On 4/30/2017 4:29 AM, unk wrote:
On Sat, 29 Apr 2017 08:35:23 -0500, dpb wrote:

On 04/28/2017 11:33 PM, dpb wrote:
On 04/28/2017 10:15 PM, DoN. Nichols wrote:
...

How will the board continue to work without backup power?

...

That's the point; it doesn't need to.

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.

Which use begs the question of why OP would leave the AC energized once
it's even remotely close to freezing, anyway...a solution looking for a
problem here.


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.
So, you do already have a thermostat that is sufficiently accurate
to do what you need.

Add a second thermostat in parallel with the first one.
Hide it or lock it so the +5 degree one can't be modified.


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.

That depends a lot on:
How long do they stay?
How cold does it get?
What keeps the pipes from freezing when it's occupied?

Use a wind-up timer that they have to wind up every day and
push the reset button that restarts the pump if they forgot.
Use a random Klixon sensor at a temp high enough that
tolerances don't matter.

If the timer runs out and the temp is below 40F or
the power is out,
shut off the pump, drain the system. Maybe even disconnect
the adjustable thermostat and use the 5F one.

This assumes that there's no downside to drain the system
and disable the pump when it's hot outside and unoccupied.
The devil is in the details.

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.

Educating the users has not worked (twice, counting the number of times
pumps broke and pipes-had-to-be-replaced). A machine you only have to
educate once.