It's actually quite simple:
Empty your entire system, disconnect all union connections, and remove all valves that aren't frost-proof. This removal should be used to inspect the valves and clean it if necessary. Then place the valve in a container of water and store it in a dark, frost-free room – changing the water if necessary during the winter months.
Why separate the union connections?
Water collects even in the threads, which can burst the thread if ice forms.
Why store the valve separately in a frost-proof container?
A PVC-U valve doesn't have a dead-space-free design, meaning there will always be water in the fitting unless you disassemble the fitting, clean it, and spray the sliding surfaces with silicone spray, with the handle at a 45° position. In this case, you don't need a water bath for your fitting.
AND - you know it from your car - the frost warning sounds as low as 4°C!
