On Sun, 30 Apr 2017 08:37:12 -0500, dpb wrote:

On 04/30/2017 6:29 AM, unk wrote:
On Sat, 29 Apr 2017 08:35:23 -0500, dpb wrote:
...

To amplify, these boards retain their settings on power-off so if
use the NO contact and the "COOL" side so closes on TSetpoint,
when power comes back on the relay won't close to restore power to
AC if the temperature is below the turn-on setpoint.

...

It's for a cottage that is available to a number of people. It has
electric heat, which is *supposed* to be set at 5 degrees above
freezing when the users leave. They are supposed to drain the water
as well.

But sometime they don't set it, and sometimes they don't drain the
water system, and sometimes the power goes out.

What I want is a system that will open a valve to drain the system
whenever

a) the power goes out, or

b) the temp gets too close to freezing.

I can do a) with a relay or contactor (which will also cut power to
the pump) and a electrically operated valve, but for b) I need a
switch that will cut power to the relay.
...

You're still going to have to have aux power to operate the valve when
power goes out for a) then.

Normally closed solenoid valve, or "power pill" valve. Don't know if
they still make the latter. Regardless, probably going to need a vent,
which can be passive but a potential PITA.