"DoN. Nichols" wrote in message
...
On 2017-04-27, dpb wrote:
On 04/26/2017 9:48 PM, DoN. Nichols wrote:
...

But he was looking for something mechanical -- or with a minimum
of electrical, so something like what I pointed to, with a couple
of
relays, could do it -- with minimal battery backup. And the
learning
curve, with all of the options which can be set in the
controllers, may
be a problem.
...

Actually, re-reading his original post, don't need battery backup,
use
the NO contact so will open on loss of power. Set the hysteresis
value
on the reclose coming back up to protect restoration if were to be
cold-enough for freeze to have occurred. That's doable with the
little
board I posted link to simply by programming the builtin firmware;
no
coding needed.

How will the board continue to work without backup power?

As for the mercury-column thermometers controllers which I
pointed to earlier are NO, closing when the temperature reaches the
set
point. But -- with the limit of 100 uA through the mercury and
moving
set-point contact, you need at least a transistor to control the
current
through the relay, and you *will* need power -- AC power supply, or
a
battery backup -- to make it continue to work during power outages.

And, since I don't know what the price is for these, I have no
idea how affordable they will be for the task -- but they are
fragile.
They *do* have a hysteresis of less than a degree F.

Enjoy,
DoN.

I just remembered the words to search for.
http://www.sensata.com/klixon/thermostat-hvac-20600.htm

-jsw