warm towel rails/radiators on hot wateronly or cen htg only

[lpkelly]

we have a sealed pressurised system with two motorised valves and using a megaflow for hotwater. when the system is on hot water only, our two towel warmers get hot, i assume as well as for convenience they provide a boiler bye pass circuit.
however when the system is on central heating only the same towel rails also get warm.
can someone explain how the towel rails can act so independently

 

[gas4you]

Sounds as if you ch zone valve is letting by, or it could be that when they were installed the pipe work to them was left connected to the old gravity hw circuit.

 

[lpkelly]

thanks dave. but why just the two towwel rails and not the other radiators?