Hello,
...new ones keep coming onto the market that sometimes promise up to 20% energy savings.
Well, colorful advertising promises fundamentally have nothing to do with the individual case. ;-) 0.01% are also
up to x% ;-)
What is correct, however, is that room thermostats prevent excessive energy supply above the setpoint (energy saving effect).
Here, one must differentiate between central room activation (lead room) and decentralized ERR depending on the heating surfaces.
Usually, there is an external, AT-controlled control loop (heating curve). The "room activation" or decentralized room thermostats compensate for the solar and internal gains not considered by this
additionally. Especially important in rooms with large window areas or possibly existing wood-burning stoves, particularly in well to very well insulated buildings!
Often, a heating system is operated "ad libitum." Occasional partial use! Here, ERR or room thermostats are indispensable, as a HA is practically overridden!
For radiators, "electronic" thermostat heads are offered. In my opinion, fundamentally very effective because they react
significantly faster than conventional radiator thermostats. The problem, however, is often the "universal" valve adapters.
The pressure of an existing, often integrated, unregulated radiator pump often leads to a
complete closure of the valve not being guaranteed! Usually a constructive adapter problem, not the closing forces!
Then you can also save yourself the effort (costs). ;-)
Best regards.