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Chimney collar confusion


OllyO

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Do I understand correctly that the 7" cast iron flue collars have an ID of 4 3/4"?  

Is it common practice that between the flue and the collar you pack the space with fire rope and then seal the top with some kind of heat proof mastic or fire cement?

I've seen this done a few times and some pics on the internet but it just seems to be a bit of a bodge/ rubbish way of doing it.

Have I got the wrong end of the stick?

what do people do who have 5" flues?

if not are there any other systems availsble?

 

Edited by OllyO
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I'm sure if you came up with a better system it would be welcomed!

Don't forget that there has to be a small amount of flexibility in the seal to allow for expansion of the flue lengthways, otherwise it could crack the stove top.

Those with 5" flues normally use an adaptor to reduce to 4 1/2" This is quite acceptable.

Fire cement is not a good idea because it is not flexible and will quickly crack. Red Plumbaflue  and stove rope or tape are OK at roof level.

Edited by Mac of Cygnet
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14 hours ago, OllyO said:

Do I understand correctly that the 7" cast iron flue collars have an ID of 4 3/4"?  

Is it common practice that between the flue and the collar you pack the space with fire rope and then seal the top with some kind of heat proof mastic or fire cement?

I've seen this done a few times and some pics on the internet but it just seems to be a bit of a bodge/ rubbish way of doing it.

Have I got the wrong end of the stick?

what do people do who have 5" flues?

if not are there any other systems availsble?

 

The problem is it's not like a house roof where you can effectively seal the roof opening with flashing tucked underneath the slates/tiles, getting a waterproof joint which allows for expansion is the reason for the "bodge".   When you think about it there isn't really another way to do it without a lot of technical complexity.  A lot of marine fixes look quite crude when you think about it - what about the traditional stern gland/stuffing box? - just some rope and grease yet most folk still use it.  On a boat it's often more important to have something simple that works even if it does appear to be from a bygone age. 

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yes I completely agree and if a system is simple, effective and easily maintained then I am all for it. I'm just not 100% convinced that there isn't an equally simple way to do this that would be better.

The most simple idea would be to make a pattern for a collar with a 5 1/2" OD.  I'm sure there would be people interested in this product.  At least then the job could be done how it is intended without reducing the flue apature etc

Ill post what I end up with

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I did see a boat once that had an interesting arrangement where the flue went straight out through the cabin roof and the "chimney" part just sat on top and had a sort of flange to stop rainwater getting in.  So the flue was a sort of sliding fit in the roof but still waterproof.  I though it was a clever way of doing it but it was a custom made thing. Again, the issue with narrowboats is the chimney is a "consumable" and the standard way of installing a flue allows you to easily and quickly replace the chimney when it wears out or falls in the cut. Our friend with the custom chimney would have to have another one made, or I suppose he could have had a spare...  

Every so often I have similar thoughts to your own about this, the problem is something else on the boat breaks/goes wrong/wears out/falls off/starts to leak and the idea gets shelved for later...  Good luck though and do keep us informed.

  

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  • 2 weeks later...

Ended up making this. Found some 6.5" OD CHS off cut welded it to a 1/4" flange I cut and tapped it for six M10 grub screws to hold/ centralise flue section with storm collar welded to it.  Cavity can be insulated if required though practically the outer tube doesn't get hot.  5" flue internally. Takes a standard 5" flue pipe on the outside. We got a nice long length as we have a mooring, can be easily removed for cruising or shorter length fitted.

Not a perfect system but much prefer it to the normal method and easily serviceable 

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On 16/10/2017 at 20:38, OllyO said:

what do people do who have 5" flues?

 

I also have a 125mm inox flue inside the boat. At the moment there is a piece of 110mm tube welded to the roof but it is not going to last for ever and I want a collar like those for narrow boats. Midland Chandlers say that their small collar is 5" Ø on the inside but I am uncertain whether it will fit inside my flue pipe. 

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