Preventing both water heaters from running at the same time

[gbandera]

Hi,

I'm sure I read in a post about someone who was running two instant electric water heaters but due to the load had a 'relay' or switching device that ensured that both would not be able to draw power at the same time.

However, I can't seem to find the post so was I dreaming or is there such a device and if so what is it called?

I realise this could be achieved manually with a DPDT switch of a sufficient rating but really need a solution that would automatically inhibit the second heater from drawing when the first one was running.

Help gratefully received, even if it's just confirming I dreamt the post

 

[vsynth]

A relay on each water heater would do this.

The coil of the relay would be energised when the heating element is powered up.

A normally closed contact on the relay in water heater 1 would disconnect the power to water heater 2 and vica-versa.