240V wiring question. Adding a wifi switch with only a 240v circuit available

Do you realize that product is not UL listed?

Here in the US there are those of us that would call this junk from the country of origin.

If it was UL listed and rated to be used with the ground I would say great put it in.

It is not listed and a non listed device could end up with really bad legal issues if someone is injured or killed.

Yes you are only controlling using the hots. Hots are not as bad as a system using the ground that is non listed.

fire could be a possibility with high amperage and junk that has not been listed or certified.

The entire assembly should be encased in a Hoffman box to prevent issues with fire. Not only the sonoff switch but the contactor.

Well you can't use a Sonoff because it's not UL listed. But really, I'd say you are making this harder than it is :)

I myself have a 240V contactor switching my water heater, also with a 240V coil, so I could use a common Intermatic run-down timer to control a water heater. However I used a UL listed timer rated for 240V to control the coil, and all the wiring was inside Class I wiring methods (EMT conduit with THHN, actually).

One of the many reasons the cheap Sonoff is not suitable is there really isn't an appropriate/safe way to enclose it in proper Class I wiring methods. After all it needs to communicate via WiFi.

So we'll do an end-run around that problem :)

We'll go back to the more readily available 24-volt coil contactor.

From the 24V coil, we'll run common thermostat wire to a plug-in 24VAC transformer. Transformer makes 24V, picks up contactor, water heater turns on.

"But wait a minute. How is that smart in any way?" Easy. You know how I said a Plug-In transformer? Plug it into any UL listed smart plug from any maker. You command the smart plug to turn on, and the transformer energizes, picks up the contactor, turns on the water heater. And Bob's your uncle!

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Do remember a couple of things about TANKED water heaters: #1 they take about an hour to warm up. #2 after they warm up, they hold heat a very long time - as long as 24 hours if the insulation is good.

So if you are thinking you'll save energy by turning it off at night, yeah, but not as much as you might think. If you're thinking you'll save energy by turning it off *all week", oh yes, you surely will!

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Heat-up time: You can actually compute this very easily using the lovely unit "the BTU": 1 BTU is the energy needed to heat 1 pound of water 1 degree F. The water heater nameplate will state the size of the tank and the size of the heating elements (in watts, which you'll need to convert to BTUs/hour), and that's all you need to derive warmup time. Note generally only ONE element runs at a time, so "4500W" is likely, 9000 is not (especially on a 30A circuit).

You can also determine the cost of warming up that amount of water from ambient. Note you pay this entire cost every time you warm it up. What you save by turning it off is the periodic "reheat cycles" as heat leaks through the insulation.

Don't let anyone tell you that you save nothing. Lots of people want to believe that because it's a convenient lie; it justifies what they already want to do. You do save something; it is quantifiable with science if you want to do some testing and crunch the numbers.

The key scientific principle is that leakage through insulation is proportional to the temperature difference.

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Lastly, beware of legionella and other very nasty bacteria that love to grow in water heaters. You are creating a perfect storm for bacteria growth by having the tank water wander between 60F and 120F. It will lie dormant at cold temps and grow like crazy during those long transition periods as it cools down. To kill that bacteria dead, you need to raise water temperature to 140F and hold it there for like an hour. Note that this is a scalding temperature, so you also need faucets with thermostatic control, such as any of the modern 1-handle joystick style faucets, or annoying twisty hotel-style shower valves. These valves will prevent output water from exceeding ~115F by mixing in cold as needed. This isn't a big ask; any modern valve has this. (obviously not the 2-faucet kinds; those are used with tankless).