well, it rather depends how hot the pipes get and how heavily loaded the cables are.

It is not a good idea to take PVC jacketed cable much above 70C, as the PVC starts to age prematurely, becoming brittle and eventually flaking off. Massive overheating - say more than 120C, causes the PVC to flow, and the wires inside to touch, and then things get a bit snap crackle and pop, as per the breakfast cereal.

But, on the other hand, most parts of typicalheating systems don't get much above 50C, as much more than that gives folk nasty burns if they touch the radiators, and wires with an air gap are a lot cooler than any pressed into intimate contact with a pipe. An air gap of a few diameters (pipe or cable, which ever is larger - probably the pipe unless you have microbore heating) is usually enough to get significantly far away in thermal terms, ideally below or to the side, rather than above, from something hot.

Without seeing it, it is unwise to be too prescriptive, but while it is certainly not good practice, unless the cables are already hot due to being fully loaded, and there is nowhere for any air to flow in a gap to convect the heat away a bit, it is not likely to be immediately dangerous.

But any cables that smell of hot plastic or have regions that show noticeable discolouration or blistering from heat should be moved as soon as possible, and where the chance arises and you have the floor up anyway, then slipping a length of the slit and slide foam insulation onto the pipe would be a good idea, if the wire cannot easily be re-routed.

https://www.screwfix.com/p/essentials-economy-pipe-insulation-15mm-x-13mm-x-1m/72357 is an example of the sort of thing - slit long-ways with knife, slip over pipe and tie on with wire or cable ties.

Mike.

well, it rather depends how hot the pipes get and how heavily loaded the cables are.

It is not a good idea to take PVC jacketed cable much above 70C, as the PVC starts to age prematurely, becoming brittle and eventually flaking off. Massive overheating - say more than 120C, causes the PVC to flow, and the wires inside to touch, and then things get a bit snap crackle and pop, as per the breakfast cereal.

But, on the other hand, most parts of typicalheating systems don't get much above 50C, as much more than that gives folk nasty burns if they touch the radiators, and wires with an air gap are a lot cooler than any pressed into intimate contact with a pipe. An air gap of a few diameters (pipe or cable, which ever is larger - probably the pipe unless you have microbore heating) is usually enough to get significantly far away in thermal terms, ideally below or to the side, rather than above, from something hot.

Without seeing it, it is unwise to be too prescriptive, but while it is certainly not good practice, unless the cables are already hot due to being fully loaded, and there is nowhere for any air to flow in a gap to convect the heat away a bit, it is not likely to be immediately dangerous.

But any cables that smell of hot plastic or have regions that show noticeable discolouration or blistering from heat should be moved as soon as possible, and where the chance arises and you have the floor up anyway, then slipping a length of the slit and slide foam insulation onto the pipe would be a good idea, if the wire cannot easily be re-routed.

https://www.screwfix.com/p/essentials-economy-pipe-insulation-15mm-x-13mm-x-1m/72357 is an example of the sort of thing - slit long-ways with knife, slip over pipe and tie on with wire or cable ties.

Mike.