Hi, I have a Vaillant ecoTEC plus boiler (in garage), Honeywell ST9400C controller and a bog standard Honeywell "dial" thermostat (in hall). The problem is that this boiler has a mind of its own and will switch the heating off and on when it likes and will not do what the controller and thermostat tell it to do. Please note this is an intermittant problem but it will occur every few days or so or after I have made a change to the controller. Typical scenario1: Cold day, heating comes on at 15:30 as per the controller, thermostat is set at about 18, the house heats up nicely and then the heating goes off about 6:30 ish. The house starts cooling and everyone is getting cold, then about an hour or an hour and half later the heating comes back on and stays on until 22:00 when the controller switches it off. During the time when the heating is off, the controller has the heating light on, the thermostat is calling for heat (I turn the dial and can hear the click at about 14 degress, then turn it back to 18 and hear it click again). The boiler however is well and truly off, no pump, radiator or flame indicator. I switch the whole thing off/on, turn the temp dial on the boiler up and down to no avail, still no heat. I have to wait for the boiler to decide. Typlical scenario2: Heating comes on and stays on, controller swicthes the heat off at 22:00, the light goes off on the controller, but that damn boiler keeps going and going sometimes past midnight whilst we're all sweating in bed. I have to either say a prayer for it to switch off or go into the garage and just manually switch the thing off. The wierd thing about this, is that it can operate for days with no problems and then decide to start playing games with me. I've also noticed that if I make a change to the normal pattern, say extend the heating by an hour one night or change the timings, then it will start to do it's own thing again. It will take 2 or 3 days to settle then it will operate normally. I've searched previously for this problem and noticed a few with the same issue. Someone mentioned that it could be controller wiring?