The term for this is 'EVU-Sperre'
Energieversorgungsunternehmen.
What you describe is off-peak tariff. Comes in different flavors. Depending on specific country it can have local names like
Economy 7(UK) or G12(PL). This is 60+ years old idea [1] so I am trying to understand the EVU-Sperre in the context of the above generic concept where price of the commodity varies in time and you decide whether you buy or not.
I did search for EVU-Sperre and googletranslated a
random article about it but I have no clue why does a distributor connect single load into this meter. Seems that they can discourage or even prevent the pump (and only pump) to turn on, even when it is cold in the house. Mind conceptually this is no different than setting two tariffs, one for €0.2/kWh and the other one for €2000/kWh so that no one sane is going to turn the pump on then, no matter how cold it is. This EVU-Sperre seems irrational at first glance, it over-complicates a simple and old idea. In case people need more heat (super cold winter, blackout, whatever), they are going to be forced to turn the resistive heating COP=1 on (because it costs €0.3/kWh whole day). Additionally you have two sets of wirings in the house with different price per kWh. One that is 0.2€ (for 18h/day) and the other one that is €0.3 (24h/day).

Where is the trick? Seems like a setback of 60yo idea.
[1] Taschenbuch für Heizung und Klimatechnik