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We're currently looking to replace our wired doorbell, but before doing so I wanted to understand the existing wiring - unfortunately I don't!

We have a transformer next to our consumer unit supplying 12V AC to the circuit, and a single push button outside the front door (which only has a single pair coming to it). Each of these accounts for 1 pair of twin wires arriving into the chime - why might there be a third pair?

enter image description here

Each of the pairs has one wire commoned together in the wire nut. From what I can tell, it seems the transformer supply is connected to the T3 terminal, with the other two on 0F.

I'm intending to replace this like-for-like, just with a nicer looking chime, so I could just reconnect everything as-is on the new one, but I'd like to know what this other wire is for! Does this imply that we might have another push button somewhere?

If I separate out the two non-transformer leads, and try one at a time to figure out which is the actual push button, am I probably better off leaving the other pair disconnected, since we have no idea where it might be going?

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    three wire pairs ... 1. power ... 2. front door switch ... 3. back door switch – jsotola Aug 22 '20 at 15:20
  • Do you have a back door or some sort of intercom system or entry system if in a multi occupancy bldg? – JACK Aug 22 '20 at 15:21
  • No, we’re completely detached with no sign of another button anywhere. Given other things we’ve found in the house, it really wouldn’t surprise me to find the wires are just buried in a wall somewhere... – James Thorpe Aug 22 '20 at 15:23
  • If you've got a rear door the wires could be buried by the trim or caulked over. – JACK Aug 22 '20 at 15:38
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    @JamesThorpe Got it .... good luck stay safe. – JACK Aug 22 '20 at 16:24
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Three pairs are most likely connected to the three other components in the doorbell system, the transformer, the "front door" pushbutton and the "back door" pushbutton.

The circuit diagram is something like this.

enter image description here

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