I have installed a new radiator but to connect it to old central heating pipes i have to drain out the entire system.The new combi boiler control has three settings.Off,central heating and hotwater and hotwater only.My question is,if i keep the control switch on hotwater position only ,can i drain out the central heating water safely while maintaing hot water available.That is only if i can not complete the job same day.The system has to be flushed out twice.Or do i have to switch the boiler off completely.Any advice needed urgently please.