I purchased a Honeywell RTH6580WF to use with my recently installed Triangle Tube Prestige Solo 110 propane boiler. I only have two wires at my existing heat-only thermostat. I understand that the c-wire is common. There is no terminal labeled "C" in the boiler, but all the grounds are to the chassis/terminal block backplane.

The existing connections at the thermostat are W, RH and a jumper between RH and RC. I contacted Honeywell customer service, and they said "call a contractor." Can I power the RTH6580WF by simply connecting the C in the new thermostat to household common, or do I need to run it back to the chassis or somewhere else of the boiler, or...? If I HAVE to add a separate transformer, where do I connect the + and - from the transformer to the thermostat?

  • 1
    Have you read this answer? If you post an image of the boiler wiring diagram (should be in the installation manual and/or inside the unit); or a link to it online, we should be able to show you where to connect the C wire. – Tester101 Nov 4 '13 at 13:08
  • @Tester101 I did look at that response, but with no labeled "C" connection on my boiler, I was leery of frying a $999 part. Wiring diagram is on page 31 of link – Jon Nov 6 '13 at 12:47

The 24V transformer is part of the control board, and so is not shown on the wiring diagram you've linked to. However, R is always one side of the transformers secondary, and C is always the other side. The thermostat needs to draw its own power, and so requires a connection to "C" to complete the circuit.

24V Transformer

Your thermostat is just a load on the 24V circuit.

Thermostat on 24V Circuit

If you don't feel comfortable modifying the wiring, it's probably best to contact a local HVAC technician.

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