  I am currently on the third flue tube as corrosion tends to get them in the end.  At least with a single wall flue inspecting the smoke affected parts is much easier than checking the inner liner of a twin-wall jobby.

 

N

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1 hour ago, pete.i said:

So my question is what is the consenus of opinion?

What’s the stove?   If it’s a modern efficient one then less heat goes up the flue so a insulated flue probably better, if it’s not as efficient (non Defra?) then single flue.

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Single wall flue inside the boat, always. Outside, twin wall with the inner snugly fitted to go inside the flue as it appears up through the roof plate, thus avoiding any chance of tar running onto the roof.

Even if it’s an efficient modern stove, still single wall, as I resent any loss of heat inside the boat/room. More heat in the room means less atmospheric heating, commonly known as wasted heat.

Edited by Stilllearning
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Twin wall gets my vote. The "science" shows it is more efficient, plus less chance of someone loosing the skin off their hand in the event it gets lent on (especially if by a door on exit/entry). Yes I know the stove gets hot and you could prolly put your head in the gas oven while you are at it, but surely tending towards safety is better?

Get the Morso kit, c£500. Not overly impressed with the roof plate but at least you get everything in one hit. The flue doesn't have greater dimensions btw - the twin wall is on the inner, so you just end ip with a smaller effective diameter.

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A bit of experimentation of single vs insulated flue.

http://www.tigerboats.co.uk/images/forum/insulated-flues.pdf

 

The result they give is counterintuitive. It might turn out to be wrong, but an answer you don't expect should always give one pause. 

 

Jen

Edited by Jen-in-Wellies
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Twin wall may be more efficient in terms of the amount of heat obtained for the amount of fuel consumed. But it must be less effective in heating the boat since the area of hot metal radiating the heat is so much less. Or to put it another way, you will need a larger stove if using twin wall flue to get the same useful heat output.

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Don't underestimate the safety advantages of not having a very hot easy to touch large pipe near the steps to the door at the front of the cabin (If that is where your stove is).

 

If you lose your footing or slip coming down the steps the instinct is to grab the nearest firm solid object to steady yourself, and it hurts when it's a scalding hot pipe. (Don't ask me how I know this).

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The key seems to be draw of the flue, mine is single with no bends and the stove draws well even with no chimney on outside as the flue gases can easily travel up it, I do use a double skinned chimney. If you have to have lots of angles to fit the stove then it may not do as well. I have seen pictures of a boat for sale which had single wall to a bend and then twin after. If the twin wall is the same outer diameter as a single wall thus having a smaller inner it is increasing the resistance to flow of the gasses and surely this would be counter productive.

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Okay thanks for that. Single it will be as long as I can find someone willing to fit it. To Robbo the stove will be non DEFRA. the only reason to buy a DEFRA approved one is if I am going to burn wood which I am not. That is is also the reason for going for a multifuel as opposed to a wood burner not to mention the hugely inflated cost of DEFRA approved stoves. MJG the stove will be at the front of the boat which is not where we, generally enter/exit the boat but I do understand what you are xaying.

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36 minutes ago, pete.i said:

Okay thanks for that. Single it will be as long as I can find someone willing to fit it. To Robbo the stove will be non DEFRA. the only reason to buy a DEFRA approved one is if I am going to burn wood which I am not. That is is also the reason for going for a multifuel as opposed to a wood burner not to mention the hugely inflated cost of DEFRA approved stoves. MJG the stove will be at the front of the boat which is not where we, generally enter/exit the boat but I do understand what you are xaying.

You will get better heat distribution if you can put it part way down

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You can fit flue-guards around the single skin flue like These  there are loads more around with a bit of searching. They tend to be used on yachts, I used to fit them when the kids were smaller as the flue was a convenient grab handle, ouch!

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3 hours ago, ditchcrawler said:

You will get better heat distribution if you can put it part way down

I tgotally agree but this is a 32 foot boat and is only a day boat. Where it is going is the only place it can go without completely rebuilding the inside.

25 minutes ago, Clodi said:

You can fit flue-guards around the single skin flue like These  there are loads more around with a bit of searching. They tend to be used on yachts, I used to fit them when the kids were smaller as the flue was a convenient grab handle, ouch!

Thanks for that Clodi. On my other boat, which I sold, I did try to make things as safe as possible. I had a 3/4 surround fire guard and a trivet on top of the stove. I haven't seen those flue guards before so they will be worth trying.  

 

UMM although a bit expensive for what they are. Whilst not so pretty I can cut some slots out of a piece of sheet tinplate and make them myself. But as I say certainly worth a look.

Edited by pete.i
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On 12/11/2018 at 11:03, MJG said:

Don't underestimate the safety advantages of not having a very hot easy to touch large pipe near the steps to the door at the front of the cabin (If that is where your stove is).

 

If you lose your footing or slip coming down the steps the instinct is to grab the nearest firm solid object to steady yourself, and it hurts when it's a scalding hot pipe. (Don't ask me how I know this).

I have fitted a vertical grab pole (floor to ceiling) to hold when using the steps. Very useful even without the stove. It is placed to prevent brushing against the flue pipe (but pole-dancers could burn their feet I guess) - and the flue is 4" single wall with a detachable long funnel on the roof.

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