Central heating in the block - Cold pipes supplying heat to the radiator

I live in a block of flats with 2 rooms. The apartment is equipped with electronic heat meters. There are one meters in each room. The radiators in the kitchen and bathroom do not have dividers and heat at full capacity. In a large room, the pipes supplying water to the radiator are hot and there is no need to turn on the radiator to keep the room warm at +24C. In the small room, the radiator is also turned off, but the pipes are cold. Unscrewing the valve causes the pipes to heat up from below. The pipes and radiator heat up in proportion to the degree to which the valve is opened.

I noticed it only yesterday and I wonder why it happens in a small room and in a large room the pipes are still hot even though I don`t turn on the radiator at all, even for a moment. Moreover, yesterday I managed to turn on the radiator for a while and warm up the pipes. I then turned it off but the pipes stayed hot until the next day. After intensively airing the room, they cooled down. I wasn`t able to repeat this procedure today. The pipes cool down immediately after the radiator is turned off.

I am asking for help in understanding this process and in understanding how such a central heating installation works. I live on the top 4th floor. The block is old and has horizontal installations - a horizontal pipe runs through all rooms near the ceiling. Until today, I thought that it was used to distribute water to the risers and from the top to distribute hot water to the apartments. Today I was very surprised when I noticed that the pipe heats up from the bottom after turning on the radiator.

Moreover, I have no idea why there is such a large radiator in the kitchen. It runs from floor to ceiling, has four pipes with a large cross-section, two small pipes run from the top, one down to the floor, the other up to a horizontal pipe. Is this, apart from the radiator, an expansion tank or a vent tank? Please let me know, or link to them, or both.

Thank you kindly.

The "risers" may be hot in the block because someone above you is using the radiator and the heat flowing to it heats the pipes in your apartment. If no one uses the heating, there is no heat circulation in this section and the pipes are cold.

Since you live on the top floor, the installation of a "thin" pipe run horizontally is used to vent the system. The mysterious radiator, as you called it, from which various pipes branch off is most likely an expansion vessel that absorbs changes in the volume of water in the installation and should be located at the highest point of the installation (it is connected to pipes such as a safety pipe, an overflow pipe, a vent pipe, etc.). The cause of a cold riser in a small room is the lack of heat distribution by your radiator and most likely by the radiators in the small room of your neighbors.

Elegant explanations, thank you very much. But I am interested in this topic and I would like to learn something more to satisfy my curiosity to the end.

How does opening the valve in the radiator cause hot water to flow upwards - into the radiator and then to the horizontal vent pipe? Where does the air and water from the vent pipe go next - is it a closed circuit?

Why is there a vent pipe and an additional vent valve in the radiator?

I understand that the flow of hot water vertically is triggered by unscrewing the radiator on the given pipe to which the radiators are connected - that is, either my radiator or that of my neighbor`s through the wall triggers the water flow.

Are such horizontal pipes under the ceiling only on the top floor or on every floor?

I have lived in several apartments and this is only the second time I have seen a similar installation with horizontal pipes. Previously, I thought I saw only two vertical pipes - are there different types of such installations?

Once again, I am asking for information, for which I thank you very much.

Below is a link to materials about expansion vessels, as well as diagrams of the heating installation:
https://www.elektroda.pl/rtvforum/topic1900517.html
If you look through them, it should clarify a lot for you.

Everything is as explained by other forum members. In fact, the pipe heats up if the neighbor next door turns on the radiator. Just like with that large radiator in the kitchen. It serves as a radiator and a tank that absorbs changes in the water volume in the installation. Thank you very much for your entries.