Altering flue to fit collar?

[Poppin]

On the new stove install, as the single wall pipe passes through the roof collar I know that I'm supposed to maintain a gap between flue and collar, which is then sealed  with rope and envirograf.

 

However due to the angle of the stove and roof, the flue almost touches the collar on one side at the very top (exit). There is a gap of maybe 1.5mm. 

 

I can improve this by moving the stove forward 1 or 2cm, but it looks a bit odd like that and the flue a tad too diagonal. Would rather avoid this.

 

What can I do? Does it matter really if the flue almost touches the collar at one edge? I've read about people cutting the pipe diagonally in this situation, but I'm unclear if that is done only leaving one side of the pipe slightly protruding the collar and the other half way down inside!

[jonathanA]

in my opinion it doesn't matter. so long as you can get a good seal between the flue and collar.  I had a similar situation on mine and I cut the flue at a slight angle so that it protruded above the collar by a couple of cm's evenly all round (if you see what I mean). Ideally you want the flue pipe to be a bit higher than the collar so it sits inside the chimney outer, but outside the chimney liner so that any gunge runs back down the flue. (i.e. the flue pipe sticks up between the chimney inner and outer) assuming a lined chimney. 

[Alan de Enfield]

Ideally you should put a 'cut & angle it' lower down the flue pipe so that it goes thru the collar vertically and centrally.

[Jen-in-Wellies]

Don't worry about it.

[Tracy D'arth]

Its totally immaterial if it touches the collar. it will never get hot enough to heat the roof to scorching temperature even with a flue fire raging, I have been there.

The cast iron collars are overkill on a steel roof, they were instigated as a heat sink on wooden roofs hence the 14kg or so of cast iron.

[magnetman]

That is good info. I did wonder why these great big lumps of cast iron existed. 

 

A welded-in flue on a steel cabin nice to have. On past boats I always fitted the cast iron lumps but occasionally had leaking problems. A welded in collar is definitely better but more complex to execute on an already fitted boat. 

 

Not sure how sprayfoam reacts to welding. 

 

 

The other approach occasionally seen is to use those orange silicone flue sealer things. That could be a solution on a curved cabin top but not sure how one then deals with having a removable terminal. 

 

Edited October 17, 2023 by magnetman

[blackrose]

1 hour ago, Poppin said:

What can I do? Does it matter really if the flue almost touches the collar at one edge? 

 

What's the gap on the other side? If it's touching on one side but you have 5mm or even 10mm on the other side that's fine. The gap isn't there to dissipate heat, it's there to allow for expansion of the flue pipe. The heat will get through to the collar anyway but it's a dense cast iron heat sink that can deal with it. Just make sure you've removed any sprayfoam from the underside of the roof for a good 6" minimum radius all the way around the collar and put some Rockwool into the area where you've removed the sprayfoam.

34 minutes ago, magnetman said:

Not sure how sprayfoam reacts to welding. 

 

It burns if the welding is in contact for any length of time. The fire rated stuff will also burn. Having had to set off a powder fire extinguisher inside the boat while a welder on the roof fitted a non standard size pipe as a collar I know how easily it catches. 

Edited October 17, 2023 by blackrose

[Onewheeler]

Not a problem, although I'd rather not have it frotting directly against the collar. So long as there is a space (filled with sealant) it should be OK.

[Poppin]

1 hour ago, blackrose said:

make sure you've removed any sprayfoam from the underside of the roof for a good 6" minimum radius all the way around the collar and put some Rockwool into the area where you've removed the sprayfoam.

 

 

 6 inches seems like a lot, I was going to replace the spray foam with a small layer or fire rated foam against the roof to stop condensation, and then some rock wool after that. The new foam would be well within the protection of the cast iron collar.

[Tracy D'arth]

37 minutes ago, Poppin said:

 6 inches seems like a lot, I was going to replace the spray foam with a small layer or fire rated foam against the roof to stop condensation, and then some rock wool after that. The new foam would be well within the protection of the cast iron collar.

Good plan. 6" is overkill. You need some insulation closer to the flue collar, say 1" to 2" away from the flue pipe at most, to avoid condensation off the roof.  Rockwool would be ideal, it will not burn.

 

Spray foam takes a direct flame to ignite it, otherwise it just goes brown and brittle. Welding on the opposite side is no problem, providing you do not burn through the steel. Done loads of welding on spray foamed hulls and containers, never lit one up.

Edited October 17, 2023 by Tracy D'arth

[BWM]

Two things that may become a problem. The first being the seal likely to fail if installed like that and this will allow black goo to leak down the flue inside the boat - this smells nasty too.

 The second is that it may prevent a liner/twin wall chimney from being fitted, due to the angle.

[pete23]

You don’t want the flue pipe touching the collar at all, no where, the flue will rot out quicker with vibration from the engine where it touches..

[blackrose]

1 hour ago, pete23 said:

You don’t want the flue pipe touching the collar at all, no where, the flue will rot out quicker with vibration from the engine where it touches..

 

I've never had that problem in 20 years of living aboard. On both boats the flue was touching the collar on one side. 4mm wall thickness mild steel flue pipe isn't going to suffer from vibration or rot though for a decade or more. There won't be any vibration of it's packed tight with glass rope. I think it's an imaginary fear.

 

11 hours ago, Tracy D'arth said:

Good plan. 6" is overkill. You need some insulation closer to the flue collar, say 1" to 2" away from the flue pipe at most, to avoid condensation off the roof.  Rockwool would be ideal, it will not burn.

 

Spray foam takes a direct flame to ignite it, otherwise it just goes brown and brittle. Welding on the opposite side is no problem, providing you do not burn through the steel. Done loads of welding on spray foamed hulls and containers, never lit one up.

 

6 inches isn't overkill and foam can ignite much easier than you think. Like many other materials charring of the foam can ignite it over time and that goes for fire rated foam too - it just takes longer. Do we even know if the builder used a proper fire rated foam? It's not worth taking the chance.

 

Yes of course you need insulation closer to the flue pipe and that's what the Rockwool is for.

 

Anyway, it's the OP's boat and the OP's life so do what you want, but I would always err on the side of caution when it comes to fire safety, especially when there's no advantage in not doing so.

 

https://rdrtechnologies.com/blog/spray-foam-insulation-fire-retardant/#:~:text=It's important to note that,such thing as fireproof insulation.

Edited October 18, 2023 by blackrose

[ditchcrawler]

All I can say is I cut a hole in the roof panel that had been sprayfoamed  and had a pigeon box welded in and the sprayfoam didn't ignite which has made me much happier, if it had been flue temperature (a lot less than welding), for several hours or days ( the welding took minutes) I don't know what the outcome would be. There was no danger of the boat catching fire as I removed the panel from the engine room roof before working on it.