Central Heating radiators not heating correctly

[MC1]

Hello, I need some help please

I have an open vented central heating system with a Potterton Profile Boiler. Probably 25 years old

I was having trouble getting all of the radiators hot at the same time, by turning the upstairs off I could get most of the downstairs hot for a while and vice versa, but the temperatures would go up and down. The pump was making very loud whirring noises, so thinking it was the pump this was replaced with a Grundfos UPS3.

The problem has persisted (it may even be worse), the pump noise is too loud even on setting 1 and the radiators are still not heating, one or 2 will get hot but the boiler cuts out quickly, it never runs for long.

I have tried

• Balancing the radiators
• Draining and refilling the system
• Replacing the automatic air vent
• feeding mains water through the flow and return pipes - this seems to run well

When the pump is turned on and off there are loud water gurgling noises in the airing cupboard, where the pump is located. There is no air in the radiators

There is a manual (ball valve) bypass - 15mm from return to flow
There is an air separator
There is a shared feed and expansion pipe.

All that has changed since the heating worked well last winter is that a bathroom radiator was removed (pipes capped off) and a towel warmer moved and replaced with a taller one.
Any suggestions gratefully received

 

[SJB060685]

Sludge or scale build up?

 

[king of pipes]

Was the water very black when it was drained ?

 

[moonlight]

Blocked pipework, and quite possibly feed. Check with a magnet.