Amazon Smart Thermostat (2021, Alexa, built with Resideo/Honeywell Home): quick sheet

Covers: Amazon Smart Thermostat sold from November 2021, alone or bundled with the Honeywell Home C-wire adapter. Set up and controlled only through the Alexa app.

Single sourceBrands: Amazon6 components

A low-cost Wi-Fi thermostat developed with Resideo (Honeywell Home). It has no microphone or speaker; Alexa control comes through other Echo devices or the app. It needs a C wire or a C-wire adapter and is for 24 V systems only. Amazon says it does not support dual-fuel heat pumps.

Safety first

Power off at the breaker before touching wires
Do not touch exposed wire ends
24 V only

How it works

Setup happens in the Alexa app, which records the wires connected and shows a 'connected wire map' under the thermostat's settings. The wire map must match what is physically on the terminals or the equipment will run wrong.

R and Rc: with one R wire it goes to R with the jumper left in place; with two R wires the jumper is moved and each wire goes to its own terminal.

Conventional systems use R, C, W, Y, G. Heat pumps use R, C, Y, G and O/B; Amazon's troubleshooting page says there should be no wire on W in that layout. Support for heat pump aux/emergency heat is listed by a retailer but is not stated clearly by Amazon, so treat aux-heat setups as 'check with Amazon's compatibility checker first'.

Without a C wire, the Honeywell C-wire adapter is installed at the equipment: at the thermostat G moves to C and Y moves to K; W (or the O/B wire) and R stay.

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Official manufacturer documentation

Always confirm values against the tech sheet for the exact model.

Amazon Smart Thermostat help: system compatibility

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. © 2026 OnCall Tech. Original content; not a reproduction of any manufacturer document.Generic part photos (not of this brand): Clothes dryer with its vent duct in a small laundry closet by Introspectivedsgn, Pexels License

Diagnostic guides for this appliance

Guide · Thermostat

Furnace / air handler 3 A low-voltage fuse keeps blowing (finding the 24 V short)

A blown 24 V fuse is a symptom of a short in the thermostat wire, outdoor unit wire or a load (contactor coil, damper, humidifier). Isolate circuits one at a time instead of replacing fuses.

8 steps ~60 minmoderate

Guide · Thermostat

Zoned system: one zone not heating or cooling (damper and zone panel checks)

If only one zone fails, the cause is almost always that zone's damper, actuator, thermostat or wiring, not the furnace or AC. Simulate a call at the panel and check whether the damper actually moves.

8 steps ~45 minmoderate

Guide · Thermostat

Field pattern: thermostat replaced, now nothing works or the wrong thing runs (fuse, system type, O/B)

Almost every post-install failure comes from one of four things: a 24 V short during install that blew the board fuse, a missing C wire, the wrong equipment type in the installer menu, or the O/B (reversing valve) setting reversed. The wrong turn is assuming the new thermostat is defective or that the furnace failed the same day; check the install before ordering anything.

7 steps ~40 mineasy

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Google Nest Thermostat (2020, mirror face, no ring): quick sheet

Google Nest Thermostat launched October 2020 (model G4CVZ, sold as GA01334-US and color variants). Not the Nest Learning Thermostat (3rd/4th gen) or the Nest Thermostat E, which have their own sheets/notes.

9 components5 common failuresGoogle Nest

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ecobee3 lite smart thermostat (EB-STATE3LT-02 and Pro EB-STATE3LTP-02): quick sheet

ecobee3 lite retail (EB-STATE3LT-02) and contractor/Pro version (EB-STATE3LTP-02). Now listed as discontinued by several distributors but very common in the field. ecobee Premium/Enhanced have their own sheet.

9 components5 common failuresecobee

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Honeywell Home T5 / T5+ Smart Thermostat (RCHT8610WF, RCHT8612WF, RTH8800WF): quick sheet

Honeywell Home / Resideo T5 Smart Thermostat RCHT8610WF (no C-wire adapter in the box), T5+ RCHT8612WF (ships with the THP9045A C-wire adapter) and RTH8800WF retail variant. T6 Pro, T9 and T10 have their own sheets.

7 components5 common failuresHoneywell Home, Resideo