Sadly the timeguard one does not appear to work with smart things. Others I have found do now appear to be able to handle the draw. I have been going around in circle asking about this, and samsung sent me from pillar to post.
you can use the Aeotec heavy duty relay which is designed for high load devices. You wire it in line to the circuit feeding the heater.
This is a Z wave plus device which works fine with smartthings. You have to make sure that you get the zwave frequency that exactly matches your hub the way frequency, so assuming that you have a UK model hub, that would be the following:
I’m not 100% sure, but I think it will run locally just as an on/off switch.
alternatively, you can get a little device which acts as a robot finger to just push the button. This is battery operated and moves your existing switch. It’s Bluetooth to its own mini hub which then talks to the cloud by Internet. It has a manufacturer provided integration with smartthings which works quite well. I use a number of these at my own house, and I wrote a review of them for the forum:
This brand is available in a number of different countries, including the US and the UK.
It will end up being somewhat less expensive than the Aeotec, but it is a cloud-based integration.
Minihub, required for ST integration:
Switchbot button pusher:
Switchbot has the advantage of not requiring any wiring so you don’t have to worry about accidentally disabling any safety features of the device. You’re literally just substituting for the finger pushing the button.
For attaching products of this type, I highly recommend Subaru which is a moldable glue that dries to a rubber like consistency. Removable but not reusable. Really simplifies projects like this.
This is why the OP would need a heavy duty switch from Aeotec instead.
The Frient is a good find. It’s much less expensive than the Aeotec, and it does handle a 15 amp load. It’s zigbee, so it will have a shorter range than the Z wave Aeotec: depending on where the device is located it might need an additional repeater, which would add a bit to the cost. The main question is the load required. Many immersion heaters are 20 amp devices, in which case again we’re back to either a heavy duty switch or something to push the button.
Another option is sonoff. They do a 15a power switch (POWR2) which will also monitor the power consumption.
Or they do a 16a TH16 which is actually a temp and humidity sensor but you can use it for your purpose and just ignore the temperature reading. They are both available on eBay in the UK and are about £15 each. They both work with Smartthings, alexa and their own app (ewelink).
BUT the first thing you need to do is check your immersion and see what power rating it is and then convert that to amps.
Power (watts) ÷ Volts (240) = Current rating (amps)