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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?

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marked as duplicate by BMitch Nov 8 '13 at 20:51

This question was marked as an exact duplicate of an existing question.

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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