Current honeywell set up can the nest smart thermo replace it?

[monc3]

Hi guys,

I've been searching the net for weeks now for any info and struggling to get someone out to change my thermostat system to the Google nest.

I've got a programmer, thermostat and a water tank receiver. Struggling to understand how the nest would communicate with the water tank if the honeywell set up is removed?

I'm being told by some that I might not be able to get the google nest learning thermostat installed. Please help.

 

[stem]

Looks like you have a Honeywell ST9400C programmer. Which as you rightly point out includes hot water control. The version of Nest that has hot water control is the Nest 3rd Gen. There is another version called Nest-e which is for combi boilers and doesn't have hot water control.

Assuming your programmer is a ST9400C as the diagram below, the wires are simply moved from there to the Nest Heat link terminals that have exactly the same function. L, N and E are straight forward, but for the heating and hot water functions, the terminology is slightly different, but can easily be worked out. For example Honeywell 'CH ON' (4) corresponds to 'Nest Heating Call for Heat' (3). Likewise, 'HW ON' (3) corresponds to 'Hot Water call for heat' (6).

Nest 3rd Gen

Nest also requires a link between Heat link terminals L, 2 and 5

The Nest thermostat is connected to terminals T1 & T2 which provide 12v for its operating power

Then the old room thermostat needs decommissioning. The receiver in your photo looks like a Honeywell unit, the most common is a BDR91, but yours looks different. However, inside there will be an N & L and two live switching terminals. The wires in the two live switching terminals need to be joined together to by-pass the old thermostat. If you need more help with this, more details of the thermostat receiver and its wiring would be needed.

If you have purchased a Nest-e that doesn't have hot water control capability, you can still wire it in to control the central heating, but the Honeywell programmer would have to remain for hot water control.

 

[monc3]

Looks like you have a Honeywell ST9400C programmer. Which as you rightly point out includes hot water control. The version of Nest that has hot water control is the Nest 3rd Gen. There is another version called Nest-e which is for combi boilers and doesn't have hot water control.

Assuming your programmer is a ST9400C as the diagram below, the wires are simply moved from there to the Nest Heat link terminals that have exactly the same function. L, N and E are straight forward, but for the heating and hot water functions, the terminology is slightly different, but can easily be worked out. For example Honeywell 'CH ON' (4) corresponds to 'Nest Heating Call for Heat' (3). Likewise, 'HW ON' (3) corresponds to 'Hot Water call for heat' (6).

View attachment 323406


Nest 3rd Gen
View attachment 323409

Nest also requires a link between Heat link terminals L, 2 and 5

The Nest thermostat is connected to terminals T1 & T2 which provide 12v for its operating power

Then the old room thermostat needs decommissioning. The receiver in your photo looks like a Honeywell unit, the most common is a BDR91, but yours looks different. However, inside there will be an N & L and two live switching terminals. The wires in the two live switching terminals need to be joined together to by-pass the old thermostat. If you need more help with this, more details of the thermostat receiver and its wiring would be needed.

If you have purchased a Nest-e that doesn't have hot water control capability, you can still wire it in to control the central heating, but the Honeywell programmer would have to remain for hot water control.

Hi Stem, gosh you sure know your stuff. Thanks so much for that. Unfortunately no one local seems to know much about the nest or remotely interested in taking this job on.

The thermostat I have bought is indeed the nest learning thermostat.

Are you able to tell me though how the nest would communicate with my water tank if the WT receiver no longer has the honeywell programmer?

The receiver at the cylinder is battery operated and has a single black wire coming from it and has a metal head on the other end which sits in a wee hole in the cylinder tank.

I've just checked my programmer and its a ST9420C.

 

[Rob]

Blind leading the blind.

Go on to the Nest website and you'll be able to find local installers.

Your set up is beyond DIY unfortunately

 

[stem]

How the nest would communicate with my water tank if the WT receiver no longer has the Honeywell programmer?

Sorry I don't understand what you mean by "WT receiver". Nest 3rd Gen is a direct replacement for the Honeywell programmer and the HW control is included as part of it. If you have some sort of remote thermostat and a receiver for hot water then the Nest would replace that, but you would need to provide details of it.

 

[stem]

Blind leading the blind.

Would you care to expand on that comment please. It's not really of any help to anyone as it stands.

 

[CBW]

Need a cylinder thermostat for hot water side to work, do you have one? (Edit - apologies, I didn’t read your reply properly).

 

[RandomGrinch]

Sorry I don't understand what you mean by "WT receiver".

The Honeywell RF2 pack 3, has an RF cylinder thermostat.
The ST9420C is the receiver for both stats.
See attached.

Your set up is beyond DIY unfortunately

As a DIY workaround (a slighty inelegant solution!), I would look to retain the ST9420C and tank sensor; having terminal 3 (HW NO) of the ST, feed terminal 5 (HW common) of Nest. The wire that was in ST9420C 3, would go into Nest terminal 6 (HW call-for-heat) - the Nest would then have timing authority, when the ST9420C retains over temperature cut-off.

The other connections to Nest, would be made appropriately, with only a Live bridge required to terminal 2 (C/H COM).

Edited for clarity (hopefully!).

 

[monc3]

Blind leading the blind.

Go on to the Nest website and you'll be able to find local installers.

Your set up is beyond DIY unfortunately

Pretty rude comment there. And yes I have messaged every local installer.

 

[monc3]

OK, the ST9420C has the same electrical connections, as the ST9400C, so the wiring is the same.

View attachment 323520


Sorry I don't understand what you mean by "WT receiver". Nest 3rd Gen is a direct replacement for the Honeywell programmer and the HW control is included as part of it. If you have some sort of remote thermostat and a receiver for hot water then the Nest would replace that, but you would need to provide details of it.

Sorry that's me being lazy. WT meaning water tank.

 

[monc3]

Need a cylinder thermostat for hot water side to work, do you have one? (Edit - apologies, I didn’t read your reply properly).

The only thing I have at the cylinder strangely is the little white box that has that black wire coming from it. No thermostat attached to the cylinder though.

 

[ericmark]

I have the same, no thermostat on the cylinder, but I am running C Plan, and you seem to be running S Plan, I can see two motorised valves. I don't understand why you want Nest Gen 3, as I am looking to move to Hive as well, as Nest has no ability to link to TRV heads.

I am not good at plumbing, so employed a plumber, who in turn employed a heating engineer, who admitted to me they had not got a clue about how to wire it up. Lucky I did, but had not realised Nest does not link to TRV heads, the instructions I had read said they did, but it seems when google took over Nest support was withdrawn.

Domestic hot water (DHW) during summer was simply timed, the floors in my house mean hard wiring is out, so needs some wireless method if using a thermostat, ½ hour 4 times a week gave me hot water to wash hands, today I use an immersion heater and iboost+ so in summer DHW heated with spare solar energy from the solar panels.

But had I not been an electrical engineer not sure how I would have got it wired up? @stem is normally good at working out what to do, but it is hard to realise what others can understand, and was shocked at the comments made by the heating engineer. Also the Nest help line seemed to just want to send out an engineer to sort problems, I had a simple problem, the central heating was turning on when not called for, it turned out due to anti legionnaires software built into Nest, which could be simply turned off, but Nest help line could not tell me that, found out what it was on here.

With Sonoff wireless relays and other devices there are ways around lack of hard wiring, but for me it was the cost, I looked at wireless options and was looking at £140, and this with no labour costs.

So I would say back to the base line, why do you want Nest? @RandomGrinch makes some good points, the wireless links are already built into the existing system. So may be having both in parallel is the way forward?

 

[monc3]

Thanks for your comment. The reason why I've chosen the nest is just purely down to the design of it. But seems like I'd have to jump through hoops to get it installed. I'm a dual fuel engineer to trade but this seems to be a bit of a foreign language to me


I do have the option to use the immersion heater but I'd rather not. I usually boost the cylinder tank on top of it being on the timer on the daily.

I'd rather stick with just the 1 device and get rid of the honeywell. It would be ideal to have the option to control my heating and water from my phone with working away often.

I've just had the whole house re-wired when I bought the place, new flooring is down etc also. So a wireless option for the cylinder is also my only option. So is the Nest out of the question for me? If so what else would you recommend?

 

[RandomGrinch]

I'd rather stick with just the 1 device and get rid of the honeywell. It would be ideal to have the option to control my heating and water from my phone with working away often.

You would do that with my suggestion.
You would set the Honeywell to a constant set temperature (for the hot water) and forget about it.
You would use the Nest to control the on and off times as you so wish, by the app.

Alternatively, to do a full replacement, you would need to add extra wiring and replace the tank stat.
And as it's an unvented tank, that would need to be done correctly.

The Nest is fine, Eric is well known for his opinion of them, that many do not share!

 

[monc3]

You would do that with my suggestion.
You would set the Honeywell to a constant set temperature (for the hot water) and forget about it.
You would use the Nest to control the on and off times as you so wish, by the app.

Alternatively, to do a full replacement, you would need to add extra wiring and replace the tank stat.
And as it's an unvented tank, that would need to be done correctly.

The Nest is fine, Eric is well known for his opinion of them, that many do not share!

Thanks for that. So basically I'd have to keep the honeywell programmer and run new wires to connect the heatlink that comes with the nest?


What if I were to ditch the nest and get the hive would I be in the same situation?


Also what is currently speaking to my cylinder anyway? Surely it's wired right down to the programmer at the boiler currently as the little receiver at the tank is wireless and only has a wire sitting on the side of the tank for the temperature?

Another question, is there something I could potentially get to replace/upgrade the wireless device at the tank that could speak to the nest?