Roger Mills (aka Set Square) wrote:

In theory, you shouldn't need it with a Y-Plan system - because there should
always be an available path for the water to take during the pump over-run
period (unlike S-Plan, where all zone valves are closed at this time). The
only proviso is that you need at least one reasonably sized (but not
massive) radiator which *cannot* be turned off - which means that it should
have 2 lockshield valves, and no TRV. There will then still be a path even
if the other rads all have TRVs, and they are all off. It sounds as if your
bathroom rad could meet this condition - but make sure that neither valve
has a handwheel on it!

Thanks for that reply Roger.

The bathroom radiator is relatively small, but I am considering moving
the TRVs from the radiator in my living room into the bathroom - since
that is where the room stat will be and I understand that the system
will not behave correctly if the room stat fights the TRVs. I may as
well have the living room radiator act as the bypass.

I assume that when the three-port valve is set to do hot water only,
that the cylinder coil will suffice as a bypass.

The pump overrun thing is a bit tricker than I thought. I had planned to
follow one of your recommendations and run a five-core cable between the
new wiring centre in the hotpress and the boiler downstairs, but it
turns out that there is not conduit all the way (it goes across a floor
as well as between walls) and tracking walls to install cables is not
something I want to get into. Instead I'm going to re-use the existing
three-core cable, which up until now has supplied mains to the pump, to
carry the SL and PL signals to/from the box, and then provide the boiler
with it's own permanent live from the existing mains connection in
downstairs. I guess that should be easy, just a matter of putting new
sleeves on the wires to ensure that it is clear they are live.