Chimney - How Hot?

[system 4-50]

Does anybody know how hot a standard single skin chimney can get where it leaves the roof collar?

[Higgs]

Does anybody know how hot a standard single skin chimney can get where it leaves the roof collar?

 

Most of the time it isn't that hot, that high up. And even in the boat, it isn't. Worst time is probably as the fire is being made and the draft door is open, and as your trying to get the fire to catch. Multi-fueled stove with 3-4mm steel flue.

Edited September 20, 2012 by Higgs

[grahame r]

Most of the time it isn't that hot, that high up. And even in the boat, it isn't. Worst time is probably as the fire is being made and the draft door is open, and as your trying to get the fire to catch. Multi-fueled stove with 3-4mm steel flue.

I'd have to disagree. When first starting the fire the flu is cold and the fire will be reluctant to draw, as the flu heats up the draw will increase and continue to raise the temperature of the chimney until you stop it by restricting the air flow, so the temperature of the flu will be at it's highest when the fire is properly established. As to the exact temperature, the fact that you need to insulate the flu from the collar and thence the combustible material on the roof/ceiling would indicate that it will be pretty high.

[Keeping Up]

I'd have to disagree. When first starting the fire the flu is cold and the fire will be reluctant to draw, as the flu heats up the draw will increase and continue to raise the temperature of the chimney until you stop it by restricting the air flow, so the temperature of the flu will be at it's highest when the fire is properly established. As to the exact temperature, the fact that you need to insulate the flu from the collar and thence the combustible material on the roof/ceiling would indicate that it will be pretty high.

 

This is only true if the requirement to do so has been determined by knowledge of that temperature rather than by uninformed guesswork.

 

(I am of course speaking as an uninformed observer, and making no comment about the statement's truth or otherwise)

[Higgs]

The flue on mine can get uncomfortable to touch. I have known diesel heater flues, hotter than I'd want to touch, as in - instant burn. I suppose it depends alot on how fierce the fire is burning. On mine, most of the time, it is usually ticking over.

[Grace and Favour]

The flue entry into the ceiling collar on our boat is insulated

 

We have a single skin flue, our stove is a 4.5Kw Squirrel

 

When running 24/7 thro' winter, you would NOT want to touch it with bare hands at any point - - I promise!

[GSer]

When i took the flue out of Cygnus the area of cabin top lining around the collar was charred due to I think insufficient air gap between the collar and the wood, and possibly because the flue had not been centered in the collar letting the flue touch the collar directly. The wood was very dehydrated with just a tinge of burnt than badly burnt though, it looked like it was only a matter of time until ignition.

[Thewatchmaker]

The flue entry into the ceiling collar on our boat is insulated

 

We have a single skin flue, our stove is a 4.5Kw Squirrel

 

When running 24/7 thro' winter, you would NOT want to touch it with bare hands at any point - - I promise!

I am sure that is how it should be.

Otherwise when burning wood you get creosote forming, due to unburnt fuel.

[bowten]

I use a twin wall,stainless steel and insulated flue from SFL.It does not get very hot and you can put your hand on it even when the stove is very hot.I have noticed I get better draw from this flue and there is less ash,the coals remain warm for a very long time and I am certainly using less fuel.When burning wood there is much less staining of the chimney or roof because the interior of the flue is much hotter.

Edited September 20, 2012 by bowten

[Proper Job]

I use a twin wall,stainless steel and insulated flue from SFL.It does not get very hot and you can put your hand on it even when the stove is very hot.I have noticed I get better draw from this flue and there is less ash,the coals remain warm for a very long time and I am certainly using less fuel.When burning wood there is much less staining of the chimney or roof because the interior of the flue is much hotter.

That's because the heat is being wasted and going up the flue rather than being emmited from it in to the cabin.

 

The higher flue gas temperature means that the products of combustion do not have a chance to condense on to cooler surfaces = no/minimal staining.

 

I recon about half of the heat produced by the fire can be emitted via the flue wall on a single skin, non insulated flue.

[grahame r]

This is only true if the requirement to do so has been determined by knowledge of that temperature rather than by uninformed guesswork.

 

(I am of course speaking as an uninformed observer, and making no comment about the statement's truth or otherwise)

I'd be amazed if "the requirement to do so" (have insulation between flu and collar) is not stipulated in the BSS requirements. I'm not an expert in these but no doubt someone who is can advise us.

[Keeping Up]

I'd be amazed if "the requirement to do so" (have insulation between flu and collar) is not stipulated in the BSS requirements. I'm not an expert in these but no doubt someone who is can advise us.

 

I'm reasonably sure there is no such requirement in the BSS (although open to correction, of course).

 

I'm also reasonably sure that my own flue isn't insulated from the collar. The collar is bolted to the steel roof which conducts the heat away so the collar never seems to get particularly hot.

 

The flue is of course well separated from the roof lining.

[bizzard]

Its really common sense to insulate the flue pipe from the collar to prevent too much heat transfer, and to allow for the pipes expansion and contraction.

Grahame's splendid stainless steel flue pipe has a uniform half inch gap through the collar stuffed with fireproof rope and goo.

[Smelly]

Its really common sense to insulate the flue pipe from the collar to prevent too much heat transfer, and to allow for the pipes expansion and contraction.

Grahame's splendid stainless steel flue pipe has a uniform half inch gap through the collar stuffed with fireproof rope and goo.

 

Why doesn't the traditional method of fire clay or rope suffice?

[blodger]

Why doesn't the traditional method of fire clay or rope suffice?

When I replaced my flue I relised I could not get perfect fire rope rings in due to the misalighnment oc collar and flue due to the camber of the roof and the collar placement in it. I should imagine most boats are likewise so that there is some flue to collar metal to metal contact and 'friction' when expanding/contracting due to use.

[Smelly]

When I replaced my flue I relised I could not get perfect fire rope rings in due to the misalighnment oc collar and flue due to the camber of the roof and the collar placement in it. I should imagine most boats are likewise so that there is some flue to collar metal to metal contact and 'friction' when expanding/contracting due to use.

 

So would you reduce the inside diameter of you flue and lose the heat from the flue pipe to compensate? That seems disingenuous to me.

[DHutch]

In normal operation ours (cast iron drain pipe) is fairly cool, crack it up a bit and it starts getting fairly warm, but it rairly hot. That said, its is suitable isolated from anything combustible, and assuming it wasnt a dull cherry im sure it would be fine, infact it would be fine dully cherry I expect, all bar burning the paint off.

[luctor et emergo]

Cast iron drain pipe? What a splendid idea

 

(Luctor is off, googling reclamation sites..)

[blodger]

So would you reduce the inside diameter of you flue and lose the heat from the flue pipe to compensate? That seems disingenuous to me.

No, you are stuck with whatever flue fits the stove. I am saying that I guess that few flues fit parallel in the collar so cannot be perfectly insulated by fireroap or heat resistant fillings/

[JDR]

I've got a 5inch 125mm flue welded in two sections in at least 3mm steel, weighs a ton but never gets too hot.After lurking on here for a while I insulated between flue and collar with firerope then cut down a length of 175mm vitreous enamel to about 5", bolted that to the collar then insulated the gap between that and the flue.I didn't have the option of fitting a brass dressing ring as the hole for the collar is on the edge of an 8' by 4' sheet so there is no flat surface. There is a small amount of heat transfer at the base but is cool where it joins collar and on a cold night is cold as is the exterior of the collar.Total cost £30.

[JDR]

Flue is actually welded in three sections (sorry can't edit crap mobile connection) to allow top section to run parallel to collar.In hindsight I would have gone for a 4" single skin, but I suspect my existing flue will outlast the boat and I can't be arsed to change it.

[blackrose]

Does anybody know how hot a standard single skin chimney can get where it leaves the roof collar?

 

My magnetic stove pipe thermometer can get up to 300 deg C at the bottom of the flue with the stove really hot. I've tried putting he thermometer at the top of the flue and it generally reads about 80 deg C lower than whatever the reading is at the bottom.

Edited September 22, 2012 by blackrose

[Mac of Cygnet]

Does anybody know how hot a standard single skin chimney can get where it leaves the roof collar?

 

I note that everyone is talking about flues, while the OP asked about a chimney - not the same thing! If he really means 'chimney', then I can only say from experience that mine gets too hot to touch soon after lighting the fire, but cools down to pleasantly warm when the stove is closed down. But it's outside the boat, outside the collar, so why does it matter?

[system 4-50]

I note that everyone is talking about flues, while the OP asked about a chimney - not the same thing! If he really means 'chimney', then I can only say from experience that mine gets too hot to touch soon after lighting the fire, but cools down to pleasantly warm when the stove is closed down. But it's outside the boat, outside the collar, so why does it matter?

I have fabricated my own chimney, (which probably won't work), and I was considering using a Dektite thingy to seal it which has a maximum temp of 200 - 250 deg C.

[furnessvale]

No, you are stuck with whatever flue fits the stove. I am saying that I guess that few flues fit parallel in the collar so cannot be perfectly insulated by fireroap or heat resistant fillings/

 

IMO whilst some misalignment would be allowable, any physical contact between pipe and collar is a no-no and the flue pipe should be kinked if necessary to allow a more even gap.

 

George ex nb Alton retired