I have a two-wire system that connects my thermostat to my Peerless boiler. I replaced my Robert Shaw thermostat with a Honeywell RTH 7500. The wires are white and black and were connected to the Robert Shaw as follows: white wire to R and the black wire to B or W the same connection. I connected the black wire to C and the white wire to R on the Honeywell RTH 7500 and programmed it for hot water or high efficiency furnace. There has been no heat produced at this zone since. My other two zones have different connections and work fine.

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If the new thermostat needs a common, it won't work with a 2 wire system. You will need to find one that works by voltage stealing or a battery.