Replacing Centaurplus C17 with nest thermostat setup

Twin and earth cable is available as both 70°C (thermal plastic) and 90°C (thermal setting) cable, in the main thermal setting is white, but not always, the only cable to be heat resistance is mineral insulated, however would not normally use that to wire up a boiler, some cable like ali-tube will not seal against gas, and even some cables one would expect to seal have been know to have problems, found dangerous chemicals in a building due to travelling through steel wire armoured cables, I had the job of fitting epoxy resin cable joints in the SWA cables to ensure hydrofluoric acid could not under fault conditions get through the cables, still have a tube of the antidote.

You can get stuffing glands for twin and earth cable, but in the main to make the cable water tight or gas tight we use round cable, oddly fire proof fittings are not necessary water tight, it is down to absorbing heat not stopping gas or water, I have had 150 mm² cables feeding a submersible pump allowing water to get into the pump there was a pin hole in outer sheave some 3 meters away from pump, and when pump was pressurised to test, pin hole was found, but to test cables one must be able to pressurise, in the main a Schrader valve is used to test, I have not seen any point on the boilers I have seen for the casing to be pressurised to test it, so visual inspection it seems is only way to check, which would include checking correct stuffing gland is used.

Although the twin and earth cables seems better the outer of the two flexes have clearly not been clamped in the heat link, and if there is a clear fault there one can assume there are also other faults, so @Chris_W does make a good point.
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