Underfloor Heating KNX Zone Control: Electrothermal Actuators, Manifold Wiring and PI Control
KNX zone control replaces conventional ON/OFF thermostats with digital setpoint management, PI room temperature control and manifold pump logic — delivering precise temperature regulation per zone and eliminating short-cycling through pulse-width modulated relay switching.
Wet UFH system overview
Wet underfloor heating uses PEX pipe loops embedded in floor screed at 150–200mm spacing. Water at 35–45°C flow temperature (supplied by a heat pump, condensing boiler or mixing valve) circulates through each loop, releasing heat uniformly across the floor surface. Each loop is controlled by an electrothermal actuator (ETA) mounted on the manifold valve — the ETA opens or closes the valve based on the room thermostat signal.
Typical UFH manifold specification
- 10–15 circuits per manifold (one circuit per room zone)
- PEX-a pipe 16mm × 2mm, loop length 80–120m per circuit
- 200mm loop spacing in screed (150mm in bathrooms)
- 35–45°C flow temperature from mixing valve or heat pump
- One ETA per manifold port — KNX relay actuator controls all ETAs
KNX UFH vs. conventional thermostats
- Digital setpoint from KNX room unit or homeserver schedule
- PI control — smoother temperature, less overshoot than hysteresis
- Floor sensor overtemperature limit (EN 1264-2: 29°C max)
- Manifold pump logic: pump only runs when zones demand heat
- Integration with boiler/HP setpoint via weather compensation
Electrothermal actuator types
Electrothermal actuators use a wax element that expands when heated (by 230V or 24V) to push open or close the manifold valve. Two types exist — Normally Closed (NC) and Normally Open (NO) — with opposite fail-safe behaviour.
| ETA type | Unpowered state | Powered state | Power loss fail-safe | Typical use |
|---|---|---|---|---|
| NC (Normally Closed) | Valve closed | Valve open (heat on) | No heat — safe default | Residential UFH (most common) |
| NO (Normally Open) | Valve open (heat on) | Valve closed | All heat on — flood risk in summer | Cooling circuits where open = cool |
Standard ETAs: Danfoss TWA-A NC (24V or 230V), Honeywell MT4-024-NC (24V), Belimo TR24A-SR (modulating, 0-10V for proportional control). For KNX relay actuator output, specify 230V NC ETAs — the relay switches mains voltage to the ETA, opening the valve when the relay closes.
Danfoss TWA-A 230V wiring to MDT relay actuator
The MDT AKD-0824V.01 8-channel relay actuator (16A channels) mounts in the KNX panel. Each channel controls one zone ETA. Wiring uses standard 3-core cable (L, N, PE) from the panel to the manifold junction box.
MDT AKD-0824V.01 channel wiring — 8-zone UFH
Panel terminal block: L (230V live) → MDT relay channel 1–8 Common (COM) N (neutral) → Manifold junction box N bus bar PE (earth) → Manifold junction box PE bus bar Per channel output: MDT relay channel N (NO contact) → Danfoss TWA-A Live wire TWA-A Neutral wire → manifold N bus bar When relay channel N closes: 230V flows through TWA-A wax element Element heats → expands → pushes valve open Valve opens within 3 minutes (TWA-A response time) ETS6 channel configuration: Output type: Switch (not blind/shutter) GA: 8/4/room_no (heating valve, DPT 1.001) Inverted: No (1 = relay closed = valve open = heat on) For 8 zones: 1× MDT AKD-0824V.01 controls all 8 ETAs
Slow response: Danfoss TWA-A requires up to 3 minutes to fully open after the relay closes (wax element heat-up time). This is normal — do not mistake slow valve response for a wiring fault. The KNX thermostat PI integral accounts for this actuator lag. After power cut, allow 5 minutes for all ETAs to return to their NC (closed) position before testing.
Modulating valve actuator (Belimo TR24A-SR)
For zones above 20m² or high-specification residential projects, modulating (0-10V proportional) UFH valve actuators provide variable flow rate per zone — improving temperature accuracy beyond the on/off capability of NC ETAs.
Belimo TR24A-SR wiring and ETS6 configuration
KNX analogue output module: MDT AMS-0816V.01 (8-channel analogue output, 0-10V) Per zone channel wiring: MDT analogue output 0-10V+ → Belimo TR24A-SR Y(+) signal GND (0V) → Belimo TR24A-SR G0(-) signal 24VAC → Belimo TR24A-SR power supply Signal scaling: 0V → valve fully closed (0% flow) 10V → valve fully open (100% flow) ETS6 group address: GA 8/4/zone_no (DPT 5.001, 0–100%) → MDT analogue output converts to 0-10V PI thermostat heating demand (0–100%) → GA 8/4/zone_no Cost consideration: Belimo TR24A-SR: ~4× cost of Danfoss TWA-A NC Justified for zones > 20m² or where ±0.3°C room temperature accuracy is required
PI thermostat in ETS6
Proportional-Integral (PI) control eliminates the temperature oscillation and overshoot of simple hysteresis thermostats. UFH has high thermal mass — the screed stores heat, making overshoot the main cause of discomfort. PI control with the correct integral time prevents this.
ETS6 PI parameters for UFH zones
Device options:
MDT SCN-HCA.02 (heating actuator, integrated PI)
ABB Logic Module PI function block
Theben RAMSES 870 KNX room unit (built-in PI)
Recommended PI parameters for wet UFH:
Proportional gain (Kp): 20% per °C deviation
Integral time (Ti): 150 minutes
Control output: 0–100% DPT 5.001 (heating demand)
Output to valve:
Option A (on/off ETAs): PWM 15-minute cycle
Relay closes for (heating demand × 15) minutes per cycle
Example: 40% demand → relay closed 6 min, open 9 min
Reduces short-cycling vs. pure hysteresis control
Option B (modulating): 0–10V analogue output
Direct proportional valve opening (Belimo TR24A-SR)
No PWM needed — actuator follows demand signal
Floor sensor limit (EN 1264-2: 29°C max floor surface):
Enable floor sensor in KNX room unit ETS6 parameters
Floor sensor input: NTC 10kΩ embedded in screed
If floor sensor > 29°C → thermostat overrides room
setpoint and reduces heating demand to 0%KNX room thermostat sensor placement
Incorrect room sensor placement is the most common cause of UFH zone temperature complaints. The KNX room unit measures the actual room temperature used by the PI controller — placement errors directly affect control accuracy.
Correct placement
- 1.5m above floor on internal wall
- Away from heat sources (radiators, appliances)
- Minimum 1m from any person's regular position
- Not in direct sunlight or window draught
- Not in dead air corner behind furniture
Avoid these locations
- Behind doors (temperature not representative of room)
- On external walls (cold bridging affects reading)
- Near supply air diffusers (cool supply air skews reading)
- Adjacent to TV or AV equipment (heat source)
- Open-plan areas: use ceiling sensor for large spaces
Manifold pump and mixing valve control
The manifold circulator pump must only run when at least one zone valve is open. Running the pump with all valves closed wastes energy and pressurises the circuit. KNX logic monitors all zone GAs and controls the pump relay accordingly.
Manifold pump logic — MDT Logic Module
Zone GAs: 8/4/0 through 8/4/7 (8-zone apartment) Pump GA: 8/5/0 (Manifold Pump relay, DPT 1.001) Relay output → pump contactor 230V coil Logic: IF any of (GA 8/4/0 OR 8/4/1 OR 8/4/2 ... OR 8/4/7) = 1 THEN GA 8/5/0 = 1 (pump on) IF all of (GA 8/4/0 AND 8/4/1 AND 8/4/2 ... AND 8/4/7) = 0 FOR 3 minutes continuously THEN GA 8/5/0 = 0 (pump off — run-on delay elapsed) Pump run-on timer: 3 minutes after last zone closes Purpose: distribute residual heat in pipes and screed Prevents thermal shock to pump (sudden stop with hot water) Mixing valve control: KNX 0-10V analogue output → mixing valve actuator (Belimo) 35°C flow in mild weather → 45°C in cold weather Weather compensation GA 8/2/0 (outdoor temp) → MDT Logic PI → mixing valve setpoint GA 8/5/1
8-zone apartment commissioning procedure
Commission the UFH KNX system in the following order: room thermostats first, then valve actuators, then pump logic, then flow balance.
| Step | Action | Pass criterion |
|---|---|---|
| 1 | Download ETS6 PI parameters to all 8 room thermostats | Each thermostat shows correct room temperature reading |
| 2 | Set each room setpoint 3°C above current temperature | Relay actuator GA = 1; ETA audible click within 3 minutes |
| 3 | Test pump logic: all zones off → wait 3 minutes | Pump relay GA = 0 (pump stops after run-on timer) |
| 4 | Open one zone → verify pump starts within 5 seconds | Pump GA = 1 immediately when first zone opens |
| 5 | All zones open at full flow — measure flow per loop | Balance manifold circuits to equal flow using manifold flow meters |
| 6 | Set room temperatures to design setpoints; monitor for 24h | All zones stabilise at setpoint ±0.5°C — no continuous oscillation |
Common problems and solutions
Room temperature oscillation (short cycling)
Cause: PI integral time too short. Solution: increase Ti from 60 to 150 minutes in ETS6 room unit parameters. Also reduce Kp to 10%. UFH has very slow thermal response — aggressive PI parameters cause overshoot and hunting.
Zones not heating despite relay GA = 1
Check 1: verify ETS6 relay channel output type = Switch (not Blind/Shutter). Check 2: measure voltage at ETA terminals — should be 230V when relay closed. Check 3: ETA polarity (some ETAs have Live and Neutral marked — connecting reversed prevents operation).
One zone stays open permanently
Most likely cause: ETA wax element stuck (mechanical fault) — replace ETA. Check via ETS6 Group Monitor: if GA = 0 but zone still flows, ETA is stuck open. Alternative cause: KNX telegram stuck at 1 (check Group Monitor for repeated telegrams from thermostat with wrong value).
Floor overheating (floor surface > 29°C)
Verify floor sensor limit is enabled in KNX room unit ETS6 parameters — parameter group "Floor sensor" must be active with maximum 29°C. Check floor sensor resistance against NTC table at room temperature. If sensor reads incorrectly, floor limit will not activate.
Need UFH KNX zone control panels built and commissioned?
We supply relay actuator panels with pre-wired ETA circuits, configure ETS6 PI thermostats and commission manifold pump logic — delivered with zone GA tables and commissioning certificates for each apartment or floor.
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