MOST RADIATORS COLD

[ns3270]

On Saturday I poured 2 litres of Sentinel X400 Sludge Remover into the system via a towel radiator, having drained off sufficient water from a drain tap downstairs.
All was fine with 19 of the 20 radiators. One (where I had added the drain tap above) was cold - it has never heated up properly but was now cold.
I turned off the 19 hot radiators hoping this would heat up the cold one. It didn't. I then turned on all 19 and they are now cold too.
I've opened the valves fully on both ends of all the radiators (I will balance when some heat comes through).
The first radiator from the pump is hot, the second is fairly hot, the third is just tepid after which all the rest are cold.
The pump was replaced 2 weeks ago and seems to be fine. There are no air locks in any of the radiators so I don't know what to do.
Could the Sentinel have caused a block somewhere if there was a lot of sludge in the system?
Should I drain the whole system now - even though the Sentinel should be there 'until satisfactory performance is restored - up to 4 weeks'?
Could there be an airlock in the pipes - how do I get rid of that?
Should I call out a heating engineer?

 

[DP]

Read the instructions!!!

 

[ns3270]

The instructions say 'where systems are badly fouled, a longer period of circulation (up to 4 weeks) is recommended.'
It doesn't say that most of the radiators will go cold while it is working. If that's the case and slowly they will get hotter then fine - but is that so?
Also if the radiators are all cold then is the water circulating at all anyway?
Am I still missing something here?