See, I'm thinking of this differently.
I'm assuming:
- The OP does not want to create an 'automation' infrastructure (just wants enough hardware to get the job done).
- Wants minimal learning hassle
- Has an existing network (all hardware will be near, or have the ability to be plugged into a local LAN)
With these assumptions I think I would suggest looking into a methodology based on the
WebRelay components. These units are a bit pricey (over $100 each), but this really would be the only 'high dollar' items, other than the Elk Relay/Contact power controlling device, that would have to be purchased AND it would solve the problem of the water heaters and air conditioners being in remote locations from each other (again provided a network cable can be run to these locations).
You can use a current sensor (my favorite is
this one as it provides a variable threshold that will give a contact closure when that threshold has been met, plus it requires no remote power), but you can also do simple methods such as a piece of cardboard in a vent with a magnetic contact (use a very small magnet) so it gives an open when the cardboard moves due to airflow (I have not done this myself, but there are posts here where this was useful).
Many ways to get a contact closure based on the air conditioner turning on.
So the theory is when one WebRelay device sees the contact closure change on its input, the other one (on a remote network location) will active its output and turn on/off the Elk contact device controlling the water heater.
suggestion.jpg
Note that if there are only one water heater and one air conditioner, only one WebRelay device would be needed. If this is the case there are other simpler methods that could be used where the expense of a WebRelay would not be needed! (I would use an
Elk-960 timer relay) to control the Elk contactor.
suggestion2.jpg
The only thing I'm not sure of is you can turn off ALL water heaters if ANY AC unit is turned on (have to double check the logic capability of the WebControl units).
The OP will also have the advantage of turning on/off any water heater from a web interface as well if needed.
Anyway, really didn't want to get into all of these details until we hear more info from the OP as I requested above.
I really don't think he wants to incorporate an automation infrastructure (Z-Wave, etc...) but could be wrong.