[Guide]Millheat hardware modification, making it open source

I will try to make a guide on how to modify a millheat heater so you can have full local control off the device. So no need to rely on Mill’s servers or API.

I have a Mill Glass Mill Glass 600W — Mill
but I think this could work on most models, since the wifi device basically is only turning on and off a relay. (Open up your device, look for labels and do some measuring)


NOTE I: If you do it like I did, there is nothing that breaks, and you can revert the modification if the heater malfunctions and you need to send it in for warranty. But still, do this on your own risk, I can’t stop you from doing mistakes that can cause a fire or hurt yourself.

NOTE II: The button panel does not work when doing this modification. I have not put to much time trying to implement this since I use Home Assistant to control the heater. (And I don’t want my wife to turn up the heat to much :slight_smile: )

NOTE III: I have tried to reverse engineer the wifi controller and the button/display controller, but I couldn’t figure out anything there. Atleast not with the posibility to revert to stock.

NOTE IV: Let me know if something is unclear or if you have suggestions on improvement.

***Note V
***: @Blern_Jalkeby Has tested this succesfully on a Mill Steel Wifi heater, he only needed to move the relay to GPIO3 and the DHT to GPIO0.

Tools and equipment required

1 esp8266 (I am using an ESP-01)
1 DHT22 (You could use the built in temperature sensor, more on that later)
Wire wrapping tool and Wire wrapping wire (If you don’t want to solder)
Small bolt with nut, or just some hot glue.

Figuring out what does what

This is a picture of the heater I have.

I opened it up and checked how it looked inside.
Here you see a overview of the inside. The top is the controller box, then two black wires goes down to the analog temperature sensor.

I opened the box up and this is how it looks inside.

There you have a relay[1], wifi controller[2], pinouts[3], button/display board[4], temp sensor connection [5] and antenna [6].
Beside the pinout there is printed some labels on the board, TX,RX,PT?,AD,GND and PWR. (Not easy to see)
By measuring these while the device was in operation I figured out that it gave 3.3VDC and the PT? is for signaling the relay.

Was easier to measure on the backside of the board.

Removing things

Since the ESP8266 I am using doesn’t have the ADC broken out, I chose to change the temperature sensor.

I also removed the antenna for the wifi controller since this won’t be used anymore. (I also reset the device so there is no chance it can connect to internet)
Then I unplugged the pinout ribbon and put it aside.

The wifi controller is soldered so I did not bother removing it.

The code

I am using ESPHOME to control my ESP8266 and integrate them with Home Assistant. You can use what you want. The code is not that special.
Be carefull when copy paste, YAML is extremely picky with indentations.
Also, if you have any questions about the code, please read up a bit in the docs before asking. https://esphome.io/

Config without any climate setup, the climate setup has to be done in HomeAssistant. This code handles errors with high temperature, faulty temperature sensor and loss of connection to HomeAssistant API.

Remember to set the temperatures to a default value you want. (One in globals also)

# Varmeovn kjøkken
  devicename: esp01_mill01

  - id: max_temp
    type: float
    restore_value: no
    #Have this a couple of degrees higher than the max temp on the thermostat
    initial_value: '26.5'
  #Used to avoid spamming
  - id: notified
    type: bool
    restore_value: no
    initial_value: 'false'

  name: $devicename
  platform: ESP8266
  board: esp01_1m

  ssid: !secret ssid
  password: !secret wpa

# Enable logging

# Enable Home Assistant API
  password: !secret api_password
  id: api_server

  password: !secret ota_password

  - platform: gpio
    restore_mode: RESTORE_DEFAULT_OFF
    pin: GPIO0
    id: relay1
    name: $devicename " Relay"
    #Reset the notification global, so it can be sent again
      - globals.set:
          id: notified
          value: 'false'

  - platform: dht
    pin: GPIO3
    model: AM2302
      id: temp1
      name: $devicename " Temperature"
        # Every time the temperature changes, check if there is something wrong
         - if:
                # Is the temperature too high?
                - lambda: 'return id(temp1).state > id(max_temp);'
                # Is NAN?
                - lambda: 'return isnan(id(temp1).state);'
                #If not connected to the API, turn off the relay, notifications
                # won't be sent since the API is not connected.
                - not:
              #Turn of the relay
              - switch.turn_off: relay1
              #Turn of the climate entity
              - homeassistant.service:
                  service: climate.turn_off
                    entity_id: climate.utleie_kjokken
              #Show a log in the esphome console
              - logger.log: Something wrong with temperature
              #Notify only one time as long as the relay hasn't been turned on
              # again
              - if:
                    #If notification hasn't been sent, to avoid spamming
                      - lambda: 'return id(notified);'
                    #Set global notified to true
                    - globals.set:
                        id: notified
                        value: 'true'
                    #Notify someone
                    - homeassistant.service:
                        service: notify.mobile_app_john_arvid_s_iphone_3gs
                         message: 'Something wrong with ${devicename}'
      name: $devicename " Humidity"
    update_interval: 10s

This config is for the generic thermostat in HA, ref: https://www.home-assistant.io/components/generic_thermostat/

- platform: generic_thermostat
  name: "Utleie kjøkken"
  heater: switch.esp01_mill01_relay
  target_sensor: sensor.esp01_mill01_temperature
  max_temp: 24
  min_temp: 3
  precision: 0.5
    minutes: 1
Wiring things

Don’t let the wire colors confuse you, I am just being cheap so I use what I have of the best lenght.

So on the ESP-01 I connected like this:
3V3 to PWR on pinout Orange
GND to GND on pinout Yellow
IO0 to PT? on pinout Purple
3V3 to VCC on DHT22 Red
GND to GND on DHT22 Black
RX to Data on DHT22 Green

Remember to test if you chose some other pins on the ESP8266, if you use the wrong pin the ESP8266 won’t boot.
Also, depending on the ESP-01 you might need to connect EN and 3V3 pin to boot normally.

The wires to the DHT22 goes the same way as the old wires, so I don’t think there would be a problem with heat.
As soon as I figure out a proper way I will put some safeguards in the code in case the temperature is to high or the readings are NAN.

I used a bolt and a nut to keep the DHT22 in place.

Then just put everything back together and you now have a device you can fully control locally with Home Assistant or some other automation tool.


Nice guide, but I think something is lost in translation. The device you pointed to is not an oven - an oven is where you cook food. This is a heater :slight_smile:

Thank you :slight_smile:
Yes got a bit lost in translation. Should be corrected now !
Maybe I should do the same on a oven next time.

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Thanks for sharing. Will look into that. Ideally I want the thermostat logic handled by the ESP8266 in case HA goes down.
How much current does it need to activate the relay?

Happy to share. Hope it will be usefull.
Yes, that is the only thing missing I think.
Not 100% sure but a datasheet I found said 200mW. The Esp-01 handles it good.
But there is some extra circuitry that handles the relay, since the relay is 12vdc and the esp gives a 3.3vdc signal.

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After going through the docs for esphomelib, this look exactly like the generic thermostat in HA

Yes, with that config you don’t have to do much in HA if you want exactly that one.
Wonder if that handles the failsafe we are talking about. I will try to test during the next week.

I have tested now and it is a step further. But it is not handling a NAN reading. I will post an issue and see if it can be solved.

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Ok, I have now updated the code with safeguards.
It will now react to no temperature reading and it will also stop when the temperature is higher than the global variable max_temp.
It will work if the heater is turned on and wireless goes down while the heater is on.

I have choosen not to handle a faulty reading, for example if the sensor says it is 0degrees celsius when it is 15. This is because I have never heard of such faults with the sensor I use. (Or any other for that matter)

I also have an idea to make a notification in HA when there are some problems. But I will do that at a later time if I feel like it.

Let me know if anyone has some feedback.

Excellent guide! I’m trying to replicate this with my Mill Steel Wifi heater and having some problems. The ESP-01 won’t boot when the GPIO0 is connected to the heaters PTC pin. It does boot when it’s disconnected and the relay works as planned when I connect the GPIO0-PTC after the boot.

Any ideas what might be the problem?

Glad that you liked the guide, thank you.

It seems like the problem is that on the Mill Steel Wifi heater the PTC is held to ground when not active. So when the GPIO0 is held low on ESP01 when booting it will go into flash mode.

You can try this:
-Move the PTC to GPIO2 (This should also cause a boot failure, but test to be sure). PS, this will probably cause the relay to kick in, either just a moment or until you power off.
-If GPIO2 causes boot failure, connect PTC to GPIO3. Then try the DHT11 on GPIO0. PS, this will probably cause the relay to kick in a short moment during boot.
-If neither works and you absolutely want to use a ESP01 you need to solder on a pin directly on the esp8266 chip. (You need to be very steady on your hand to do this and you need a fine tip solder iron). But I would recommend getting a ESP-12 since it has more pins broken out.


Moving the DHT to IO0 and PTC to IO3 worked! Aweome, might consider buying some more Mills for the country cottage now!

Great! Glad you got it working. Will make another note about this in the guide.
Yeah, when I managed to modify the heater this easy I am also thinking about buying more.

Btw, if you are using OTA, could you test if that is working?

Hi john-arvid. Have you opened up a Mill non WIFI heater?
I suppose that adding an ESP with temp sensor is doable as those heaters also have an electronic thermostat.


Hi @Lucky70,

No I don’t have a non WIFI heater from Mill, but as you say, it should be doable.
The first thing that comes to mind is that it probably don’t have a DC circuit so you would have to add a power supply to it.
I would rather suggest using a smart plug from a vendor that uses the ESP8266 chip. Check this site for ideas: https://blakadder.github.io/templates/

Then you don’t have to bother with problems that might occur when implementing something totally different.

Hi @john-arvid ,
Im going to modify my mill heater with a faulty WIFI unit.
Afther some serching i found this thread and a thread in a norwagian forum.
My suggestions is that you check it out.
Short summary of what Jan-Tore in the norwegian thread did:
He remove the stock mill wifi controller and replace it with an EPS01. The EPS01 connects to the button/display board on serial.
Leaving the display / buttons wokring and removing the need for the DHT22.

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Hi @JDolven,
Thank you for bringing that to my attention. I will try to implement it in my code if I get my head around the code Jan-Tore wrote, since it looked like the code was only partial.
It looks exactly like what I need to make this implementation better.


To send commands to the button/display it’s possible to use the the function calls on line 5, 6 and 8. They work but reading the response don’t work. It is possible I connected the ESP01 wrong. I tried to replicate the connecting form the picture in the thread. Not sure how to convert form Arduino to ESPhome :stuck_out_tongue:

I was succesfull using parts of the code and a wemos D1 mini to replace the HF-LPT120A. Talking to home assistant using MQTT. I need to look more into ESPhome to make it a custom component. I can post the code to git or pastebin if your intrested.

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Great work!
Yes I would like to see the code.
I am currently not in a position to work and test any code (maybe in a month or so), but I can look and make some plans.

If you haven’t seen this already, this can help you start: https://esphome.io/custom/uart.html