Should the underfloor heating controller be kept permanently open? A question of understanding

Hello, we will be building soon and have a heat pump with underfloor heating. Now I often read on the internet that the valves should always be open so that the heat pump does not have to work against the closed heating. Now I am asking, how does that work? I set the room thermostats to e.g. 21 degrees. As long as the room is below 21 degrees, everything is fine. But if I have more, either because the heating has warmed the room above 21 degrees or due to external heat, the valve closes.
So what is the "special" thing about the statement that the valves should always be open? As I said, up to 21 degrees they are always open anyway. Maybe you can help with that?
Regards and thanks
Pascal

The keywords are flow temperature and heating curve. The heat pump heats the water in the heating system. The temperature of the water is called flow temperature (from the heat pump towards the underfloor heating). This flow temperature is determined, for example, based on the outside air temperature. However, not entirely; it can be set, for example, 0 degrees outside = 25 degrees flow temperature. If the heating curve is set incorrectly, the heat pump "produces" a flow temperature that is too high. The underfloor heating heats the room more than necessary, the room controller closes the heating circuit.

The goal is to adjust the heating curve so that the heat pump reaches exactly the flow temperature needed for the desired temperature in the house. This saves energy. In addition, it is gentler on the heat pump.

Warning, this is strongly simplified and abbreviated. There are lengthy explanations about this online.

We bought a house with a heat pump and now I have similar thoughts, as I believe the elderly people who owned the house didn’t have much technical understanding. Does anyone maybe have a link on how to proceed to optimize the heating curve etc?

Read through the pages in [Haus_technik_dialog] - you can spend hours there with questions on topics like these

PS: why can't I actually write the term as one word? The post is then automatically changed to [HausbauDialog]?!

Thermostats shorten the time during which the underfloor heating is available for heating, as they switch it on and off. Conceptually, a shortened heating time is nothing other than if you were to install a small radiator instead of a large one.

To reach 21° in the house, a small radiator would have to be operated at a higher flow temperature (e.g., 35°) than a very large radiator (e.g., 26°). At an example outdoor temperature of +2° (air as the source), the temperature lift that the (air) heat pump must provide would be: a) with thermostat: 35° - 2° = 33K b) without thermostat: 26° - 2° = 24K

The occasionally ignored property of heat pumps: the smaller the temperature lift, the more efficient the heat pump is at producing the same (!) amount of heat. Therefore, as large and continuously operated heating surfaces as possible increase the efficiency/performance factor of the heat pump and reduce electricity consumption for the same result. For this reason, it is, for example, sensible to heat rooms such as central hallways continuously.

In practice, thermostats are often accompanied by a heating curve that is much too high, a buffer tank that absorbs arbitrarily high temperatures, and an oversized heat pump. The hydraulic balancing and planning of the heating surfaces in the rooms can be neglected with this setup. The heating engineer has little work with the planning, and the builder is happy because it gets warm and he can set the temperature as he pleases. The downside is unnecessarily high energy costs.

Alright, but at some point I have to regulate it, either through external heat or if it really gets warmer than 21 degrees? Without a thermostat, wouldn't it eventually get too warm automatically, even if only slowly? So you can definitely turn off a well-adjusted underfloor heating system at 21 degrees, even if it only approaches that temperature slowly? My thought would be to set the thermostat to 21.5 degrees, so that the underfloor heating can comfortably operate as set up to 21 degrees, and if it gets warmer due to external heat, the controller just switches off at 21.5. Is this line of thinking correct?