Combi and thermostat wiring help

6 Mar 2004
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United Kingdom
Hi all,

I'm just wondering if you could give me some help with understanding the wiring on my combination boiler and thermostat.

I have a supply direct from the CU for the combi, this feeds into a SFCU.
There is a T&E to the thermo stat which is situated downstairs.
The wire from the combi is 4Core and Earth. Blue, Brown, Black and Black with the earth.

Here is a picture of the combo wiring.

1) Can anybody help me with what the terminals 3 and 4 are in the combi (The two black wires)

2) What wires for the 4 core and Earth connect to the live and neutral to the thermostat

thanks in advance.
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Make and model of boiler?
Make and model of thermostat?
Are you fitting a new thermostat, or replacing an old one?
I know its a valiant, i will have to check the model number tonight.
The thermostat is a time guard trt033 digital it is a new one

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The boiler previously had a programable stat. The wiring has had to be removed and replaced.

The wiring to the stat itself is not the problem it is at the other end at the combi. Within the combi you have the L,N, E and then terminal 3 and Terminal 4, both of which are black wires. When the tennants left, they chopped wires and i have managed the salvage what they cut.

There were two sets of wires going to the thermostat. Each wire was T&E. I presumed this was because it was a programmable one.

The four core was connected to the two thermostat wires as follows:
4 Core blue (4 core) to black (Therm) brown (4 core) to red (therm) and Black (4 core) to red (therm) and Black (4 core) to Black (therm)

Hope this helps

:confused: :confused: :confused:
That thermostat only requires 2 wires, which correspond to the the black wires at the boiler.

Boiler - Thermostat
3 - L
4 - SL

The other 2 wires (blue/brown) are the power supply for a mains powered thermostat.
However as you have chosen a shonky battery operated thing, they are not required.
The brown and blue are the permanant supply to the boiler.

The two black wires are a common and a return for any external controls to the boiler.

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