Customer Replaced Thermostat, Now No Heat: Decision Tree

Why this matters

A customer-installed thermostat is the most common cause of a sudden no-heat call, because the wiring conventions that look obvious to a homeowner are not. The same colored wires mean different things across equipment types: a heat pump's reversing-valve and auxiliary-heat terminals have no analog on a simple gas furnace, and a missing common wire breaks every modern smart thermostat. Wire color is a convention, not a standard, and an installer who reused an old conductor for a new purpose can leave a green wire on a call terminal or a yellow on common. When heat stops after a thermostat swap, the fault is almost always at the new stat: a miswired terminal, a missing C-wire workaround that backfeeds, a swapped heat-pump reversing valve, or a configuration set for the wrong system type. Your job is to restore correct terminations and settings without assuming the equipment failed, because energizing the wrong terminals can short the transformer or damage a board.

Symptom presentation

The customer installed a new thermostat and now has no heat. Presentations:

• Thermostat powers up but the heat call produces nothing at the equipment.
• Heat pump runs but blows cold air on a heat call (reversing valve miswired).
• Auxiliary or emergency heat does not engage.
• Thermostat is dead or resetting, indicating a power or C-wire problem.
• The blower runs continuously or not at all on a heat call.

Quick checks

• Identify the equipment type first: gas or oil furnace, electric furnace, straight cool with electric heat, or heat pump. The correct wiring depends entirely on this.
• Open the thermostat and photograph the terminations. Compare to the equipment's wiring and the new stat's terminal map.
• Confirm a common (C) wire is landed if the new stat needs continuous power. A missing C often drives homeowners to backfeed or jumper incorrectly.
• Check for a blown low-voltage fuse on the control board, a sign of a short from miswiring.
• Verify the new thermostat's system-type configuration matches the equipment.

Isolation tree

1. Low-voltage fuse on the board blown? A miswire shorted R to C or crossed terminals. Find and correct the short, replace the fuse, and re-verify before powering up.

2. Heat pump blows cold on a heat call? The reversing-valve wire (O or B) is miswired or the stat is configured for the wrong valve energization. Correct the O/B termination and the stat's reversing-valve setting.

3. Gas furnace produces nothing on a heat call? Verify W is landed and calls. Jumper R to W at the furnace board (with the stat removed) to confirm the furnace heats. If it does, the fault is at the stat wiring or config. If it does not, the furnace has its own problem.

4. Auxiliary or emergency heat absent on a heat pump? The W or E and auxiliary terminals are miswired or the stat lacks the strips configured. Correct the aux/emergency terminations and settings.

5. Thermostat dead or rebooting? No continuous power. A missing C-wire, a power-stealing setup that cannot support the stat, or a broken wire. Land a proper C or add an adapter that provides one.

6. Blower runs continuously on a heat call but no heat? On a furnace, the G terminal may be jumpered to R or the fan setting forced on while W never calls. On a heat pump, a fan-only configuration with no compressor or aux stage produces room air with no heat. Re-examine the G wiring and the stat's fan and stage settings.

Confirming diagnosis

• Equipment confirmation jumper: with the stat disconnected, jumper the appropriate terminals at the equipment (R to W for heat, R to G for fan, R to Y for cool) to prove the equipment responds independent of the stat. This separates a wiring problem from an equipment problem cleanly.
• Voltage checks: confirm 24V between R and C at the stat. Verify the call voltage appears on the correct terminal when the stat calls.
• Reversing valve: on a heat pump, confirm the O/B output state matches the configured cooling-or-heating energization for that equipment.
• Configuration: review the stat's system-type, heat-pump-vs-conventional, and stage settings against the actual equipment.
• C-wire integrity: with a meter on R and C at the stat plate, confirm a steady 24V that does not collapse when the stat boots. A reading that sags as the stat draws current reveals a power-stealing setup that cannot support the device.
• Terminal-by-terminal map: lay the new stat's terminal labels next to the old base and the equipment board, and confirm each conductor lands where its function belongs rather than where its color suggests.

Remediation

Re-terminate the thermostat to match the equipment type and the new stat's terminal map, correcting any crossed or backfed wires. Establish a proper C-wire if the stat requires continuous power, using an existing unused conductor, an add-a-wire adapter, or a transformer kit rather than a power-stealing hack that destabilizes the stat. Set the stat's configuration for the correct system type, reversing-valve energization, and number of heat stages. Replace any blown low-voltage fuse only after the short is found. Confirm heat, auxiliary or emergency heat, fan, and cool all respond correctly, and that a heat pump delivers warm air on a heat call. Counsel the customer that thermostat wiring is equipment-specific and that the wire colors are not standardized. Close by cycling a full heat call and verifying the supply air warms and the correct stages engage.

References

• ACCA Standard 4 (Maintenance of Residential HVAC Systems), control and thermostat verification scope.
• NFPA 54 (National Fuel Gas Code), control-circuit context for gas-fired heating appliances.
• UL 60730, safety standard for automatic electrical controls including thermostats.
• AHRI Standard 210/240, heat-pump operating-mode context relevant to reversing-valve control.