On Wed, 26 Apr 2017 10:02:26 +0000 (UTC), unk wrote:

I want to cut AC power if the temperature drops below a close-to-freezing
temp, and I want it to work even if there's no power (takes care of the
power fails first then system freezes then power is restored - bang goes
pump motor). This needs to be accurate to about 1-2 degrees.

Check out solar water heating controllers. They're designed to
activate/deactivate pumps based on multiple sensors or snap switches,
including freeze sensors. You can reverse the contacts if needed with
an external relay.