John Rumm wrote:
John Stumbles wrote:

thus a bypass is needed for an S-plan system but not Y-plan (where
the 3-way
mid-position valve provides a path via the HW cylinder)

Could you not still get into a problem situation though with Y plan?

If there was no call for heat from the cylinder stat, but there was
from
the room stat, the 3 port valve would have switched to the heating
only
position and eliminated the path through the cylinder. Meanwhile if
there were TRVs on all rads (including the one in the room with the
stat), then they could all close off but leaving the main stat
calling
for heat and no path through the CH circuit.

That's why you have 2 lockshield valves on the rad(s) in the room with
the thermostat.