Hi All,

I have a Y-Plan system with a Potterton PrimaF boiler, a Horstmann
CentaurPlus C21 programmer, 2x 2-port valves, a tank stat and a Danfoss
TP7000 room thermostat. I have just noticed that my boiler is switching
itself on despite the fact that the programmer for HW & CH is set to
off. I have done a little "basic" investigation as follows:

1, When I turn on the CH and/or HW via the programmer or room
thermostat the boiler, valves and pump appear to work correctly.

2, If I turn off the CH&HW via the programmer (and thermostat), the
boiler and pump continue to turn on/off on their own. When this
happens, both the 2-port valves remain closed and the radiators
(upstairs only) start to get hot (I'm guessing this is happening via
some kind of bypass pipe/value due to both valves remaining closed and
in "normal" operation the system is not zoned in any way to allow me to
heat upstairs radiators only).

Is there an easy way for me to determine where the fault lies without
replacing parts? My hunch is that it's the programmer at fault and I'm
going to try and borrow one to test my theory)

The system has been working okay for some time now, the only things
changed in the last 3-4 years have been adding the TP7000 2 years ago
and a new pump 8 months ago.

Any suggestions for me to try would be most welcome! :-)

Regards,

LN.