Thermostats

Nest, ecobee, Honeywell and more: wiring, C-wire, error codes and heat-pump setup.

4 field manuals · 57 error codes · 13 guides · 4 brands

Field service manuals

Manual · Thermostat

Google Nest Learning Thermostat (3rd and 4th gen): quick sheet

Nest Learning Thermostat 3rd generation and 4th generation (released 2024). 24 V systems only.

4 components2 common failuresGoogle Nest

Manual · Thermostat

ecobee Smart Thermostat Premium / Enhanced and the Power Extender Kit: quick sheet

ecobee Smart Thermostat Premium and Enhanced (current lineup), plus older ecobee3 lite / ecobee4 that use the same Power Extender Kit (PEK). 24 V AC systems only.

3 components2 common failuresecobee

Manual · Thermostat

24 V HVAC thermostat control wiring: field service manual

Residential 24 VAC forced-air furnaces, air conditioners, air handlers and heat pumps (single and multi-stage). Not for line-voltage or millivolt systems.

5 components4 common failuresAll

Manual · Thermostat

Smart thermostat installation & troubleshooting: field service manual

Wi-Fi thermostats for 24 VAC systems: Nest Learning/Nest Thermostat, ecobee, Honeywell Home T-series/Lyric, Sensi, Amazon Smart Thermostat. Line-voltage models (e.g. for baseboard heat) are covered only by the safety notes.

3 components5 common failuresGoogle Nest, ecobee, Honeywell Home / Resideo, Emerson Sensi, Amazon

Diagnostic guides · 12

Guide · Thermostat

Auxiliary heat runs too much or all the time

Aux heat should only help when the heat pump cannot keep up. Constant aux usually means wrong wiring/settings (aux on the heat pump terminal, emergency heat left on), big setback recoveries, or a heat pump that is not producing heat.

4 steps ~45 minmoderate

Guide · Thermostat

Heat pump blows cold on heat (or warm on cool): O/B setting

The O/B terminal drives the reversing valve. Most brands energize it in cooling (O); Rheem and Ruud typically energize it in heating (B). A wrong O/B setting reverses the modes. Check the equipment diagram, not wire color.

4 steps ~20 mineasy

Guide · Thermostat

Indoor fan won't turn off from the thermostat

Start at the thermostat: fan set to ON instead of AUTO, or a setpoint the system can never reach. If the thermostat is right, the cause is in the equipment: a stuck fan relay, a limit switch keeping the blower on, a shorted G wire or a control board fault.

4 steps ~40 minmoderate

Guide · Thermostat

Installing a C-wire adapter (Nest Power Connector, ecobee PEK and similar)

An adapter mounts at the furnace/air-handler board and lets the existing wires carry both the calls and constant 24 V. Check compatibility first: dual-transformer, fan-coil and communicating systems are commonly excluded.

5 steps ~45 minmoderate

Guide · Thermostat

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.

5 steps ~60 minmoderate

Guide · Thermostat

Smart thermostat offline / Wi-Fi problems

Separate network problems (router, credentials, internet) from power problems: many smart thermostats switch the radio off when batteries or borrowed power run low. Read the brand's code first.

5 steps ~30 mineasy

Guide · Thermostat

System short cycles (turns on and off quickly)

Short cycling can be a thermostat placement or power-stealing problem, but often it is the equipment tripping on a safety (high limit, frozen coil, float switch). Watch one full cycle and see who ends it: the thermostat or the equipment.

5 steps ~60 minmoderate

Guide · Thermostat

Thermostat blank screen / no power

A dead display is usually a missing 24 V supply, not a bad thermostat. Work from batteries and breakers to the furnace door switch, condensate float switch, board fuse and transformer. A board fuse that blows again points to a shorted thermostat or outdoor wire.

6 steps ~45 minmoderate

Guide · Thermostat

Thermostat calls but heat or cooling does not start

Split the problem in two: is the thermostat sending 24 V on the call wire, and is the equipment responding to it? A jumper test at the board answers that in minutes.

5 steps ~40 minmoderate

Guide · Thermostat

Thermostat reads or holds the wrong temperature

Compare the thermostat reading with a reference thermometer at the same spot. If the reading is wrong, look at location, drafts and sensors; if the reading is right but the room never reaches setpoint, the equipment is undersized, starving for air or faulty.

4 steps ~30 mineasy

Guide · Thermostat

Thermostat setpoint changes by itself

Most 'ghost' changes come from features, not faults: a program schedule resuming after a temporary hold, learning schedules, eco/away modes, utility demand-response events, or another person or app. Check the thermostat history before replacing it.

4 steps ~20 mineasy

Guide · Thermostat

Thermostat shows WAIT or flashing 'Cool On'

Most thermostats hold the compressor off for a few minutes after it stops or after power returns, to prevent short cycling. That delay is normal. If the message lasts well past the delay or the system never starts, check mode, breakers, filter-related lockouts and wiring.

5 steps ~20 mineasy

Installation · 1

Install · Thermostat

Thermostat replacement (low-voltage)

Most central HVAC thermostats are 24 V. Kill power to the HVAC system, photograph and label the existing wires, and confirm compatibility (C wire for most smart thermostats). Line-voltage (120/240 V) thermostats for baseboard heat are a different product and need a matching line-voltage model.

5 steps ~45 mineasy

ecobee · 9 codes

ecobee · Thermostat

Alert 1000

Low (cold) temperature alert

Thermostats

Field service manuals

Google Nest Learning Thermostat (3rd and 4th gen): quick sheet

ecobee Smart Thermostat Premium / Enhanced and the Power Extender Kit: quick sheet

24 V HVAC thermostat control wiring: field service manual

Smart thermostat installation & troubleshooting: field service manual

Diagnostic guides · 12

Auxiliary heat runs too much or all the time

Heat pump blows cold on heat (or warm on cool): O/B setting

Indoor fan won't turn off from the thermostat

Installing a C-wire adapter (Nest Power Connector, ecobee PEK and similar)

Replacing an old mercury or 2-wire thermostat

Smart thermostat offline / Wi-Fi problems

System short cycles (turns on and off quickly)

Thermostat blank screen / no power

Thermostat calls but heat or cooling does not start

Thermostat reads or holds the wrong temperature

Thermostat setpoint changes by itself

Thermostat shows WAIT or flashing 'Cool On'

Installation · 1

Thermostat replacement (low-voltage)

ecobee · 9 codes

Emerson Sensi · 2 codes

Google Nest · 34 codes

Honeywell Home / Resideo · 12 codes

Official manufacturer support