Honeywell T40 Terminal 4

[OnionAbroad]

Hi All.
I'm looking to replace my Honeywell T40 with a Hive system. When checking the current thermostat, the wires are in terminals 1/3/4 and earth.
In all documentation I've seen and forum posts, it always mentions terminals 1/2/3 and earth.
I haven't yet checked the backend of the wires into the boiler, but can anyone suggest why this terminal 4 might be used?

 

[CBW]

It does seem odd and would recommend checking. What type of system do you have?

 

[RandomGrinch]

As above, it may be best check what system you have, and to examine the wiring connections at the other end of the cable.
Currently, you don't have a neutral connection to the T40; you'll need one for the Hive.

 

[Harry Bloomfield]

Honeywell T40 Wiring

I am looking to replace an old Honeywell T40, but I am a little concerned how it is wired currently. I believe terminals 3 and 1 are connected the wrong way around. Assuming red is live and yellow/green is switched live. Any opinions? How could I test it? Thanks[/img]

www.diynot.com

 

[Hairymelon]

There’s no neutral in no 2. It must switch between 1and 3 and the wire in no 2 is in the junction box but not connected to anything and hopefully made safe.
1 is common 2 is neutral 3 is heating call ( 4is for air con call )
Sorry missed harry bloomfields post

 

[OnionAbroad]

Thanks all.
Indeed i did forget that important info. It's a WB Greenstar 34CDI Combi.
Will check the backend in the next day or so.

 

[OnionAbroad]

Suppose it might be relevant to note, when checking voltages, there's 240 when I check earth/3, and earth/1 when calling for heat.
This #4 never really does anything, perhaps is a red herring.
That being said, I've found a manual page for this thermostat, and while the printed circuit diagram doesn't list terminal 4 being used, there is a handwritten note that "white in 4"... but no description of what for.

 

[RandomGrinch]

This #4 never really does anything, perhaps is a red herring.

That was one of my thoughts. It would be useful if it is a wire that is just 'parked' there.
If that was the case, you could re-utilise it as a neutral; but that's some wishfull thinking!
...as said before, further investigation is necessary!

 

[Harry Bloomfield]

Suppose it might be relevant to note, when checking voltages, there's 240 when I check earth/3, and earth/1 when calling for heat.
This #4 never really does anything, perhaps is a red herring.
That being said, I've found a manual page for this thermostat, and while the printed circuit diagram doesn't list terminal 4 being used, there is a handwritten note that "white in 4"... but no description of what for.

It's perhaps a terminal which closes on a rise in temperature, for cooling systems?

 

[OnionAbroad]

Hrrm having removed the outer case and had a check, but not at the actual service panel.
Luckily, yes the white IS indeed not connected to anything so just a red herring.
However, ..I suppose first to ask, is the earth being exposed a risk (as shown)? I imagine if it's earthing then it's safe anyway, but still seems dodge.
Either way, it would leave me with an earth, supposed switched live in 3 (or N), and a live in 1. Nothing for terminal 2.

 

[OnionAbroad]

So as it stands, would it be safe to simply cut off the power via the breaker at the consumer unit, remove the service panel on the combi, and just replace the whole cable run from the service panel to the wall location? It's become pretty clear now that where the neutrals/earths/lives and SW lives go.
It notes "isolate the mains" before removing the service panel cover. does hitting the breaker qualify as that?

 

[CBW]

Not sure how close it is to the house, but perhaps piggy back the LNE and take switch wires using 5 core from boiler to inside, or from fused connection unit?

 

[OnionAbroad]

Thanks @CBW ,The above grey sheathed (looks like 4 strand) from boiler to thermostat is about 5m secured along the garage ceiling and through simple hole to the inside, if ease of access was what you were checking

 

[RandomGrinch]

suppose first to ask, is the earth being exposed a risk (as shown)? I imagine if it's earthing then it's safe anyway

It is wrong, but would normally be 'relatively' safe, however...
Whoever parked that grey wire in the 'stat at the other end is a plonker!
Currently, whenever the boiler isn't calling for heat, the end of that grey wire is live, with the posibility of shorting to the case, or the exposed earth:

As I said earlier, if the neutral is available near to this connection block, the grey could be reutilized as the neutral needed for the Hive.
However, given the wiring we have seen so far, and these cut off cables:

I would say it's worth checking the connections in the service panel.
Although your boiler will need checking for safety by an RGI.

 

[OnionAbroad]

Cheers for the heads up @RandomGrinch
To be fair I have the heating guys coming in for a separate matter in a few weeks. Perhaps I'll just get a new cable run set up, get the hive receiver installed, and then just let the engineer spend 5-10 mins wiring it to the boiler, just in case.... especially since a lot of this seems botched in some way and it'll have to be inspected anyway.