KNX Temperature Sensors: Room, Outdoor, Floor and Pipe Types
Temperature measurement is the input for every heating and cooling control loop in a KNX project. Wrong sensor placement or sensor type leads to offset control, comfort complaints and energy waste that is difficult to diagnose after handover. This guide covers every sensor type, NTC vs PT1000 characteristics, DPT 9.001 parameters, placement rules and HVAC mode switching.
Sensor types
| Type | Element | Housing | Typical range | Primary use | Accuracy |
|---|---|---|---|---|---|
| Room thermostat | NTC thermistor | Wall-mounted room unit | 0–50°C | Room temperature control, setpoint input, HVAC mode switching | ±0.5°C |
| Outdoor sensor | NTC thermistor | IP54 weatherproof wall housing | -40–80°C | Heating curve compensation, weather-based control, heat pump setpoint | ±0.5°C |
| Floor temperature | NTC or PT1000 in conduit | Probe in floor screed | 0–60°C | Underfloor heating floor temperature limiting | ±0.1–0.5°C |
| Pipe temperature | Clamp-on NTC or immersion PT100 | Pipe clamp or immersion thermowell | -20–120°C | HVAC supply/return temperature monitoring, heat meter input | ±0.5–1°C |
| Duct temperature | NTC or PT100 probe | In HVAC duct, threaded fitting | -40–80°C | AHU supply air temperature, combined with humidity sensor | ±0.3°C |
DPT 9.001 — the KNX temperature data type
All KNX temperature values are transmitted as DPT 9.001 — a 2-byte floating-point format with a range of -671°C to +671°C and a resolution of 0.01°C. Understanding transmission parameters prevents unnecessary bus load.
DPT 9.001 — temperature format and transmission
Format: DPT 9.001 (2-byte float, unit: °C)
Range: -671°C to +671°C
Resolution: 0.01°C
Transmission triggers (configurable in ETS):
On change: send telegram when value changes by ≥ 0.5°C (default threshold)
→ reduces bus telegrams vs periodic; recommended for room sensors
Cyclic: send telegram every N minutes regardless of change
→ default 60 minutes; use for supervisory monitoring
On request: only transmit when another device reads the group address
→ rarely used for sensors; more common for actuator status
Typical ETS configuration:
Change threshold: 0.3°C (sensitive spaces) to 1.0°C (outdoor sensor)
Cyclic period: 60 minutes (for SCADA logging continuity)
Combined: send on change AND cyclic — both conditions activeFor outdoor sensors, set the change threshold to 0.5–1.0°C — outdoor temperature changes slowly and a tighter threshold generates unnecessary bus telegrams. For room sensors in PI-controlled zones, 0.3–0.5°C threshold gives the control loop sufficient resolution without flooding the bus.
NTC vs PT1000
The two most common resistive temperature sensor elements used in KNX installations differ in cost, accuracy, linearity and application suitability.
NTC (Negative Temperature Coefficient)
- Characteristic: resistance decreases as temperature increases (non-linear curve)
- Accuracy: ±0.5°C typical — adequate for room and outdoor sensing
- Linearity: non-linear — requires calibration curve in the sensor transmitter
- Cost: low — €2–€15 for the element; room units use NTC internally
- Lead resistance: susceptible to cable length errors — compensate for cables >10m
- Best for: room thermostats, outdoor sensors, simple temperature monitoring
PT1000 (Platinum Resistance)
- Characteristic: resistance increases linearly with temperature (IEC 60751)
- Accuracy: ±0.1°C (class A) — required for floor heating screed temperature
- Linearity: linear — simpler transmitter electronics, more consistent across units
- Cost: higher — €15–€50 for element; transmitter module additional
- Lead resistance: less sensitive — 4-wire connection eliminates cable resistance entirely
- Best for: floor temperature sensors in underfloor heating, pipe temperature, process monitoring
Floor heating screed probes must use PT1000 (or PT100 for 3/4-wire connections). The non-linearity of NTC at screed temperatures (25–35°C) introduces calibration errors of up to 1–2°C, which causes overshoot of the 28°C floor temperature comfort limit and potential floor damage.
Room thermostat placement
Room thermostat placement is the most common source of HVAC control offset in KNX installations. A sensor in the wrong location reads 2–3°C different from the actual occupant experience, resulting in either overheating or insufficient comfort.
Correct: Mount on interior partition wall at 1.5m from floor
Interior walls are not affected by outdoor temperature variation. The 1.5m height represents average seated/standing occupant experience and avoids floor-level cold draughts and ceiling-level heat stratification.
Avoid: Avoid external walls
External walls in poorly insulated buildings can be 3–5°C colder than interior walls due to thermal bridging. A sensor on an external wall reads too cold → system overheats to compensate.
Avoid: Avoid direct sunlight
Direct solar radiation on the sensor housing causes false high readings. Even indirect sunlight through a window can raise the sensor by 2–4°C. Mount on north or east-facing walls where direct sun cannot reach the unit.
Avoid: Avoid HVAC supply outlets
Mounting within 1.5m of a supply diffuser or fan coil blows conditioned air directly across the sensor, creating feedback instability. The system reads the supply air temperature (controlled output) not the room temperature (what it should be controlling).
Correct: Near the main door is preferred
The zone near the main entrance represents the average occupant path and provides a consistent measurement point. Occupants entering from a corridor bring a temperature disturbance that stabilises quickly at this location.
Correct: For rooms wider than 8m: use two sensors averaged in logic
Temperature stratifies horizontally in large open-plan spaces. Two sensors — one on each long wall — with their readings averaged in a KNX logic block gives a representative zone temperature. Configure the logic block average function in ETS.
Outdoor temperature sensor
The outdoor sensor feeds the heating curve calculation for weather compensation — as outdoor temperature drops, the heating system increases flow temperature proportionally. Incorrect outdoor sensor reading translates directly to under or over-heating of the building.
Outdoor sensor mounting rules
- Orientation: north facade only — direct sunlight on sensor causes false high readings of 5–15°C
- Height: 2–3m above ground — avoids frost pocket that forms near the ground on clear nights
- Roof runoff: position below eaves overhang but protected from direct rainwater from roof drainage
- Cable: shielded 2-core to KNX transmitter module (e.g. Thermokon SR04) in the panel
Applications for outdoor temperature in KNX
- Heating curve: outdoor temp → KNX logic → Modbus setpoint to heat pump (Nibe, Viessmann)
- Summer cutoff: outdoor temp > 18°C → disable heating mode across all zones
- Window interlock: outdoor temp < 10°C → inhibit window opening automation
- Combined unit: Elsner P04 KNX-S provides outdoor temp + wind + rain + lux on a single device
Floor temperature limiting
Underfloor heating (UFH) systems must limit floor surface temperature for both occupant comfort and floor material protection. KNX logic compares the floor PT1000 sensor reading against configured limits and overrides the zone actuator regardless of the room thermostat setpoint.
| Floor type | Maximum floor temperature | Reason |
|---|---|---|
| Below seating / desk areas | 27°C | Occupant discomfort above 27°C when feet are stationary — EN 1264-2 |
| Standing / walking areas (office, corridor) | 29°C | Thermal comfort limit for standing occupants |
| Bathroom / wet area (tiles) | 33°C | Tile adhesive and grout tolerance; barefoot comfort limit |
| Bathroom / wet area (underfloor max) | 35°C | Absolute maximum for most UFH system design (hydronic) |
| Hardwood flooring | 27°C | Wood expansion/contraction and adhesive integrity limit |
| Laminate flooring | 28°C | Manufacturer warranty limit for most laminate products |
Floor temperature limiting logic — KNX ETS
Floor sensor GA: 3/1/5 (DPT 9.001, PT1000 transmitter output)
Room setpoint GA: 3/1/1 (DPT 9.001, room thermostat)
Zone actuator GA: 3/2/1 (DPT 1.001, UFH valve open/close)
Logic block (ETS or MDT Logicmodule):
IF floor_temp (GA 3/1/5) > 28.0°C:
→ Force zone actuator (GA 3/2/1) = CLOSED
→ Suspend normal PI control output
ELSE:
→ Normal PI control: compare room_setpoint vs room_temp
→ Zone actuator follows PI output
Hysteresis:
Actuator re-opens when floor_temp drops below 26.5°C
(1.5°C hysteresis prevents rapid cycling of the zone valve)KNX product examples
MDT SCN-RTT55.02
Room thermostat with display
NTC internal sensor. 55mm form factor (standard flush box). Display shows measured temperature and active setpoint. Supports Comfort/Standby/Economy/Protection mode switching via DPT 20.102. 3 separate setpoint group objects.
Thermokon SR04 PT1000
DIN rail transmitter — 4 PT1000 inputs
Accepts 4 external PT1000 probes. Each input configurable as floor sensor, pipe sensor or outdoor sensor. DPT 9.001 output per channel. Bus-powered, 2TE DIN rail. Ideal for floor heating zones with multiple screed probes.
Elsner CARMA KNX-S
Combined room sensor
NTC room temperature + CO2 + VOC + humidity + lux + presence. All values on separate KNX group addresses. Wall mount, flush box. Eliminates multiple separate sensors in premium residential and office fitouts.
Siemens QAX34.1
Room unit with humidity
NTC temperature + relative humidity. ETS6 parameter set. HVAC mode switching via DPT 20.102 from rotary switch on unit. Compatible with Synco and KNX heating controllers.
Jung 4154 FU
Design room thermostat
Flush-mount room unit in Jung LS, CD, A, AS ranges. NTC sensor. Setpoint dial or electronic adjustment. Comfort/Economy/Standby mode DPT 20.102. Compatible with standard 55mm flush boxes.
Setpoint DPT and write mechanism
Room temperature setpoints in KNX use the same DPT 9.001 format as the measured temperature value. The setpoint is written to a separate group address by a KNX room thermostat, wall panel or from a visualisation interface. The actuator logic compares setpoint vs measured temperature and calculates PI control output.
Setpoint group address structure — typical zone
GA 3/1/1 → Room temperature measured (DPT 9.001, °C)
← MDT SCN-RTT55.02 temperature output
→ MDT AKH-0400.02 fan coil controller measured temp input
GA 3/1/2 → Room setpoint (write) (DPT 9.001, °C)
← MDT SCN-RTT55.02 setpoint output (21.0°C)
← KNX IP panel override (visualisation write)
→ MDT AKH-0400.02 fan coil controller setpoint input
GA 3/1/3 → HVAC mode (DPT 20.102)
← MDT SCN-RTT55.02 mode output (0=Auto, 1=Comfort, 2=Standby, 3=Economy, 4=Building protection)
← KNX time switch (schedule: workday = Comfort, night = Economy)
← Presence detector (occupied = Comfort, absent = Standby)
→ MDT AKH-0400.02 HVAC mode inputComfort vs Economy vs Night mode
KNX thermostats operate in four HVAC modes defined by DPT 20.102. The mode determines which setpoint temperature is active — each mode has a separate setpoint group address. Mode switching is driven by time switches, presence detectors, manual buttons or a combination.
| Mode | DPT 20.102 value | Typical setpoint (heating) | When active |
|---|---|---|---|
| Comfort | 1 | 21°C | Occupied hours — presence detector or time schedule |
| Standby | 2 | 19°C | Temporarily unoccupied — occupant left room briefly |
| Economy (night) | 3 | 16°C | Unoccupied night period — time switch setback |
| Building protection (frost) | 4 | 7°C | Extended unoccupied — holiday, shutdown mode |
Presence detector integration
A KNX presence detector can switch HVAC mode automatically: presence detected → Comfort mode (DPT 20.102 = 1); absent after hold time → Standby mode (DPT 20.102 = 2). The night time switch then overrides to Economy (DPT 20.102 = 3) regardless of presence detector state.
Mode priority
Building protection mode (DPT 20.102 = 4) must always override all other modes — configure the KNX actuator to accept Building protection write from the central panel regardless of local room thermostat mode. This ensures frost protection during building shutdown without local override risk.
Sensor integration in your KNX panel
We select and pre-wire KNX sensors based on your space requirements — presence detectors, CO2 sensors and temperature sensors integrated into the panel design with correct DPT mappings verified before delivery.
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