Bleed valve

[Sally2000]

We're going to flush out our radiator system as it hasn't been done for many years and the radiators are not all heating as they should. We are quite happy with the procedure and have Sentinel chemicals ready.
There's just one question that I hope you can answer. There is a bleed valve on one of the pipe loops, which looks as if it should be automatic. Is this sure to work or is there anything we should do? The black cap on the top seems fixed. Should it be loosened?
Thanks in advance for any advice!

 

[terryplumb]

Black cap should be loose . May have been closed because it leaks. You should replace it whilst system is drained out.

 

[Gasguru]

I can almost guarantee that 99% of radiators not heating properly is NOT due to sludge...the problem is either poor balancing, let-by on diverter/mid-position valves or something else etc.
Cleaning a system without fully understanding the system can kill off a modern boiler.
What's the boiler make and model.

 

[Sally2000]

Thanks for the advice Gasguru. The radiators haven't been flushed for the 20 years we have been here and the lower ones have gradually got colder. We haven't changed anything in that time, so we think it deserves to be cleaned out and new inhibitor put in.

 

[Sally2000]

Black cap should be loose . May have been closed because it leaks. You should replace it whilst system is drained out.

Does that black knob just unscrew? It doesn't seem to want to move. It's all very clean so I don't think it's been leaking at all.

 

[terryplumb]

It won't leak if the cap on top is screwed down tight, nor will it be able to do what it is supposed to,which is to automatically expel air from the system. Just replace with a complete new valve.

 

[Sally2000]

We'll do just that. Thank you!