Discovered on March 8, 2011

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Thermostat Wiring Protip

When upgrading an old 2-wire mercury-switch thermostat to a digital one, connect the existing wires running from your furnace to the W and RH wiring posts in the new thermostat.

For two wire systems, the thermostat is simply acting as a relay. This means you don’t need to worry about the polarity. However, convention states that you should connect the white/red wire to W and the black wire to RH.