Digital thermostat mounted on a concrete wall

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R, Rh and Rc: when to keep the jumper and when a system has two transformers

Most forced-air systems have one 24 V transformer at the furnace or air handler, and R and Rc are tied together (a factory jumper or an internal link in the thermostat). Some homes, usually a boiler or older furnace for heat plus a separate air handler or add-on AC, have two transformers: one for heat on Rh/R and one for cooling on Rc. Joining two transformers through the thermostat can blow fuses or burn transformers. Decide which you have before landing wires, and pick a thermostat that supports separate Rh and Rc when there are two.

Cross-checked ~30 min moderateBrands: All

Before you start

Turn off power to all HVAC equipment (furnace, boiler and air handler) before moving thermostat wires.
Live voltage checks at the thermostat are 24 VAC, but the equipment cabinets you trace wires into contain 120/240 V: keep covers on or work with power off.
Never use the thermostat jumper to 'test' by joining two transformers.

Diagnostic steps

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