Some rads are cold

[LeeDowner]

I have 7 rads and one ladder style towel heater.

3 of the 4 downstairs rads are cold including the pipes leading to the valves, valves are fully open. 1 of the downstairs rads is only 4 metres from the boiler and that one is hot.

The towel heater and two other upstairs rads are hot but one rad is just warm.

I've bled all of them.

So is it thick sludge blocking the pipes and I need inhibitor to clear it? If so how do I do that?

I have a Ariston combi boiler.

Thanks, Lee.

 

[LeeDowner]

UPDATE.

My living room rad which was cold is now warm at the top but cold at the bottom, both valves are fully open and one is thermostatic, the boiler has a dial for how hot I want the heating water to be and that is set to the highest.

 

[andytw]

If you have thermostatic valves then it would be worth removing the valve heads and make sure the valve pin pushes up and down.
Sometimes they get stuck in the down position and just need a little easing to get them free.
There are even some plastic heads by Myson that break and hold the metal pin closed.

 

[terryplumb]

Lee ,has this problem recently surfaced,with all the rads that don't heat suddenly not heating together? Or had it happened over a period of time ,starting with one rad then a while later a second etc ,not heating up ?

 

[John D v2.0]

Turn off all the radiators that are hot and see if the other ones get hot. It could be that all your water is going through those rads and your system is not balanced.

 

[dilalio]

When was system last drained, cleaned and inhibitor added? Ever?
If you’ve had it for 5+ years and never been done then do that and see what state the system water is in. Flush through with clean water a couple of times, then add a cleaner for a week, then dump that and refill with inhibitor. If it persists you need a Powerflush.