Making My Water Heater SMART!

Hello. Keen DIY-er here. I needs some advise on the following project. 

Trying to come up with a cheap and affordable way to turn my Water Heater, which has a electrical immersion heater, into a 'smarter device'. This is because the property in question often goes unoccupied for a few weeks. The requirement here is as follows; 

- I want to be able to turn it on and off remotely.   

- I want to be able to create On/Off schedules in order to optimise efficiency. 

- I want to be able to bypass the 'Smart' capabilities of the system in case of a fault or loss of connectivity. 

Here's what I'm thinking. 

- Use a Smart (WiFi) Plug to trigger the system. I have a few of these plugs at home already. They are very reliable and work well with Apple Homekit. 

- Use the Smart Plug to switch on 2 Solid State Relays (Fotek variety. High spec. Like a SSR-40 AA 40A AC 80-250V to AC 24V-380V SSR)

- Leave the existing Dual Pole manual switch wired in to act as a backup, in order to override the Smart system if it does not work for whatever reason.  

I'm not 100% sure of the wiring of the Dual Pole switch. I'm concerned that there could be an instance where both systems are triggered on at the same time allowing current flow along 2 paths. 

Also, my existing Dual Pole has a LED to indicate if power is on. I could replace this with a Dual Pole switch that doesn't have a LED, or a single pole switch. My understanding of using the Dual Pole switch is that it is safer as it fully isolates the load. Positive and neutral. This is also why I used 2 Solid state relays instead of just one.   

Please see attached a crude picture of the idea. It's not a virus, I promise. It's a link to a jpeg on WeTransfer. I don't know how to post pictures to the forum.  

https://we.tl/t-m17Yumj1ke

Any suggestions, connects, much appreciated. 

Regards

I'm not sure that leaving a "home made" device in an empty property may not be sensible if something went wrong ................

If I understand your requirement wouldn't buying a product off the shelf like this do the job?

https://www.heatingcontrolsonline.co.uk/optimium-wifi-immersion-heater-timer.html

Johnnydt said:

Positive and neutral.

Click to expand...

There is no positive with AC.

Don't use two seperate relays to switch the line and neutral, that is asking for a disaster if/when the line relay fails. 

Don't connect this in parallel with the 20A DP isolator, connect it after it with an additional, cleraly identified override switch if required. Anyone working on the system in the future may switch off the 20A DP switch thinking that isolates the system only to die from an electric shock when the WiFi setup switches on.