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Flue Pipe Rusted Through


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Our stove was fitted 9 years ago and as liveaboards it has had a lot of use. The join of the flue to stove is filled with fire cement and where the flue passes through the collar the gap was packed with fire rope and fire cement. Over the years we've had the occasional weep of water running down the flue but nothing great.

Lit the fire yesterday and smoke came out of the bottom of the collar into the boat. With a torch I could see a large, rusty void. Scraping the fire cement away this morning the pipe has completely rusted through. So a new flue needs fitting. For the moment I've rolled a sheet of aluminium and forced it down the flue to give a mechanical join between the two bits and then forced GunGum into the crack. 

Now I'm assuming that moisture/rain/condensation has found its way through the fire cement so the exposed bit inside stays damp and rust the pipe through. So when the new pipe is fitted is it better to use heat resistant silicone for the collar to give a fully waterproof joint. Or will it still rust through from the inside? How long can one expect a flue to last on a liveaboard burning smokeless fuel, not coal or wood?

 

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IMG_20170412_110056.jpg

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For the flue/roof collar joint I wouldn't use fire cement. Either fire rope or the glass fibre rope for sealing flues, plus as little silicon as possible to make a reasonably waterproof joint. (In our case, most of the time the joint would either have the chimney on and a fire going, or be covered with a cap, so doesn't need to be fully waterproof)

I feel the roof collar/flue joint should be sliding, if possible.

On Kelpie, the flue is solidly into the collar, and the top collar comes off the stove over the course of a year or so, and needs re packed with high temp silicon. A previous (shared) boat had a similar problem, but the result there was a stove hanging by the flue and dangling above the hearth 

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How long they last is down to several factors, but if you go stainless steel it will not rust but expensive, or do what I did and get a length of steel tube with a decent wall thickness of about 4mm and spray it with stove black and it will last years longer than the 1mm vitreous ones.

Neil

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1 hour ago, Neil Smith said:

How long they last is down to several factors, but if you go stainless steel it will not rust but expensive, or do what I did and get a length of steel tube with a decent wall thickness of about 4mm and spray it with stove black and it will last years longer than the 1mm vitreous ones.

Neil

I would guess this was around 4mm when new. Certainly not vitreous enamel. Steel or cast iron.

We rarely take the chimney off.

Edited by pearley
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42 minutes ago, pearley said:

We rarely take the chimney off.

It's curious then as to how rainwater can get between the collar and the flue. Rain runs down the outside of the chimney, over the collar and onto the roof. It should never be able to get anywhere near the flue. 

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Mine corroded just like the picture. My guess was that there is something in the flue gases and residues which causes it.

Having said that, there could have been a gap between the collar and roof seal, which allowed water in - thus nothing to do with the chimney.

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3 minutes ago, Richard10002 said:

there could have been a gap between the collar and roof seal, which allowed water in - thus nothing to do with the chimney.

That's what I was wondering. 

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I don't think it's a failure of the seal which is the problem. What happens is that the bit of pipe going through the collar is always colder, as the collar and roof act as a heatsink. You get condensation from the flue gasses on the inside at that point, and the acids etc dissolved in the condensation rots the pipe from the inside out. Eventually the pipe perforates and the condensation gets into the gap between the pipe and the collar and starts to rot it outside-in as well. At this point you start to get the weeps down the outside of the pipe in the boat.

 

Ours went in the same way last year, and was rebuilt by a chap at Kings Lock in Middlewich. As the pipe has two angled joints and a change in diameter, it would have been expensive to build a new on from scratch, the rest of it, below the collar and top bend, was fine.

 

MP.

 

  • Greenie 2
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49 minutes ago, MoominPapa said:

I don't think it's a failure of the seal which is the problem. What happens is that the bit of pipe going through the collar is always colder, as the collar and roof act as a heatsink. You get condensation from the flue gasses on the inside at that point, and the acids etc dissolved in the condensation rots the pipe from the inside out. Eventually the pipe perforates and the condensation gets into the gap between the pipe and the collar and starts to rot it outside-in as well. At this point you start to get the weeps down the outside of the pipe in the boat.

Well that all certainly makes sense :)

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11 hours ago, WotEver said:

It's curious then as to how rainwater can get between the collar and the flue. Rain runs down the outside of the chimney, over the collar and onto the roof. It should never be able to get anywhere near the flue. 

We do use a liner inside the chimney so I guess some rain water will get channeled down the gap and onto the seal on top of the collar.

 

11 hours ago, MoominPapa said:

I don't think it's a failure of the seal which is the problem. What happens is that the bit of pipe going through the collar is always colder, as the collar and roof act as a heatsink. You get condensation from the flue gasses on the inside at that point, and the acids etc dissolved in the condensation rots the pipe from the inside out. Eventually the pipe perforates and the condensation gets into the gap between the pipe and the collar and starts to rot it outside-in as well. At this point you start to get the weeps down the outside of the pipe in the boat.

 

Ours went in the same way last year, and was rebuilt by a chap at Kings Lock in Middlewich. As the pipe has two angled joints and a change in diameter, it would have been expensive to build a new on from scratch, the rest of it, below the collar and top bend, was fine.

 

MP.

 

All that makes sense. I hope they can repair ours by just welding a new section on. Won't know until Tuesday as our trusted engineer is on holiday.

 

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15 hours ago, pearley said:

How long can one expect a flue to last on a liveaboard burning smokeless fuel, not coal or wood?

Our last one lasted about 14 years, but is a straight pipe, so not too expensive to change. 

Edited by rusty69
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Stainless steel flue pipes will also get attacked by corrosion near the top which will eat through it, albeit not as quickly as mild steel. A way of stalling this off for longer is to allow a layer of hard clag to build up inside which seems to have become inert and it will line and protect the steel to some degree.  Sweep soot away regularly though. Of course there will come a time when you have to remove the layer of clag because its getting too thick and restricting the bore too much. I've found with mine that to have the pipe poking up out of the collar by about 3'' and using the same size pipe for the chimney lightly clamped on top of it with a homemade wide clamp ''like a piston ring clamp'' ideal. I've never bothered with those daft fancy chimneys that overlap the outside of the collars orifice, which slowly corrode the top of the collar and flue pipe and oozing out goo.

Edited by bizzard
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I discovered last summer that my flue had corroded in a similar manner . The very top of the flue , where it entered the roof collar had rusted away . I asked at the local boatyard and they cut 6 inches off the top of the existing flue pipe and welded an offcut of the same diameter to replace the top of the flue . Cost me £30 . Once done it was refitted into the existing roof collar which needed a fair bit of surgery all done with a hammer and chisel to remove what is technically classed as " crud " . New pipe fitted and then i was advised to stuff the gap between flue & collar with fibre glass rope which was awful stuff to work with . 

A cheap job - i think i got off lightly 

cheers

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Ours lasted 6-7 years in our previous boat, and although not corroded into holes was definitely weakened where it entered the roof collar. Probably could have got another 3-4 years out of it before it corroded through completely. 

I would say 10 years would be a good guess for longevity but no doubt some will go on a lot longer. That was burning a mix of wood and smokeless as full time liveaboard. Worth making it part of an annual check on stove and flue condition, including poking it around the roof collar with screwdriver to check integrity. I'm sure several boaters are unaware of the risk of this problem. 

Reason for early replacement was we needed a new flue for the epping in the back cabin, so it made sense to replace the lounge flue with a new one and use the good middle section to make a new rear flue pipe.

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