Here in Stockholm it gets below minus 10 deg C regularly. All aircraft that are left outside unattended have the water system drained. We drain ours when the forecast is Minus 5degC or lower.
But aircraft that are parked on the F pier or Terminal 2, can have conditioned air supplied continuosly, and in that case the water is not drained.
One night a B737 had the heater hose fall off, and was not noticed. The tank froze and we had to park the aircraft in the SAS hangar for 8 hours to defreeze it!