"Andy Hall" wrote


I've installed and worked on both and migrated from a vented to a sealed
system.

.............snip

Many thanks once again for such an informative response Andy.
Can I prevail on your time/kindness and ask you to look at the schematic
http://www.thesculls.freeuk.com/CHScheme.jpg please?
This shows the flow from the boiler to the vent and the relative position of
the make up feed.
It looks like I do indeed have the worst case when the hot water motorised
valve is shut. With the valve shut, both the pump and the boiler are
"between" the fill and vent connections. ie fill to pump suction, vent after
pump and boiler.
To get around this, can you see any problem with moving the fill to the
point where the water tees from the flow/vent to the h/w cylinder?
This looks like it would create an arrangement close to a combined fill/vent
but not quite.
(The schematic shows system items at approx the right relative elevations).

Thanks again

Phil