2 of my radiators (upstairs) are barely getting luke-warm. I’ve shut the lock-shields down on all the other rad’s in the house and the temperature doesnt change so im fairly sure its not a balancing issue (?)
The rads are also cold at the bottom & luke warm at the top – so i suspect there is a build up of sludge / corrosion in the rads (?)

Are the DIY sludge treatments effective? Or am I as well fitting 2 new radiators? (i figure for the cost of a pressure treatment I could fit new rads myself).

Since I’ve lived in the house the system has always been run with corrosion inhibitor. Obviously before that I cant be sure.

I drained the rads, took them off the wall and carried them carefully outside, then hosed them out. Ideally take the plugs out of the top so that you can hose diagonally.

Check the TRV is working too, often they stick over summer. A spray of WD40 and a whack is normally enough to free them up if you take the plastic head off.

Thanks chaps. Already done a hose clean and checked / swapped the TRV for one that definitely works. Still no joy.

Check the TRV is working too, often they stick over summer. A spray of WD40 and a whack is normally enough to free them up if you take the plastic head off.

This.

It’s not the twisty plastic knob but the little brass pin in the valve that sticks. Unscrew the plastic bit and you will see the brass pin. Pull it up gently and when fully extended it will allow the flow of water into the radiator. Spray WD40/Gt85 on the pin and work up and down before refitting the plastic knob.

Once you have a buildup of sludge you need to flush the radiator or system to get rid of it. The treatment only really prevents it.

Same thing happened here, two rads not even coming on. Increased the boiler pressure to just under 2 bar and it did the trick.

swap the rad and the valves… might not cure but at little cost will set you on a path..

We had this about a year ago. Turned out to be blocked pipe going to the radiator – it was plastic, but not the right kind and had degraded internally until the pipe was almost totally blocked.

We had similar when we moved into our new house. One radiator wouldn’t get warm. I shut all the others off and still nothing happened. I drained the whole system and I was amazed how filthy it was inside. The water came out black with big chunks of corrosion. I took most of the radiators off and flushed them through many times in different orientations until the water run crystal clear. I also added a cleaner to the system and ran it for a few weeks and again the water was black when i drained it. I filled it back up with just water and ran for a few days and drained it but again it was black. I think i ended up draining and filling about 4 times and when the water was considerably clearer i added another cleaner and ran it for about a month. When I drained it this time it was still filthy but considerably better than it had been. The radiator still wouldn’t get warm though. i traced the pipework back to the boiler and came to the conclusion that there was a blockage in the bit of pipe work i couldn’t get to. I cut the pipe as far back as I could and fed some stiff wire rope up as far as i could but i think i came to a 90deg elbow or similar but the wire wouldn’t go any further even when i spun it with a drill. In the end i made up a piece of pipe with a plumbers pressure gauge with a schrader valve and attached a bike pump. I opened everything else on the system and pumped it up. I heard something go bang and then water came pouring down. This solved the blockage and after another cleaner treatment worked perfectly.
We recently had a new boiler installed and I asked for a power flush to be done (and i believe the warranty asks for one) before the new stuff was installed. I have also had a magnetic filter added for piece of mind.
The lessons i learned from all this is look after your central heating system with proper inhibitor and don’t use microbore which blocks easier than 15mm pipe.