Hydraulic short circuit? Return flow of radiator upstairs warmer than supply flow

Thank you for the information. Then I think I understood it. I see the problem in the second pump. It would probably be best to have this removed. Otherwise, let's see what happens if it is simply switched off. However, the thermostat in the distributor is still not clear to me.

The problem is most likely the pump installed in excess in the underfloor heating circuit if there are no additional valves on the supply and return lines. You can imagine it like this: The UWP in the heat pump generates a differential pressure between the supply and return (in the heat pump) of 100mbar. According to the pressure drop in the individual circuits (radiator and underfloor heating on the ground floor), you normally have the distribution of the volume flow, e.g., 40/60%. If you now have an additional pump in the underfloor heating that creates an additional pressure difference at this point between supply and return, with the given wiring you have a flow of the radiator circuit in the wrong direction. How is the pump controlled? If this does not cause a control error, I would first switch it off, otherwise deactivate the heating circuit assigned to the pump in the control so that only the main pump runs. In the correct direction, you can also flow through UWPs without much resistance.

Good morning,

@Rotor: the thermostat in the distributor (which is attached directly to the fixed value control set) is intended for temperature control of the underfloor heating circuit, but since I use the same supply temperature throughout the entire system, it does not really serve a purpose.

: The circuits are not really separated; there is an outlet on the air-water heat pump which then goes to the underfloor heating and radiators (this can be seen in the photo). Since the second heat pump on the underfloor heating now creates a suction in the supply of the entire system and a certain pressure on the return side, the overall system only works moderately, in any case it creates back pressure in the radiator circuit. The pump of the underfloor heating is only controlled by a characteristic curve; there is no direct connection to the air-water heat pump. There is a PWM cable, but currently I do not know whether and how I can control the signal via the air-water heat pump.

I will try deactivating the pump and hope that the pressure distributes "well" across the underfloor heating and radiator circuits. Yesterday I found out that the installer must have incorrectly set the fixed value control set because the entire return flow of the underfloor heating was returned to the main return (bypass of the fixed value control set was set to 0), which of course created a high pressure in the return. I will observe this over the next few days and report whether these measures have resolved the problem!

Many thanks for the great hints; despite a certain understanding of the issue, sometimes you just cannot see the forest for the trees. ;)