Oleg Tarasov built a local smart thermostat for an OpenTherm-compatible gas boiler using an ESP32, an OpenTherm interface, ESPHome, Home Assistant, DS18B20 temperature sensors, and PID control. Instead of simply switching the boiler on and off, the system can request a central-heating flow-temperature setpoint and expose boiler information such as modulation, flame status, temperatures, and faults.
Important current update: Tarasov’s original 2024 configuration relied on an external ESPHome component. New installations should start with ESPHome’s native OpenTherm component, which is now documented and integrated into ESPHome.
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What problem did the project solve?
Basic boiler thermostats generally provide on/off demand. They may start the boiler when a room is cold, but they do not necessarily provide local visibility into the boiler’s flow temperature, modulation level, flame state, or diagnostic information. Vendor smart thermostats can add remote access, but may introduce cloud dependency and proprietary hardware.
Tarasov’s goal was a more open, locally controlled system for critical home infrastructure. His thermostat connected the boiler to Home Assistant through an ESP32 running ESPHome, while keeping the core temperature-control logic close to the heating equipment.
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The project was built around a Buderus Logamax U072, according to the project coverage and original material. That model should be treated as the example installation—not proof that every boiler or heating system will behave identically.
Read the Hackster project coverage.
OpenTherm versus an on/off thermostat
OpenTherm is a low-voltage communications standard used between a compatible boiler and a controller. A simple thermostat usually provides a demand contact. An OpenTherm controller can exchange structured commands and status data with the boiler, including a requested heating-water temperature and, where supported, operating information.
| Capability | On/off thermostat | OpenTherm controller |
|---|---|---|
| Heat demand | Yes | Yes |
| Boiler flow-temperature setpoint | Usually no | Yes |
| Modulation telemetry | No | Often, boiler-dependent |
| Flame status | No | Often, boiler-dependent |
| Fault and diagnostic data | Limited | Varies by boiler |
| Home Assistant integration | Requires additional hardware or integration | Directly achievable with ESPHome |
Compatibility is not all-or-nothing. A boiler may support OpenTherm but expose only some of the protocol’s possible data items. Pressure, domestic-hot-water temperature, modulation, fault details, and other registers depend on the boiler’s implementation. Consult the service documentation before designing the dashboard.
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System architecture
The main signal path is:
Room sensor → ESP32 running ESPHome → OpenTherm adapter → boiler
Home Assistant communicates with the ESP32 over the ESPHome API:
ESP32/ESPHome ↔ Wi-Fi and API ↔ Home Assistant
The ESP32 can read a local room sensor, calculate a heating demand, send the appropriate OpenTherm request, and publish controls and telemetry to Home Assistant. The boiler remains responsible for its own burner, pump, flame-supervision, pressure, and overheat safety systems.
ESPHome’s current OpenTherm implementation acts as an OpenTherm master. It is not a passive gateway. An existing thermostat generally cannot simply remain connected in parallel. If preserving another thermostat or monitoring an existing controller is essential, use a separate gateway architecture designed for that purpose.
Hardware used in Tarasov’s build
- An ESP32 development board, identified in the configuration as
mhetesp32minikit. - A custom carrier or interface board.
- An electrical OpenTherm adapter.
- DS18B20 temperature sensors.
- A suitable power supply, enclosure, and wiring.
- Home Assistant with ESPHome.
- An OpenTherm-compatible boiler and heating installation.
Tarasov also made several screw-in temperature probes by adapting inexpensive gauge-thermometer housings and installing DS18B20 sensors inside them. That is a project-specific fabrication technique, not a requirement. Preassembled waterproof DS18B20 probes are simpler for many installations.
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The heating system included wall-mounted radiators, heated-floor loops, an indirect domestic-hot-water tank, and separate heating and hot-water branches. The boiler handled much of the low-level burner logic and gave hot-water heating priority. Other installations may have direct DHW, no DHW circuit, multiple zones, weather compensation, or proprietary extensions.
Choosing the OpenTherm interface
Do not connect the boiler bus directly to ESP32 GPIO pins. The boiler interface requires a suitable electrical bridge or isolated adapter. ESPHome lists several compatible options, including the DIYLESS Master OpenTherm Shield, Ihor Melnyk’s OpenTherm Adapter, and the Jiří Praus OpenTherm Gateway Arduino Shield.
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Check the adapter’s voltage requirements, isolation, power supply, signal direction, and wiring before connecting it to a boiler. Ihor Melnyk’s documentation describes compatibility with 3.3 V and 5 V controllers and provides the relevant controller-to-adapter wiring relationship.
Original project configuration
Tarasov’s original base configuration looked like this:
esphome:
name: boilotron
friendly_name: Boilotron
esp32:
board: mhetesp32minikit
framework:
type: arduino
logger:
level: INFO
logs:
component: ERROR
sensor: WARN
opentherm.output: INFO
api:
encryption:
key: "<your key>"
ota:
password: "<your password>"
wifi:
ssid: !secret wifi_ssid
password: !secret wifi_passwordThe original configuration also included:
external_components:
- source: github://olegtarasov/esphome-openthermThis declaration explains how the 2024 build obtained OpenTherm support at the time. It should not be copied automatically into a new installation. ESPHome now documents a built-in component, and the older arthurrump/esphome-opentherm repository is archived and points readers toward the improved native implementation.
Current ESPHome starting point
A current configuration should begin with the native component:
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in_pin: GPIOXX
out_pin: GPIOYYReplace the placeholder GPIOs with pins appropriate to the chosen board and adapter. The labels can be confusing: they are defined from the microcontroller’s perspective. In many adapter designs, the adapter’s out connects to the ESPHome hub’s in_pin, while the adapter’s in connects to the hub’s out_pin. Verify this against the adapter documentation rather than relying on the names alone.
Current implementation details worth checking during setup include:
sync_modedefaults tofalse.sync_mode: truemay help when frames are intermittently invalid, particularly with Dallas/1-Wire sensors.- Some boilers need initialization parameters.
- Multiple OpenTherm buses require explicit IDs.
- Unsupported boiler registers may remain unavailable even when the YAML is valid.
Controls and telemetry
Tarasov’s system exposed central-heating and hot-water controls in Home Assistant. The original configuration included:
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- Central-heating setpoint,
t_set. - Domestic-hot-water target,
t_dhw_set. - Central-heating enable,
ch_enable. - Domestic-hot-water enable,
dhw_enable. - Relative modulation level,
rel_mod_level. - Boiler feed temperature,
t_boiler. - Central-heating active, DHW active, flame-on, fault, and diagnostic binary sensors.
The exact values available depend on the boiler. Treat missing telemetry as a compatibility limitation until the boiler documentation and logs show otherwise.
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Tarasov used these project-specific setpoint ranges:
output:
- platform: opentherm
t_set:
id: boilotron_ch_setpoint
min_value: 40
max_value: 60
zero_means_zero: true
number:
- platform: opentherm
t_dhw_set:
id: boilotron_dhw_setpoint
name: "Hot Water target temperature"
min_value: 20
max_value: 60
restore_value: true
initial_value: 60The 40–60 °C central-heating range was chosen for Tarasov’s boiler and radiator system. It is not a universal setting. Underfloor heating may require substantially lower water temperatures and separate mixing, manifold, pump, and high-limit controls.
How the PID controller works
Instead of switching the boiler between no demand and one fixed flow-temperature request, the PID climate controller adjusts the requested heating-water setpoint in response to room temperature.
- When the room is below target, the controller raises the requested boiler temperature.
- As the room approaches target, it reduces the request.
- Near the target, the deadband and output limits reduce unnecessary demand.
- When demand is removed, the configuration should explicitly send an appropriate zero or disabled state.
Tarasov’s example used:
climate:
- platform: pid
id: boilotron_boiler_pid
name: "Central heating"
heat_output: boilotron_ch_setpoint
default_target_temperature: 23
sensor: boilotron_temp_room
visual:
min_temperature: 15
max_temperature: 30
temperature_step:
target_temperature: 0.5
current_temperature: 0.1
control_parameters:
kp: 0.76394
ki: 0.00050
kd: 0
output_averaging_samples: 10
deadband_parameters:
threshold_high: 0.5
threshold_low: -0.5
kp_multiplier: 0
ki_multiplier: 0.15
kd_multiplier: 0.0
deadband_output_averaging_samples: 15Those coefficients were tuned empirically for his installation. They are a starting point, not manufacturer-approved values. Tarasov disabled derivative control because small room-temperature changes made the derivative term react too strongly and reduce boiler demand prematurely. Another building, sensor, or heating circuit may need different settings.
Sensor filtering and OpenTherm timing
The project updated temperature sensors every 30 seconds, calculated a sliding-window moving average across five readings, and used a heartbeat filter to provide the last known value to the PID controller once per second.
This separates measurement smoothing from control-loop responsiveness: the sensor value is not allowed to jump around with every reading, while the PID component can still receive a regular input.
There is a practical trade-off. Dallas/1-Wire activity can interfere with OpenTherm timing on some installations. If communication becomes unreliable:
- Temporarily disconnect the 1-Wire sensors.
- Increase sensor update intervals.
- Enable
sync_mode: true. - Reduce unnecessary logging and polling.
- Reintroduce sensors one at a time.
Build procedure
1. Confirm the boiler protocol
Find the boiler’s installation or service manual and verify that the terminals are genuine OpenTherm terminals. A two-wire connection is not automatically OpenTherm; some boilers use proprietary buses or simple dry contacts.
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Confirm whether the boiler supports master control, which DHW functions are available, whether the existing thermostat must be removed, and which registers the boiler implements.
2. Choose and inspect the adapter
Use a purpose-built OpenTherm adapter or a correctly designed isolated interface. Confirm the ESP32’s 3.3 V logic compatibility, power requirements, input/output direction, and any interrupt or GPIO requirements.
3. Establish a minimal ESPHome device
Install ESPHome using the current official procedure for your Home Assistant installation type. Tarasov used the Home Assistant add-on route, but menu labels and availability differ between Home Assistant OS and other installation methods.
First flash only Wi-Fi, logging, encrypted API access, and OTA. Confirm that the ESP32 builds, boots, joins the network, appears in Home Assistant, and can be updated before adding OpenTherm.
4. Add OpenTherm
Add the native opentherm: hub with verified GPIO mapping. On startup, the logs should show communication rather than repeated invalid-frame or no-response errors.
5. Add supported boiler entities
Add only controls and sensors that the boiler is known to support. A valid configuration does not guarantee that every OpenTherm value will be returned.
6. Add a local room sensor
A room sensor connected to the ESP32 is preferable when basic heating must continue during a Wi-Fi or Home Assistant outage. Place it away from radiators, direct sunlight, doors, and other local heat sources.
7. Add filtering and PID gradually
Begin with manual setpoint control. Once communication, room sensing, and enable states work independently, add smoothing and then PID. Tune for the room’s thermal mass, radiator size, boiler minimum modulation, sensor position, flow-temperature limits, TRV behavior, and DHW priority.
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Deliberately test Wi-Fi loss, Home Assistant shutdown, ESP32 reboot, OTA failure, sensor disconnection, OpenTherm cable disconnection, boiler faults, power restoration, and restored switch states. The system should fail predictably, and invalid temperature readings must not become uncontrolled heating demand.
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Compatibility and safety checklist
- Verify OpenTherm support in the boiler documentation.
- Do not assume all OpenTherm registers are implemented.
- Use a suitable electrical bridge; never connect the boiler bus directly to GPIO.
- Confirm whether the original thermostat must be disconnected.
- Use a local temperature sensor if local fallback matters.
- Define behavior for invalid, missing, or stale sensor data.
- Use conservative initial temperature limits.
- Do not treat radiator and underfloor-heating circuits as interchangeable.
- Preserve mixing valves, manifold controls, pumps, high-limit protection, and boiler safety systems.
- Do not bypass flame supervision, pressure controls, overheat protection, or certified service procedures.
- Follow local rules and use a qualified heating professional where required.
Troubleshooting
No OpenTherm response
Check adapter input/output direction, GPIO numbering, power, grounds, boiler terminals, and whether the boiler actually uses OpenTherm. Disconnect any existing thermostat if the installation requires a single master. Test with the smallest possible ESPHome configuration before adding sensors and automation. Power down before rewiring.
Intermittent invalid frames
Suspect 1-Wire timing conflicts, electrical noise, marginal power, incorrect pins, or excessive polling. Disconnect Dallas sensors temporarily, increase update intervals, reduce logging, verify wiring, and try sync_mode: true.
The boiler heats continuously
Check whether a nonzero t_set is being sent continuously, whether a heating-enable state was restored after reboot, whether PID output can reach zero, and whether the deadband is configured correctly. Verify zero_means_zero behavior and test manual control before re-enabling PID.
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Possible causes include excessive PID gains, poor sensor placement, a slow thermal response, an overly broad water-temperature range, noisy readings, or TRVs closing unexpectedly. Start by reducing proportional and integral gains, keeping derivative control disabled, increasing averaging, and retuning after the system reaches steady state.
Heating stops when Home Assistant is offline
Move essential room sensing and fallback control into the ESPHome device rather than depending entirely on Home Assistant automations. The precise level of offline operation depends on how the configuration is written; ESPHome connectivity alone does not automatically reproduce every Home Assistant automation locally.
Build, buy, or use a gateway?
| Approach | Best for | Main trade-off |
|---|---|---|
| DIY ESPHome controller | Home Assistant users wanting local telemetry and customization | Requires wiring, tuning, maintenance, and recovery planning |
| OpenTherm adapter plus ESP32 | Low-cost builders who already have development hardware | Not a complete thermostat and may require soldering |
| OpenTherm gateway | Installations that need a separate gateway architecture or existing-controller coexistence | Different firmware and integration model; capabilities vary |
| Prebuilt OpenTherm thermostat | Users wanting less fabrication | Less control over firmware and fallback behavior |
| Manufacturer or installer-supplied control | Warranty, certification, support, or rental-property priorities | May be less open or locally customizable |
Commercial hardware can reduce fabrication without removing compatibility work. The DIYLESS Master OpenTherm Shield is a low-cost bridge for an existing ESP8266 or ESP32. Ihor Melnyk’s adapter provides published wiring and schematic information. The Jiří Praus shield is another option listed by ESPHome, particularly for gateway-oriented designs. Product stock, pricing, and compatibility change, so check the manufacturer or vendor page before ordering.
DIYLESS also lists prebuilt OpenTherm thermostats, but a prebuilt product will not necessarily reproduce Tarasov’s exact ESPHome, Home Assistant, PID, and local-fallback architecture.
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Why this project remains useful
Tarasov’s build demonstrates what OpenTherm adds to a local smart-home system: not just a remote switch, but a communications path into the boiler’s supported controls and status data. It also illustrates the parts that are harder than the wiring diagram suggests—boiler-specific compatibility, sensor placement, timing conflicts, PID tuning, fallback behavior, and safe installation.
For a new build in 2026, use the native ESPHome OpenTherm component, verify the boiler and adapter independently, keep essential control logic local, and treat the original PID values and temperature limits as installation-specific examples. The result can be a powerful local thermostat, but it is not a universal drop-in replacement or a substitute for the boiler’s certified safety systems.
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