Honeywell Home T6 Pro programmable thermostats (TH6210U, TH6220U, TH6320U): quick sheet

Covers: Honeywell Home / Resideo T6 Pro programmable thermostats on the UWP mounting plate: TH6210U (1H/1C or 1-stage heat pump), TH6220U (up to 2H/2C conventional or 2H/1C heat pump) and TH6320U (up to 3H/2C heat pump). Wi-Fi and Z-Wave variants have different power options.

Cross-checkedBrands: Honeywell Home, Resideo6 components

A pro-channel thermostat set up through numbered Installer Setup (ISU) codes. Most callbacks are setup mistakes: wrong system type, O/B reversed, backup heat type wrong, or compressor protection delay mistaken for a fault.

Safety first

Turn off HVAC power before wiring.
Line-voltage (120/240 V) thermostats are not compatible; never connect T6 Pro to line voltage.

How it works

Enter installer setup from the home screen by holding the Menu and + buttons for about 3 seconds, then select ISU. Change values with + / -, Select to move forward, Home to save and exit.

Key settings: ISU 200 system type (conventional forced air default, heat pump, radiant, cool only); ISU 218 reversing valve (O energized in cool by default, or B energized in heat); ISU 255 backup heat source for heat pumps (electric or gas/oil); ISU 230 fan control in heat (equipment or thermostat).

ISU 387 compressor protection sets a minimum off time (default 5 minutes). During this delay the Cool or Heat indicator flashes; it is normal, not a fault.

Non-Wi-Fi models can run on AA batteries or 24 VAC (C wire). The Smart Wi-Fi T6 Pro requires 24 VAC on C.

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Official manufacturer documentation

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

Honeywell Home thermostat wiring help

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): White retro-style refrigerator beside a kitchen sink cabinet by Pavel Danilyuk, 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

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

Other thermostats manuals

Manual · Thermostat

Honeywell Home T9 / T10 smart thermostats with room sensors: quick sheet

Honeywell Home T9 (RCHT9610WF, RCHT9610WFSW sensor bundles) and the related T10 Pro. Wi-Fi touchscreen thermostats with wireless smart room sensors.

6 components5 common failuresHoneywell Home, Resideo

Manual · Thermostat

Emerson / Copeland Sensi smart thermostats (Sensi, Sensi Lite, Sensi Touch, Touch 2): quick sheet

Sensi Wi-Fi thermostats: classic Sensi (ST55 style) and Sensi Lite, Sensi Touch (ST75) and Sensi Touch 2. The Sensi brand moved from Emerson to Copeland; support pages live on both domains.

6 components5 common failuresSensi, Emerson, Copeland, White-Rodgers

Manual · Thermostat

Honeywell / Resideo TrueZONE zone panels (HZ322, HZ432): quick sheet

TrueZONE HZ322 (up to 3 zones, 2-heat/2-cool conventional or heat pump) and HZ432 (up to 4 zones, 3-heat/2-cool) residential forced-air zone panels with a discharge air temperature sensor (DATS).

6 components5 common failuresHoneywell Home, Resideo