Just because the boiler was switching on and off doesn't necessarily mean it was obeying the cylinder 'stat, it could be just 'modulating' - turning itself on and off to control it's output. (As the cylinder heats up, so will the hot water return. So the boiler needs to provide much less heat to this water before it sends it around again. Ergo the boiler may need to shut itself off for a short while. In this case, the pump would keep running.) I also had problems configuring Nest, it used terms like system boiler and mine is not a system boiler the pump is exterior, and it switched on the domestic hot water at what seemed random times, it turned out is was anti legionaries and it could be disabled, but took me ages to find out why it was doing it. It's often the case that a standard cable L, N, E is used to connect a room thermostat and the N conductor is often used as the switched live. It should really have a brown sleeve over it (or red in the case of older installations) to indicate that it is being used as a live, but not all do, as you have discovered.

Hi Graham. When you say the boiler is permanently 'on', what's it supplying - the hot water or the radiators? So hot water off on the programmer and hot water off on the tank thermostat but connect to the grey wire on motorised valve to turn DHW off, default is DHW on.My question is, as we have had to move some appliances around the kitchen, the st699 is now awkwardly behind the fridge freezer. Ok, try this; power off, and move the lever from one extreme to t'other a few times. What does it feel like? Does it move smoothly against spring pressure? Both ways? Does it make a faint 'whirring' sound both ways? Where does it 'settle' when released? Thanks, Sam. Could this resistor (R2) be affected so's its resistance is lowered? Usually when a resistor becomes damaged - overheated, etc - the resistance increases, but I wonder if something could have happened in this case to lower the value to the extent it keeps the boiler/pump running after the valve moves? Nah, I don't think so either... Just thinking aloud Not seen a Y Plan using gravity, in the main the DHW is the default mode, and the tank thermostat turns on the boiler for DHW and the tank thermostat when satisfied powers the grey wire which will cause the motorised valve to open all the way so only have central heating, the white wire powers the valve half way if no grey but all the way if grey also live. I have the same problem with mine and I just moved in to new house has this device. I tried everything but it still works for both of them at the same time.

The wires at the Honeywell ST699 move to the Hive terminals that have exactly the same function. Where there are two wires in a terminal both wires make the transfer and stay connected together. Graham, the motorised valve wiring - and what happens inside - is a lot more complicated than I thought! There's apparently a resistor/diode thingy going on which holds the valve in the CH positon even when the CH is turned off. The resistor is then meant to introduce enough of a resistance to turn the pump and boiler off. Yeah, I don't understand that either.However, the ST699 has two simple switches inside, one for heating and one for hot water, and so does the Hive. From an electrical point of view they are the same, so you are simply exchanging one switch for another identical one. For exchange purposes, what's on the other end of the wiring is irrelevant really, provided that the ST699 was installed and working correctly in the first place. Hi, I recently replaced a 27 year old Honeywell ST699 Programmer with a new ST9400c. Whilst system delivers Heating and Hot Water, it will not deliver Hot Water when HW only is selected (despite green LED being illuminated) i.e. boiler does not fire. My wiring changes were as follows: So i will also need to move the programmer when i upgrade it, but im not sure what cable to use to extend the length.