A simple boiler or electric heater can behave like a full smart thermostat using:

• An ESP32
• A temperature sensor (DS18B20 or SHT45)
• A relay for on/off control
• Home Assistant for schedules, modes and UI

This guide shows how to build a room or boiler thermostat with:

• ESPHome “climate” entity
• Hysteresis (to avoid rapid on/off cycling)
• Manual & automatic modes
• Time-based schedules in Home Assistant

Both DS18B20 (wired, accurate, great for pipes/tanks) and SHT45 (room temp/humidity) are covered.

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1. Hardware Overview

Typical setup:

• ESP32 DevKit
• One of:
  • DS18B20 (waterproof probe for tank / pipe / floor)
  • SHT45 breakout (room air temperature + humidity)
• Relay module (dry contact or contactor coil like in boiler article)
• 4.7 kΩ resistor (for DS18B20 data pull-up)
• 10 kΩ resistor (optional if using SHT45 breakout without onboard pull-ups)
• USB power supply
• DIN enclosure or wall box if installed permanently

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2. Wiring

2.1 DS18B20 + Relay (Boiler or Electric Heater)

Temperature probe (1-Wire bus):

ESP32        DS18B20
3.3V   ----> VDD
GND    ----> GND
GPIO4  ----> DQ (data)

3.3V ---- 4.7kΩ ---- GPIO4

Relay control (example: GPIO26):

ESP32       Relay
3.3V  ----> VCC
GND   ----> GND
GPIO26 ----> IN

Relay COM/NO → boiler thermostat input or contactor coil circuit

2.2 SHT45 + Relay (Room Thermostat)

SHT45 on I²C:

ESP32        SHT45
3.3V   ----> VCC
GND    ----> GND
GPIO21 ----> SDA
GPIO22 ----> SCL

Relay wiring is identical (GPIO26 → IN).

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METHOD 1 – ESPHome Climate-Based Thermostat

ESPHome can expose a full climate entity (thermostat) to Home Assistant.
The ESP32:

• Reads the sensor
• Applies hysteresis / min run time
• Decides when to turn the relay on/off

Home Assistant only sees a thermostat card with setpoint and mode.

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3. ESPHome Smart Thermostat (DS18B20 Example)

3.1 Base ESPHome YAML

esphome:
  name: esp32-smart-thermostat
  platform: ESP32
  board: esp32dev

wifi:
  ssid: "YOUR_WIFI"
  password: "YOUR_PASSWORD"

logger:
api:
ota:

3.2 DS18B20 Temperature Sensor

dallas:
  - pin: 4

sensor:
  - platform: dallas
    address: 0x1234567890ABCDEF
    name: "Boiler Temperature"
    id: boiler_temp
    filters:
      - median:
          window_size: 7
          send_every: 3
          send_first_at: 1
    update_interval: 10s

(Use dallas.read_sensors or omit address to auto-detect; you can let ESPHome print the address in logs.)

3.3 Relay Switch

switch:
  - platform: gpio
    id: boiler_relay
    pin: 26
    restore_mode: ALWAYS_OFF

3.4 Climate “Bang-Bang” Thermostat

climate:
  - platform: thermostat
    name: "Boiler Thermostat"
    sensor: boiler_temp
    default_target_temperature_low: 45 °C
    default_target_temperature_high: 50 °C

    # Single-stage heating control
    heat_action:
      - switch.turn_on: boiler_relay
    idle_action:
      - switch.turn_off: boiler_relay

    min_heating_off_time: 300s   # avoid rapid cycling
    min_heating_run_time: 300s
    min_idle_time: 120s

    visual:
      min_temperature: 30 °C
      max_temperature: 70 °C
      temperature_step: 1 °C

    # Optional: supported modes
    supported_modes:
      - HEAT
      - OFF

What this does:

• Exposes climate.boiler_thermostat in Home Assistant
• Uses hysteresis between target_temperature_low and target_temperature_high
• Enforces minimum run/idle times to protect the heater

You can later adjust the target range directly from Home Assistant.

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4. ESPHome Room Thermostat (SHT45)

For room air control (radiators, electric panel heater, underfloor):

4.1 SHT45 Sensor

i2c:
  sda: 21
  scl: 22
  scan: true

sensor:
  - platform: sht4x
    temperature:
      name: "Room Temperature"
      id: room_temp
    humidity:
      name: "Room Humidity"
    update_interval: 10s

4.2 Climate Config

switch:
  - platform: gpio
    id: room_heater_relay
    pin: 26
    restore_mode: ALWAYS_OFF

climate:
  - platform: thermostat
    name: "Room Thermostat"
    sensor: room_temp
    default_target_temperature_low: 20 °C
    default_target_temperature_high: 21 °C

    heat_action:
      - switch.turn_on: room_heater_relay
    idle_action:
      - switch.turn_off: room_heater_relay

    min_heating_off_time: 180s
    min_heating_run_time: 300s

    visual:
      min_temperature: 16 °C
      max_temperature: 26 °C
      temperature_step: 0.5 °C

    supported_modes:
      - HEAT
      - OFF

Now the room thermostat:

• Heats up to ~21 °C, cools down to ~20 °C (1 °C hysteresis)
• Shows as a standard thermostat in Home Assistant (supports setpoint sliders, modes, graphs, etc.)

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5. Home Assistant – Using the Climate Entity

Once flashed, ESPHome auto-registers the thermostat:

• climate.boiler_thermostat or climate.room_thermostat
• sensor.boiler_temperature or sensor.room_temperature

5.1 Thermostat Card in Dashboard

type: thermostat
entity: climate.room_thermostat

Or an entities card:

type: entities
entities:
  - climate.room_thermostat
  - sensor.room_temperature

From here you can:

• Change the setpoint
• Set mode to HEAT / OFF
• See history of room temperature and relay state

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6. Scheduling in Home Assistant

ESPHome handles hysteresis & relay control; Home Assistant handles schedules and modes.

6.1 Simple Time-Based Schedule (Comfort / Eco)

Example: daytime 21 °C, night-time 18 °C.

automation:
  - alias: "Room Thermostat – Daytime Comfort"
    trigger:
      - platform: time
        at: "07:00:00"
    action:
      - service: climate.set_temperature
        target:
          entity_id: climate.room_thermostat
        data:
          temperature: 21

  - alias: "Room Thermostat – Night Eco"
    trigger:
      - platform: time
        at: "23:00:00"
    action:
      - service: climate.set_temperature
        target:
          entity_id: climate.room_thermostat
        data:
          temperature: 18

ESPHome automatically uses that setpoint with its built-in hysteresis.

6.2 Presence-Based Control

Lower the setpoint when nobody is at home:

  - alias: "Room Thermostat – Away Mode"
    trigger:
      - platform: state
        entity_id: person.you
        to: "not_home"
    action:
      - service: climate.set_temperature
        target:
          entity_id: climate.room_thermostat
        data:
          temperature: 16

  - alias: "Room Thermostat – Back Home"
    trigger:
      - platform: state
        entity_id: person.you
        to: "home"
    action:
      - service: climate.set_temperature
        target:
          entity_id: climate.room_thermostat
        data:
          temperature: 21

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METHOD 2 – MQTT + Home Assistant “Generic Thermostat”

If you prefer to keep the ESP32 simple, you can:

• Publish temperature only from the ESP32
• Let Home Assistant implement the thermostat logic using generic_thermostat

In this mode:

• ESP32 is just:
  • One temperature sensor
  • One MQTT switch for the relay

Home Assistant handles hysteresis, modes, scheduling.

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7. ESP32 MQTT Entities

7.1 Temperature Sensor Topic

ESP32 publishes:

{"temperature": 21.8}

to e.g. home/room/thermostat.

Home Assistant:

mqtt:
  sensor:
    - name: "Room Thermostat Temperature"
      state_topic: "home/room/thermostat"
      value_template: "{{ value_json.temperature }}"
      unit_of_measurement: "°C"
      device_class: temperature

7.2 Relay as MQTT Switch

mqtt:
  switch:
    - name: "Room Heater Relay"
      command_topic: "home/room/heater/set"
      state_topic: "home/room/heater/state"
      payload_on: "ON"
      payload_off: "OFF"
      retain: true

ESP32 listens on .../set, toggles relay, and reports to .../state.

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8. Home Assistant Generic Thermostat

climate:
  - platform: generic_thermostat
    name: "Room Thermostat"
    heater: switch.room_heater_relay
    target_sensor: sensor.room_thermostat_temperature
    min_temp: 16
    max_temp: 26
    ac_mode: false
    cold_tolerance: 0.3     # hysteresis below setpoint
    heat_tolerance: 0.3     # hysteresis above setpoint
    min_cycle_duration:
      minutes: 5
    initial_hvac_mode: "heat"
    away_temp: 16

This:

• Exposes a standard climate.room_thermostat
• Uses built-in hysteresis defined by cold_tolerance / heat_tolerance
• Ensures min 5 minutes per cycle

All scheduling / presence automations from section 6 apply here as well.

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9. Optional Extras

• Humidity-based boost: With SHT45, use high humidity as a trigger to raise temperature slightly in bathrooms.
• Window sensor lockout: If a window is open, force the thermostat to OFF or reduce setpoint.
• Energy monitoring: Combine the thermostat with a PZEM or smart plug to estimate heating cost and efficiency.

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Summary

An ESP32, a single sensor (DS18B20 or SHT45) and a relay are enough to build a full-featured smart thermostat:

• ESPHome approach: the thermostat logic runs entirely on the ESP32, exposed as a climate entity.
• MQTT + generic_thermostat: ESP32 stays dumb; Home Assistant does all the logic.

Both support:

• Modes (heat/off)
• Hysteresis & minimum runtime to protect equipment
• Time & presence-based schedules via Home Assistant automations

This makes it easy to upgrade boilers, electric heaters or room panels into fully integrated, centrally managed heating zones.