Daikin Altherma 3 DCOM-RW Heat Pump

Modbus RTU

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Last updated: 03. 2026

Requires external gateway
Daikin DCOM-LT/MB

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.

Looking for more standalone devices? See the standard Altherma 3 template 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

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

DCOM-LT/MB wiring detail and DIP switch address reference table

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

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)

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’).

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

DCOM-LT-MB

Read (module)

var errreg := MODBUSR(A, 21, Int16);
IF( errreg = 1, 
ADDERROR(MODBUSR(A, 22, String, 1) + " " + MODBUSR(A, 23, int16) ));
IF( errreg = 2, 
ADDWARNING(MODBUSR(A, 22, String, 1) + " " + MODBUSR(A, 23, int16) ));

Service Attributes

Oběhové čerpadlo běží

SWITCH(MODBUSR(A, 30, int16),0, "VYP", 1, "ZAP", 32766 ,"Nedostupné", "Chyba");

Chod kompresoru

SWITCH(MODBUSR(A, 31, int16),0, "VYP", 1, "ZAP", 32766 ,"Nedostupné", "Chyba");

Chod pomocného topného tělesa

SWITCH(MODBUSR(A, 32, int16),0, "VYP", 1, "ZAP", 32766 ,"Nedostupné", "Chyba");

Dezinfekční provoz

SWITCH(MODBUSR(A, 33, int16),0, "VYP", 1, "ZAP", 32766 ,"Nedostupné", "Chyba");

Rozmrazování/spuštění

SWITCH(MODBUSR(A, 35, int16),0, "VYP", 1, "ZAP", 32766 ,"Nedostupné", "Chyba");

Teplý start

SWITCH(MODBUSR(A, 36, int16),0, "VYP", 1, "ZAP", 32766 ,"Nedostupné", "Chyba");

Provozní režim

SWITCH(MODBUSR(A, 38, int16), 1, "Vytápění", 2, "Chlazení", "Chyba")

3cestný ventil

SWITCH(MODBUSR(A, 37, int16), 0, "Prostorové vytápění", 1, "TUV", "Chyba")

Teplota výstupní vody předřazeného PHE

MODBUSR(A, 40, Int16)/100 + " °C";

Teplota výstupní vody předřazeného el. topného tělesa

MODBUSR(A, 41, Int16)/100 + " °C";

DHW Reheat On/Off Switch

Triggers domestic hot water reheat cycle

Service Actions

Set DHW Reheat Temperature

DHW Reheat On/Off

Read switch state

MODBUSR(H, 12, Bool)

Write switch state

MODBUSW(H, 12, Bool, St)

Service Attributes

Požadovaná hodnota op. ohř. TUV

MODBUSR(H, 10, Int16);

Service Actions

Hodnota opětovného ohřevu TUV

Parameters: Teplota (30–60 t)

MODBUSW(H, 10, Int16, t);

Space Heating/Cooling On/Off Switch

Master switch for space heating and cooling operation

Operation Mode Multi-value Switch

Operation mode with setpoint service actions — Auto, Heating, or Cooling

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

Operation Mode

Read switch state

MODBUSR(H, 3, Int16)

Write switch state

MODBUSW(H, 3, Int16, Mu)

Service Attributes

Požadovaná hodnota hlavního ohřevu výstupní vody

var reg := MODBUSR(H, 1, int16);
SWITCH(reg, 32766 , "Nedostupná funkce", reg + " °C");

Požadovaná hodnota hlavního chlazení výstupní vody

var reg := MODBUSR(H, 2, int16);
SWITCH(reg, 32766 , "Nedostupná funkce", reg + " °C");

Požadovaná hodnota vytápění při regulaci teploty pokojovým termostatem

var reg := MODBUSR(H, 2, int16);
SWITCH(reg, 32766 , "Nedostupná funkce", reg + " °C");

Požadovaná hodnota chlazenípři regulaci teploty pokojovým termostatem

var reg := MODBUSR(H, 2, int16);
SWITCH(reg, 32766 , "Nedostupná funkce", reg + " °C");

Service Actions

Vstupní voda - ohřev

Parameters: teplota [°C] (25–55 t)

MODBUSW(H, 1, Int16, t);

Vstupní voda - chlazení

Parameters: Teplota (5–22 t)

MODBUSW(H, 2, Int16, t);

Pokojový termostat - vytápění

Parameters: Teplota (12–30 t)

MODBUSW(H, 6, Int16, t);

Pokojový termostat - chlazení

Parameters: Teplota (15–35 t)

MODBUSW(H, 7, Int16, t);

Flow Rate Variable Read-only

DHW Booster Mode Switch

Enables auxiliary/booster heater for domestic hot water

Liquid Refrigerant Temperature Temperature Sensor Read-only

Outside Air Temperature Temperature Sensor Read-only

DHW Temperature Temperature Sensor Read-only

Domestic hot water tank temperature

Return Water Temperature Temperature Sensor Read-only

Quiet Mode Switch

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

Download Template

Sources

DCOM-LT/MB Reference and Installation Manual (English)
www.daikin.eu 2026-03-18
PDF
DCOM-LT/MB Register Map (Czech, from submodule)
2026-03-18
PDF
Daikin Altherma 3 DCOM-LT/MB Modbus Registers (GitHub Community)
github.com 2026-03-18