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Is it just the case that in laymans terms it's the hot water but to a plumber it's a loop connected to the boiler that supplies the towel rail.
I'm pretty sure ours is hot in the summer but I've never really felt the need for a towel to be warm in the summer and they usually dry quite well between uses.
Airhead-
So on a combi system you always need one radiator thats a bypass thats always on. This rad you dont put a thermostat on so it is only on. Thats what your towel rad could be and without seeing it Id put that a good guess from experience.
But also the best thing about a combi is that you can have heating and hot water completely separate, so as Ive said already if your heating is off and your radiator is on theres an issue but without seeing the actually way your system is or you might have a heat only boiler or a system boiler theres so many variations and things that could be the cause of the problem.
A radiator will never be piped into the hot water and if it is, then it should be changed as thats wrong.
konastab01
Honestly I’ve never heard of the hot water heating a rad unless it’s been piped wrong.