Digital thermostat mounted on a concrete wall
Generic photo: Alexander Mass · Pexels License

Replacing an old mercury or 2-wire thermostat

First identify the system: low-voltage 24 V, millivolt (wall furnace/gas fireplace) or line-voltage (120/240 V baseboard). Most smart thermostats only fit 24 V systems. Old dial/lever units often contain a mercury bulb that must be recycled, never trashed.

Cross-checked ~60 min moderateBrands: All

Before you start

  • Line-voltage thermostats (thick wires, often marked 120/240 V, on baseboard heaters) carry lethal voltage: breaker off, verify dead, use only line-voltage-rated thermostats
  • Never put a 24 V smart thermostat on a line-voltage circuit
  • Mercury: keep the old thermostat intact; do not open or remove the glass bulb

Diagnostic steps

Unlock all 5 steps with expected values

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OnCall Tech is a reference tool for trained technicians and can make mistakes. Always disconnect power and confirm with the manufacturer's documentation before you work.

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Replacing an old mercury or 2-wire thermostat · OnCall Tech