Anti Vac Valve
After replacing a anti vac valve should there be any noise or fissing at all once the valve cuts off or what is the norm for this, there seems to be a little fissing noise going on just wondering if that was normal.
There should be absolutely no hiss or fizzing from the anti vac valve. If there still is anything like that, you need to re-replace
Once they pop shot when the boiler reaches pressure they make no noise (perhaps a bit of water droplets boiling off for a few seconds).
Maybe there is a bit of crud caught in the seal or the seal between the valve/boiler is leaking. If you can fire up machine while looking at the valve it should be obvious where it's coming from. Beware live power etc etc.