Honeywell ST699

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I opened up the Honeywell wiring hub near the boiler power switch to try to get a better idea of what the wires were doing This has lead me to think that while I'm replacing valves anyway I may as well use the opportunity to fit Thermostatic valves to all the upstairs radiators. I always thought I would need to replace the boiler system to get these but (unless I'm very mistaken) it's just a case of swapping the valves on the existing system. Not so worried about downstairs but our upstairs rooms are very prone to overheating and I think TRVs would really help and save money. The bypass provided a path from flow to return if both valves are closed buit the pump is still running. The manual valve should be replaced by an automatic bypass valve.

Is it a DIY job or something a professional should do, baring in mind I don't want to replace the whole system, just the timer/thermostat of the existing system? They've both got what looks like a switch with A - B settings, the one on the pipe running down past the cylinder is set to B, and the one on the pipe running into the cylinder is set to A, someone has also written on this one with a marker pen putting a C next to the A and an O next to the B (presumably meaning open and closed?). I've attached pics of the ST9400C wiring before and after the swap (ST9400c Wiring 1) and a pic of the remaining wires in the ST699.

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The only reason I wasn't going to bother with downstairs was because we don't really have trouble with the heat levels downstairs, if anything it's hard to keep it warm even when heaters are on constantly as it is a large open plan space and has a very large bay window at one end and French doors at the other. Whilst Hive has an earth terminal it isn't actually connected to anything. Hive doesn't need an earth connection itself. The terminal is just there to 'park' any earth wires that happen to be present and keep them together so that they are safely out of the way of the other terminals. Yep like that - they're called 'Lifestyle 2-port motorised valve', there is one underneath the pump on a pipe that doesn't go into the cylinder, and one on a pipe that come off the other pipe and goes into the cylinder about halfway up it.

I've been doing a lot of research on heating systems recently, and for those that have seen my other topic I've been having trouble with a cold radiator which I think will need new valves. My question is how difficult would it be to replace just the timer and thermostat (with a wireless one) without touching anything else in the system? Would I need to replace it with a Honeywell model or are all the wires like for like? If you have a S-Plan system with a single heating zone 3 for HW-On and 4 for CH-On, nothing to either 1 or 2.

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We have good water pressure in the house so I don't think a combi would be a problem, however that's a couple of years off yet (or until the current one dies!). Each Google Nest product works out how to make your home better. So imagine what they can do when they work together. If Nest Protect detects carbon monoxide, the Nest Thermostat can turn off your boiler, because it's a common source of CO leaks. And when the Nest Protect senses smoke, it can turn on Nest Cam to help find out where it came from. Also on your prior ST699 connections. The hard link from L to 5 means that when the hot water was "off" power would have gone across to 3 (CH-On)

to the terminals of the dual channel Hive that have exactly the same functions. So N goes N, L to L, Hot water on, to Hot water on etc... Using "ST9400c Wiring 1.jpg" going left to right can you indicate which core went into which connection on the ST699

Why settle for old technology, when Google Nest can do it all?

You had a hard wired link between Live, 5 (common for CH) and 6 (Hot water on). That's why I thought your hot water was permanently on. There was also a hard link between 3 (CH on) and 7 (HW off). Then there were connections to all four terminals for CH on/off and HW on/off which is not normal. So I wonder if the other end of those wires go to the expected places. I would really like to get rid of the thermostat in the hall altogether for cosmetic purposes and I would love to have more flexible control over the heating than the draconian timer we have now that has one setting for all days for both the heating and the hot water at the same time.

If so, you have a fully pumped system, but it's not being used to its full ability because the ST699 has not been wired correctly. Fortunately the changes I mentioned (motorized valves!) will not be necessary. It's just a case of installing a new programmer, correctly wired, and a thermostat. With the older type of systems, (without motorised valves) it's not possible to have the heating 'on' without the hot water being 'on' as well. These are usually older systems installed before 1980's, but can be upgraded. Changing the Hive to gravity mode just makes it automatically turn the hot water on whenever the heating is selected on.Whilst wire colours do necessarily mean anything on a central heating system, the combinations you have are a bit unusual. For example blue being used for CH on & HW on and brown for hot water off. That's based on your indication above about wiring on ST699.