Nothing to do with the original question, really, but I am always intrigued by the way heating installers, and as a result the public also, talk of X number of radiators to heat a house. Radiators vary enormously in size and output. If my house had been fitted with the wrong size where some of mine are very large, then either it would take an age to bring the respective rooms up to temperature or they would never get there. Mind you, at the time the installers that quoted based their choices on 20 degrees in the lounge and 15 degrees in most of the rest - and I doubt they correctly assessed the matter of windows or insulation. I suspect that serious attention to both of these would be more cost effictive than eliminating circulation as described. My instinct would have been to leave circulation to run through the room of main use (lounge) with a room thermostat regulating it and have the towel rail on a thermostatic valve. Buildersmate is correct that it seems perverse to pump unlimited heat into the bathroom. Perhaps you can reduce the flow to an acceptable level by throttling it down at the valve that must be on either in or out pipe.