Repairing wiring to an outside chimney fan

[Chris Green]

Continuing from the other thread....

I now have good access to the chimney fan and its wires.

The original installation had a (not very) weatherproof plastic junction box with the wires from it to the fan itself in a length of flexible spiral metal trunking.

The immediate cause of "it needs mending" was that it tripped the RCD which turned out to be because the (not very) weatherproof junction box was full of water.

I need to replace the trunking as well as the connections between the (ordinary PVC) downlead and the wires to the fan which are (by the look of them) some sort of heatproof wire.

I can crimp and/or solder the actual connections and heatshrink them but I'm not sure what to use to provide overall protection. The old spiral metal trunking is very corroded after only a year or so, not very suitable I think. It has to cope with a very nasty combination of seriouse exposure to the weather and heat and smoke from the chimney.

What sorts of reasonably heatproof, non-corroding, flexible trunking are available? It probably only has to cope with 100 degrees or so, I doubt it the air/smoke coming out of the chimney is much hotter than that.

I'll need a metre or so of it, small diameter, around 1cm I'd guess.

[newshound]

How about this

OK it's ten times what you need

[Chris Green]

It's too fat and it's only galvanised, I don't think it will survive very long. I had looked at Screwfix and Toolstation before I asked the question! :-)

[Steve Walker]

You can get 10mm, in stainless-steel.

[newshound]

7mm bore is quite small for feeding typical "flex" through. Not sure why OP thinks galvanised might be in trouble, is it actually inside the flue?

[Chris Green]

It runs across the outlet from the flue, can't be avoided as it's a centrifugal fan that blows out 360 degrees.

I think the very corroded original one was galvanised, that's why I'd prefer to go for something better if I can. 7mm bore should be OK, it's probably what the original was.

[newshound]

Understood.

[Brian]

Use high temperature cable typically used for soldering irons for the short run in the conduit. It is flexible and temperature resistant. The fan can?t be that powerful so the current should be low.

It should fit through a 7 mm bore.

[Chris Green]

The cable isn't the issue, the existing wires (they are actually separate wires) are perfectly sound and good for the temperature. I can't (easily) get to where they're connected to the motor so it makes sense not to change them. They're long enough that the connection to the downlead is in the cool so I can (for example) crimp and heat shrink that. I just need something to protect the 'separate wires' from mechanical damage.

[alan_m]

Teflon fuel hose??? Safe to 250C Example

[Brian Gaff (Sofa]

You say galvanised is an issue, but if its been done right its very good. I have lots of bits of metal that have been outside for 20 years or more which are still rust free, however those with that slightly gold appearance I'm assuming Cad and pass plated, go south in about five years. Brian

[Tim Lamb]

Get someone to make up a suitable length of pyro?

[charles]

You won't get a pyro termination through a 7mm hole. You'd have to pot the ends on site once it was through the hole. Yes, I can still remember learing how to deal with pyro in 1960. Haven't done it often though.

[Chris Green]

Buut not in the extremely corrosive stuff that comes out of a chimney.

[Chris Green]

I think I may be able to do 'D-I-Y' pyro, I have some 10mm copper pipe and I don't think there's going to be a problem with it being rigid. I'll just have to make very sure the wire is kept still inside the pipe and doesn't damage itself.

[charles]

How are you going to create the insulation?

[Tim Lamb]

In message [e-mail]>, charles [e-mail]> writes

Back in 1960 when I was learning about this stuff, Admiralty switchgear was often connected in solid copper/rubber insulation/woven asbestos outer!

[Chris Green]

These are three straightforward insulated singles like you'd have in a conduit, they have high temperature insulation rather than ordinary PVC, that's all. They'll be fine inside a smooth copper pipe, I just need to protect them at the ends of the pipe.