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In the old days when I was more up to date with this sort of thing the thermostats did not always switch the load directly but used an intermediate relay.
As Brian appropriately points out, a good approach for X10 control of water heaters is to use a suitably high current contactor, controlled by a wire-in appliance module, all tucked into a suitable electrical enclosure...
As Brian appropriately points out, a good approach for X10 control of water heaters is to use a suitably high current contactor, controlled by a wire-in appliance module, all tucked into a suitable electrical enclosure. The contactor would be a two pole device with a 120 VAC coil.