KNX Forced Control and Locking

Forced control tells the actuator what to do and allows nothing else. Locking merely tells it to stop listening, leaving its state to the coincidence of the moment in which it was locked.

At a window contact on a heating actuator the difference is immediately visible. With forced control the valve moves to a defined value and stays there, whatever the room controller sends afterwards. With a lock it simply stays where it was, and with the window open that may be full heating.

For anything safety-related, forced control is therefore the right route: wind alarm, frost protection, fire, interlocking. It prescribes a defined state instead of freezing an unknown one.

A normal switching command is one bit: on or off. Forced control needs one statement more, namely whether the force applies at all. That is exactly what DPT 2.001, "1-bit controlled", is for: one bit carries the value, the other says whether it is enforced.

This allows four states to be sent. Force off and the actuator carries on normally, or force on with the value it should hold. A lock object, by contrast, is an ordinary 1-bit object, because it knows only two states.

Some actuators also offer forced control as a pure 1-bit object. The forced state is then fixed in the parameters and the telegram only switches the force on and off. That is simpler to wire and less flexible, because the target state can afterwards only be changed in the ETS.

What an actuator does after the force falls away is a separate setting, and it is readily overlooked during parameterisation. Common options are: hold the state it had under force, return to the state from before, or track the value that arrived on the normal object in the meantime.

With a wind alarm this is the difference between a blind that stays up after the storm and one that moves back into shading by itself. Either can be right, but only one is intended, and which one is decided in the ETS, not at the device.

As soon as several functions can override the same device, a ranking is needed. Wind alarm must rank above shading, frost protection above night setback, and a fire above everything. Most actuators come with a fixed order for this; it is in the manual and is not negotiable.

Anyone who does not know it and instead uses two forced objects in parallel gets behaviour that depends on the order of the telegrams. That is the kind of fault which never occurs on the building site and does occur in the first storm.