Aluminium radiators cold bottom-left. Can they suffer from sludge buildup?

[Italianconnectio]

We have just moved into a house with existing combi boiler, and aluminium radiators.
The radiators are attached to 12mm copper piping top-right (flow-side) and bottom-right (lockshield). They all have the same symptom: they are mostly warm (not boiling), but cold in the bottom-left quarter of the radiator.

A plumber cam round and quoted over £1,200 to install a more powerful pump/expansion tank system as he said the piping was too narrow and probably restricted flow to the radiators too much. He did not check the actual radiators for cheaper alternatives such as balancing or sludge. Oh, and he said no guarantees it would work!
-When I spoke to the plumber about the possibility of sludge in the radiators, he said that as the rads were made of alumiunium, there would be no sludge.

-There are 10 radiators on the system, so I have tried turning off 9 of the radiators: this makes the one open radiator warmer, but its bottom-left quarter is still only lukewarm. Both the flow and return pipes are hot.
-After searching on this website, I carried out the instructions for balancing the one open radiator, but this does not fix the problem either.

is it true that sludge could not be the problem? is the plumber right with his £1000 pump/expansion tank idea?

Thank you so much for valuable help!

[Italianconnectio]

jomifl, I double-checked the "'cools a bit by the time it reaches the zones" and I think i was wrong: I was feeling the rads.. but the piping itself (both flow and return) are too hot to touch for more than 1 second.

I've taken a photo of the where the 22mm piping becomes lots of 12mm..
https://picasaweb.goo...g#5679684060598344754
The column on the right is the flow, the column on the left is the return. Below those two, below the taps, there is a T junction going to the second zone. (I guess).
Only 2 rads are turned on. All of the 12mm piping off the right column (flow) are too hot to touch more than 1sec. All of the 12mm piping off the return columnn, are also too hot to touch apart from one near the top.
(I presume these are heating up by convection).
The valve that is attached to the left hand column is connected to the thermostat. Does this valve open/close both the flow and return? to the right of the valve you can see a white cap - this can be unscrewed but I am not sure what it does.
I'm not sure that any of this helps narrow down causes?
is the airlock scenario still a possible cause?

Thanks!
Melvin

[jomifl]

Melvin, I am not familiar with the motorised valve shown, It could be that it closes the supply and return simultaneously, but it is more likely that it bypasses the supply into the return pipe when the rads aren't needed. It is possible that the white cap covers a valve which can be adjusted to control the bypass flow rate. It may be the valve is stuck in bypass mode.
Re. airlocks, if the pipes to and from the boiler are vertical and run straight down to the boiler it is unlikely that airlocks are a problem. Is the pump built into the boiler or is it separate? Some pumps have variable speed, if this is the case with yours ,is it on maximum?

[jomifl]

Although aluminium radiators do not form sludge in the same way as steel rads. the oxidation products of aluminium are very voluminous and may be coating the inside of the rad. If as you say the return pipe from a rad is hotter than the rad then there must be some internal problem. Have you tried draining one of the rads to see if it is clogged in any way?

[Italianconnectio]

the pipes do not run vertically from the boiler as the photos you see is 2 rooms across and 1 floor up from the boiler. I have no idea what path the tubes take.!!

The pump is built into the boiler. It has 3 settings: off, slow, fast. It is on the faster setting, which is the recommended setting for the boiler anyway.

No I haven't tried draining one of the rads.. I asked the plumber to do it many times but he didn't seem keen (obviously preferring to go for the expensive, no-guarantees solution!!).
Is this something I can try on my own? OR do you need to be at least 2 to do this? (parquet floors, not carpets..)

Behind the white cap is a valve that I could try and turn with a spanner.. should I try it and see if anything happens? (hoping not to flood the place here!!)

[Italianconnectio]

with only 2 rads on, about 1/4 of the rad is almost as hot as the pipes. Then 1/2 is hot but you can keep your hand on it no problem, and 1/4 is just tepid.

[jomifl]

If the pipes connect to the rad. on opposite sides at the bottom you should be able to remove the rad. without too much difficulty. There are several posts on AB that explain how to do it with hardly any water loss at all.