Radiator or piping clogged

[Z]

We bought a house some Months ago with a central heater. The old boiler (from 1974) was not working so we got a brand new one installed.
All radiators are fine but two, I'm not sure what to do about these so I'm asking here.

If I bleed water from both of them (a lot, almost half a bucket), they become warm but one one radiator the return pipe is cold all the time so it appears as if something is clogged within the radiator itself and not the piping. There's also an area at the bottom left of it that doesn't get warm at all. On the other one, the entire radiator becomes warm if I bleed it. The 'bleed screw' is located at the return pipe, but the return pipe is cold immediately after the screw as if the radiator is off.
There's no air in the system, there's a steady flow of water once I turn the bleed screw.

I don't know the last time the old boiler was used, it had been broken for many years but the previous house owner told me the oil on the tank is from 1985 - 1986 (tested and found OK), so it can be many years ago.

Question is, is there anything to do with this if the pipes are clogged up, but to replace the pipes themselves?
Are there any tricks I can do to perhaps 'force' the water flow through the pipes, or is this strictly a plumber job?

 

[taydo]

did the plumber flush the system through what did he have to say when he fitted the system.

it could be an airlock, try turning off the surrounding radiators and see if it works

 

[boilerbeasty]

Are all the radiators that old as well ? They may be well rusted, and dissolved into sludge which has formed in your radiator.
The second radiator gets warm because the water is able to flow through it. If it's return is blocked, there may be no flow through at all, so it won't have warm water running through.

Plumbers may offer a 'powerflushing' service. It can be a bit pricey, but ought to be thorough and give a good spring clean to the system. I think you can hire machines to do it yourself if you really want to.But just for these two rads, if you feel confident, you can take the radiator off the wall, carry it (carefully, with the ends sealed) into the garden and blast it through with a hosepipe. Does it have any little rust marks on the radiator ? They can get quite corroded, thin and form 'pin holes' which will leak water. In this case you might as well buy a new radiator (50 quid-ish).

 

[Z]

Thanks for the information.
I guess the radiators are as old as the house, but they look fairly clean and well maintained, there's no rust or corrotion at all to be seen on them.
The plumber didn't flush the system, he just stated that "these two are cold" and adviced me to bleed them in order to see if it was air in them that caused this.

He told me to turn the boiler way up and turn off all radiators but these two. I tried that yesterday, but could only have it that way for an hour - it's cold outside.
But if this is a trick, can it work and if so, for how long should I run on these two in order to see some result (yeah, silly question, I know)?

 

[taydo]

this might be a silly question but are the valves open both sides of the rads.

in answer to your question results should be seen within the hour really.

 

[Z]

this might be a silly question but are the valves open both sides of the rads.

in answer to your question results should be seen within the hour really.

I don't know much about radiator types, but all of the radiators in-house, have only one valve. If you're talking about the feed/return pipes, I wouldn't expect them to be too tight or something?