Auf Deutsch lesen: README.de.md
Documentation of how I made my old Junkers/Bosch gas boiler smart by replacing the Junkers TRQ 21 modulating room thermostat with a Shelly Plus Uni on the 1-2-4 interface. Four heating levels (Off / Low / Medium / High), room temperature via DS18B20, ready to integrate with Home Assistant, MQTT or the Shelly Cloud.
Compatible with older Junkers/Bosch boilers that use the 1-2-4 interface – e.g. ZWR, ZSBE, ZWN, ZWA series.
Disclaimer: This is my personal documentation, not a build-along tutorial. If you mess with your boiler, you should know what you're doing – recreate at your own risk.
Instead of the modulating control of the TRQ 21, the Shelly Plus Uni is used to implement four discrete heating levels – High, Medium, Low and Off. The circuit is deliberately wired so that if the Shelly fails (unpowered, broken) the boiler keeps heating at full power instead of going off: frost damage to heating and water pipes is much more expensive than temporary overheating, which is limited by the radiator thermostatic valves anyway.
The two switchable outputs of the Shelly are used in all four combinations – active pulldowns between terminal 2 (signal) and terminal 4 (GND) pull the voltage down step by step from the failsafe full-load level:
| Level | Output 1 | Output 2 | active pulldown between 2 ↔ 4 |
|---|---|---|---|
| High | off | off | none (terminal 2 floating) |
| Medium | on | off | 5.1 kΩ |
| Low | off | on | 4 kΩ (2 kΩ + 2 kΩ in series) |
| Off | on | on | 5.1 kΩ ∥ 4 kΩ ≈ 2.22 kΩ |
Each level corresponds to a fixed voltage at terminal 2 and therefore a fixed burner power – the matching voltage levels are documented further down under Voltages and measurements.
The TRQ 21 is a modulating room temperature controller for older Junkers/Bosch gas boilers. It measures the room temperature, compares it to the setpoint and tells the boiler via an analog control signal how much to heat.
The TRQ 21 is connected to the boiler via a three-wire Junkers interface. The name comes from the terminal numbers in the wiring diagram: terminals 1, 2 and 4 are used (terminal 3 exists on the strip but is not connected).
| Terminal | Meaning | Description |
|---|---|---|
| 1 | +24 V DC | supply voltage from the boiler to the controller |
| 2 | control signal | analog feedback voltage from the controller |
| 4 | GND / ground | common reference ground |
The boiler applies its 24 V supply voltage between terminal 1 and terminal 4 and measures what voltage the controller returns on terminal 2. This voltage directly controls burner modulation:
- below ~7 V: burner off
- up to ~15 V: burner power increases up to 100 % (higher voltage
produces no additional power)
Failsafe behavior: If no controller is connected – terminal 2 left open – the boiler interprets this as a full-load request and heats at 100 %. This circuit deliberately leverages that behavior as frost protection: if the Shelly fails, the boiler automatically falls back to full power.
Instead of a TRQ 21, a Shelly Plus Uni now sits at the 1-2-4 interface. The circuit uses no active electronics – the Shelly does not drive terminal 2 "analog", it only modifies the voltage divider formed by the boiler's internal pull-up and external pulldown resistors:
- No permanent pulldown: terminal 2 is left floating in the idle state. The boiler's internal pull-up pulls it to +21 V – so the boiler heats at full power on any fault (frost-protection failsafe).
- Pulldowns via the Shelly: the two switchable outputs each connect a resistor between terminal 2 (signal) and terminal 4 (GND). When the outputs are not switched on – or the Shelly is unpowered – the pulldowns are automatically disconnected. The smaller the effective pulldown, the harder terminal 2 is pulled toward GND – and the lower the burner power.
Voltage at terminal 2 for the four heating levels, measured on my boiler:
| Pulldown 2 ↔ 4 | Voltage at terminal 2 | Level | Burner behaviour |
|---|---|---|---|
| none (floating) | 21.4 V | High (failsafe) | full power |
| 5.1 kΩ | 10.5 V | Medium | on, modulated |
| 4 kΩ (2 kΩ + 2 kΩ) | 9.3 V | Low | on, ignites cold |
| 2.22 kΩ (5.1 kΩ ∥ 4 kΩ) | 6.4 V | Off | shuts off |
The values were measured on my boiler – they may vary slightly between models.
Modulating burners typically have a hysteresis – the voltage at which a cold burner ignites is higher than the voltage at which a running burner shuts off. For my boiler the measurements yield:
- Cut-off threshold: ~6 V → running burner shuts off
- Hysteresis zone: ~6 – 9 V → burner stays on if already running, but does not restart from the off state
- Re-ignition threshold: ~9 V → cold burner ignites reliably
This drives the choice of R_b: R_b is deliberately picked large enough (4 kΩ) so that the Low voltage (9.3 V) sits above the re-ignition threshold – the burner therefore ignites reliably from the off state even when Low is selected. A smaller R_b such as 3 kΩ would push Low down to 7.8 V, right into the hysteresis zone: the burner would stay on if already running but would not restart by itself from the off state.
Full measurement series:
| Pulldown 2 ↔ 4 | Voltage at terminal 2 | Burner behaviour |
|---|---|---|
| none (open) | 21.4 V | on, full power |
| 10 kΩ | 14 V | on (modulation saturation zone) |
| 5.1 kΩ | 10.5 V | on, modulated |
| 4 kΩ | 9.3 V | ignites cold reliably |
| 3 kΩ | 7.8 V | stays on, won't ignite cold |
| 2.22 kΩ (5.1 ∥ 4) | 6.4 V | shuts off (from running) |
| 2 kΩ | 5.9 V | shuts off (from running) |
| 1.88 kΩ (5.1 ∥ 3) | 5.7 V | shuts off (from running) |
| 1 kΩ | 3.5 V | shuts off (from running) |
In addition to the two switching outputs, the Shelly Plus Uni has an analog voltage input (0–30 V DC). This allows logging the voltage between terminal 2 and terminal 4 directly – i.e. exactly the signal the boiler modulates with. This makes it possible to verify at any time which level the boiler is actually in, and to monitor both the boiler and the circuit remotely.
With the TRQ 21 gone, its built-in room temperature measurement is also
gone. This job is taken over by a DS18B20 connected directly to the
Shelly.
The temperature reading can later be used in an automation to decide
which output should be switched.
The exact wiring is shown in the schematic.
The complete wiring is shown in images/wiring.png
(editable source: images/wiring.drawio):
At the top of the image are the four boiler terminals. Only 1, 2 and 4 are used; terminal 3 is left open.
- Terminal 1 (24 V) powers the Shelly via VAC1.
- Terminal 2 (signal) is pulled towards terminal 4 (GND) on demand through the pulldown resistors switched by the two Shelly outputs. The same line also feeds the Shelly's ANALOG IN pin for voltage measurement.
- Terminal 4 (GND) is the common ground for the boiler, the Shelly's power supply, the DS18B20 and the switched pulldowns.
The Shelly Plus Uni has a 10-pin connector with color-coded wires. Six of the ten wires are used in this circuit:
| Pin | Label | Wire color | Use |
|---|---|---|---|
| 1 | VAC1 | red | → boiler terminal 1 (+24 V) – Shelly power |
| 2 | VAC2 | black | → boiler terminal 4 (GND) – Shelly power |
| 3 | ANALOG IN | grey | → boiler terminal 2 (signal) – voltage measurement |
| 4 | SENSOR VCC | yellow | → DS18B20 red wire (sensor power) |
| 5 | DATA | blue | → DS18B20 yellow wire (1-Wire data line) |
| 6 | +5 VDC | - | unused |
| 7 | GND | green | → DS18B20 black wire (sensor ground) |
| 8 | COUNT IN | - | unused |
| 9 | IN 1 | - | unused |
| 10 | IN 2 | - | unused |
In addition, the Shelly has two potential-free relay outputs. Each switches its resistor between terminal 2 and terminal 4 (pulldown):
| Output | switched pulldown between terminal 2 ↔ terminal 4 | Heating level |
|---|---|---|
| both off | open (no pulldown) | High (failsafe) |
| OUT 1 | 5.1 kΩ | Medium |
| OUT 2 | 4 kΩ (2 kΩ + 2 kΩ in series) | Low |
| OUT 1 + OUT 2 | 5.1 kΩ ∥ 4 kΩ ≈ 2.22 kΩ | Off |
The whole circuit is wired with Wago lever-nut connectors – only the DS18B20 is soldered. All components fit inside the original TRQ 21 wall bracket:
To replace the TRQ 21 visually, I modeled a 3D-printable cover that clips onto the existing wall bracket of the original thermostat – no new holes. The Shelly Plus Uni and all wiring fit underneath.
Layout inside the cover: the Shelly belongs at the top, the DS18B20 at the bottom, next to the ventilation slits. The Shelly runs slightly warm – its waste heat rises up and away from the sensor, while fresh ambient air is drawn in through the slits at the bottom and across the DS18B20. This way the Shelly's own warmth does not skew the room-temperature reading.
Mounted on the original wall bracket, with the Shelly and wiring tucked underneath:
The print files live in 3d-printing/:
cover.stl– ready-to-slice modelcover.png– preview rendercover-wall-mounted.jpg– installed on the wall
- Driving the Junkers 1-2-4 interface – mikrocontroller.net (German) – detailed discussion with measurements, voltage and current characteristics
- Control Junkers gas heating (1-2-4 interface) with Home Assistant – Home Assistant Community – practical implementation with an ESP8266 (PWM) and opto-coupler
- Schematic of the Junkers ZSBE 16-3 board – roter-unimog.de (German) – original schematic of a compatible boiler including terminal layout
- TRQ 21 W user manual (PDF) – ManualsLib (German) – manufacturer documentation of the original controller
- KNX user forum: TRQ 21 T and ZWR 18-3/24-3 boiler (German) – discussion about KNX integration of the same interface