Floor Heating + KNX Zone Control
Wet underfloor heating with individually controlled zones, KNX room thermostats and pump management logic. The complete design guide for EU residential projects.
How wet underfloor heating works
Wet (hydronic) underfloor heating circulates hot water through plastic pipes embedded in the floor screed. A manifold — typically Uponor, Rehau, or Danfoss — distributes hot water from the heat source (gas boiler, heat pump) to individual floor loops, one per room zone.
Each loop is controlled by an electro-thermal actuator mounted on the manifold. The actuator is a normally-closed valve: when a 24V or 230V signal is applied, the valve opens and hot water flows through the loop. When de-energised, the valve closes and the zone cools down.
⚠️ Normally-closed vs normally-open actuators
Most EU floor heating actuators are normally closed (NC) — valve open when energised. In a KNX system, this means a zone call for heat = actuator output ON = valve open. If power fails, all valves close — no flooding risk, but also no heating. Some systems use normally-open actuators for fail-safe heating (valve stays open if power is lost) — check the actuator datasheet before wiring.
Common floor heating actuators
| Brand / Model | Voltage | Type | Power | Note |
|---|---|---|---|---|
| Uponor X-77 | 24V AC/DC | NC electro-thermal | 2W | Standard residential, M30 thread |
| Danfoss TWA-A | 24V AC | NC electro-thermal | 2W | M30×1.5, very quiet |
| Rehau 13720 | 24V AC | NC electro-thermal | 2W | M30×1.5, common in CEE |
| Honeywell V8043F | 230V AC | NC electro-thermal | 3W | Legacy 230V installations |
| IMI Hydronic TA-SOCL | 24V DC | NC motor-driven | 1W | Fast response, PWM-controllable |
| Watts RT-24V | 24V AC | NC electro-thermal | 2W | Budget option, M30×1.5 |
KNX actuator outputs for 24V floor heating
A KNX multi-channel output actuator provides the 24V AC switched outputs to drive floor heating actuators. Common choices:
MDT
16-channel 24V AC output actuator. Each channel independently switched. Rated 0.5A per channel. Perfect for manifolds with up to 16 zones. DIN rail.
Hager
12-channel 24V AC actuator. 0.5A per channel. KNX heating controller with valve protection function (prevents sticking). Popular in CEE.
ABB SA/S 16.16.6.1
16-channel 24V AC binary output. Part of ABB i-Bus KNX system. Also available in 12V DC variant for some North European installations.
Zennio
4-channel fan coil + heating actuator. Handles 2 or 4-pipe fan coil units AND up to 4 heating zone outputs in one device. Hotel and apartment use.
KNX room thermostats and PI control
A KNX room temperature controller (RTC) measures the room temperature and calculates the valve open time needed to reach the setpoint. Two control modes are available:
2-point (on/off) control
Simple hysteresis: if temperature < setpoint − 0.5°C → output ON. If temperature > setpoint + 0.5°C → output OFF. Simple to configure. Works adequately for large zones but causes temperature oscillation (±0.5–1°C).
PI control (Proportional-Integral) ✓ Recommended
Calculates valve on-time as a percentage (0–100%) every cycle (e.g. 15min). E.g. 60% call: valve open 9min, closed 6min. Eliminates oscillation. KNX object: DPT 5.001 (0–255 = 0–100%). Temperature accuracy: ±0.2°C.
The MDT AKU-1616.03 and Hager TXA212B both support PWM (pulse-width modulation) control on each channel — they receive the PI controller output (0–100%) and automatically switch the 24V output for the correct proportion of the cycle time.
Heating pump management with KNX logic
The heating pump should run only when at least one zone is calling for heat. Pump run-on when all zones close wastes energy and heat. A simple KNX logic block handles this:
KNX logic — pump control
Zone 1 valve state ─┐ Zone 2 valve state ─┤ OR logic → Pump enable signal Zone 3 valve state ─┤ (with 3-min on-delay) Zone N valve state ─┘ Pump ON delay: 0 min (start immediately when zone opens) Pump OFF delay: 3 min (run-on to clear residual heat) KNX objects: Zone valve states → DPT 1.001 (1-bit) per zone OR logic result → DPT 1.001 Pump actuator → DPT 1.001 binary output
In ETS, this logic is typically implemented in the MDT Logikmodul (SCN-LOGIC.01) or equivalent. The 3-minute off-delay prevents the pump from cycling on/off rapidly when zones are borderline on their setpoints.
Setback scheduling and presence integration
| Mode | Setpoint | Trigger | Typical saving |
|---|---|---|---|
| Comfort (occupied) | 21°C | Presence or schedule | Baseline |
| Standby (short absence) | 18°C | No presence 30 min | 10–15% |
| Economy (night) | 17°C | Night schedule 23:00–06:00 | 20–25% |
| Frost protection (vacation) | 13°C | Vacation mode from KNX | 40–50% |
KNX weekly timer sends HVAC mode commands to all room thermostats simultaneously. Presence detection (KNX PIR sensors or phone-based presence via Home Assistant) adjusts modes in real time. With heat pump integration (see heat-pump-knx guide), tariff-based pre-heating further reduces energy cost.
Floor heating panel design
We design KNX panels with 16+ zone actuator outputs, PI thermostat integration, and pump management logic — fully documented and CE certified.
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