Honeywell ST699

Product Information

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 PF Community Forums General Off-Topic Chat For Sale & Wanted Make a suggestion about the forum in general

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: The old room thermostat can't simply be disconnected as this would leave the heating wiring 'open circuit' and the heating will not operate, so, the two live wires that used to go to the thermostat then need to be joined together to complete the circuit. This can be done where the thermostat was by putting them in a junction box, or at their origin, and then the old thermostat cable can be disconnected completely. The Honeywell ST699B-1022 Electronic programmer is part of the Honeywell controls range at PlumbArena. 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. Ahh..............interesting. It was in the middle. I pushed it to the "manual" end a few times and allowed it to return to the middle. Then when I switched the main switch on, the pump ran for a second or two then stopped, which sounds to be what should happen.......................?But I see a post like yours, and I think of my central heating, I have two pumps, two motorised valves, the domestic hot water and bathroom radiator is thermo syphon, no control other than boiler run time as to domestic hot water, and closest plan is a C Plan, I still am not sure of pipe layout, but I have got a wiring diagram of the system. 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. 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?

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.Hi Graham. When you say the boiler is permanently 'on', what's it supplying - the hot water or the radiators? 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. of the current 3 wires in the wall with the (neutral maybe?) with its box off but as shown in the photo previously all were just wired into seperate boxes. Any idea which to connect to get the circuit closed again? 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. Thanks Les. I have a multi meter and I'm reasonably competent at electrical stuff but I have little understanding of what signals what and when when it comes to central heating systems.