Solid wood parquet on underfloor heating

Let's just briefly summarize. Yes, it is better to glue down the parquet, otherwise the impact sound insulation/air layer acts as insulation. Possibly a bit more energy is lost downwards, since it doesn't move upwards as easily.

BUT: In real life, you won't be able to measure whether you have glued or floating parquet. Opening the windows three times has a greater impact on energy consumption.

BUT: Glued parquet sounds more valuable when walking on it, as it is firmly connected to the floor. That alone is reason enough to glue down the parquet. The 50 cents per month saved on heating costs don't bother me at all ;)

So you mean that with a fixed flow temperature and a fixed room temperature, the indoor temperature with an air layer will always be lower than without, no matter how long you wait?

Correct! (Obvious limitation: AT < RT < flow temperature and the pump's volumetric flow must also be the same.)

Where does the energy go then?

It stays in the water, in other words: the return flow is warmer. (or it disappears into the floor slab, but probably a rather small effect). Now one might say, well, if it stays inside, then no higher consumption has arisen. Here comes the point that we have to raise the flow temperature to achieve the desired RT. This causes greater losses and the heating system (especially the heat pump) operates less efficiently.

If it sits in the warm heating pipe, it wants to go into the cold room.

And exactly that is made more difficult by certain floor coverings and laying methods.

The faster it slips from the room through the house wall, the faster you have to heat so that it doesn’t get colder inside. If it slips from the heating pipe into the room as fast (or faster) as it migrates from the room outside, then "everything should be fine."

Fast and slow is somewhat difficult as a concept here. But basically correct. Again, some floor coverings slow down heat transfer, and to then achieve the same speed you have to send in a higher flow temperature to "reach the same speed."

The 50 cents per month saved on heating costs don’t bother me at all ;)

Basically correct. But if it is an issue, it should be answered properly and not dismissed as laziness. Depending on the floor covering and laying method, several degrees of flow temperature difference can result. It is said about 2.5% more consumption per degree of flow temperature increase. Whether that is relevant for oneself should then be decided individually and informed. I just find it strange when the spacing of the loops is calculated and optimized to the last detail, but the floor covering is suddenly unimportant, even though there are differences between tiles and vinyl versus parquet and carpet in the range of a factor of ten in heat transfer.

as I understood you, you would prefer to install it as a floating floor because you want to do it yourself and don’t want a completely hard floor, a so-called resilient floor :D. The most important thing is that you have heard from various experts that you can do it either way and that you are not committing a cardinal error. In that respect, I would go the way that I can implement myself, because it is important to me to do it myself or it would be fun for me. You can approach this (any) topic from many different angles, here most recently from the subject of temperature etc. Likewise, you can derive it from the unused glue in your house, in my opinion there is no right/wrong here. Since the forum is in my opinion very (often too much for me) technology- and calculation-heavy, it could be that the mere calculation regarding the effect of your underfloor heating would guide you towards gluing. But what would you answer to the person who explains to you exactly which nastiness is in the large amounts of glue that your family would probably inhale afterwards. Therefore, I would do what suits me better and for me that would definitely be the floating installation. The statement above that the other way is "better" seems too general to me, because it always depends on what I want to achieve. Until November 2021 we lived on about 200sqm of floating installed Meister parquet (2nd choice/1A-quality) and I really wouldn’t know anything that should have seriously bothered me about it. There was underfloor heating below and in between was the Meister insulation material with aluminum foil content. Like every product or processing, every side has advantages and disadvantages; you should simply determine what you prefer. Against gluing spoke for us that I would never sand finished oak parquet!!! Why should I? We had lived with a dog on pine floorboards and we didn’t sand that floor either, I also didn’t want large amounts of glue in the house. You just have to make clear to yourself that wood is still wood with all its properties, but not everyone wants that and therefore there are, for example, tiles in wood look, a contradiction that I cannot understand like vegan sausage. My son lives on glued parquet and I find it so hard, as if I were walking on tiles, which I don’t like. With floating installation the floor does not float or bounce, it lies perfectly flat on the floor. It is also additionally loaded by furniture etc. If you drop something it is not immediately broken as on a hard glued floor and it feels much softer to walk on than on the glued floor. Generally, I find the idea of gluing wood on cement somehow strange, but that’s just in my head. Here in the house, for example, we have screwed floorboards and the joists underneath are all laid floating. We probably could have screwed them as well but I liked the idea and it works quite well. Yes, sometimes it creaks or joints change but for heaven’s sake it is a natural product that I want, how can I then be bothered when nature shows itself this way. With the Meister parquet we sometimes had a walking noise in one room that sounded like the aluminum foil underneath. It never bothered us but the supply temperature. Take an underlayment without this aluminum foil, although I really find these microdetails unnecessary. I knew about the expansion joint too, but we were rather "bold" about it. I would consider whether to even make something deliberately beautiful out of this specification, like laying part in a different direction or in places where it fits a section with nice carpet or tiles etc., let your imagination run or use a nice metal strip where it makes sense. For me, with a tile I also like to see a joint just as with wood I can sometimes hear a knot or creaking, the often popular violation of material properties just to adapt to my ideas is not my thing.

Ok, if that is the case, then you have convinced me. But then I wonder why this actually plays no role in the heating load calculation. At least in mine, it didn’t matter whether I specified laminate or tiles. And here it is even about the same covering (maybe even the same) and only about an air layer that is presumably less than 1mm thick.

Where there is insulation, it moves slowly; where there is a good heat conductor, faster.

With good insulation, it doesn't move at all. As an example, an electrical cable is mentioned. Basically, it has nothing to do with speed, but with the amount of energy transferred. Because steel transmits a lot of energy, the heating element feels hot faster. It simply transmits more energy from the heating water to the air. An insulated floor cools down faster on top than the amount of energy coming from below. Because the energy is not slower but less. In the worst case, the energy is then insufficient.

Ok, if that is the case, then you have convinced me. But then I wonder why it actually doesn't play a role in the heating load calculation. At least in mine, it didn't matter whether I specified laminate or tiles.

The covering also has no influence on the "load". Rather the entry. So it does have an influence on the design of the underfloor heating and is taken into account there. Usually, however, for simplicity and due to lack of precise data, only a distinction is made between tiles and parquet.