In an earlier contribution to this discussion,
jashton wrote:

Hi. I have a problem in my new house. It's a 'standard' gas boiler
feeding the heating and a DHW tank. There's a 3-way solenoid valve on
the exit from boiler to control flow to DHW, CH or both.

More likely, a 3-port mid-position motorised valve.

There was a DHW tank thermostat fitted but missing the cable. The
input for the thermo, in the controller panel, had been bypassed so
that DHW is heated to the same temp as CH water, ie too hot for me.
The problem is that, when I wired the DHW thermo back in, when it
reaches the set temp the hot water cuts off as expected but so does
the CH.
Is this down to the way the programmer and controller panel have been
installed? The panel has a number of wires around the edge, some of
which have been clipped. I assume they are left intact/clipped to
select various options during installation. I don't have instructions
to decipher though.
Thanks.

You have a wiring problem. Your system needs to be wired as per the Y-Plan
diagram in http://content.honeywell.com/uk/homes/systems.htm (with the
possible exception that the pump may need to be controlled by the boiler
rather than being wired exactly like the diagram).

The important thing is that the cylinder stat *must* have 3 contacts - and
be of the change-over variety - and all 3 wires must be connected. When the
HW is hot, the stat *must* provide a HW-satisifed signal, which must be
connected to the 3-port valve's grey wire. The valve needs to turn the
boiler on when only CH is required, and cannot do so unless its grey wire is
powered.

There should also be a HW-off connection to the programmer. If this is
connected correctly, you may find that the CH works when you turn HW off at
the programmer. This may offer a short-term solution while you sort out the
stat wiring.
--
Cheers,
Set Square
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