I am trying to connect the T5+ smart thermostat to my oil based furnace. I see there are only two used wires at the thermostat R And W but 3 additional cores available. My boiler is a old Weil McLain AB/468. Can you help where I can find the C wire terminal on this furnace/boiler so I can complete the install for the thermostat. I would be very grateful. I’ve added couple of pictures that I how will help

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  • Can you post the wiring diagram for your boiler, or photos of the wiring for that matter? Oct 19, 2018 at 1:13
  • I’ve added a couple of picture to my priogional post. Can you see if they will help?
    – Harj
    Oct 23, 2018 at 16:39
  • Can you post a photo of the other end of the bottom cable in your top photo? Oct 23, 2018 at 22:49
  • I’ll start fresh Incase I haven’t explained properly. Which is likely. Sorry. Basically My current thermostat has total of 5 cables but only two cables connected(bottom pic) in R and W terminals. The other 3 are just spare/loose in the wall. From there they travel towards (top picture) orange connectors, two that are connected to transformer. And red wound around the cable itself. The cables from orange connectors go to boiler via that two core brown cable.. That’s all there is in terms of connections. When I switched the transformer circuit breaker. Can you help? Contact number 19147213750
    – Harj
    Oct 25, 2018 at 21:51

1 Answer 1


Welcome Harj!

Looks like your boiler is steam. If it has all of the original equipment, it doesn't have a C connector. Even with new equipment, it probably doesn't have one. If it does, it will be right near where the thermostat wires connect to the boiler. You'll either need to get a plug-in transformer to get power from or you might be able to tap into a 24v transformer at the boiler if there is one.

This might help you if you want to use a existing transformer: https://www.doityourself.com/forum/boilers-home-heating-steam-hot-water-systems/514502-need-c-wire-boiler.html

  • Thank you so much. I’ve added a couple of pictures one of the thermostat and the other I believe is the transformer by the furnace. With r and c terminals on them. Can I just connect the spare leg going to the thermostat to that of the transformer labelled c?
    – Harj
    Oct 23, 2018 at 16:40
  • By spare you mean the common coming back from the theromstat, yes, you should be able to. Oct 24, 2018 at 15:41

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