Installing smart thermostat on a solar water heater

Hello, I have a thermosiphon water heater (Solar water heater) of this sort:

The tank has a resistor inside that can be turned on for rainy days when there’s not much sun to heat the water.

The tank has a simple “dumb” thermostat of this sort
I would like to be able to:

  1. Know the current temperature in the tank
  2. Possibly turn it off / on via HA

I am not sure I can replace this thermostat (might need to empty the tank, also probably void the warranty), so goal #1 is the most important for me. #2 is a nice to have.

Any ideas on ways to achieve this?

Thanks in advance

All you need is a temperature probe (for example a ds18b20 type which are sold already in inox tubes with thread) and a Relay together with the brains of the operation: A esp with esphome :rocket:


That way it’s also possible to have the thermostat directly on the esp so it’s fail safe (can work independently from HA) while still being perfectly integrated (local push :trophy: via native api :muscle: ) in HA at the same time. :arrows_counterclockwise:

It should also be possible to get ready made devices with this specs like a Sonoff TH10/TH16 maybe.

You could use (Hardware) Thermostat you have already as a “fall back” if you like and just set it to the highest possible temperature you plan to heat up your system when powered from the outlet instead of the sun :sun_with_face:

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Thanks for your answer.

I am new to the HA ecosystem and also to hardware you mentioned.

So far, all i have is a mini pc running HAOS. I would therefore need to buy some ESP32 board in that case, plug the probe into it (?) And program it to work with HA through ESPHome ?

Regarding the probe, where do you think it goes in to the tank ?

Overall that sounds interesting, would love to try it. Do you have any recommendation for a ESP32 board ?

Essentially, yes. Both (the cheaper) ESP82xx and the ESP32 (typically with bluetooth) are compatible with esphome. You could also split to have one esp with the temperature probe and another one to switch the load (this way you could maybe use of a ready made smart plug - for example from athom or kaufha). Disadvantage from that solution is that the thermostat function will than be depending on HA and the moments your mini pc for example is restarting because of an update the thermostat will not work.

A short getting started guide (with more information linked) is to be found here:

This type of hot water tanks usually have various in/outlets with threads. You want to “find” a “free” (closed) one - typically on the the upper part (were hot water accumulates) which also hosts the hot water outlet.

Might be like here: One on the bottom and two on the top (cold water in - hot water out and a extra one):
This will be on both sides as usually there will be a water circuit for the fresh water and another one which is passing through the panel itself and exchange their heat in the tank.
In worst case (no free thread left) you need to switch the thermostat for a temperature probe. Maybe you want to check if your heating element features a (internal) cut off temperature - because in that case your hardware thermostat might be completely obsolete.

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Wow thanks a lot for the detailled answer. Looks like I have enough reading material to get started.

I haven’t’ thought of the option to find an available outlet. The ones on the tank come with bigger threads, so I will have to fit the temp probe thread to the bigger outlet thread, I guess?

And if I go with a ready device like the Sonoff one (example TH Origin - SONOFF Official) I can buy a probe and replace the thermostat that came with the tank with the Sonoff one and the probe. Problem is that I couldn’t find such a long probe, they are all quite shorter, but maybe I can push it into the tank with the flexible cable…

(can only upload one image per thread)

In that picture it looks like the heating element is actually mounted together with the thermostat on a socket which then is screwed into the tank? :nut_and_bolt:

As the graphic shows no pipes (or connections) I expect to be 4 pipe in/outlets on the other side - maybe one more free thread can be found there? :wrench:

As an alternative it might be possible to mount a (massive) T-piece on the hot water outlet to allow the mounting of a temperature probe. But depending on the size of the thread (to small) it might be limiting the water flow :potable_water:

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hello ,
i have one question maybe you guys can help me out , i have a solar water heater with a TK-7 solar controller with bottom fixed sensor for non pressurized system , and this is the water level water temperature sensor probe , any idea what device i should use for the water temp and level sensor to integrate it into home assistant ?