Install a Low-Voltage Thermostat

Kill 24V, map and photo the terminals (mind the C-wire), set O/B and AUX/E for heat pumps, level-mount, configure equipment type, then prove every mode.

Approx. 45 min

Before you start

•Know the equipment type the stat controls: conventional gas/AC, single- or two-stage, or a heat pump (which needs O or B reversing-valve config and AUX/E heat).
•Confirm there is a common (C) wire or a plan to provide 24V common (existing C, an add-a-wire/PEK, or a power kit) — a modern Wi-Fi stat needs continuous 24V.
•Replacement stat compatible with the system stages and voltage (this is 24VAC low-voltage, not line-voltage baseboard).

Tools & materials

Small screwdrivers + wire strippers · Torpedo level · Multimeter (verify 24VAC R–C, confirm de-energized) · Phone camera + labels for the existing wiring · Drill + anchors for the new base

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Power down & document

Kill the 24V power
Switch off the air handler/furnace at the breaker (and the unit's service switch). The 24V control transformer is fed from the equipment — leaving it live risks shorting R to a control wire and blowing the transformer or its fuse.
Tools: Multimeter
⚠ Don't short R to any output (Y/W/G) on a live system — it pops the low-voltage fuse or the transformer.
Photograph and map the existing terminals
Photo the old base and label each wire to its terminal before removing anything: R/Rc (24V hot), C (common), Y/Y2 (compressor stages), W/W2 (heat stages or AUX), G (fan), O/B (reversing valve). Note any jumper between R and Rc.
Target: Every conductor labeled to its letter; record the R↔Rc jumper if present

Wire the new base

Land the C-wire (don't skip it)
Land C to give the new thermostat continuous 24V. Without C, smart stats 'power steal' through the equipment and cause chattering relays, ghost calls, and dropouts. If there's no spare conductor, use an add-a-wire adapter (PEK) at the air handler or repull cable.
Target: Confirm ~24–28 VAC across R and C with power restored
Configure O/B and AUX/E for heat pumps
On a heat pump, the reversing valve is energized in cooling (O) on most brands or in heating (B) on others (e.g. some Rheem/older). Land it on O or B and set the stat's reversing-valve config to match. Wire AUX (auxiliary/strip heat) and E (emergency heat) so backup heat works when the compressor is locked out or failed.
Target: O = valve energized in COOL (most); B = valve energized in HEAT; AUX/E landed for backup strip/elec heat
⚠ Get O/B right — a wrong reversing-valve setting makes the system heat when it should cool (and vice versa).

Mount & configure

Mount the base level
Fasten the new base to the wall, level (mercury bulbs are gone but a level mount still looks right and seats the screen). Don't pinch or kink the conductors; seal the wall hole behind the base so drafts don't fool the sensor.
Tools: Torpedo level
Configure the equipment type in the stat
In the thermostat's installer settings, declare the system type (conventional vs heat pump), number of stages, fan control (gas vs electric), and the O/B polarity. A miscofigured equipment type will drive the wrong outputs no matter how the wiring is landed.
Target: Installer settings match the actual equipment: system type, stages, fan, O/B

Prove every mode

Test each call and confirm the right output energizes
Restore power and exercise the system: a fan-only call (G) should run the blower; a cooling call (Y) should start the compressor (and energize O/B for a heat pump); a heat call (W) should fire heat; emergency heat (E) should bring on backup. Confirm the reversing valve actually shifts in the right mode.
Target: G→blower, Y→compressor, W→heat, reversing valve shifts correctly per mode; no short-cycling