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Buderus SSB condensing boilers (built-in Modbus RTU) Modbus register map
Updated 29 October 2026
Short answer
Buderus SSB floor-standing condensing boilers have a Modbus RTU slave on their own controller. Everything is a 16-bit holding register: burner state, status and error code, pump outputs, supply/return/DHW/flue temperatures, firing rate, counters, lockout and blocking history, cascade dependents, and the CH, DHW and outdoor-reset setpoints. Writes need a write-enable bit set just before each write.
This page lists 50 Modbus registers for the Buderus SSB condensing boilers (built-in Modbus RTU) — 10 writable — with the table, 0-based wire address, data type, word order, scale, units and access of each, re-typed from Buderus’s own free documentation (linked below) and checked against it.
- Manufacturer
- Buderus
- Model
- SSB condensing boilers (built-in Modbus RTU)
- Equipment type
- Boiler
- Registers
- 50 (10 writable)
- Transport
- Modbus RTU
- Default unit ID
- 1
- Default serial
- 9600 baud, 8N2
- Documented addressing
- 0-based (the number printed is the address on the wire)
- Also known as
- SSB, Buderus SSB boiler, Bosch SSB, 6720818450
Integration notes: Buderus SSB condensing boilers (built-in Modbus RTU)
Integration in brief
Wire Modbus + and - to the MOD BUS terminals of the boiler's terminal strip, set the master to 9600 8-N-2 and read holding registers 100-142 with function 03. To change a setpoint, first write register 99 = 1 (Write Enable), then write the value x10 within 4 seconds.
Minimum registers to map
Addresses are 0-based wire addresses. Enough to monitor and run the unit; map more as the job needs.
| Register | Name | Type | Why | Access |
|---|---|---|---|---|
| HR 101 | Status | UINT16 | Boiler status | Read |
| HR 102 | Error code | UINT16 | Error code | Read |
| HR 120 | Supply temperature | UINT16 | Supply temperature | Read |
| HR 121 | Return temperature | UINT16 | Return temperature | Read |
| HR 140 | Firing rate | UINT16 | Firing rate | Read |
| HR 502 | CH set-point | UINT16 | CH setpoint | Read/write |
| HR 503 | DHW set-point | UINT16 | DHW setpoint | Read/write |
| HR 99 | Control register | BITMAP16 | Write enable / lockout reset | Read/write |
Enable Modbus control & writes
Writes are refused unless register 99 bit 0 (Write Enable) is set. The bit clears itself after each write sequence or after 4 seconds, so set it again before every write.
Communication loss & watchdog
If no Modbus read or write arrives for more than 4 seconds the control register 99 is cleared. No other comm-loss behaviour is documented.
Byte order & addressing
- Only functions 03, 06 and 16 are supported - no input registers or coils.
- Setting any undefined bit in register 99 clears the whole register.
- Lockout reset (99 bit 14) works only while the boiler is in lockout.
Network setup
- Slave address (default 1) and stop bits (1 or 2, default 2) are set with the 894PB tool or LabVision software.
- Baud is fixed at 9600, 8 data bits, no parity.
- RS-485 two-wire, ground optional: Modbus + and - go to the MOD BUS pair on the boiler terminal strip (terminals 3-4).
Watch out for
- Do not leave Write Enable set when you are not writing; the document warns that registers can be corrupted.
- Changing register 98 changes the units of every temperature you read and write.
Buderus SSB condensing boilers (built-in Modbus RTU) Modbus registers
Address = 0-based wire address (the documented number is shown under it when it differs) · Order: ABCD = high word first, CDAB = low word first · Scale: engineering value = raw × scale.
50 of 50 shown
| Table | Address | Name | Description | Type | Order | Scale | Units | Access | Values / bits |
|---|---|---|---|---|---|---|---|---|---|
| HR | 98doc 98 (0x0062) | Unit selection | Bit 0 picks °C or °F, bit 1 bar or psi; needs Write Enable first. | BITMAP16 | Read/write | bit 0 = Temperature unit (°C / °F); bit 1 = Pressure unit (bar / psi) | |||
| HR | 99doc 99 (0x0063) | Control register | Set bit 0 before every write; bit 14 resets a lockout. | BITMAP16 | Read/write | bit 0 = Write Enable (cleared automatically after a write or after 4 s); bit 14 = Controller reset (only in lockout; clears itself) | |||
| HR | 100doc 100 (0x0064) | State | Burner sequence state. | UINT16 | Read | 0 = RESET_0 (start-up init); 1 = RESET_1 (start-up); 2 = STANDBY_0 (standby / idle); 3 = PRE_PURGE (init); 4 = PRE_PURGE_1 (pre purging); 5 = SAFETY_ON (test safety relay on); 6 = SAFETY_OFF (test safety relay off); 7 = IGNIT_0 (ignition); 8 = IGNIT_1 (ignition); 9 = BURN_0 (burning); 10 = SHUT_DOWN_RELAY_TEST_0 (init); 11 = SHUT_DOWN_RELAY_TEST_1 (testing safety and gas valve relays); 12 = POST_PURGE_0 (init); 13 = POST_PURGE_1 (post purging); 14 = PUMP_CH_0 (init); 15 = PUMP_CH_1 (pumping for CH); 16 = PUMP_HW_0 (init); 17 = PUMP_HW_1 (pumping for DHW); 18 = ALARM_1 (lockout); 19 = ERROR_CHECK (blocking); 20 = BURNER_BOOT (rebooting); 21 = CLEAR_E2PROM_ERROR; 22 = STORE_BLOCK_ERROR; 23 = WAIT_A_SECOND | |||
| HR | 101doc 101 (0x0065) | Status | What the boiler is doing. | UINT16 | Read | 0 = STANDBY; 10 = ALARM (lockout); 14 = BLOCK (blocking error); 15 = FROST_PROTECT; 16 = CH_DEMAND; 17 = RESET_STATE; 18 = STORAGE_DEMAND; 19 = DHW_TAP_DEMAND; 20 = DHW_PRE_HEAT; 21 = STORE_HOLD_WARM; 22 = GENERAL_PUMPING | |||
| HR | 102doc 102 (0x0066) | Error code | Active lockout or blocking error, see error table. | UINT16 | Read | 1 = Ignit Error (3 failed ignitions) - lockout; 2 = Gv Relay Error - lockout; 3 = Gv Relay Not Open - lockout; 4 = Gv Relay Not Close - lockout; 5 = Safety Relay Error - lockout; 6 = Safety Relay Open - lockout; 7 = Safety Relay Closed - lockout; 11 = Blocking Too Long Error (blocked > 20 h) - lockout; 12 = Fan Error - lockout; 13 = Ram Error - lockout; 14 = Wrong E2prom Signature - lockout; 15 = X Ram Error - lockout; 16 = E2Prom Error - lockout; 17 = E2Prom Error C - lockout; 18 = E2Prom Error Cal Table - lockout; 19 = State Error - lockout; 20 = Rom Error - lockout; 21 = Rom Error C - lockout; 22 = Air Sw Not Open - lockout; 23 = 15Ms Xrl Error - lockout; 24 = Air Sw Not Closed - lockout; 25 = T Max Lock Error (external overheat) - lockout; 26 = Stack Error - lockout; 27 = Flame Out Too Late Error - lockout; 28 = Flame Error 1 (flame before ignition) - lockout; 29 = 20Ms Xrl Error - lockout; 30 = 41Ms Error - lockout; 31 = Too Many Flame Failures - lockout; 32 = Flow Switch Not Closed Error - lockout; 33 = Flow Switch Not Open Error - lockout; 34 = Flag Byte Integrity Error - lockout; 35 = Ad Hi Cpl Error - lockout; 36 = Ad Lo Cpl Error - lockout; 37 = Register Error - lockout; 45 = WD Error Ram - blocking; 46 = WD Error Rom - blocking; 47 = WD Error Stack - blocking; 48 = WD Error Register - blocking; 49 = WD Error Xrl - blocking; 50 = Refhi Too Lo Error - blocking; 51 = Refhi Too Hi Error - blocking; 52 = Reflo Too Lo Error - blocking; 53 = Reflo Too Hi Error - blocking; 54 = Flame Error 2 - blocking; 55 = Water Level Detect - blocking; 56 = Water Level Meas - blocking; 57 = Low Water Cutoff Error - blocking; 58 = Low Water Pressure Error - blocking; 59 = Low Water Pressure Sensor - blocking; 60 = Flue Gas Error - blocking; 61 = Return Temp Error - blocking; 62 = Blocked Drain Error - blocking; 64 = Wd Freq Error - blocking; 65 = Phase Error (hot/neutral reversed) - blocking; 66 = Net Freq Error - blocking; 67 = Faulty Earth Error - blocking; 68 = WD Communication Error - blocking; 70 = Overheat Error - blocking; 72 = T Supply Open - blocking; 73 = T Return Open - blocking; 76 = T Dhw Out Open - blocking; 80 = T Supply Shorted - blocking; 81 = T Return Shorted - blocking; 84 = T Dhw Out Shorted - blocking; 86 = T Flue Shorted - blocking; 87 = Reset Button Error - blocking; 93 = Appliance Selection - blocking; 107 = Gas Pressure Error - blocking; 108 = Flue Press Error - blocking | |||
| HR | 110 | CH pump | 0 or 100 % (or 0-100 % if modulating). | UINT16 | × 0.1 | % | Read | ||
| HR | 111 | DHW pump | 0 or 100 % (or 0-100 % if modulating). | UINT16 | × 0.1 | % | Read | ||
| HR | 112 | General pump | 0 or 100 % (or 0-100 % if modulating). | UINT16 | × 0.1 | % | Read | ||
| HR | 120 | Supply temperature | In °C or °F per register 98. | UINT16 | × 0.1 | °C/°F | Read | ||
| HR | 121 | Return temperature | In °C or °F per register 98. | UINT16 | × 0.1 | °C/°F | Read | ||
| HR | 122 | DHW temperature | In °C or °F per register 98. | UINT16 | × 0.1 | °C/°F | Read | ||
| HR | 123 | Flue gas temperature | In °C or °F per register 98. | UINT16 | × 0.1 | °C/°F | Read | ||
| HR | 124 | System (heat exchanger) temperature | If fitted; °C or °F per register 98. | UINT16 | × 0.1 | °C/°F | Read | ||
| HR | 140 | Firing rate | Burner power output. | UINT16 | × 0.1 | % | Read | ||
| HR | 142 | Flame (ionization) current | UINT16 | × 0.1 | µA | Read | |||
| HR | 201 | Failed ignition counter | UINT16 | Read | |||||
| HR | 202 | Flame failure counter | UINT16 | Read | |||||
| HR | 203 | Burn hours CH | UINT16 | h | Read | ||||
| HR | 204 | Burn hours DHW | UINT16 | h | Read | ||||
| HR | 220 | First lockout in history | History slot 1 of 16 (220-235); not in time order. | UINT16 | Read | 1 = Ignit Error (3 failed ignitions) - lockout; 2 = Gv Relay Error - lockout; 3 = Gv Relay Not Open - lockout; 4 = Gv Relay Not Close - lockout; 5 = Safety Relay Error - lockout; 6 = Safety Relay Open - lockout; 7 = Safety Relay Closed - lockout; 11 = Blocking Too Long Error (blocked > 20 h) - lockout; 12 = Fan Error - lockout; 13 = Ram Error - lockout; 14 = Wrong E2prom Signature - lockout; 15 = X Ram Error - lockout; 16 = E2Prom Error - lockout; 17 = E2Prom Error C - lockout; 18 = E2Prom Error Cal Table - lockout; 19 = State Error - lockout; 20 = Rom Error - lockout; 21 = Rom Error C - lockout; 22 = Air Sw Not Open - lockout; 23 = 15Ms Xrl Error - lockout; 24 = Air Sw Not Closed - lockout; 25 = T Max Lock Error (external overheat) - lockout; 26 = Stack Error - lockout; 27 = Flame Out Too Late Error - lockout; 28 = Flame Error 1 (flame before ignition) - lockout; 29 = 20Ms Xrl Error - lockout; 30 = 41Ms Error - lockout; 31 = Too Many Flame Failures - lockout; 32 = Flow Switch Not Closed Error - lockout; 33 = Flow Switch Not Open Error - lockout; 34 = Flag Byte Integrity Error - lockout; 35 = Ad Hi Cpl Error - lockout; 36 = Ad Lo Cpl Error - lockout; 37 = Register Error - lockout; 45 = WD Error Ram - blocking; 46 = WD Error Rom - blocking; 47 = WD Error Stack - blocking; 48 = WD Error Register - blocking; 49 = WD Error Xrl - blocking; 50 = Refhi Too Lo Error - blocking; 51 = Refhi Too Hi Error - blocking; 52 = Reflo Too Lo Error - blocking; 53 = Reflo Too Hi Error - blocking; 54 = Flame Error 2 - blocking; 55 = Water Level Detect - blocking; 56 = Water Level Meas - blocking; 57 = Low Water Cutoff Error - blocking; 58 = Low Water Pressure Error - blocking; 59 = Low Water Pressure Sensor - blocking; 60 = Flue Gas Error - blocking; 61 = Return Temp Error - blocking; 62 = Blocked Drain Error - blocking; 64 = Wd Freq Error - blocking; 65 = Phase Error (hot/neutral reversed) - blocking; 66 = Net Freq Error - blocking; 67 = Faulty Earth Error - blocking; 68 = WD Communication Error - blocking; 70 = Overheat Error - blocking; 72 = T Supply Open - blocking; 73 = T Return Open - blocking; 76 = T Dhw Out Open - blocking; 80 = T Supply Shorted - blocking; 81 = T Return Shorted - blocking; 84 = T Dhw Out Shorted - blocking; 86 = T Flue Shorted - blocking; 87 = Reset Button Error - blocking; 93 = Appliance Selection - blocking; 107 = Gas Pressure Error - blocking; 108 = Flue Press Error - blocking | |||
| HR | 260 | First blocking in history | History slot 1 of 16 (260-275); not in time order. | UINT16 | Read | 1 = Ignit Error (3 failed ignitions) - lockout; 2 = Gv Relay Error - lockout; 3 = Gv Relay Not Open - lockout; 4 = Gv Relay Not Close - lockout; 5 = Safety Relay Error - lockout; 6 = Safety Relay Open - lockout; 7 = Safety Relay Closed - lockout; 11 = Blocking Too Long Error (blocked > 20 h) - lockout; 12 = Fan Error - lockout; 13 = Ram Error - lockout; 14 = Wrong E2prom Signature - lockout; 15 = X Ram Error - lockout; 16 = E2Prom Error - lockout; 17 = E2Prom Error C - lockout; 18 = E2Prom Error Cal Table - lockout; 19 = State Error - lockout; 20 = Rom Error - lockout; 21 = Rom Error C - lockout; 22 = Air Sw Not Open - lockout; 23 = 15Ms Xrl Error - lockout; 24 = Air Sw Not Closed - lockout; 25 = T Max Lock Error (external overheat) - lockout; 26 = Stack Error - lockout; 27 = Flame Out Too Late Error - lockout; 28 = Flame Error 1 (flame before ignition) - lockout; 29 = 20Ms Xrl Error - lockout; 30 = 41Ms Error - lockout; 31 = Too Many Flame Failures - lockout; 32 = Flow Switch Not Closed Error - lockout; 33 = Flow Switch Not Open Error - lockout; 34 = Flag Byte Integrity Error - lockout; 35 = Ad Hi Cpl Error - lockout; 36 = Ad Lo Cpl Error - lockout; 37 = Register Error - lockout; 45 = WD Error Ram - blocking; 46 = WD Error Rom - blocking; 47 = WD Error Stack - blocking; 48 = WD Error Register - blocking; 49 = WD Error Xrl - blocking; 50 = Refhi Too Lo Error - blocking; 51 = Refhi Too Hi Error - blocking; 52 = Reflo Too Lo Error - blocking; 53 = Reflo Too Hi Error - blocking; 54 = Flame Error 2 - blocking; 55 = Water Level Detect - blocking; 56 = Water Level Meas - blocking; 57 = Low Water Cutoff Error - blocking; 58 = Low Water Pressure Error - blocking; 59 = Low Water Pressure Sensor - blocking; 60 = Flue Gas Error - blocking; 61 = Return Temp Error - blocking; 62 = Blocked Drain Error - blocking; 64 = Wd Freq Error - blocking; 65 = Phase Error (hot/neutral reversed) - blocking; 66 = Net Freq Error - blocking; 67 = Faulty Earth Error - blocking; 68 = WD Communication Error - blocking; 70 = Overheat Error - blocking; 72 = T Supply Open - blocking; 73 = T Return Open - blocking; 76 = T Dhw Out Open - blocking; 80 = T Supply Shorted - blocking; 81 = T Return Shorted - blocking; 84 = T Dhw Out Shorted - blocking; 86 = T Flue Shorted - blocking; 87 = Reset Button Error - blocking; 93 = Appliance Selection - blocking; 107 = Gas Pressure Error - blocking; 108 = Flue Press Error - blocking | |||
| HR | 300 | Dependent 01 state | Cascade dependent boiler, state table codes. | UINT16 | Read | 0 = RESET_0 (start-up init); 1 = RESET_1 (start-up); 2 = STANDBY_0 (standby / idle); 3 = PRE_PURGE (init); 4 = PRE_PURGE_1 (pre purging); 5 = SAFETY_ON (test safety relay on); 6 = SAFETY_OFF (test safety relay off); 7 = IGNIT_0 (ignition); 8 = IGNIT_1 (ignition); 9 = BURN_0 (burning); 10 = SHUT_DOWN_RELAY_TEST_0 (init); 11 = SHUT_DOWN_RELAY_TEST_1 (testing safety and gas valve relays); 12 = POST_PURGE_0 (init); 13 = POST_PURGE_1 (post purging); 14 = PUMP_CH_0 (init); 15 = PUMP_CH_1 (pumping for CH); 16 = PUMP_HW_0 (init); 17 = PUMP_HW_1 (pumping for DHW); 18 = ALARM_1 (lockout); 19 = ERROR_CHECK (blocking); 20 = BURNER_BOOT (rebooting); 21 = CLEAR_E2PROM_ERROR; 22 = STORE_BLOCK_ERROR; 23 = WAIT_A_SECOND | |||
| HR | 302 | Dependent 01 error code | Cascade dependent boiler, error table codes. | UINT16 | Read | 1 = Ignit Error (3 failed ignitions) - lockout; 2 = Gv Relay Error - lockout; 3 = Gv Relay Not Open - lockout; 4 = Gv Relay Not Close - lockout; 5 = Safety Relay Error - lockout; 6 = Safety Relay Open - lockout; 7 = Safety Relay Closed - lockout; 11 = Blocking Too Long Error (blocked > 20 h) - lockout; 12 = Fan Error - lockout; 13 = Ram Error - lockout; 14 = Wrong E2prom Signature - lockout; 15 = X Ram Error - lockout; 16 = E2Prom Error - lockout; 17 = E2Prom Error C - lockout; 18 = E2Prom Error Cal Table - lockout; 19 = State Error - lockout; 20 = Rom Error - lockout; 21 = Rom Error C - lockout; 22 = Air Sw Not Open - lockout; 23 = 15Ms Xrl Error - lockout; 24 = Air Sw Not Closed - lockout; 25 = T Max Lock Error (external overheat) - lockout; 26 = Stack Error - lockout; 27 = Flame Out Too Late Error - lockout; 28 = Flame Error 1 (flame before ignition) - lockout; 29 = 20Ms Xrl Error - lockout; 30 = 41Ms Error - lockout; 31 = Too Many Flame Failures - lockout; 32 = Flow Switch Not Closed Error - lockout; 33 = Flow Switch Not Open Error - lockout; 34 = Flag Byte Integrity Error - lockout; 35 = Ad Hi Cpl Error - lockout; 36 = Ad Lo Cpl Error - lockout; 37 = Register Error - lockout; 45 = WD Error Ram - blocking; 46 = WD Error Rom - blocking; 47 = WD Error Stack - blocking; 48 = WD Error Register - blocking; 49 = WD Error Xrl - blocking; 50 = Refhi Too Lo Error - blocking; 51 = Refhi Too Hi Error - blocking; 52 = Reflo Too Lo Error - blocking; 53 = Reflo Too Hi Error - blocking; 54 = Flame Error 2 - blocking; 55 = Water Level Detect - blocking; 56 = Water Level Meas - blocking; 57 = Low Water Cutoff Error - blocking; 58 = Low Water Pressure Error - blocking; 59 = Low Water Pressure Sensor - blocking; 60 = Flue Gas Error - blocking; 61 = Return Temp Error - blocking; 62 = Blocked Drain Error - blocking; 64 = Wd Freq Error - blocking; 65 = Phase Error (hot/neutral reversed) - blocking; 66 = Net Freq Error - blocking; 67 = Faulty Earth Error - blocking; 68 = WD Communication Error - blocking; 70 = Overheat Error - blocking; 72 = T Supply Open - blocking; 73 = T Return Open - blocking; 76 = T Dhw Out Open - blocking; 80 = T Supply Shorted - blocking; 81 = T Return Shorted - blocking; 84 = T Dhw Out Shorted - blocking; 86 = T Flue Shorted - blocking; 87 = Reset Button Error - blocking; 93 = Appliance Selection - blocking; 107 = Gas Pressure Error - blocking; 108 = Flue Press Error - blocking | |||
| HR | 303 | Dependent 01 firing rate | Cascade dependent boiler power output. | UINT16 | × 0.1 | % | Read | ||
| HR | 306 | Dependent 02 state | Cascade dependent boiler, state table codes. | UINT16 | Read | ||||
| HR | 308 | Dependent 02 error code | Cascade dependent boiler, error table codes. | UINT16 | Read | ||||
| HR | 309 | Dependent 02 firing rate | Cascade dependent boiler power output. | UINT16 | × 0.1 | % | Read | ||
| HR | 312 | Dependent 03 state | Cascade dependent boiler, state table codes. | UINT16 | Read | ||||
| HR | 314 | Dependent 03 error code | Cascade dependent boiler, error table codes. | UINT16 | Read | ||||
| HR | 315 | Dependent 03 firing rate | Cascade dependent boiler power output. | UINT16 | × 0.1 | % | Read | ||
| HR | 318 | Dependent 04 state | Cascade dependent boiler, state table codes. | UINT16 | Read | ||||
| HR | 320 | Dependent 04 error code | Cascade dependent boiler, error table codes. | UINT16 | Read | ||||
| HR | 321 | Dependent 04 firing rate | Cascade dependent boiler power output. | UINT16 | × 0.1 | % | Read | ||
| HR | 324 | Dependent 05 state | Cascade dependent boiler, state table codes. | UINT16 | Read | ||||
| HR | 326 | Dependent 05 error code | Cascade dependent boiler, error table codes. | UINT16 | Read | ||||
| HR | 327 | Dependent 05 firing rate | Cascade dependent boiler power output. | UINT16 | × 0.1 | % | Read | ||
| HR | 330 | Dependent 06 state | Cascade dependent boiler, state table codes. | UINT16 | Read | ||||
| HR | 332 | Dependent 06 error code | Cascade dependent boiler, error table codes. | UINT16 | Read | ||||
| HR | 333 | Dependent 06 firing rate | Cascade dependent boiler power output. | UINT16 | × 0.1 | % | Read | ||
| HR | 336 | Dependent 07 state | Cascade dependent boiler, state table codes. | UINT16 | Read | ||||
| HR | 338 | Dependent 07 error code | Cascade dependent boiler, error table codes. | UINT16 | Read | ||||
| HR | 339 | Dependent 07 firing rate | Cascade dependent boiler power output. | UINT16 | × 0.1 | % | Read | ||
| HR | 502 | CH set-point | Central heating setpoint; °C or °F per register 98. | UINT16 | × 0.1 | °C/°F | Read/write | ||
| HR | 503 | DHW set-point | Hot water setpoint; °C or °F per register 98. | UINT16 | × 0.1 | °C/°F | Read/write | ||
| HR | 504 | Reset curve boiler design | Supply temperature at outdoor design condition. | UINT16 | × 0.1 | °C/°F | Read/write | ||
| HR | 505 | Reset curve boiler mild weather | Supply temperature at mild outdoor condition. | UINT16 | × 0.1 | °C/°F | Read/write | ||
| HR | 508 | Warm weather shutdown | Outdoor temperature above which heating stops. | UINT16 | × 0.1 | °C/°F | Read/write | ||
| HR | 509 | Reset curve boiler maximum | UINT16 | × 0.1 | °C/°F | Read/write | |||
| HR | 510 | Reset curve boiler minimum | UINT16 | × 0.1 | °C/°F | Read/write | |||
| HR | 511 | Night setback | UINT16 | × 0.1 | °C/°F | Read/write |
Notes on this map
- Scaling follows the document's data-type table: temperatures, pressures, voltages, flame current and percentages are sent x10 (one decimal). Registers marked 'automatic conversion' use it; state, status and error codes and the counters do not.
- Temperatures follow the unit chosen in register 98 (°C or °F) for both reads and writes; the document does not say which bit value means which unit or what the default is, so check a known reading first.
- The document never says whether values are signed. Outside temperature (125) and the outdoor reset-curve points (506 Outdoor Mild Weather, 507 Outdoor Design) can be below zero, so they are left out rather than guessed; the other temperatures are typed unsigned.
- Left out: successful ignition counter 200 (printed with 'resolution 16', meaning not explained); CH mode 500 and DHW mode 501 (range '0..x' with no value list).
- Only the first slot of the lockout and blocking histories is listed; slots 2-16 follow at consecutive addresses (lockouts 221-235, blockings 261-275). The history is not in order of occurrence.
- Dependent registers 300-339 report the cascade dependents (01-07) in the same state/error code tables; the document leaves their access column blank, so they are listed read-only.
- Left out: the 'time after lockout' (236-251) and 'time after blocking' (276-291) hour stamps. They are marked 'automatic conversion' yet hours are not one of the x10 data types in the document's data-type table, so their scaling is unclear; the burn-hour counters 203-204 carry no such mark.
Check it against the live device. A register map tells you what the controller can expose; the unit in front of you may differ by firmware or options. Read a few registers and compare them with the unit’s own display; for a 32-bit value the float and byte-order decoder shows all four word orders at once, so a wrong word order or scale is obvious. New to this? Read what a register map is and why addresses are off by one.
Questions about the Buderus SSB condensing boilers (built-in Modbus RTU)
What Modbus registers does the Buderus SSB condensing boilers (built-in Modbus RTU) have?
This map lists 50 registers: 50 holding registers. 10 of them can be written. Registers vary with firmware, options and configuration, so confirm against the device.
Are the Buderus SSB condensing boilers (built-in Modbus RTU) Modbus addresses 0-based or 1-based?
The manufacturer's document numbers registers 0-based (the number printed is the address on the wire). Every address on this page is the 0-based wire address you put in the request; the documented number is shown under it where it differs. The document prints decimal register numbers with their hex form (98 = 0x0062) and says to use them as printed when the master counts from 0x0000, and to add 1 only if the master counts from 0x0001. So register 100 is wire address 100.
What are the minimum Modbus registers to integrate the Buderus SSB condensing boilers (built-in Modbus RTU)?
Start with HR 101 Status (Boiler status); HR 102 Error code (Error code); HR 120 Supply temperature (Supply temperature); HR 121 Return temperature (Return temperature); HR 140 Firing rate (Firing rate); HR 502 CH set-point (CH setpoint); HR 503 DHW set-point (DHW setpoint); HR 99 Control register (Write enable / lockout reset).
How do I enable Modbus control of the Buderus SSB condensing boilers (built-in Modbus RTU)?
Writes are refused unless register 99 bit 0 (Write Enable) is set. The bit clears itself after each write sequence or after 4 seconds, so set it again before every write.
What happens when the Buderus SSB condensing boilers (built-in Modbus RTU) loses Modbus communication?
If no Modbus read or write arrives for more than 4 seconds the control register 99 is cleared. No other comm-loss behaviour is documented.
Where is the official Buderus SSB condensing boilers (built-in Modbus RTU) Modbus documentation?
The source is Modbus for Buderus SSB Boilers - Installation and Service Instructions for Contractors (6720818450 (2015/10) US Rev.0), revision 2015-10, published free by Buderus at https://www.bosch-homecomfort.com/us/media/country_pool/service/technical_guides/6720818450_ssb_boiler_modbus_installation_instructions_en_10.2015_us_us.pdf.
More Buderus Modbus register maps
Source: Modbus for Buderus SSB Boilers - Installation and Service Instructions for Contractors · 6720818450 (2015/10) US Rev.0 · rev. 2015-10. Register numbers, types, scaling and units are reproduced as published; descriptions are our own.