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I'm in the process of figuring out how to swap my existing thermostat for a new WiFi thermostat. Naturally it's connected to my home's boiler by 2 really old cloth wrapped wires. Examening the wires, it does look like I it should be feasible to run a new wire which also hooks into the C screw on my boilers transformer, but I'm the sort who likes to act cautiously on this sort of thing of course, so I was hoping someone could give me a quick yes/no on my reasoning, as well as see if there's anything I missed.

Anyways, on the technical side, the furnace I'm hooking the thermostat too is a Weil-McLain boiler (Link https://www.weil-mclain.com/products/eg-gas-boiler), and the transformer in it I'm looking to tie into is this one here: (http://www.supplyhouse.com/Weil-Mclain-510-312-166-Transformer-120-24V). There's curently a wire running off the C connection on the transformer, it looks like to the water fill unit, does that raise any concerns? Anyways, thanks for any advice.

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  • What model of boiler do you have? – ThreePhaseEel Nov 2 '17 at 22:29
  • I've also seen some thermostats come with instructions for hooking up a second plug-in transformer to power the thermostat without needing to mess with the furnace/boiler wiring. – JPhi1618 Nov 3 '17 at 18:19
  • The "C" marking on a furnace controller is generally the "common" lead from the 24vac transformer, using a black wire. It may be connected to several different units that are then activated by devices switching the other 24vac lead (e.g., the Y lead for cooling, W lead for heating). – Upnorth Nov 4 '17 at 6:23
  • Ok, sorry for the delay in replying, the precise boiler model is a Weil McLain EG-65 series 4 – Brendan odonnell Nov 4 '17 at 18:03
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How old is the home? It is not impossible that the cloth wiring could be asbestos based, so work safely.

As long as the wires are NOT 120V wires, you should be fine. Some newer thermostats (like the Nest Version 3) do not require the C.

You can run a new wire, if you do so, and it is as easy as you imply, you could always replace the old wire.

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