RDF660 thermostat for EC motor control

A technical note on the Siemens RDF660 room thermostat and control of suitable electronically commutated motors.

RDF660 room thermostats and EC motors

The original product news discussed the growing use of electronically commutated (EC) fan motors in hotel and commercial fan-coil units.

Why EC fan control matters

EC motors may support efficient and quieter operation. A continuous 0–10 V control signal can provide more gradual fan-speed regulation compared with simple on/off or three-speed control.

Product variants

The archived text refers to the RDF660T version with scheduling and RDF660MB/MM with Modbus connectivity. Confirm precise interfaces and order codes in current Siemens documentation before specifying them.

RDF660 variants described in the announcement

Model	Archived description
RDF660T (S55770-T434)	Standalone room thermostat for two- or four-pipe fan-coil installations with EC motor control and weekly schedule
RDF660MB/MM (S55770-T433)	Standalone variant for two- or four-pipe applications with Modbus communication

In the original application, a modulating 0–10 V fan-speed output was presented as a means of improving comfort and noise compared with conventional three-speed fan operation. Actual efficiency and acoustic results depend on the fan-coil installation, motor, operating profile and controller settings.

Check the current Siemens datasheet and replacement status before specifying these historical part numbers.