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Topic Title: Design temperature for boiler cable running adjacent to heating pipes
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Created On: 13 October 2017 08:04 AM
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 13 October 2017 08:04 AM
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TimJWatts

Posts: 421
Joined: 07 August 2013

Hi,

Sorry - couple of quick-fire queries, due to my lack of experience - just after a second opinion. Would like to get this done as nicely as possible:

Bit of a finger in the air, but is it worth going up to 85C PVC heating cable for a conduit run adjacent but under heating pipes in a confined space?

or would 70C PVC singles be sufficient, more than likely? Singles would be easier to manage - conduit will have bends. I'll terminate this in a metal surface box, and joint and gland 85C heating flex in for them to take to the boiler for the last 2 feet.

Pipe run over doorway

Black = conduit with cables
Red = hot water
Blue = cold water


I am preparing for a GasSafe firm to install heating and new boiler at the house I'm renovating. So I'm laying in the controls for them.

I plan to put the conduit horizontally at the lowest point, ask them to run a cold pipe directly above. May be space for the thinnest of pipe lagging - not sure, it's tight.

This will be boxed over, but open at one end.

Likely the heating pipes will not exceed 65C and the hot water 55C but heating flow *can* go to 90C on this boiler if someone cranks the knob up.

Cheers for any random thoughts... I *know* it's a bit of "how long is a piece of string..."

Tim

Edited: 13 October 2017 at 08:23 AM by TimJWatts
 13 October 2017 10:15 AM
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davezawadi

Posts: 3962
Joined: 26 June 2002

It is a bit of string Tim, but just put the conduit under the heating pipes and you should be safe. You should understand that the effect of excessive temperature is not to melt the insulation but to speed up its deterioration due to loss of the plasticiser, but this is not harmful to the insulation properties, it just makes it rigid like uPVC window frames rather than flexible. In this cable you will have no significant internal heating from current flow, so the core temperature will be much the same as the environment which will never get anywhere neat 70C, even if the heating pipes themselves get to 90C, again which is unlikely even if the water temp at the boiler is that high. Domestic cables almost never suffer serious heat damage, even if wrapped around the heating pipes (which is quite common when the heating is added after the electrics!). Nice picture though, I wish we got those more often from some of the others.

-------------------------
David
BSc CEng MIET
david@ZawadiSoundAndLighting.co.uk
 13 October 2017 10:42 AM
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TimJWatts

Posts: 421
Joined: 07 August 2013

Hi David,

Thank you for your thoughts - I feel happy about this now

Best, Tim
 13 October 2017 11:14 AM
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mapj1

Posts: 9717
Joined: 22 July 2004

Indeed - and if you ever drop a wire offcut into your tea, or ideally someone else's, or even pour boiling water onto cable directly, it does not melt like butter leaving bare copper, indeed once its been cleaned up, its hard to tell it was ever hot.
Another aspect of this is that when you re-arrange the adiabatic equations for very fast faults you can see a final temperature is reached of well over 100 degrees.

-------------------------
regards Mike
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