Digital thermostat mounted on a concrete wall
9 components covered

Field manual · Thermostat

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

Covers: 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.

Cross-checkedBrands: Google Nest9 components

The budget Nest: a round mirror-finish face with a touch strip on the right side instead of the rotating ring, and a radar presence sensor behind the glass. It does not learn a schedule on its own (you set one), it runs on 24 V systems only, and it has fewer terminals than the Learning models. The most common service calls are star (*/OB) connector confusion on heat pumps, power problems on systems without a C wire, and the wrong equipment answers given during app setup.

Safety first

  • Turn HVAC power off before moving wires
  • Never connect this thermostat to line-voltage (120/240 V) heat
  • Never put two wires in one connector

How it works

Setup is done in the Google Home app, which asks which wires are in the old thermostat and then draws a custom wiring diagram. The thermostat then detects which connectors have wires and asks you to confirm the function of anything on the star connector.

The star connector is also labeled O/B. On this model it can carry only one of three functions: Y2 (second-stage cool), W2 (second-stage heat) or O/B (heat pump changeover valve). Only one wire may ever go into it, so a heat pump with a second compressor stage or a second heat stage cannot use all of those at once.

Rh and Rc are bridged internally; no jumper wire is needed and only one wire goes in each connector. Systems with a single R wire just use one of them.

For heat pumps the thermostat manages auxiliary heat automatically. Heat Pump Balance (Max Comfort default, Balanced, Max Savings, or Off for a manual lockout) uses local weather from Wi-Fi to decide when aux heat may run; there is no outdoor sensor input. Emergency Heat is a manual switch in the app and on the thermostat.

Power is drawn from the HVAC 24 V circuit. Heating-only, cooling-only, zoned and heat pump systems are the ones most likely to need a C wire or the Nest Power Connector; small AAA batteries are listed by retailers as a backup only.

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Official manufacturer documentation

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

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): Digital thermostat mounted on a concrete wall by Alexander Mass, 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

Other thermostats manuals

Manual · Thermostat

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
Manual · Thermostat

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
Manual · Thermostat

Honeywell Home FocusPRO N100 non-programmable thermostats (TH1110U4000, TH1210U4001, TH1320U4002): quick sheet

FocusPRO N100 series on the UWP wall plate: TH1110U4000 (1H/1C, heat pump without aux), TH1210U4001 (heat pump 1 compressor + aux, or 1H/1C conventional, dual fuel), TH1320U4002 (heat pump up to 2 compressor stages + aux, up to 2H/2C conventional, dual fuel). Successor family to the older FocusPRO 5000/6000 contractor thermostats.

6 components5 common failuresHoneywell Home, Resideo
Google Nest Thermostat (2020, mirror face, no ring): quick sheet · OnCall Tech