Pump over/flushing

[simondiy]

Hallo,I have a few basic questions.
CH/HW system is a pumped system ,vented(wallstar boiler)
1 The last two rads stay cold at the bottom so I assume they need flushing/cleaning.Is there a best system cleaner/additive?(sludge removal)
2 CH on its own with pump on low power no problem,pump on 2nd or third power setting then water is pumped out of the vent.HW on its own,no problem,but if HW and CH on together then water comes out of the vent.The top of the vent pipe to the top of the water level is @13",I have seen a figure of 18" as a minimum so should I extend the vent higher,if so by how much?
Maybe the 3 way valve needs replacing?
Please ask for more info if needed.
Thanks

 

[hi-spec plumbing heating]

fernox / sentinel x400

whats the bottom of the f/e tank like for silt/sludge and colour ?

does sound like a restricted heating circuit

 

[simondiy]

The f/e tank water looks quite clear but on the sides/bottom there appears to be layer of "goo" which looks like its peeling off in a layer.
After the water comes out of the vent pipe the water turns a cloudy dark (black?)colour.

 

[hi-spec plumbing heating]

The f/e tank water looks quite clear but on the sides/bottom there appears to be layer of "goo" which looks like its peeling off in a layer.
After the water comes out of the vent pipe the water turns a cloudy dark (black?)colour.

good enough sign that it needs a clean.

 

[simondiy]

Thanks for the replys,looks like I'm busy this weekend
Some more info on the vent/feed pipe.I had just the CH on and after @20 mins the rads were hot (apart from the last two)so I felt the pipes in the airing cupboard and the feed pipe was hot but the vent pipe was cold(up to 6 feet then the pipes go into the roof).Later on with just the HW on the vent was hot and the feed cold.I am confused
Please can someone explain what is happening.
Cheers