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[wheelsea]

I am trying to control the 24 volts power into a thermostat , so that when there is a power outage the a/c unit will shut down and stay shut when the power returns. I want to be able to either manually turn it back on or give it a command thru AH software. My house is on a generator that will not handle the a/c load. Will the UM506 do this or will it stay latched on in a power failure?
 Appreciate any comments or another way to work around this.

[Brian H]

Did a test with a UM506. In the Continuous mode. If it is on and the power is removed; I unplugged it; the switch in it stays on and will be on; at power return. The only thing that does not come back on is the Beeper if it was enabled and on at power loss.

[Oldtimer]

Although I can't claim to be familiar with every single one of them all the appliance modules I am familiar with are latching and remember their previous status after a power failure.

On the other hand lamp modules don't remember their pre power failure status.  Unfortunately they are not recommended for non lamp loads, although many people use them that way.  Try powering your 24 volt HVAC transformer with one but do it so that nothing, including you, will be harmed if it blows or overheats.

If that works make sure the thermostat won't trigger the module on when it's off by playing with the temperature setting.  If it does trigger it try my 22k bypass resistor suggestion posted elsewhere here.

If you've gotten this far then you need to worry about accidental dimming of the module.  I'd suggest putting it on its own house code and, if you're going to control it with AHP, use an applance module icon instead of a lamp module icon.

If anything about this worries you don't do it but do tell us what your level of electronic fabrication experience is. There are none X10 solutions to this problem that could be used with an X10 module but you'd need to build the device yourself.

[steven r]

I think you would be safest with a latching relay that you could reset manually or by X10.

[Charles Sullivan]

I think your best bet is to wire a double pole relay (with 24V coil) so that one contact  is wired to "self-latch" the relay so long as it's powered up, but will drop out once power is lost (and stay off when power is restored).  Then wire a UM506 in _momentary mode_ across this contact to actuate the relay.

[steven r]

I think your best bet is to wire a double pole relay (with 24V coil) so that one contact is wired to "self-latch" the relay so long as it's powered up, but will drop out once power is lost (and stay off when power is restored). Then wire a UM506 in _momentary mode_ across this contact to actuate the relay.

Funny you should happen to post that while I was working on my crude drawing.

Ok here's one one my crude drawing showing how you could do it with a 24V double pole relay.

                    o
o-----------o
               !  \ o--------------------o
               !
2             !   o
4             o-----------------o
V               \ o---------+---o *              ---> To AC
        -----mmmm------!
        !  24V relay
        !   
o-----+--------------------------------o


*Reset point - You can connect a universal module here in momentary mode to reset relay.

Relay is powered through one of its own contacts and stays on only while power is on. When power fails, a reset is required to latch the relay again.

BTW...Does you generator power all your outlets. i.e. Are any not on the generator? If so they could be be used to automate a reset when power returned. Does X10 run when outlets are powered by the generator?

[-Bill- (of wgjohns.com)]

Since it sounds like it's going to be a "hack" anyway, here is my suggestion:

Use a double-pole 24 volt relay.
Connect one pole in series with the 24 volt thermostat so that the thermostat gets power when the relay is energized.
Connect the other pole in series between the positive 24 volt supply and the relay coil so that the coil will connect to the positive 24 volt supply when the relay is energized.
Connect the other end of the relay coil to the negative (or ground) side of the 24 volt supply.
Connect a momentary normally-open push-button between the positive 24 volt supply and the positive side of the relay coil.

When power is first applied, the relay will be de-energized, and no power will flow to the thermostat.
When you press the push-button the relay will be energized, supplying power to the thermostat and to the relay coil, holding it energized until the 24 volt supply is cut off (like in a power outage).

[-Bill- (of wgjohns.com)]

Looks like great minds think alike Charles and steven r! 

But you were quicker on the trigger.

[steven r]

Looks like great minds think alike Charles! 

But you were quicker on the trigger.

I bet I started my drawing before he posted. 
He just sneaked in while I was trying to make all my lines meet.

[-Bill- (of wgjohns.com)]

Looks like great minds think alike Charles! 

But you were quicker on the trigger.

I bet I started my drawing before he posted. 
He just sneaked in while I was trying to make all my lines meet.

Yeah, but my text description makes your drawing easier to follow.   

[steven r]

...Yeah, but my text description makes your drawing easier to follow.   

Both Charles's summary and your technical manual do help my crude illustration.

[-Bill- (of wgjohns.com)]

...Yeah, but my text description makes your drawing easier to follow.   

Both Charles's summary and your technical manual do help my crude illustration.

Hey, at least you made an illustration, I gave up on that and just used text!

[steven r]

...Hey, at least you made an illustration, I gave up on that and just used text!

Well if a picture is worth a 1000 words, do I win? 
I don't have a way to really draw a diagram and post it so I have to be creative to make my crude drawing. Your description definitely helps clarify my drawing.
Hopefully its clear enough for wheelsea.

[-Bill- (of wgjohns.com)]

Hopefully its clear enough for wheelsea.

That's the important part.

[wheelsea]

Hey everyone thanks for the info so far. One thing I just thought of would be to replace my service switch at the A/C air handler with  an XPS2 switch, since the transformer for the thermostat side of A/C receives power through this.Am I correct to assume that the XPS2 will behave like a lamp module and stay off until I either manually turn it on or turn back on with a  command from AHPro.
Thanks, look forward to your thoughts on this approach.