Despite being a clever design, they couldn't be more obtuse, could they?

I'm trying to work out why, this morning, the central heating came on
despite the timer control being set to off and the thermostat turned
down. Hot water was fine, I think, and the thermostat on the cylinder is
certainly activating the boiler when appropriate. Controller is standard
old thing with a timer and "off-cont-twice-on" settings for HW and CH.

I assume it must be the 3-port valve (Honeywell Sundial Y plan), but
once all the pipes everywhere are hot it seems actually rather hard to
test it without waiting for everything to cool down again in order to
see where heat is being delivered when I mess about with the controls.


If I think the motor has gone, then if I disconnect all the wires should
it stay in "how water only" mode?

Also, I can't find out what the manual override lever on the valve is
for!!! Is it supposed to set it to "heating only", "hot water only", or
"both on"??

Michael