wiring a 24v thermostat with a twist

[simmo490]

Hi All,

New home owner here needing a little bit of advice please.

i Have a new property with a 230V Honeywell 2 wire thermostat. I intend to fit a 24V "nest" thermostat which can also run the 2 wires.

I have a honeywell "aube" to step the 230V down to 24V. I have located the junction box for my boiler, Power switch and thermostat all on a chock block.

Can someone please have a look at my pics. how would i wire in this relay. The thermostat i think i can work out just not the relay. Any help is great.

I think i can just swap and add in to the chock block but unsure which way. thanks

Untitled

• simmo490
• 20 Jun 2013
• 1

Untitled

• simmo490
• 20 Jun 2013
• 1

Untitled

• simmo490
• 20 Jun 2013
• 1

 

[EFLImpudence]

Edited -

I think I may have misunderstood.

Where does the 24V come from?

 

[simmo490]

I am not sure how these work really all I know is the thermostat I am installing is a 24v used in America and all the people on forums that fit them in the UK have to use this "subs" to step from 230 to 24v

 

[ericmark]

If you have a new property then likely you will have thermostatic radiator valves and the boiler with be condensate type and the electric thermostat job is to switch off the system during the summer months so the pipe work is not heated. The electric thermostat does nothing during the winter months all control is by thermostatic valves.

To use the new thermostat it would not replace the main boiler thermostat but would control special thermostatic radiator valves which would need changing for a special type when can be connected to the electric thermostat.

My son has been looking into these but in the UK has only been able to source the wireless type which work though a computers Wifi system and there is so much wireless stuff in the house he does not really want to add more.

In some countries for example USA they use hot air rather than water system and as a result the TRV we use will not work and the "Nest" thermostat is really designed for those systems where it would be very hard to control each room independent.

If you have a hot air system then you should seriously consider changing. The first house I had used hot air and it was a failure in the UK. There was very little one could do to control heat room to room and was really designed for an open plan house not the type we live in but those found in Ideal Home magazines.

Yes there are UK system which will connect to your smart phone and integrate with the alarm and has a memory but it is not the "Nest" that is for hot air systems which don't have the control associated with the standard UK system installed today.

 

[ericmark]

As a PS I did look for the system my son talked about. He explained how it integrated into the alarm system heating only rooms in use and going into a stand-by mode when you leave the house.

However I could only find valves like these

and the reports were rather damming. Seems they work OK but are noisy. These had a remote control and there were others with settings done on the valve its self. However they replace your existing TRV head they are not the whole valve.

Maybe this should be linked to plumbing section as it is more their field?

 

[ban-all-sheds]

I am not sure how these work really all I know is the thermostat I am installing is a 24v used in America and all the people on forums that fit them in the UK have to use this "subs" to step from 230 to 24v

That's a mad, and no doubt eye-wateringly expensive way to buy a 24V transformer to allow the use of a 24V controller.


I don't know what percentage (it'll have lots of 9s in), but most boilers in this country need volt-free programmers, so if you've got a 24V control signal, all you need is a very small 24V relay, not a galumphing great thing capable of switching a 22A resistive load.

And what's the point? It's a pretty dumb device, unsuited to European CH systems, if you've bought the US version it won't know where it is, and therefore won't know what time it is, what the weather is going to be, etc., and there's no guarantee, as far as I can see, that you'd be able to install new software for it.

All the reviews which rave about it are from gadget freaks/sites/magazines who go on about how cool it is, and how awesome, seemingly because of its styling and tablet/smartphone interface.

And how does it work with a 2-wire system? Does it still turn the heating on when it's battery goes flat, just so it can recharge it? That'll be fun if we ever get a summer.

Have you read these: http://www.avforums.com/forums/home-appliances/1692238-nest-v2-easily-compatible-uk-systems.html

//search.diynot.com/forum_sear...&since=any&forum_id=7&hasimage=0&topiclimit=0 ?

 

[bernardgreen]

i Have a new property with a 230V Honeywell 2 wire thermostat. I intend to fit a 24V "nest" thermostat which can also run the 2 wires.

Why ? Because it looks cool is not a sensible reason.

What happens if it mis-understands your life style and puts the heating on when you don't need it.

On its own it can only measure the temperature in one room and then use that to decide if all rooms are to be heated or not. On its own it CANNOT control the temperatures in individual rooms. In short it gives virtually no improvement over the thermostat you already have but adds a lot of complication.

Apparently the Nest system can be expanded to control the temperatures in each room with each room having its own time program. That will be expensive, complicated to program and, using wireless comms prone to the adverse affects of interference. It also requires the boiler to be able to function correctly both when the heating load is only that of the smallest room and when all rooms are demanding heat.

PS I am developing a wired controlled system with thermostats in each room and motorised valves on radiators all monitored and controlled by a central controller / programmer.

A friend is installing an off the shelf system with wired ( data network ) thermostats in each room. Getting it to work properly and reliably is giving him a lot of problems and he is an experienced engineer.

 

[simmo490]

Thanks all I thinkwithout going in to the above I don't have trv fitted in the house.

I know all the pros and cons of having this fitted.

Does anyone actually have any suggestions on wiring.

Thanks

 

[bernardgreen]

As B A S has pointed out, the RC840T is designed to switch up to 22 amps ( 5 Kwatt ) of ELECTRIC heating elements.

http://www.aubetech.com/manuel/2/RC840T.pdf

Depending on the type of contact protection in the RC840T it is not impossible that the leakage current across the contact will be enough to keep the boiler firing. In short using this item to control a GAS heating system may result in the boiler being continuously on. ( Though as the RC840T is restricted to switching resistive loads it may not have any contact protection in which case it should be able to control a gas boiler ).

Before precise advice on wire to wire installation can be given more information will be needed.

What heating system plan do you have, S plan, Y plan or something else. Are there motorised valves like this ?

Untitled

• bernardgreen
• 21 Jun 2013
• 2

 

[simmo490]

Hi am I looking at the bottom of the boiler for these. Thanks

 

[stem]

I'm not understanding your photos as I can't follow some of the wires from the terminals to the cables from which they come.

You mark what looks like a brown / blue twin and earth cable as "Live Socket" There is not usually a socket associated with a heating controls. Do you mean that this is the supply cable to the heating from a socket?

Then you mark what looks to be a three core cable, or is it four? "To Boiler" please explain what each of the cores is connected to at the boiler. e.g. permanent live supply / switched live supply / neutral.

Finally you mark a Red / Black cable as "To thermostat" yet a photo that appears to be the thermostat shows different coloured wires.

 

[bernardgreen]

Hi am I looking at the bottom of the boiler for these. Thanks

They need not be in or even close to the boiler. If you have a hot water tank they might be near that.

To explain the problem facing you. Often the thermostat controls a zone valve and that valve, when open, instructs the boiler to fire up. ( firing a boiler with the zone valve(s) closed is not a good idea as the boiler will have to shut down on over heat )

 

[simmo490]

Hi thanks for your reply

The blue and brown I believe come from the switched spur

The boiler has a 4 core but unsure how these are connected internally

The black and brown wire go to the thermostat as well as a non connected grey and earth

The nest thermostat can take many configurations so maybe the grey wire will be if some use with power? Not sure

Thanks

 

[ban-all-sheds]

I know all the pros and cons of having this fitted.

TBH it beggars belief that any sane, intelligent person who really knew the cons of trying to use a Nest on a 2-wire control system in the UK would want to have one, but there ya go....

Does anyone actually have any suggestions on wiring.

You know which wires bring power into that junction box, which ones take it to the thermostat and boiler so that they will function and which are the ones connecting the boiler control terminals to the thermostat contacts.

And you have a diagram showing which terminals are which on your RC840T.

On the basis that you, just like everybody else in the world, MUST NOT attempt electrical wiring unless you GENUINELY UNDERSTAND how it works and what you are doing, I strongly suggest you spend a bit of time learning what thermostats do and how boilers are controlled.

I don't have trv fitted in the house.

Installing them would almost certainly be a better way to spend your money than buying a Nest.

 

[ericmark]

New home owner here needing a little bit of advice please.

Seems I have miss-read the above not a new home but a new owner?

Step one is to work out what you have already. I will assume a water system so start with boiler.

Early boilers where very simple they heated water when electric supply was switched on and stopped when switched off. All the control was external to the boiler using motorised valves to either simply divert water to domestic hot water or central heating and in some cases no control for domestic it was thermo-syphon. Although one can fit TRV to these often the system will run anyway and the electric thermostat is the main control together with a timer.

As time went on we got the combi-boiler which these we did not need a water store for domestic hot water it was built into the boiler. This also meant that all the valves were also built in often with timer fitted on the boiler its self.

Then we went to condensating this was a big move as the return water as well as output water temperature was monitored and if the return water gets too warm it will shut down the boiler, also rather than simple on/off the flame hight is altered by monitoring the return water temperature depending on the boiler this may also go a step further with anti-cycle software all designed to go with the TRV and the use of TRV's is really a must.

Each system can be improved but step one is working out what you have. I use a programmable thermostat rather than a programmer and thermostat so rather than simple on/off it changes the temperature in the main room for different times of day. But I have an open plan house this may not work where you have a corridor and doors into each room.

So also explain what your house is like.

However from what you have said it would seem the unit you have selected is not suitable.