The Wallport wrote:

Don't understand the full meaning of the term "Two channels" but if
this means ability to control hot water on/off and heating on/off then
yes it could do this but there was only one set of times available for
on/off.

Two channels means as you suspect - the ability to control hot water and
heating independently. Your older timer may only have the ability to
have one or two sets of on/off times that can be used for both CH and
HW. The modern digital beasties will usually allow CH and HW to be
switched on and off independently at different times if required.

Don't fully understand the Y plan but I did trace the wires to a

Y plan is where you have the flow pipe from the boiler going to a three
port valve. From there it connects to two pipes - one to the HW cylinder
heating coil, the other to the radiators.

junction box and the hot water off wire does connect to a valve which
I presume directs the boiler output away from the hot water circuit
when it's off. I don't understand what happens when the heating is
switched off but the water is switched on.

Depends on the type of valve - the modern 3 port "mid position" valves
can be switched to 3 positions - A, B and A+B - so you can have the
boilers flow directed to either the HW, the CH, or split between both.
You can normally spot these since they will have a cable with five wires
going to the valve.

The other type you may find is a simpler diversion valve which has only
3 wires and only supports A or B but not both. With this type of valve
the HW is usually wired to take priority - it gets the full boiler flow
until its thermostat is satisfied. At which point either the boiler
shuts down, or the valve switched to the CH position if the CH is on at
the timer and the room stat is demanding heat.

Anyway, all seems well for the moment - unless anyone is going to warn
me of something I've not realised!

You will find out soon enough ;-)


--
Cheers,

John.

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