Radiators get warm when only hot water selected

[RichardRichard]

If the hot water motorised zone valve is on, but the central heating motorised valve is Off, the radiators still get a little warm.
The hot water primary return is about two feet from the boiler and there are central heating returns between the hot water return and the boiler.
A plumber tells me this causes the hot water return to pressure the rads making them warm. However, if the central heating motorised valve is off, how can the water circulate to the radiators? I might warm the pipes by conduction but not radiators some way off. So what is going on?

 

[45yearsagasman]

There should be no CH returns after the HW return. The HW return must be the last connection. You probably have reverse circulation. You can either turn off the affected rads or have it repiped correctly. View media item 32248It appears that your plumber knows what he's talking about.