Replace Honeywell ST6400C With Nest

26 Oct 2020
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United Kingdom
Hello all,

Now that I’ve had an unvented cylinder fitted, I’m considering having a Google Nest (3rd Generation) thermostat fitted.

I’ve read a few threads on here by others who also wanted to replace a Honeywell ST6400C controller (with room thermostat, hot water cylinder thermostat and a heat only boiler), but have a few questions that you might be able to help:

1) An electrician has quoted £150 to fit it. That was more that I expected but might have had unreasonable expectations. Is this about right?

2) Having removed the cover of the ST6400C (attached), I seem to have more wires that other photos I have seen on this forum. I get the first three are E,L and N, but does this look right?

3) Reading the installation guide for Nest, it seems possible (and not too hard) for a competent DIYer to fit it. As I don’t need the room thermostat anymore (will use the wireless feature), is it as simple as working out which connections go to which on the new Heat link?

Any help would be much appreciated.


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I think you have a y plan system so heating on, hot water on and hot water off wires in programmer, so wiring appears ok. I do think you had unreasonable expectations, and prices can vary up and down the country. Competent DIYer should be able to sort yes. Nest is only wireless through phone app, still has power via wires or usb plug.
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Thanks Chris. Yes, I should have said, I only have a single motorised valve for water and heating (under the hot water cylinder), which I think makes it a Y Plan rather than S.

I am tempted to try to fit the Nest myself, is there an easy way to know which current wire will map to the Nest?
Nest instructions are pretty good, and the rear of Honeywell should indicate which wires go where.
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Thanks again. I’ve had a look at the Honeywell wiring diagram, and this is what I have so far....

Current Honeywell (ignoring L,N & E):

1 - HW Off
2 - CH Off (Not used on my installation)
3 - HW On
4 - CH On

One the Nest Heatlink (again, ignoring L,N & E and the 12V supply for a wired thermostat), there are only 3 connections:

1 - Satisfied
2 - Common (Optional)
3 - Call for Heat

Any ideas which connections map to which, based on the above?


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Err, what nest is it you have? That looks like it could be for a nest e?
is the Nest Gen 3 diagram for the UK, there is also Nest e and there is Nest for USA but the diagram you show not seen before. Nest is basic designed for hot air systems, with hot air we have vents in the walls pumping out hot air and vents in internal doors so the hot air can return to heater, so all house is controlled by one device, in the UK we do have some homes with hot air and some open plan homes, but in the main we have homes with radiators and internal doors, and Nest only controls the heat in one area, it does not have the ability to link to more than one thermostat, most high end thermostats used in UK link to other thermostats be it wall mounted with EPH or radiator mounted with TRV heads.

So with Nest house design must be so one thermostat can control whole house, so has to be fitted in a room which is fastest to cool, you can adjust heat times with the lock shield valves on the radiators so you can set which room is the slowest to heat, what you can't do is make any adjustments on the heating system which alters cooling rate, in the main with most homes the hall cools fastest, so traditionally we fit the thermostat in the hall, which means very careful setting of the hall radiator TRV head, because the hall has a door to outside, so we want the hall to both be the slowest room to heat or other rooms in house will not have time to heat up, and also fastest room to heat as it needs to replenish the heat lost to outside.

So by setting the hall TRV a little lower to the wall thermostat we get fast heating until the TRV starts to close which replenishes heat when door to outside opened then it slows the heating rate to allow rest of house to catch up, the question is where does a thermostat with all clever algorithms fit into that system? Some how the thermostat needs to know if doors both exterior and interior are open or closed or connect to some remote sensors to tell it the temperature of other rooms, which Nest does not have.

Even Hive one of the cheapest wall thermostats has the ability to connect to TRV heads, the day Google took over Nest and withdrew support for the Energenie TRV heads it became unsuitable for most UK homes. Yes it can work, I have Nest Gen 3 in my own hall, but most the others work better.
If you go on to the nest website you can put your post code in and it gives you a local nest approved installer. I was quoted £80. But the poor chap ended up having to do so much remedial work from the monkey who fitted my boiler that I gave him a decent tip. He was here for 3 hours.

I’m not entirely convinced my install is right as my radiators appear to get warm when the hot water is on so it might not be wired to the valve correctly.
@Chris_W - I’ve not yet purchased the Nest, but you are quite right, I was looking at the Nest E instructions, when I should have looked at the Nest 3rd Gen ones.

Kindly, @ericmark posted the right schematic! And thanks for the background information, very interesting, especially about the thermostat position.

@motorbiking - Thanks, £80 for the install was closer to my expectations. I’ll give the Nest Website a go today and see who I can find.

I am still intrigued if I can do this myself. With the schematic supplied, I think:

- Honeywell Connection 1 (HW Off) - Where does this go?
- Honeywell Connection 2 (CH Off) - Not used
- Honeywell Connection 3 (HW On) —> Nest Connection 6 (Call For Water Heat)
- Honeywell Connection 4 (CH On) —> Nest Connection 3 (Call For Heating Heat)

Is that right and where does Connection 1 go to on the Nest?

Thanks again for all you help, this really is a wonderful forum!
Nest or any other on/off or change over thermostat I feel easy to wire, but I am an electrician so I would, what is a lot harder is the whole system and getting the balance right between money spent and the result.

A Drayton Wiser system or a Honeywell Evohome system are likely the bees knees, but a programmable TRV which is wifi linked is going to set one back around £60, I have seen non wifi linked for £10 although before brexit, but the problem is not so much the wiring up, but setting up and getting the right compromise.

I would guess my oil bill is around £500 per year, I would say a system is good for around 10 years, and a clever system is unlikely to save more than £100 per year, so the upper limit is around £1000, and 14 x 60 for TRV heads is £840 plus £200 for thermostat and it is clear you can spend more to save money than it is going to save you within the life time of the system. Clearly if it also improves comfort then maybe worth spending a little extra, but step one is decide how far to go.

Now for me with oil fired a little easier, but with gas the big question is will what I am going to do still allow the boiler to extract the latent heat? With gas this is a big issue, modern gas boilers to extract the latent heat would as analogue devices, the TRV gradually opens and closes, the boiler either with ebus (OpenTherm) or return water temperature slowly increases or decreases output, clearly when you leave the home or go to bed you may want to switch off for a time, but with the boiler running at 25% output it is far more economical to running 100% for a ¼ of the time.

So step one is look at the boiler and decide if the boiler you have can be controlled in an analogue method using wall thermostats? Not all can, some can old be controlled using the return water temperature, then you look at the aim over say 3 years, so you may say best long term option is two linked TRV heads, six non linked but still programmable TRV heads, two standard TRV heads, and two wall thermostats, then you look at which to do first which may be down to when a room is to be decorated.

I did this with mothers house, OK never completed, she died before that happened, but I decided 4 programmable TRV heads which would link to a master wall thermostat, it went wrong, as Nest Gen 3 removed their support for using Energenie TRV heads, however I actually found I did not need wall thermostats, once the programmable TRV heads were set up, the rooms were near as spot on as I could hope, living room still had small problem but that was the bay window causing that not the central heating control. All other rooms were within 0.5°C of target temperature.

This house still trying to adjust, the big problem with most homes is the radiator has two controls, the TRV and the lock shield valve, and without °C setting on the TRV, it is a bit of guess work which to tweak. What I found with mothers house, was once the TRV displayed target and current, I had the info to tell me is lock shield should be opened or closed, and once the lock shield was set, could use old TRV's quite successfully.

This house however I have a problem, selected hall for main wall thermostat, and the hall is too slow to cool, heating time you can adjust with the lock shield valve, but cooling not much one can do, so some times 4 am wake up cold, as bedroom has cooled but not hall. The cure would be a thermostat like Hive with a linked TRV head which can bring on the central heating even if the room where the wall thermostat is placed is warm enough, I made an error getting Nest as there are no linked TRV heads.

However three years ago when I started Hive did not have linked TRV heads, so cross fingers and hope Nest will also bring out some linked TRV heads.
Thanks @ericmark. I have (briefly) looked into Wifi TRVs, but as you suggest, the cost put me off.

We do have “normal TRVs” and spent some time balancing the radiator input and outputs, along with getting the temperatures of each room about right.

The attraction of Nest was more the ability to remotely control the heating (assuming we ever get out of Lockdown), as we would often go away and come back to a cold house without warm water.
I do see how the geofencing can help, I also use geofencing, or at least I have not disabled it. What would be nice is some thing like a which report saying which is best, however which is now an advertising system for suppliers that pay them, so we have no way to work out which system is best.

Reading what the manufacturers claim it seems Drayton Wiser has the edge. The problem is reaction time, I set my mothers TRV head to change from 16°C over night to 20°C in the morning at 7 am, but it was nearly 11 am before the room was sitting at 20°C, so I cheated and set it to 22°C then 20°C at 8 am and it worked.

The Drayton Wiser TRV head claims to use algorithms to work out how long it takes to heat the room, so can re-heat the room fast, but not tried, however it does not matter what the wall thermostat does, it will not heat room if the TRV is not open, so if the TRV is set at 20°C then however much the wall thermostat tries it can take a couple of hours to reheat the room.

We can cheat so if you know arrival time will be 4 to 7 pm at 4 pm the TRV goes to 22°C so when the wall thermostat turns on it is already open and one can guess by 7:30 pm it can return to 20°C, but we are back to the compromise, and you will need programmable TRV heads.

The eQ-3 programmable TRV head was £10 before brexit, I got the bluetooth version for £15, the Terrier i30 was a little more expensive, but prices have gone daft since brexit, think the eQ-3 is made in Germany, however still much cheaper than the wifi versions, one can clearly sit at work and get your phone out and put the heating up an hour before you leave work, however I would forget, odd I would remember in summer to check room temperature and turn on AC with phone, but would forget to turn on heating, so looking at speed.

Now for set times it was easy, staggered, kitchen, dinning room, living room, bedrooms. So rooms heated in the order used, but this does not really work with geofencing.

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