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 TapHome template provides 20 devices covering temperature monitoring, space heating/cooling control, DHW management, quiet mode, defrost status and weather-dependent setpoints. For an advanced template with service attributes, service actions and module-level error handling, see the DCOM-RW variant.
Hardware connection
DCOM-LT/MB gateway overview
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)
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.
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).
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
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 |
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 yellow (synchronizing) to solid green when ready.
Device capabilities
The two templates share a common set of registers but differ in scope. The main template provides comprehensive monitoring and control with 20 devices including defrost status, quiet mode, DHW reheat control and additional temperature sensors. The DCOM-RW template adds module-level error handling, service attributes for system status, and service actions for advanced setpoint management with parameter validation.
Temperature monitoring
All temperature sensors use the comp14 format (signed 16-bit value divided by 100 for °C). The templates handle 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
- 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
- Liquid Refrigerant Temperature (A:45) — refrigerant liquid line temperature
- Flow Rate (A:49) — water flow rate in l/min (main template) or m³/h (DCOM-RW)
Space heating and cooling control
- Space Heating/Cooling On/Off (H:4) — master switch for space heating and cooling
- Heat Pump Mode (H:3) — operation mode: Auto, Heating, or Cooling
- Weather Dependent Mode (H:53) — selects between Fixed, Weather Dependent, Fixed+Scheduled, or Weather Dependent+Scheduled setpoint modes
- Weather Curve Setpoint (H:1) — leaving water main heating setpoint, range 25–55°C
- Weather Curve Offset (H:54) — weather dependent setpoint offset, range -10 to +10°C
The DCOM-RW template extends the Operation Mode device with 4 service attributes reading current setpoints (leaving water heating/cooling, room thermostat heating/cooling) and 4 service actions for writing individual setpoints with defined ranges.
DHW (Domestic Hot Water) control
- DHW Setpoint (H:10) — reheat setpoint temperature, range 30–65°C
- DHW Reheat On/Off (H:12) — triggers DHW reheat cycle
- DHW Booster Mode (H:13) — enables auxiliary/booster heater for DHW (DCOM-RW only)
The DCOM-RW template adds a service attribute on DHW Reheat showing the current setpoint and a service action for setting the DHW reheat temperature (30–60°C).
System status monitoring
- Unit Error (A:21) — error status: No Error, Fault, or Warning
- Compressor Run (A:31) — compressor running status (ON/OFF)
- Circulation Pump (A:30) — circulation pump running status (ON/OFF)
- Disinfection Operation (A:33) — legionella disinfection cycle status (ON/OFF)
- 3-Way Valve Position (A:37) — valve position: Space Heating or DHW
- Defrost/Startup (A:35) — defrost or startup cycle status
- Quiet Mode (H:9) — quiet mode operation switch
The DCOM-RW template exposes additional module-level service attributes: Booster Heater Run (A:32), Hot Start (A:36), actual Operation Mode (A:38), and two additional temperature readings (Leaving Water Temp pre PHE and pre BUH).
Additional capabilities
The DCOM-LT/MB also exposes registers for leaving water main cooling setpoint (H:2, range 5–22°C), room thermostat heating/cooling setpoints (H:6/H:7), 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). Some of these are available through service actions in the DCOM-RW template; the rest can be added in a future template update.
The DCOM-RW template provides additional control capabilities. The leaving water cooling setpoint and room thermostat setpoints are accessible via service actions on the Operation Mode device, even though they are not standalone devices.
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)
- Check P1P2 cable connections between DCOM-LT/MB and the Altherma indoor unit
- Verify the Altherma unit is powered on and operational
- Allow up to 8 minutes for initial synchronization — if the Status LED remains yellow for more than 10 minutes, the DCOM will retry automatically
- If Status LED flashes red (timeout), verify P1P2 wiring and restart the DCOM
RS-485 LED red (communication errors)
- Check RS-485 wiring: + (terminal 3) to A+/D+, - (terminal 4) to B-/D-, C (terminal 5) to common
- Verify the Modbus slave address on DIP switches SW1.3 and SW1.4 matches TapHome configuration
- Confirm communication parameters: 9600 baud, 8 data bits, no parity, 1 stop bit
- 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’).
