Valiant Ecotec 428

[Razor900]

The system really should have had separation between the boiler side and radiator side.

Using both pumps and a low loss header would have stopped the resistance of the heat exchanger being a factor but as its all working you can relax

 

[fezster]

That's the route I went down, after advice on this forum and lots of struggling. Meant I could buy 2 standard domestic pumps, rather than use a much more expensive commercial pump, and the whole system just behaves better overall (especially as I have multiple zones).

 

[Gasguru]

At some point, I'll measure temperatures of flow/return

Pull up D40 and 41 on the boiler display...that'll give you the flow and return temps across the heat exchanger.
Ideally you want 15 to 20 degrees...much more of a drop and you'll run into S53 errors and the boiler will sit around forever.
Remember a heating system can only be deemed fully functional at the design temperatures ie. when it's -1 or -3 whatever outside and the desired
indoor temperatures are achieved...hot rads on a mild day doesn't prove anything.