The Danfoss TWA-A (Thermal Wax Actuator — Actuator) is the most widely installed floor heating zone valve actuator in European KNX installations. The TWA-A NC (normally closed) 24V version is the KNX standard: with 24V DC power removed, the valve closes — a fail-safe behaviour that stops heating if the KNX system loses power. Each TWA-A controls one zone circuit on a manifold (typically 8-12 manifold ports per heating circuit zone manifold). In a standard 6-zone underfloor heating system: 6× TWA-A 24V NC + one 8-channel KNX heating actuator module (MDT AKK-08.01 or ABB SA/M8.230.1) provides zone-by-zone KNX control.

TWA-A specification

Danfoss TWA-A NC 24V (article 088H3101): 24V AC/DC supply, normally closed (valve closes when power removed), IP21 rated, 3-wire connection (L, N, Earth), mounting: M30×1.5 threaded union on manifold valve body. Power consumption: 2.5W in operating phase (valve opening against wax actuator spring), 0.5W holding power when open. Opening time: 3 minutes from closed to fully open (thermal wax expansion). Closing time: 3 minutes from open to fully closed (spring return). Important: the 3-minute transition time must be accounted for in KNX PI controller timing — do not use pulse widths shorter than 5 minutes for TWA-A valves or they never fully open before closing again.

KNX heating actuator module wiring

MDT AKK-08.01 heating actuator (8 channels, 24V DC, DIN-rail, 4 modules wide): each channel = one relay contact switching 24V DC power to one TWA-A valve. Panel wiring: MDT AKK-08.01 terminal L+ (24V DC from PSU) → each TWA-A L terminal via separate cable. Neutral/negative from PSU → all TWA-A N terminals via neutral busbar. MDT AKK outputs switch the 24V DC positive — output contact in series between 24V bus and TWA-A terminal. Cable: 3-core 0.75mm² (or 2-core where earth not required for 24V SELV circuits). Run from panel to underfloor manifold location. Max distance: 30m at 0.75mm² for 2.5W load — voltage drop under 1V.

ETS6 configuration

In ETS6: add MDT AKK-08.01 to project topology. Per channel: heating mode = PWM (pulse-width modulation, 0-100%), PWM cycle time = 15 minutes (minimum for TWA-A thermal actuator — shorter cycle causes excessive valve cycling and reduced actuator life). Alternatively: PI control mode internal to AKK-08.01 (reads temperature sensor GA and room setpoint GA, calculates valve opening percentage). Group addresses: for each zone channel: GA_Valve_Zone1 (DPT 5.001, 0-100% valve opening), GA_Temp_Actual_Zone1 (DPT 9.001, actual room temperature from KNX room thermostat), GA_Setpoint_Zone1 (DPT 9.001, target room temperature). PI controller parameters: P-band 5K (temperature difference for full opening), I-time 180 minutes (integral time), dead band 0.3K (hysteresis).

Pump control integration

Uponor manifold with 6 zones: when all 6 TWA-A valves close, the circulation pump should stop to avoid pump cavitation and energy waste. MDT AKK-08.01 has a pump output contact (channel 9, normally open, closes when any zone valve is open). Wire pump output contact → 24V DC pump relay → Grundfos UPS circulation pump start/stop input. KNX GA: GA_Pump_Status (DPT 1.001, 1 = pump running) → connect to pump status binary input channel on MDT BE binary input module → monitoring via KNX room controller display.

Dew point protection for cooling

For KNX cooling mode with underfloor circuits (radiant cooling): dew point protection is critical — surface temperature must stay above room dew point to prevent condensation. Elsner P04-KNX-GPS provides outdoor humidity and temperature → calculates dew point and publishes DPT 9.001 GA. KNX function block: if surface temp (from floor sensor) < (dew point + 2°C) → close all cooling valves and send alarm GA. Floor sensor: PT1000 or NTC resistance thermometer wired to KNX analog input module (MDT AMS-0816.01) — compare floor temperature against dew point GA in ETS6 logic comparator block.