Replacing a Honeywell T40 thermostat.

[SVRich650]

Hi

I have a Worcester Greenstar 24i Junior boiler and the mechanical clock on the front no longer keeps time accurately, rather than replacing that clock I was wondering if I could put a programmable thermostat in and use that to control the boiler.

At the moment we have a honeywell T40 thermostat and I was thinking of something like the Salus RT500RF instead. The T40 have three cables wired into it and red and a blue one as well as and earth. Would I be able to just do a straight swap or would there be a lot more too it than that?

Any help would be greatly appreciated.

Thanks.

 

[stem]

The T40, if installed correctly requires the following connections:

Live supply connected to terminal 1 (usually red wire)
Neutral connected to terminal 2 (usually blue wire)
Switched live connected to terminal 3 (usually yellow wire)

If you have all three of these connections, you have what you need for a swap with a Salus RT500RF. The old mechanical clock can simply be left in place and set to permanently 'on'

However, unless you have failed to mention a yellow wire, it would seem that of the three wires you mention, one is an earth wire, but may not being used as such. If so this is wrong, earth wires (ie green and yellow) should only be used for earth connections really.

It may be a case of the installer using any old bit of cable he can get hold of rather that purchasing some three core and doing the job properly. But the wire colours you mention without a yellow are a bit odd.

 

[SVRich650]

The one I have assumed is earth is wired to a terminal with the earth symbol on it, the other two are numbered. I wouldn't be surprised it the bloke who did it hasn't done it properly tbh as he wasn't the best.

I'll grab a photo when I get home.

 

[SVRich650]

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• SVRich650
• 22 Jan 2015
• 1

 

[denso13]

You need 3 wires plus earth for that one - it isn't wired correctly.

 

[D_Hailsham]

The earth wire is being used as an earth, so you do not have enough wires to fit a wireless stat's receiver in that location. However there's no reason why you shouldn't fit it somewhere else. Near the boiler, or near the motorized valves (if you have a hot water cylinder).

Alternatively you could fit a wired programmable stat in the same location as the existing stat. They are battery powered and only need two wires to work.

 

[johnscts]

Hi

I have a Worcester Greenstar 24i Junior boiler and the mechanical clock on the front no longer keeps time accurately, rather than replacing that clock I was wondering if I could put a programmable thermostat in and use that to control the boiler.

At the moment we have a honeywell T40 thermostat and I was thinking of something like the Salus RT500RF instead. The T40 have three cables wired into it and red and a blue one as well as and earth. Would I be able to just do a straight swap or would there be a lot more too it than that?

Any help would be greatly appreciated.

Thanks.

 

[johnscts]

Agree with the the previous response. I tested one I installed for my son and the + (line goes to 1) the neutral goes to 2 and the switched live connects to 3, and the earth is labelled and connect to the earth simple (very left)live will be the red lead or brown depending on the connection from the Honeywell control box. Neutral should be blue on 2 and the switched live on three. If you have a screw driver with an neon just touch it on the various terminals. You will see that 1 and 4 will be live - as you turn up your stat you will see position 3 become live with 1 and 4 now being 0v
Ps hope this did not confuse whatever you do d not canner neutral to any other terminal but 2

 

[SVRich650]

Thanks all.

You need 3 wires plus earth for that one - it isn't wired correctly.

It seems to work

So do I have a live, neutral and earth or is the blue actually a switched live? I have looked at the wiring for the Salus RT500 (non wireless one) and it needs a live and a switched live so if that is what I have there then is looks to be a straight swap.

 

[stem]

You don't have a neutral connection, which you would need for your proposed replacement if you plan to fit the wireless receiver in the place of the existing T40. The blue is indeed being used for the switched live as you suspect.

The existing set up will work, but not as well as it should do. There is an 'accelerator' built in to the T40 which is there to improve the accuracy of these mechanical thermostats. Without a neutral on terminal 2, that part of it won't be working.

You can still fit a wireless stat/programmer to your system. The beauty of these is that the wired receiver part can be installed at a suitable location to connect to the necessary wiring and then the actual thermostat part located to suit.

 

[SVRich650]

Thanks, the wiring diagram for the RT500 just shows Live and Switched live and an earth parking dooda:

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• SVRich650
• 23 Jan 2015
• 1

Which looks to be what I currently have.

 

[stem]

In your original post you referred to installing a Salus RT500RF. The manual I have for that quite clearly shows that it needs a neutral.

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• stem
• 23 Jan 2015
• 1

However, it would appear that you have now changed to an RT500 which will indeed work with the wiring you have and does not need said neutral.

 

[SVRich650]

Ah, Sorry After D_Hailsham mentioned a wired one should work I did say I was now looking at the RT500. I don't really need a wireless stat.

Thank you very much for your help.

 

[stem]

OK fine, then you're good to go

 

[Nicholas Moore]

I also have a T40 thermostat and would like to replace with AC95. This requires a live connection, a neutral connection and two connections for dry contact. Any help appreciated.