Underfloor heating with KNX zone control offers precise per-room temperature management and energy efficiency that far exceeds conventional thermostat wiring. A typical residential installation with 8–12 heating zones uses KNX room thermostats with PI (proportional-integral) controllers, thermal actuators on the manifold, and a KNX scheduling logic that adjusts setpoints based on time of day, occupancy, and outdoor temperature. This article covers the complete hardware chain from manifold actuator to KNX room controller, including pump overrun protection, anti-freeze logic, and summer shutdown.

Manifold actuator and KNX switching output hardware

A 10-way underfloor heating manifold uses 24V AC normally-closed (NC) thermal actuators (e.g., Honeywell T4050, Caleffi 654000, Siemens STA23). NC type means the actuator closes the valve when de-energised — providing fail-safe frost protection if KNX power is lost. Each actuator connects to a KNX heating actuator group: MDT SCN-10TP.01 (10 outputs, 24V AC, DIN rail). The MDT heating actuator supports PWM (pulse-width modulation) output mode for modulating valve position — output 1 = valve fully open, output 0 = valve fully closed, with a configurable cycle time (15 minutes recommended for thermal actuators to avoid mechanical wear). In ETS6, configure each MDT output channel: Output type = Heating PWM, cycle time = 15 min, DPT = DPT 5.001 (0–100%). Each output connects to one heating zone manifold actuator.

Room thermostat and PI controller configuration

Room thermostats: MDT SCN-RT4.01 KNX room controller provides a built-in PI controller. Configuration parameters in ETS6: P-band (Xp) = 4°C (typical for underfloor heating), Integration time (Tn) = 240 minutes, Dead zone = 0.5°C. The PI controller output is DPT 5.001 (0–100% valve demand) sent to GA 8/1/[zone]/2 every 5 minutes or on change > 1%. Group addresses per zone follow this scheme: GA 8/1/[zone]/0 — Actual room temperature (DPT 9.001, read from NTC sensor or external KNX temp sensor). GA 8/1/[zone]/1 — Setpoint temperature (DPT 9.001, set by schedule or manual override). GA 8/1/[zone]/2 — Valve demand % (DPT 5.001, output from PI controller to MDT actuator). GA 8/1/[zone]/3 — Operating mode (DPT 20.102: 0=Auto, 1=Comfort, 2=Standby, 3=Economy, 4=Protection). For 10 zones, main group 8 sub-group 1 contains 40 group addresses total.

Pump overrun timer and anti-freeze protection

Heating distribution pump control: KNX switching actuator (MDT SCN-1600.02 or AKU-1616.02) drives pump contactor via GA 8/0/0 DPT 1.001. Pump overrun logic (implemented in KNX logic module or IP-Symcon): when the last heating zone sends valve demand = 0 (all zones satisfied), start a 15-minute timer. After 15 minutes with all zones at 0%, send GA 8/0/0 = 0 (pump off). If any zone demand > 0 during the timer, reset timer. This prevents thermal stratification and pump cavitation on sudden load drop. Anti-freeze logic: a KNX outdoor temperature sensor (GA 8/2/0 DPT 9.001) feeds a logic block. If outdoor temperature < 5°C and any zone setpoint is in Protection mode (DPT 20.102 = 4), the logic overrides the zone setpoint to a minimum of 18°C on GA 8/1/[zone]/1. This ensures pipes in poorly insulated areas do not freeze even in extended absences.

Occupancy setback, weekly scheduling and summer shutdown

Occupancy setback: PIR sensor (KNX, e.g., Jung 3360) sends DPT 1.001 to GA 8/1/[zone]/4 (occupancy). Logic block: occupancy = 0 for 20 minutes → subtract 2°C from active setpoint on GA 8/1/[zone]/1. Occupancy = 1 → restore comfort setpoint. The 20-minute delay prevents setback from triggering during short absences. Weekly setpoint schedule (ETS6 time controller): Weekday 06:30 → all zones: DPT 20.102 = 1 (Comfort, 21°C setpoint). Weekday 23:00 → DPT 20.102 = 2 (Standby, 18°C setpoint). Weekend 08:00 → Comfort. Weekend 23:00 → Standby. Absence mode: KNX visualisation panel (Gira Homeserver or MDT Glass Touchpanel) allows "Holiday" mode selection → all zones set to Economy (16°C) for duration. Summer shutdown: logic block monitors outdoor temperature sensor GA 8/2/0. If daily maximum outdoor temperature > 22°C for 3 consecutive days (counter incremented by day-end evaluation logic), the heating system switches to Summer Mode: all zone operating mode forced to Protection (15°C anti-freeze only), pump control disabled, boiler flow setpoint set to minimum. Summer mode auto-reverts when outdoor temperature drops below 18°C for 2 consecutive days.