Then I’ll speak as a mixed installation owner. However, our house is not a KFW building but from 1990. I often wonder how such a mess could have been made. I installed a new heating system last year and am still trying to find the ideal adjustment, as you have to properly coordinate the flow temperature, heating ramp, mixer control, and underfloor heating. Of course, mixers and condensing boilers today automatically control many things, but that is exactly the problem. I have thermostats on the radiators. So they work quite dynamically. If I now change something in the individual room control of the underfloor heating, the system takes a few days to adjust the entire system back to the new state. Therefore, I removed the thermostatic valves on all UFH heating circuits and now only control 3 of the 9 circuits. This works quite well for me, and the different control loops don’t escalate each other. But it occupied me from November to February.
Of course, we need significantly more energy, therefore also higher flow temperatures than a new building; we also have a fairly open building design. Nevertheless, the underfloor heating causes significantly more effort in operation “just” for warm feet. To make matters worse, there were also isolated radiators installed next to the underfloor heating on the ground floor. We quickly dismantled those (except in the bathroom).
Of course, the underfloor heating has a lower flow temperature than the radiators, but that doesn’t help with the supply, since the radiators need at least 60°C. The mixer then reduces this for the underfloor heating. That is why the system is also more expensive, as a second pump, a mixer, and appropriate control must be installed.
If you had the option to go completely with underfloor heating (don’t know why that shouldn’t be technically possible), I would consider that because it’s effective, simple, and unobtrusive. But if the choice is only pure radiators or a mixed system, then definitely not the mixed system. It only creates more effort without additional benefit.
That you therefore feel like you have underfloor heating upstairs is, in my opinion, also not correct. Of course, the floor upstairs is not as cold as on the ground floor because the room below is heated. But there is still easily a 10-15°C temperature difference (depending on the floor covering). Underfloor heating makes the most sense in the bathroom, in the bedroom, and possibly in the living room. The first two rooms are upstairs for you, so underfloor heating doesn’t make sense, especially since you can quickly turn off the radiators when using the fireplace, while you cannot turn down the underfloor heating during that time. Although this doesn’t save costs, you won’t be heating the house twice.