Arduino Paloma Tankless Water Heater Control

Hi everyone,

Now that my Vera Arduino GW is working and the first sensors are doing their job I’m starting to look into a bit more complicated automation task. Below I try to describe what I’m planning to do and which questions are still open. Please feedback and feel free to suggest alternatives and code (i’m still in the infancy of arduino coding…)

I’m planning to tackle my Paloma Waterheater next.
The issue with this heater - aside from lack of documentation - is interfacing it with vera as it uses a, to my knowledge, proprietary bus system between the externally mounted heater and the internal control panel (MC-117).

So my plan is to hack the control panel - literally - by opening the back and attaching Relais to the momentary switch buttons inside the panel.

The panel has the following buttons/displays to interact with:

  • momentary push button to toggle power on/off
  • led which indicates power on/off
  • momentary push button to increase temperature
  • momentary push button to decrease temperature
  • 2 segment led display which shows the temperature

Implementation Phase 1:

  • control power on/off via arduino connected reed relais
  • indicate on/off status in vera based on control panel LED on or off (update is either toggled via vera or if switched locally on the panel)

Open questions:

  • best way to read the led on/off status and feed it back to vera. Is it possible to read the led high low status directly on the arduino (issue i guess is that the panel and the arduino are on separate power circuits). Or use a reed relais across the led to switch 2 pins on the arduino?

Implementation Phase 2:

  • increase / decrease temperature from vera - same as the power button using reed relais
  • read the temperature and feed it back to vera
  • new set temperature need to be compared with existing temperature and translated in required button pushes

Open Questions:

  • how to read the temperature off the 2 segment LED on the panel? So far no idea how to get this done
  • what needs to be done on the Vera side plugin to create the new heater device (power on/off, temp up and down, temperature info)?

Looking forward to get some comments.


Hi idefix,

I had plans to hack my Daikin a/c control panel in a similar fashion.

My thoughts:

  1. See if you can buy a new control panel first, so should you let the smoke out, you can replace it without spending potentially weeks of cold showers!

  2. If you steal the power from the control panel for the arduino, then i think you should be fine to read the logic level for each of the leds . Alternatively, a optocoupler wired in parallel with the indicator led.

For phase 2:
To increase /decrease, you could monitor the button presses for up and down, and report this back to Vera .
Then to change the value from Vera, use relays for each button (see relay actuator with button sketch)

As for reading the 2x7digit displays… you may be able to find a circuit to do this ( without needing 14pins )…or you could try to sync what vera thinks via the up/down presses with whats shown on the display.

Does the display default to a particular temp at startup? if so then you could “reboot” the controller and arduino when they get out of sync, by setting the default temp in your sketch.

Hi Gregl,

thanks for the reply.
Good points. I might be able to “steel” the power from the panel.
I will try this out once the new panel arrives - which will still take a few weeks :frowning: .

Syncing the up/down with vera might create trouble especially as the panel does not reset but stay at the previous set temperature when powered up.
Worst case I could read the digits by logic state of the individual LED…sounds painful though - especially if I need to use optocouplers… :o

If it’s possible to add your own temperature probe you could skip hacking the display :slight_smile:

Unfortunately this is not possible :frowning:

Sent from my GT-I9505 using Tapatalk

It has begun… ;D
Just got a spare control panel to hack apart and see what needs to be done.
Also checked the original control panel voltage = 12V. So I will try and use that to power the arduino receiver first and see how it goes (finger crossed).

Step1 (hope I find time to buy some reed relay’s tomorrow):

  • On/off switch controllable by vera
  • On/off status feedback to Vera by tapping from the power LED of the panel

Step 2 will be to get temp up/down buttons implemented and a Vera Device customized so that those can be used.

Fingers crossed… ??? ???

The On/Off button as well as the Temp Up and Down buttons are now hooked up via reed relays to the arduino board.
This was the easy part.

I hit a bit of a roadblock with the status LED of the heater panel which I wanted to use to feedback to vera whether the heater is on or off.

The issue is as follows:

  • The only wire that connects the control panel to the heater is a 2 wire 12V “data bus”. When I connect 12V (external power supply) only to the panel an error message blinks.
  • The heater panel is sealed so in order to access the circuit board I’m cutting holes in the back at the approximate location of the push buttons and LED. This makes measuring and tracing of the circuit very very difficult.
  • The Arduino Nano needs to be powered by 5v as else the 3.3v will be disabled so I’m unable to connect it directly to the 12V data bus which powers the panel.
  • As a result I believe I need to use a reed relay which is switched by the panel in order to update the arduino board with the on/off status of the panel.

I think I found the LED at the back of the board. When off I measure around 5V from LED to negative data bus wire. When on, the voltage drops to around 1.5v - I assume this means that ground is switched through once the heater is on.
If I connect a reed relay across the LED to the negative data-bus wire, the LED will be dim when the heater is off and light up brighter once the heat is on. Measuring on the read relay shows that it is closed when the heater is off = 5v across but when the heater is on it alternates very fast between closed and open.

As access to the board is an issue (only option would be to dremel the complete back open), I’m bit at a loss of how I can get the present on/off status over to the Arduino.

Ideas and suggestions are very welcome 8) Thanks

I would start with trying to understand how the controller updates the LED display with the set point temperature. It seems to me a lot of what you want is tied to set point.

I’s start with the manufacturer and tell them that you are trying to create an auxiliary display. They may share with you the specifications on how that display gets updated. On the other hand, they may tell you that doing so will void your warrantee and not be helpful at all!

Is there a connection for an auxiliary display already?

I do not see any interface for an aux panel.
Also I’m trying to just read the on off state of the panel. Hence the idea to use the power state LED to update vera on on/off state by attaching a relay to the led in the panel and use it to switch the high/low state on the arduino

You may not need to attach a relay for that. The Arduino has the ability to create an interrupt on either of two pins. If you measure the voltage across the LED, it may be enough for your Arduino to pick up that change of state. I’d look for a voltage coming out of the controller before the current limiting resistor for the LED. If it is 5V, you are very lucky.

More on Interrupts. with Arduino.

The relay may need more current than the LED driver can supply. You may be better-off using a high-sensitivity optically-coupled isolator (kind of like a solid-state relay). This would give you a pull-down on its output when the LED is on so is easy to connect to an Arduino. If the LED is being pulsed (very likely) you will need to deal with that in your Arduino code.

I hadn’t thought of that option.

Also, there is a base sketch for a nicely sensitive light sensor that on Hek’s site. The hardware is highlighted there too, @idefix. That may suit your requirements…

the sketch is all ready. If you get that responding to the led switching On/Off you have come a long way.

This could be a good project. I have been gathering more ideas on water heater. Right now, I am planning to set up a more improvised water heater in my home.

Just curious if you ever got this working. I have a takagi water heater that has an external panel with LED display as well as on/off push button and temperature lower and raise buttons. I wanted to connect an arduino so I could control power on/off and raise and lower the temperature remotely.

if you were successful i’d like to know which relays you used and how you were able to read the state of the LED. In the takagi case it provides info on the current water temperature and also any error codes.

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