Radiators that don't bleed?

[dkayton]

I have read quite a few of the posts here. I have a two-zone heating system that also heats hot water. The downstairs zone and hot water are working fine. The upstairs zone used to work fine. Now, however, the radiators upstairs at one end of the house are not getting hot. They do seem to get warm occasionally.

Anyway, when I read about problems similar to this, they either say the system isn't balanced or I should bleed the radiators.

For the first point, if the system isn't balanced, why did it used to work?
For the second point, the radiators in my house don't have any valves of any kind. They have a pipe that comes up from the floor on one side and goes back down on the other side, with fins to dissipate the heat. It seems completely sealed, except at the furnace where there is a water spigot for each zone.

Can anyone please help get these radiators working again?

 

[taydo]

dont no about your country but in the uk in the top left or right hand side of the rad should be a square nut about 5mm this is the bleed valve

 

[dkayton]

I looked several times on the right side, the left side, in the back, etc. There are no bleed valves of any kind. They are very low baseboard radiators that extend the length of the room but are only about 25cm high.