Install and final design Low Cost Immersion Heater Solar Divert

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Background

Why pay lots for second rate low-quality home automation with poor software support and service. When a DIY solution is safer, simpler, amd from low cost, high reliablity, widely used parts and has no dependency on third-party support or the internet!



Background

We have an immersion heater. Not used much until we had solar installed. We found it was a good way to use up surplus solar. We are on a scheme where it is assumed we use 50% of our solar generation. We have no export meter just a generation meter. This is fine as we have a couple of Powerwall2 and an EV. While flipping a switch on / off was not a big issue I thought it would be good to automate

  1. Automation is fire and forget
  2. Seasonal variations simple with Node-Red and big-timer
  3. I wanted to try Tasmoda on a Sonoff Basic
  4. I wanted to use a 20AMP contactor dual pole as it would be designed for high power switching and used industrially Vs Sonff which while more than capable was not really a commodity except in the hacker community. 
  5. I wanted low cost (cheaper than the 16AMP Sonoff) and modular so when parts fail I can quickly replace the faulty part from any electrical supplier
  6. High safely professional parts
  7. Advances software control both on the device (Timers, State, MQTT, firmware upgrades, all on Wifi) and Node-Red support via MQTT and web services
  8. Installation would be serviceable by standard electricians
  9. No switches on the device except for emergency isolation
  10. Improved the current fused, switched spur

Updated Design

I was not happy with the heat in the existing controls for the immesion heater which was via a  13Amp fused switched spur

  • The fuse fot hot as the immersion heater might run for hours/days
  • 13amp fuse not intended for such heavy loads
  • No longer a recommended setup
To cover these concerns and the concerns above I changed the design adding

  1. Low-cost dual pole 63Amp Rated Isolator
  2. 16 Amp MCB
  3. 20 Amp Contactor controlled by hardware unmodified (except soldering a programming header and flashing Tasmoda WHEN NOT MAINS connected)
  4. Also changed the wiring to look like a normal consumer unit
    1. Live cable (a spur off ring main and  16 Amp cable)  rear entry to unit and direct connection to the top of the isolator
    2. Live from isolator output runs only to 16 Amp MCB
    3. Neutral from isolator output runs to neutral bus bar
    4. Second bus bar used for earth
    5. The output from 16Amp MCB feeds Sonoff and Contactor (Ferrules used)
    6. Sonoff and contactor are off (no Live) if MCB breaks
    7. MCB, Sonoff and contactor are isolated Live and Neutral if Isolator OFF
    8. All wire colour coded
    9. 16 AMP wire used for all wires except for the power to and from the Sonoff. Here a lower rates multicore copper standard cable with ferules was used.

Siting

Replace the existing fused switched spur with a 30Amp junction box with Ferules on the leads from the dimension heater (cut to trim the frayed ends) Unit is IP57 dustproof.



Photo of the final unit (cover open)



Cover closed


Thermals

The contractor and MCB are slightly warm (this is normal)
Isolator and junction box show no signs of heating even after prolonged use

Software

Tasmoda for Sonoff Control. Much more advanced than Sonoff firmware, and my custom web server that I use sometimes. Also has local device schedule to act as a safety default. Would work even if the Access point failed, the home network or the internet failed.




Battery Integration

This flow checks the state of the battery during hours of sunlight and powers on the immersion heater when the battery hit 90% state of charge.