Cross posting from the Weather thread. From my pool guy:
The next couple of days are forecasting a hard freeze in our area. To protect your pool equipment the most important preventive measure is to run the pool pumps continuously while temperatures are near and below freezing. This includes all pumps: Main, Cleaner and Water Features.
Pools with electronic control boards typically have freeze protection built in and will come on automatically as the temperature approaches freezing. Some manual timers have a freeze protection feature (a black knob below the timers to select a temperature). Regardless of your system, we recommend you verify your pumps are running to avoid any freeze damage to your equipment.
Pools with hot tubs and automatic valves will alternate the valves to keep water moving in both sets of pipes. If you have manual valves that you turn when using the hot tub, I would partially turn them, ~45 degrees, to keep water flowing in both sets of pipes.
In addition to running your pumps, you may also cover your pipes. Care should be taken to not cover the motors, heaters or any electronic components that could pose a fire hazard.
It is not recommended to run the heater as it will have little effect.