ABV always flowing, Smart valves and overheating boiler

13 Apr 2016
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United Kingdom
Just in case anyone has some ideas to point me in the right direction!

The system: Vaillant EcoTec 418 system boiler with unvented tank. S-plan (2 zone, CH and HW). Grundfos 3-speed pump (set to 2). Recently added smart valves (Drayton Wiser) to 5 out of 9 rads, so boiler load is reduced (all other rads have TRVs). Last week I replaced 6 rads to more modern rads.

The issues: recent boiler service shows overheating evidence in the exchanger (boiler is 4 years old and previous services were not as thorough... so this may be pre-smart valves and pre ABV retrofit).
Since the new rads have been installed the honeywell ABV lets water through even on the max setting (0.6) and when CH is open.
On fire up, the boiler struggles to modulate low enough after the first few seconds... once in a while it cuts out as the temp overshoots.
I had set the boiler to 14kW output. Should I go lower? Or is this the wrong approach?

The ABV loop is very short (can't dissipate heat well during anticycling). I'm thinking this might be the cause of the overheating evidence, so I am experimenting with forcing CH valve open all the time (ABV still leaks a lot, but heat dissipates better).
Is this a terrible idea? Will it break the valve?

I'm guessing I either have a faulty ABV, or the water can't flow through the system fast enough (it makes a lot of sloshing noise though). Or boiler too powerful for autumn conditions since most rads choked by TRVs or smart valves? (but surely even with all rads shut, the CH loop should offer less resistance than the ABV?)

Any thoughts greatfully received!
thank you
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