you can drain a combi from a radiator, either from a drain point or by removing a radiator af fitting a hose to the valve. You need to establish whether the combi is pressurised or tank fed (majority are pressurised) If the system is pressurised just drain the system there's no need to turn off any water supply. To re-pressuirise there'll be a combi fill on or near the boiler, it may be an internal combi fill or two valves under the boiler that will have to be connected with a flexible filling loop. Fill to about 1.5 bar and vent radiators, re-fillin until all rads are full and the pressure is between 1 and 1.5bar (may nee to open Auto air vent and possibly need to vent pump.
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