It's more complex than that. The main loop is a two-pipe system but there is a hydraulic shunt in every apartment that separates the two systems entirely, i.e. without the local pump running everything stays cold because the water in the main loop only goes into the shunt and back into the return pipe. It's a ridiculous setup but apparently the planner (this was only built three years ago) was worried about the pressure drop of the pipework and radiators and chose to split the system using the shunts. The thermostats control the local pumps (on/off).
Potable water is heated using a heat exchanger and another speed-controlled pump in each apartment, basically a tankless water heater minus the burner/electric element.
The whole system is garbage and I'd love to rework it but my uncle (who owns half the house) seems to be happy and doesn't want to change anything. Due to the hydraulic shunts the main pumps always need to run at 100% which makes them awfully noise and the noise carries through the pipes which happen to run right next to my bed. The system runs at 80 degrees C 24/7, heating up the rooms during summer and making everything expensive to run. Moreover, the pumps for the potable hot water are even noisier and the noise travels through the pipes into every single radiator.
A friend of mine is looking into the hydraulic side of the issue and might be able to figure out something affordable but we'll see.
