Room thermostat wiring

Gredawarha · 12 Apr 2020 at 09:17

Morning all

Last week our central heating stopped working. The boiler is still firing and heating the water cylinder but the radiators are not coming on. So I did some research and I found I was able to manually activate the radiators by manually moving the switch on the motorised valve in the airing cupboard.

No on the basis that I think the likely issue is either the room thermostat or the valve itself I purchased a replacement room thermostat. Since we move into the house two years ago the thermostat has on occasion made some buzzing noises so it is my likely culprit.

The existing thermostat is a Drayton RMT230 and is wired as per the image below:

As I understand it the blue connected to screw 4 is neutral the brown to screw 1 is the permenant live and the final wire (despite being yellow and green the colour for earth) connected to screw 2 is the switched live.

The replacement part is a Salus RT200 and has wiring as per the image below:

If I have understood the wiring in the original thermostat correctly the blue neutral cable goes to the top far left scre marked N, the brown to the middle of the three marked L and the final green and yellow to the third marked SL.

I think my understanding is consistent with the manual:

https://www.plumbase.co.uk/link/1/h032967_23215_t.pdf

I would appreciate a sanity check before I do the change over.

Many thanks

Darren

SPArton · 12 Apr 2020 at 09:46

I agree with your plan.

Blue to N
Brown to L
Yellow/Green to SL

SPArton · 12 Apr 2020 at 09:46

Let us know how it goes.

Gredawarha · 12 Apr 2020 at 09:56

Savan_P said:
I agree with your plan.

Blue to N
Brown to L
Yellow/Green to SL

Thanks, will wait for any further responses but of course I will update the thread when done. I am hoping it is the thermostat and the replacement solves the issue. Don't need the heating right now but don't want to end up getting toward the end of the year and it not be fixed.

ZG002 · 12 Apr 2020 at 11:50

The electrician in me says, yeah that will work but you are powering a unprotected cable. You are missing a core to do the job properly.

That green and yellow is designed to be your cpc and should under no circumstances be used as a switched live.

So either run a new cable with the correct number of cores (3 + cpc)

or buy a wireless stat and wire the receiver in next to the wiring centre.

You will no doubt ignore this and replace as is.

Danm54 · 12 Apr 2020 at 12:05

You don't always need the neutral on mechanical room thermostats.
Honeywell for example works fine without a neutral, you just need a live and switched live.

You'd need to alter the wiring in the wiring centre but that's an option to do the job properly without running a new cable.

Gredawarha · 12 Apr 2020 at 12:06

ZG002 said:
The electrician in me says, yeah that will work but you are powering a unprotected cable. You are missing a core to do the job properly.

That green and yellow is designed to be your cpc and should under no circumstances be used as a switched live.

So either run a new cable with the correct number of cores (3 + cpc)

or buy a wireless stat and wire the receiver in next to the wiring centre.

You will no doubt ignore this and replace as is.

Hello

Thanks for your comments.

I think running a new cable would be the best thign to do but I am not sure I could succefully do that? If I did I guess I would somehow attach one end of hte new cable to the other end of the existing cable and pull it through at the thermostat end?

I did wonder about a more advanced wireless setup but cost aside I might be wrong but would I not need to do something with the existing thermostat and wiring or would the wireless thermostat effectively override the existing thermostat and cabling?

Judging by the age of the thermostat, the boiler, the wiring and what we know of the previous owners of the house I am pretty confident that the cabling has been in place as installed (allbeit wrongly) for many years without issue so yes I am inclined to simply replace the thermostat even if that is not the best way of doing it.

Gredawarha · 12 Apr 2020 at 12:10

Danm54 said:
You don't always need the neutral on mechanical room thermostats.
Honeywell for example works fine without a neutral, you just need a live and switched live.

You'd need to alter the wiring in the wiring centre but that's an option to do the job properly without running a new cable.

Yes, as I undertand the RMT230 came in various editions despite having the same model number. Mine appears to be the model with the resistor that warms the switch and therefore needed the neutral, as I understand it this model had less temperature swing due to the resistor rather than the model without neutral.

As I undertand it although I am going from a mechanical thermostat to a digital thermostat both the original and the new require live, switched live and neutral so need to alter the wiring in the wiring centre.

Danm54 · 12 Apr 2020 at 12:22

You don't specifically need the neutral for the resistor, but as you say it helps with keeping a steady room temp.
If it's digital it will need the neutral to power the display though.

I'd personally wire it back as it was and swap the cable out when the room gets decorated.

Or go for an RF version and fit the new stat over the hole your old one will leave.

Gredawarha · 12 Apr 2020 at 12:25

Bit more research and it seems that it is not that uncommon for installers to lazily use the earth as a live when they shouldn't have. I've seen a number of forum threads where people have found similar to mine, and the general concensus is that this is poor work not in accordance with regulations.

Gredawarha · 12 Apr 2020 at 12:26

Danm54 said:
You don't specifically need the neutral for the resistor, but as you say it helps with keeping a steady room temp.
If it's digital it will need the neutral to power the display though.

I'd personally wire it back as it was and swap the cable out when the room gets decorated.

Or go for an RF version and fit the new stat over the hole your old one will leave.

Yes that is how I understood it too. The neutral in the new unit powers the display and thermostat with the permentant live and switch to call for heat when required.

Danm54 · 12 Apr 2020 at 12:42

Gredawarha said:
Yes that is how I understood it too. The neutral in the new unit powers the display and thermostat with the permentant live and switch to call for heat when required.

It's not normally a permanent live for a mechanical stat, it's a switched live from the programmer.

Is the link correct for your new stat as that seems to suggest you don't need the neutral and it doesn't have a display.

Gredawarha · 12 Apr 2020 at 12:58

Danm54 said:
It's not normally a permanent live for a mechanical stat, it's a switched live from the programmer.

Is the link correct for your new stat as that seems to suggest you don't need the neutral and it doesn't have a display.

I think you might be confused, probably my fault. The link is the manual for the old thermostat which is a RMT230 which is mechanical with a dial, the new unit is a Salus RT200 which is an electronig thermostat with digital display.

The manual for the Salus is here:

https://salus-controls.com/files/RT200-Manual.pdf

Darren

ZG002 · 12 Apr 2020 at 12:58

You could go for a two wire stat with battery to power the display.

Check out the Dryton Digistat+

Only requires two wires (leaving your cpc to be a cpc) and the battery powers the stat.

You would need to make a slight change in your wiring centre but we could help with that.

Gredawarha · 12 Apr 2020 at 13:16

ZG002 said:
You could go for a two wire stat with battery to power the display.

Check out the Dryton Digistat+

Only requires two wires (leaving your cpc to be a cpc) and the battery powers the stat.

You would need to make a slight change in your wiring centre but we could help with that.

Thanks, might see if I can return the Salus and order a 2 wire unit.

Always learning, many thanks.

Danm54 · 12 Apr 2020 at 13:29

Gredawarha said:
I think you might be confused, probably my fault. The link is the manual for the old thermostat which is a RMT230 which is mechanical with a dial, the new unit is a Salus RT200 which is an electronig thermostat with digital display.

The manual for the Salus is here:

https://salus-controls.com/files/RT200-Manual.pdf

Darren

Ha, my bad.
Yea I'd wire that up the same as your old one, I'd assume their Mains Live to be a switched live from your programmer.
The LED only comes on when it's calling for heat so you'd need your programmer to be on then anyway.

Chasman · 2 Dec 2023 at 05:37

I'm replacing installing a Nest Gen 3 as a room thermostat on a boiler only system.

I'm not planning to use the Heat Link, hot water only comes on twice a day via the timer by the boiler, and works fine.

We're on the old school wiring colours red, blue, yellow and yellow/green.

The old stat has 4 numbered terminals which doesn't help!

Can anyone tell me which connections I need for which colour on the Nest?