Added device Mill panel heater generation 2 (#996)

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Author: owangen

src/docs/devices/Mill-Panelheater-gen2/desolder.jpg

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+---
+title: Mill panel heater (generation 2)
+date-published: 2025-01-20
+type: misc
+standard: global
+board: esp8266
+project-url: https://github.com/owangen/esphome/tree/dev/esphome/components/mill_panelheater_gen2
+made-for-esphome: false
+difficulty: 5
+---
+
+## General Notes
+
+Mill panel heaters are available in various versions. This component is specifically designed for **Generation 2 panel heaters** (not compatible with portable heater models). Have a look here to check which generation you have https://millheat.zendesk.com/hc/en-us/articles/360000252071-Which-generation-do-I-have.
+If you have a Generation 3 panel heater (which comes with a built-in ESP32), check out this component: https://github.com/ssieb/esphome_components/tree/mill/components/mill_heater
+
+To use this component with a Generation 2 panel heater, you’ll need to replace the HF-LPT120A module with an ESP8266.
+![Desolder](./desolder.jpg "Desolder the LPT120A chip")
+
+HF-LPT120A desoldered
+
+![Removed](./removed.jpg "Removed the LPT120A chip")
+
+Solder the ESP8266 in place.
+Refer to the **GPIO Pinout section** for detailed instructions
+
+![Solder](./soldered.jpg "Soldered 8266")
+
+## GPIO Pinout
+
+| PIN (mill board) | ESP8266    | Description              |
+|------------------|------------|--------------------------|
+| +3.3V            | VCC        | Power input (3.3V)       |
+| GND              | GND        | Ground                   |
+| WTX              | TX (GPIO1) | UART transmitter         |
+| WRX              | RX (GPIO3) | UART receiver            |
+
+## Configuration
+
+```yaml
+substitutions:
+  devicename: "millzero_heater"  #Lower case name
+  friendly_name: "Mill Zero heater"
+
+esphome:
+  name: "${devicename}"
+
+external_components:
+  source:
+    type: git
+    url: https://github.com/owangen/esphome
+  components: [ mill_panelheater_gen2 ]
+
+esp8266:
+  board: d1_mini
+
+wifi:
+  ssid: !secret wifi_ssid
+  password: !secret wifi_password
+  ap:
+
+captive_portal:
+
+# Enable logging
+logger:
+  baud_rate: 0 # Deactived to prevent conflict with UART, sending UART to mill controller does not work if enabled
+
+# Enable Home Assistant API
+api:
+
+ota:
+
+uart:
+  id: uart_bus
+  tx_pin: GPIO01
+  rx_pin: GPIO03
+  baud_rate: 9600
+
+climate:
+  - platform: mill_panelheater_gen2
+    name: Mill heater
+```
+
+## Credits
+
+This component is based on the work by https://github.com/JDolven/Replacing-HF_LPT120A-in-a-millheat-heater and https://github.com/trondsundt/MillHeat-ESPHome-NodeMCU