Pump and radiator problem

[Badger2020]

We recently had a plumber out to clean our header tank and replace a bit of piping in our airing cupboard that had a blockage in. (They had done a powerflush a year ago without cleaning the header tank so I think muck had got into the system again as it was very sludgy in the tank) After the header tank was cleaned he showed me the tank and it looked fine. Now 2 weeks on it looks like soapy water in there (pic supplied). And despite our radiators getting hot upstairs, downstairs they stay cold apart from when I turn the pump up to it's maximum underfloor heating setting (we don't have underfloor heating). And even then still 2 of the radiators struggle to push out any kind of heat. I'm going to get the plumber out again but I'm suspecting either the pump is struggling after having to deal with sludge in the system in the past and / or there is still sludge in the system?

 

[ianmcd]

when I turn the pump up to it's maximum underfloor heating setting (we don't have underfloor heating).
What ?

 

[Badger2020]

The constant speed setting that's used for underfloor heating.

 

[D_Hailsham]

Which pump do you have?

 

[Badger2020]

Which pump do you have?

A DAB Evosta