Power supply for room units: AC or DC?

In the UI... data sheets there is required power supply of 24 V AC. Sometimes, there is only a 24 V DC or even 12 V DC power supply in the panel, and we know that the room units do operate and communicate normally even at those power voltages when tested on our desk. So, is it possible to supply the room units by DC voltage in regular operation?

It depends on the utilisation of the inputs and outputs of the unit. The inputs (DI) operate reliably at 24 V, while their function is not guaranteed at 12 V. This means that if neither inputs nor outputs are used, 12 V DC is OK as a power voltage, and the unit works as expected: temperature and humidity are measured, settings of values and states is possible, and values and states are indicated on the LCD display.

The outputs use triacs as switching elements. A triac (TRIode Alternating Current switch) is used to control alternating current. In simple words, after activation it keeps the output open until the power current hits zero. This normally does not happen at direct current power, and the output would be kept active even after deactivation of its control signal.

As a result, units containing no inputs nor outputs (UI010, UI010BL, UI0x1, UI0x1BL) or those which do have DIs/DOs but do not use them may be powered by 12...24 V AC/DC. If only inputs are used at the room units, 24 V AC/DC is required. As soon as triac outputs are used, the units must be powered by 24 V AC.

When using DC power, G is connected to positive, and G0 to negative potential.