--- title: "Daikin Altherma 3 DCOM-RW Heat Pump" description: "Daikin Altherma 3 R, H MT or H HT via the DCOM-LT/MB gateway on Modbus RTU: heating, cooling and DHW control, temperatures and system status in TapHome." url: "https://taphome.com/en/compatibility/daikin-altherma-3-dcom-rw/" language: en lastmod: 2026-10-08 generator: hugo ai: topics: ["modbus-rtu","heat-pump","rs485","dip-switch","p1p2-bus","weather-dependent","dhw","service-attributes","service-actions"] questions: ["How to integrate Daikin Altherma 3 with TapHome via Modbus using the DCOM-RW template?","What does the Daikin Altherma 3 DCOM-RW TapHome template monitor and control?","How to connect DCOM-LT/MB gateway to TapHome via RS-485?","How to configure the DCOM-LT/MB DIP switch for Modbus address?","What service attributes and service actions are available in the DCOM-RW template?","How to control DHW reheat and space heating on Daikin Altherma 3 via TapHome DCOM-RW?"] --- # Daikin Altherma 3 DCOM-RW Heat Pump Communication: Modbus RTU - [Daikin Altherma 3 (Standard Template)](https://taphome.com/en/compatibility/daikin-altherma-3/) - [Daikin Altherma 3 Product Page](https://www.daikin.eu/en_us/products/daikin-altherma-3.html) The Daikin Altherma 3 is an air-to-water heat pump series covering Altherma 3 R, Altherma 3 H MT and Altherma 3 H HT models. TapHome connects to the heat pump through the **DCOM-LT/MB** Modbus RTU gateway, which interfaces with the Altherma via the P1P2 bus and exposes registers over RS-485. This is the advanced DCOM-RW template with 10 devices, module-level error handling via readscript (ADDERROR/ADDWARNING), 10 module-level service attributes for system status monitoring, and device-level service actions for setpoint management with parameter validation. Switches use Bool data types for reliable control. > [!TIP] > Looking for more standalone devices? See the [standard Altherma 3 template](https://taphome.com/en/compatibility/daikin-altherma-3/) with 20 individual devices covering defrost status, weather-dependent setpoints, compressor diagnostics and additional temperature sensors. ## Hardware connection ### DCOM-LT/MB gateway overview ![DCOM-LT/MB gateway component layout showing terminals, LEDs and DIP switches](https://taphome.com/images/compatibility/daikin-altherma-3-dcom-rw/wiring-diagram-p001_hu_f5d69e28ad5222cd.jpg) The DCOM-LT/MB gateway (model S34-001) mounts on a DIN rail and has the following connections: - **Terminals 1–2 (A)** — Power supply: 15–24 VDC regulated, minimum 100 mA - **Terminals 3–5 (B)** — RS-485: + (terminal 3), - (terminal 4), C/common (terminal 5) - **Terminals 10–11 (E)** — P1P2 bus connection to Altherma indoor unit - **SW1 (J)** — 8-position DIP switch for Modbus address and operating mode - **LEDs (P, Q, R)** — Status, ACNET (P1P2), RS485 indicators ### RS-485 wiring Connect using a **3-wire RS-485 connection** between DCOM-LT/MB and TapHome: - **Terminal 3 (+)** → A+/D+ on TapHome RS-485 - **Terminal 4 (-)** → B-/D- on TapHome RS-485 - **Terminal 5 (C)** → GND/Common on TapHome **Cable specification:** - Type: screened twisted pair (Belden 8761 or equivalent) - Minimum cross-section: 0.33 mm² - Resistance: ≤ 60 Ω/km - Max bus length: **1000 m** - Max devices on bus: 32 (including Modbus master) > [!WARNING] > The common wire (terminal 5/C) **must** be connected between DCOM-LT/MB and TapHome. Without a proper common connection, communication errors will occur. Shield should be earthed at one location only. > [!NOTE] > **Termination is not required** at 9600 baud for network lengths up to 1000 m. The DCOM-LT/MB has internal failsafe biasing, so no external bias resistors are needed. If termination is used, a failsafe biasing circuit must be added as well. ### P1P2 connection Connect the P1P2 terminals (10, 11) to a compatible Altherma LT Master Remote Controller (e.g., MM4 or equivalent). > [!WARNING] > **Altherma 2 compatibility:** The DCOM-LT/MB can only be used with Altherma 2 via the Remcon/LT LAN adaptor. Do NOT connect DCOM directly to Altherma 2 (EKRUCBL / EKRUEHBL) units. ## Configuration ### DIP switch addressing ![DCOM-LT/MB wiring detail and DIP switch address reference table](https://taphome.com/images/compatibility/daikin-altherma-3-dcom-rw/wiring-diagram-p003_hu_7dc5fc3bf6d653b1.jpg) The 8-position DIP switch **SW1** configures the Modbus slave address and operating mode: | Switch | Function | |--------|----------| | SW1.1 + SW1.2 | Smart Grid / operating mode select | | **SW1.3 + SW1.4** | **Modbus slave address (1–4)** | | SW1.5 | Smart Grid function (with SW1.1, SW1.2) | | SW1.6–SW1.8 | Reserved | **Modbus address selection (SW1.3 and SW1.4):** | SW1.3 | SW1.4 | Address | |-------|-------|---------| | OFF | OFF | 1 (default) | | OFF | ON | 2 | | ON | OFF | 3 | | ON | ON | 4 | ### Communication parameters | Parameter | Value | |-----------|-------| | Protocol | Modbus RTU | | Baud rate | 9600 | | Data bits | 8 | | Parity | None | | Stop bits | 1 | ### Synchronization After power-up, the DCOM-LT/MB synchronizes with the Altherma Master unit. This process can take **up to 8 minutes**. During synchronization, register reads return the special value 32765 ("Wait for value"). **Special return values:** | Value | Meaning | |-------|---------| | 32765 | Waiting for value — synchronization in progress | | 32766 | Register unavailable in current configuration | | 32767 | Register not supported by connected unit | > [!TIP] > After initial power-up or restart, allow up to 8 minutes for DCOM to fully synchronize before troubleshooting missing register values. The Status LED changes from flashing yellow (synchronizing) to solid green when ready. ## Device capabilities This template provides 10 devices with Bool data types for switches, module-level readscript with ADDERROR/ADDWARNING for error handling, 10 module-level service attributes for system status monitoring, and device-level service actions for setpoint management with min/max validation. Flow rate is reported in m³/h (not l/min). ### Temperature monitoring All temperature sensors use the comp14 format (signed 16-bit value divided by 100 for °C). The template handles this conversion automatically. - **Outside Air Temperature** (A:44) — outdoor ambient temperature - **DHW Temperature** (A:43) — domestic hot water tank temperature - **Return Water Temperature** (A:42) — heating circuit return water temperature - **Liquid Refrigerant Temperature** (A:45) — refrigerant liquid line temperature - **Flow Rate** (A:49) — water flow rate in m³/h Additional temperature readings are available as module-level service attributes: Leaving Water Temp pre PHE (A:40) and Leaving Water Temp pre BUH (A:41). ### Space heating and cooling control - **Space Heating/Cooling On/Off** (H:4) — master switch for space heating and cooling - **Operation Mode** (H:3) — operation mode: Auto, Heating, or Cooling The Operation Mode device includes 4 service attributes reading current setpoints (leaving water heating/cooling, room thermostat heating/cooling) with 32766="Unavailable" handling, and 4 service actions for writing individual setpoints with defined ranges: - Set Leaving Water Heating Setpoint (H:1, 25–55°C) - Set Leaving Water Cooling Setpoint (H:2, 5–22°C) - Set Room Thermostat Heating Setpoint (H:6, 12–30°C) - Set Room Thermostat Cooling Setpoint (H:7, 15–35°C) ### DHW (Domestic Hot Water) control - **DHW Reheat On/Off** (H:12) — triggers DHW reheat cycle - **DHW Booster Mode** (H:13) — enables auxiliary/booster heater for DHW The DHW Reheat device includes a service attribute showing the current DHW reheat setpoint (H:10) and a service action for setting the DHW reheat temperature (30–60°C). ### System status monitoring (module-level service attributes) The module readscript provides error handling with ADDERROR/ADDWARNING based on register A:21 (Unit Error). The following system status values are exposed as module-level service attributes: - **Circulation Pump Running** (A:30) — pump status (ON/OFF/Unavailable) - **Compressor Run** (A:31) — compressor running status - **Booster Heater Run** (A:32) — booster heater status - **Disinfection Operation** (A:33) — legionella disinfection cycle status - **Defrost/Startup** (A:35) — defrost or startup cycle status - **Hot Start** (A:36) — hot start status - **3-Way Valve** (A:37) — valve position: Space Heating or DHW - **Operation Mode (actual)** (A:38) — actual operation mode: Heating or Cooling - **Leaving Water Temp pre PHE** (A:40) — leaving water temperature before plate heat exchanger - **Leaving Water Temp pre BUH** (A:41) — leaving water temperature before backup heater - **Quiet Mode** (H:9) — quiet mode operation switch ### Additional capabilities The DCOM-LT/MB also exposes registers for weather dependent cooling LWT offset (H:55), unit error code decoding (A:22 text14 format + A:23 sub-code), and room temperature reading (A:50). These can be added in a future template update. ## LED status indicators The DCOM-LT/MB has three LED indicators: | LED | Normal | Error | |-----|--------|-------| | **Status** | Solid green — synchronized, no fault | Solid red — unit fault; slow red blink — timeout | | **ACNET** | Green flash — normal P1P2 communication | Red flash/solid — P1P2 errors | | **RS485** | Green flash — normal Modbus communication | Red flash/solid — RS-485 errors | At power-up, all LEDs change from red to green, then the Status LED begins flashing yellow while waiting for Altherma Master synchronization. ## Troubleshooting ### Status LED yellow (waiting for Master) 1. Check P1P2 cable connections between DCOM-LT/MB and the Altherma indoor unit 2. Verify the Altherma unit is powered on and operational 3. Allow up to 8 minutes for initial synchronization — if the Status LED remains yellow for more than 10 minutes, the DCOM will retry automatically 4. If Status LED flashes red (timeout), verify P1P2 wiring and restart the DCOM ### RS-485 LED red (communication errors) 1. Check RS-485 wiring: + (terminal 3) to A+/D+, - (terminal 4) to B-/D-, C (terminal 5) to common 2. Verify the Modbus slave address on DIP switches SW1.3 and SW1.4 matches TapHome configuration 3. Confirm communication parameters: 9600 baud, 8 data bits, no parity, 1 stop bit 4. For long cable runs, verify cable impedance and consider adding termination ### Registers returning 32765 This indicates the DCOM is still synchronizing with the Altherma Master. Wait up to 8 minutes after power-up. If the value persists, check the Status LED — it should be solid green when synchronization is complete. ### Register H:9 behavior (Quiet Mode vs DHW Reheat) Register H:9 has dual functionality depending on the DCOM operating mode: in Normal Mode, official documentation defines it as "DHW Reheat On/Off"; in Sequencer Mode, it functions as "Quiet Mode Operation". The TapHome templates use it as Quiet Mode based on community verification. Actual behavior may vary depending on firmware version and DCOM configuration. ### Unit error code decoding When Unit Error (A:21) reports a fault, the error code is available in register A:22 as a 16-bit value encoding two ASCII characters. To decode: extract LSB (value mod 256) and MSB ((value - LSB) / 256), then convert both to ASCII characters. For example, value 21816 decodes to "U8" (MSB=85='U', LSB=56='8'). ## Available devices DCOM-LT-MB Module Service Attributes | | | |----------------------------|---------------------------------------------------------------------------------------------------| | Circulation Pump Running | reads A:30 (Int16); SWITCH: 0=OFF, 1=ON, 32766=Unavailable | | Compressor Run | reads A:31 (Int16); SWITCH: 0=OFF, 1=ON, 32766=Unavailable | | Booster Heater Run | reads A:32 (Int16); SWITCH: 0=OFF, 1=ON, 32766=Unavailable | | Disinfection Operation | reads A:33 (Int16); SWITCH: 0=OFF, 1=ON, 32766=Unavailable | | Defrost/Startup | reads A:35 (Int16); SWITCH: 0=OFF, 1=ON, 32766=Unavailable | | Hot Start | reads A:36 (Int16); SWITCH: 0=OFF, 1=ON, 32766=Unavailable | | Operation Mode (actual) | reads A:38 (Int16); SWITCH: 1=Heating, 2=Cooling | | 3-Way Valve | reads A:37 (Int16); SWITCH: 0=Space Heating, 1=DHW | | Leaving Water Temp pre PHE | reads A:40 (Int16) → /100 conversion; leaving water temperature before plate heat exchanger in °C | | Leaving Water Temp pre BUH | reads A:41 (Int16) → /100 conversion; leaving water temperature before backup heater in °C | DHW Reheat On/Off Switch Triggers domestic hot water reheat cycle Register: H:12 Bool Values / States: ON · OFF Service Actions | | |----------------------------| | Set DHW Reheat Temperature | Space Heating/Cooling On/Off Switch Master switch for space heating and cooling operation Register: H:4 Bool Values / States: ON · OFF Operation Mode Multi-value Switch Operation mode with setpoint service actions — Auto, Heating, or Cooling Register: H:3 Int16 numeric Values / States: Auto · Ohřev · Chlazení Service Actions | | |--------------------------------------| | Set Leaving Water Heating Setpoint | | Set Leaving Water Cooling Setpoint | | Set Room Thermostat Heating Setpoint | | Set Room Thermostat Cooling Setpoint | Flow Rate Variable Read-only Register: A:49 Int16 Unit: m³/h numeric DHW Booster Mode Switch Enables auxiliary/booster heater for domestic hot water Register: H:13 Bool Values / States: ON · OFF Liquid Refrigerant Temperature Temperature Sensor Read-only Register: A:45 (implicit) Temperature Unit: °C numeric Outside Air Temperature Temperature Sensor Read-only Register: A:44 (implicit) Temperature Unit: °C numeric DHW Temperature Temperature Sensor Read-only Domestic hot water tank temperature Register: A:43 (implicit) Temperature Unit: °C numeric Return Water Temperature Temperature Sensor Read-only Register: A:42 (implicit) Temperature Unit: °C numeric Quiet Mode Switch Register: H:9 Bool Values / States: ON · OFF **Connection:** Modbus RTU • 9600 baud• 8N1 • Slave ID: $\[SlaveId] Possible improvements (10) - H:2 Leaving Water Main Cooling Setpoint — Cooling setpoint 5–22°C — implemented via service action on Operation Mode device - H:6 Room Thermostat Control Heating Setpoint — Room thermostat heating setpoint 12–30°C — implemented via service action on Operation Mode device - H:7 Room Thermostat Control Cooling Setpoint — Room thermostat cooling setpoint 15–35°C — implemented via service action on Operation Mode device - H:55 Weather Dependent LWT Cooling Offset — Weather dependent cooling leaving water setpoint offset -10–10°C — not exposed in this template - A:22 Unit Error Code — Error code (2 ASCII chars, text14 format) — partially covered by module readscript ADDERROR - A:23 Unit Error Sub Code — Error sub-code 0–99 — partially covered by module readscript - A:32 Booster Heater Run — 0=OFF, 1=ON — available as module-level service attribute, not as standalone device - A:36 Hot Start — 0=OFF, 1=ON — available as module-level service attribute - A:38 Operation Mode (actual) — 1=Heating, 2=Cooling — actual (vs. requested H:3). Available as module service attribute - A:50 Room Temperature — comp14, °C × 100 — room temperature reading. Not exposed in this template ## Sources - DCOM-LT/MB Reference and Installation Manual (English) [www.daikin.eu](https://www.daikin.eu/content/dam/document-library/installation-manuals/ctrl/bms/dcom-lt-mb/DCOM-LT-MB_reference%20and%20installation%20manual_English.pdf) 2026-03-18 [PDF](https://taphome.com/compatibility/devices/daikin-altherma-3-dcom-rw/manufacturer/DCOM-LT-MB_reference_manual_EN.pdf) - DCOM-LT/MB Register Map (Czech, from submodule) 2026-03-18 [PDF](https://taphome.com/compatibility/devices/daikin-altherma-3-dcom-rw/manufacturer/DCOM-LT-MB_registers.pdf) - Daikin Altherma 3 DCOM-LT/MB Modbus Registers (GitHub Community) [github.com](https://github.com/budulinek/arduino-altherma-controller/issues/4) 2026-03-18 ## Related Documentation - [Modbus Integration](https://taphome.com/en/compatibility/modbus-guide/) - [Modbus](https://taphome.com/en/docs/integration/modbus/) - [Simple configuration using templates](https://taphome.com/en/docs/integration/modbus/templates/) - [Modbus scripts and prefetch](https://taphome.com/en/docs/script/modbus-scripts/) - [Modbus functions](https://taphome.com/en/docs/script/modbus/) - [Modbus data types](https://taphome.com/en/docs/script/modbus-data-types/)