I have an Ideal Classic boiler?

I only have my water on (my heating is off) yet yesterday when I put my heating on for a few hours,I noticed that when I turned it off the radiator in my bathroom (which I can not regulate the tempertaure of as it isnt fitted with a temp control) was still on.
When I turn the hot water off it does go off but when I put my hot water on it comes back on.No other radiatirs are on.