Digital thermostat mounted on a concrete wall
7 components covered

Field manual · Thermostat

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

Covers: 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.

Cross-checkedBrands: Honeywell Home, Resideo7 components

A retail touchscreen Wi-Fi thermostat set up through the Resideo / Honeywell Home app. It must have 24 V common; the T5+ adds a small C-wire adapter that mounts at the equipment and converts the G and Y wires so a 4-wire home can power it. Most callbacks are adapter wiring mistakes, setups on systems with no G or no Y wire, and wrong system type or O/B setting.

Safety first

  • Leave HVAC power off for the whole adapter install
  • Never use on line-voltage heaters
  • Do not land adapter leads between a zone panel and the equipment

How it works

The thermostat requires 24 VAC on C. If no spare conductor exists in the wall, the THP9045A adapter is installed between the thermostat cable and the equipment control board. It only works when both a G and a Y wire are present.

With the adapter, at the thermostat the old G wire is moved to C and the old Y wire is moved to the K terminal; R and W stay where they are. At the equipment the adapter's leads go to the board's R, C, G, Y and W terminals. The adapter then splits the K signal back into fan and compressor calls.

On zoned systems the adapter goes between the thermostat and the zone panel, never between the zone panel and the equipment.

System type, heat pump changeover (O/B) and backup heat are chosen during app setup or in the thermostat's advanced setup. The installation manual lists up to 2 cool/compressor stages and 0-1 aux/emergency stages for heat pumps.

S and U terminals (sensors, accessories) are not supported on this model.

Components & tests

Unlock all 7 components, specs, service mode and teardown

Full library + AI assistant + the OnCall Tech app for iPhone & Android. $34.90, one time.

Get lifetime access

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

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