I would suspect one of the switches is sticking and causing the intermittent fault you describe that makes the boiler stay on occasionally.

The st699 controller is volt free, so often there are two link wires to work with 230 volt supply, and boiler can be fed from the com or NO contacts with other contract being connected to line, with Hive the link to 230 volt is internal it has not able to run volt free, so there is no way to say which terminal on the st699 controller is the output so you can't say which terminal on the old goes to the new unit. I googled the direct replacement (after looking at the cost of a ST699) and the advice from Honeywell was the ST9400C, so I bought one. The problem is that sometimes with thermostat wiring a blue or black wire is often used for one of the live connections.It just seems odd there are 2 wires into the Neutral on the Honeywell and I wonder if the boiler not turning off sometimes is caused by a NC terminal not being connected. The engineer scheme is here to support you and your clients, simply save yourself time and money on finding parts and gain access to our massive catalog of over 110,000 parts. It consists of a 7 day CH and HW programmer, with complete independence between CH and HW, and a thermostat which communicates wirelessly with the programmer, so it can be located in another room. The use of three blue cores, two brown cores and one red core coming into the ST699 is a recipe for confusion. Tbh, we are going to struggle to take this any further unless you are prepared (and capable) to get a grip with the wiring and get your meter stuck in.

If Nest Protect detects carbon monoxide, the Nest Thermostat can turn off your boiler, because it's a common source of CO leaks. Yes, the picture of the ST699 is now - those are the linking cables left in after removing the other cables. The pump was powered from the room thermostat, so the boiler would run when central heating was required through the normal closed contacts of the DWH thermostat.My concern is that according to the ST699 installation PDF I linked above there should only be 4 cores connected (L, N, 3 and 8 ) but you had 6 instead (L, N, 3, 4, 6 and 7). This was prompted by removing the old thermostat that I thought wasnt working and doing nothing to find it was infact setting the heat of the house and plunged me (and begrudging partner) into a cold house situation. One may be the mains supply from the fused connection unit and the other the supply out to the boiler or wiring centre. The purpose of the second resistance (R2) is to prevent 240V getting onto the boiler (and lighting it) when CH is satisfied but the valve is still held in the CH only position. The programmer is suitable for surface or switch box mounting and comes complete with a built-in battery back-up.

Just investigated more closely and the right black cable on the bottom is the one leading down to the programmer, it has a yellow ( seems to be the earth), brown ( live ) blue ( neutral ) red white and black ( didn't see that downstairs ).

Unfortunately I'm not sure how to identify my heating system ( it is oil fired), I'll try to find out. 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. I thought the job was done however it seems that the heating still won’t come on… can’t tell if the hot water is working as when I turn the shower on it is hot but we do have a hot water tank. The other small radiator at the other end is back to back with the kitchen radiator but not sure if that would affect the TRV reading. You may need to gently move the lever each way to see where it's 'gone' ie: the mechanism inside the unit doesn't always 'pull' the lever along with it, but you should find the lever has gone loose if the thingy inside has moved.

The only difference is that the ST699 only provides time control, but Hive incorporates both time and temperature control in its software, so the old room thermostat becomes redundant. 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.

A room thermostat would probably be better, but I was mainly looking for quick (and cheap) fixes to improve the current system. Yes as a default I will just replace the wires to the exact layout of the Honeywell , utilizing the L N 3 and 4 terminals on the Hive. From an electrical point of view they are the same, so you are simply exchanging one switch for another identical one. Not so worried about downstairs but our upstairs rooms are very prone to overheating and I think TRVs would really help and save money. I have noticed however, that the old wiring to the ST699 that is redundant, wasn't as per the diagram I saw in one of the posts.