Is your bathroom rad permanently on?

[slowmotion]

I plumbed the bathroom radiator/towel rail in parallel with the hot water primary circuit. When the water is being heated by the boiler, it's on.

 

[stephec]

Some CH systems use a radiator as the bypass in case all the TRVs close but water still needs to be able to circulate, so the boiler flow thermostat knows what's happening. This may be that radiator (it's usually the bathroom one, as extra heat there is rarely unwelcome). Others have a separate bypass pipe in the airing cupboard. I've had both.

This is how our central heating was installed around ten years ago.

 

[rogerzilla]

I plumbed the bathroom radiator/towel rail in parallel with the hot water primary circuit. When the water is being heated by the boiler, it's on.

I thought ours was plumbed this way but it's actually just a leaky diverter valve allowing some flow in the CH circuit when the HW is on. Only enough to warm up the highest rads, i.e. the towel rails. It's a handy misfeature.