There is actually a very practical reason why bathroom radiators are connected in the hot water circuit. When HW only is on, the heated water circulates from the boiler to the HW tank and back to the boiler. Note that this in NOT the hot water that comes out of your taps but is hot water that is passed through pipes in your HW tank which then radiate heat into the cooler water contained in the tank (THIS is the water that comes out of your hot taps). The, now cooler, water in the heating pipes then returns to the boiler to be re-heated and recirculated. When the water in the tank reaches a set temperature, controlled by the thermostat, the boiler is automatically switched off, along with the pump that drives this circulation. In a typical HW system all the pipework is well insulated, so when the HW is satisfied, you end up with hot water in the heating pipes throughout the circuit, including in the hottest part which is inside the boiler. If the HW tank is at or near required temperature, then the water in the heating circuit will not have been cooled down significantly because the 'water jacket' will be at or near the same temperature. This can cause considerable problems, as the boiler then potentially becomes a superheated pressure vessel. The idea of passing the water through the radiators in the return pipework on this circuit is that you do not then get heated water re-entering the boiler because he radiators help ensure the returning water is cooler. There is another practical use of this type of circuit - you get warm radiators in the bathroom during the summer at times when you are likely to be taking a bath.
