First 2 Radiator Fins Heating Up Only: Venting, Water Flow, Block Apartment 4th Floor

Hello.
I searched the forum for similar topics, but I couldn't find the same one, so I'm writing here how the situation looks like.

Only the first 2 ribs are hot, then cold. When I unscrew the vent at the radiator, nothing happens, nothing hisses and the water does not flow either. When I unscrew this bottom plug and drain the water, it finally comes out warm and the radiator down the entire length is warm, but for a while. I don't know what it is. I live in a block of flats on the 4th floor, before it was ok. Then the radiator kept airing up every now and then, but it was possible to vent it, now it's completely dead. How does the water spread in this radiator? Pressure too weak? Any bubbles on the 3rd rib? What can be? I will add that the radiators were recently replaced.

Clogged radiator from 2 ribs. In the spring, dismantle and pressure flush. If that doesn't help, it needs to be replaced.

If the first 2 ribs are really hot, this is a typical symptom of air pockets. To vent the radiator, you need to close the valve in front of the radiator (the one on the supply branch, i.e. the one for temperature control - the valve on the return - if there is one - is to be opened) and unscrew the vent. Air should escape through the unscrewed vent, and when it completely comes out of the radiator, water will start to come out through the vent. Turn off the air vent when only water comes out of it, without "snorting" (i.e. without air residues) and then unscrew the valve in front of the radiator (on the supply). It is not known whether this vent on the riser (with a butterfly valve) is not clogged, but do not unscrew it under pressure (so as not to make a geyser in the room).

Low water level in the system.
The first 2 ribs are warm because the circulation pump "pours water through them"
The water level is below the inlet to the radiator, while the pump gives enough pressure to raise the level slightly so that the water is poured into the partially empty radiator.

Check the air vent with the butterfly valve, report the low water level in the radiator to the administrator.

myestro, the radiator is new so I think it's not clogged because what? especially since it was hot beforehand. Zbigniew, I tried to vent in all ways, but the air does not come out, as I wrote above, and the water does not come out with the vent unscrewed all the way. I think strain is the closest to the correct answer. Because it was 2 days ago that the radiator was cold from 2 ribs then it started and was warm all night. So I think it's something with the pressure. I will contact the admin and we'll see

Added after 3 [hours] 55 [minutes]:

mystery solved. Some sensor did not turn on from pumping water. There was little in the way.

Hello. I wanted to join the topic because I have a very similar problem. I live in a duplex house. Recently, the radiator in the bedroom (on the first floor) heats only 2 ribs. I bled it according to the instructions above and it didn't help. I do not know how it is in the water in the circuit, because recently I allowed (until it overflows) and everything was fine, and then I poured a few buckets of water that were dripping from the pipe drained from the compensating vessel. And this happens every time you try to add water to the circuit. The eco stove works without a problem, the pump is full. It heats everywhere, only on this one radiator not!!! Any suggestions???

Is this the highest radiator? Probably the expansion vessel is ended with a vent instead of a pipe. Take a photo of the expansion vessel.

Yes the radiator is on top. The vessel is finished with a pipe - when replacing the stove with an eco one and modifying the installation about a year ago, the vessel was moved higher.

But interestingly, I fought with the stove - I increased the temperature on the stove without the pump to 80C and let the pump run to the max. I left the set temperature on the controller at 60C (interestingly, the thermometer attached to the stove shows 75-80C) and the radiator has been heating up for half an hour I don't know why it helped.

But I still have a question that came to my mind while browsing the net: Do you have to turn off and cool down the stove to allow water to circulate? Because opinions on this subject are divided

kamwod1986 wrote:

Is it necessary to turn off and cool down the furnace to allow water to circulate? Because opinions on this subject are divided

You can have it warm and you can have it cold. On a warm one, it should be done in moderation, rather slowly, so that very cold water does not suddenly flow into the heated boiler in large quantities. However, when supplementing water in a cold system, you need to know that later, after firing up, much more water will flow out of the expansion vessel - in an open system.

I also have this problem, I bought 2 aluminum radiators (previously they were panel) and both heat as in the picture - the circulation pump is on the 3rd gear and it does not change anything.
What could be the cause?
Radiator with bottom connection, maybe reverse connection?

Maybe it is connected incorrectly (was this heater factory designed for such a connection). In general, these bottom connections are not the best - they often cause a decrease in the efficiency of the radiator. It may also have air in it (the bottom connection means that when there is air in the system and it enters the radiator, it has no way to get out, even if the pump stops and you have to manually remove it with the air vent). I do not exclude the reverse connection (there are such crosses that will help to change the connections).