THX9421R5021WW/U — Honeywell Home Prestige IAQ 2-wire thermostat with RedLINK technology: a 7-day programmable high-definition color touchscreen thermostat with a white front and white sides, selectable at installer setup for residential or light commercial applications.
The Prestige IAQ provides control of temperature, humidification, dehumidification and ventilation for up to 4 heat / 2 cool heat pump systems or up to 3 heat / 2 cool conventional systems. The thermostat itself carries only R and C terminals — this is what “2-wire” refers to. All equipment, IAQ and sensor connections are made at the Equipment Interface Module (THM5421R1021), which communicates with the thermostat over RedLINK 2.0 wireless at 900 MHz. The EIM provides the W-O/B, Y, Y2, G, W2-Aux 1, W3-Aux 2, A-L/A terminals, three sets of universal IAQ contacts (U1, U2, U3) and four assignable sensor inputs (S1–S4) for indoor and outdoor temperature sensors, discharge and return air sensors, or dry contact devices. Features that depend on those sensor inputs — Delta T alerts and diagnostics, the Delta T installer test and the performance log — therefore require the EIM.
The system is RedLINK 2.0 compatible, so it can be extended with the Wireless Outdoor Sensor, Wireless Indoor Sensor, Portable Comfort Control, RedLINK 2.0 Internet Gateway, TrueZONE zoning panel with wireless adapter, Vent Boost Remote and Entry/Exit Remote. Up to 10 customizable service reminders can be set, based on date or on outdoor temperature, and the thermostat keeps an alerts log, a user interactions log and a performance log.
Specifications
| Part number / color |
THX9421R5021WW — front white, side white |
| Thermostat terminals |
R, C only (equipment wiring lands on the Equipment Interface Module) |
| Powering method |
Common wire only |
| System capability |
Up to 4 heat / 2 cool heat pump; up to 3 heat / 2 cool conventional |
| System types |
Gas, oil or electric heat with air conditioning; warm air, hot water, high-efficiency furnaces, heat pumps, steam and gravity; cool only |
| Power consumption |
Full brightness 3.0 VA; 1/2 brightness 2.1 VA; backlight off 1.9 VA |
| RedLINK 2.0 communication |
900 MHz frequency range; devices re-establish communication within 6 minutes after AC power resumes |
| Heating setting range |
40 to 90°F (4.5 to 32°C) |
| Cooling setting range |
50 to 99°F (10 to 37°C) |
| Temperature sensor accuracy |
±1.5°F at 70°F (0.75°C at 21.0°C) |
| Humidification setting range |
10% to 60% RH |
| Dehumidification setting range |
40% to 80% RH |
| Humidity display range |
0% to 99% |
| Humidity sensor accuracy |
±5% RH from 30% to 50% RH at 75°F (24°C) |
| Clock accuracy |
±1 minute per month at 77°F (25°C); ±2 minutes per month over the operating ambient range |
| Changeover |
Manual or Auto, selectable |
| System setting |
Em Heat – Heat – Off – Cool – Auto |
| Fan setting |
Auto – On – Circ – Follow Schedule |
| Operating ambient temperature |
32 to 120°F (0 to 48.9°C) |
| Operating relative humidity |
5% to 90%, non-condensing |
| Shipping temperature |
−20 to 120°F (−28.9 to 48.9°C) |
| Physical dimensions |
4-1/2 × 3-1/2 × 7/8 in. (115 × 88 × 22 mm) |
| Mounting |
Mounts directly on the wall with screws and anchors provided; fits a horizontal 2 × 4 in. junction box |
Installation notes
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This part number is the thermostat only. In Resideo’s Prestige Kits table, the Equipment Interface Module and sensors are supplied with the Y-prefix kits, not with THX9421R5021WW. Order the EIM (THM5421R1021) and any sensors separately, or buy a kit.
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The EIM is what drives the equipment: the thermostat has only R and C terminals, so an Equipment Interface Module (or a compatible RedLINK zoning panel) is required for the thermostat to control HVAC equipment.
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Not a Wi-Fi thermostat on its own: the Prestige communicates over RedLINK 2.0 at 900 MHz. Remote access over the internet requires the THM6000R7001 RedLINK 2.0 Internet Gateway, which is included only in the YTHX9421R7001WW kit.
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ENERGY STAR qualification is conditional: per Resideo, current models of Prestige with date code 1745 or later, used with the THM6000R7001 white gateway, are ENERGY STAR rated — it is not a property of the bare thermostat.
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Delta T diagnostics need the EIM and cannot be used on a zoned system, because Delta T is affected by damper position and one zone may call for heat while another calls for cool.
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EIM location: the Equipment Interface Module mounts on the HVAC equipment or on a wall in the equipment room, while the thermostat mounts in the living space.
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Demand Response: qualification requires the EIM with a date code of 1745 or higher used with the THM6000R7001 gateway.
Related models
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YTHX9421R5085WW — Prestige IAQ kit: this thermostat plus the THM5421R1021 EIM and two 50062329-001 discharge/return sensors.
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YTHX9421R5101WW — same kit plus a C7089R1013 wireless outdoor sensor.
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YTHX9421R7001WW — kit with EIM, discharge/return sensors, THM6000R7001 RedLINK 2.0 Internet Gateway, C7189R1004 wireless indoor temperature/humidity sensor and C7089R1013 wireless outdoor sensor.
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THX9321 Prestige 2.0 — the non-IAQ sibling; unlike the 2-wire THX9421 it carries full equipment terminals (R, Rc, C, W-O/B, W2-Aux/E, Y, Y2, G, A-L/A, K, U1, U2) on the thermostat itself.
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THX9421R5021BB / SG / WG — black, silver/gray and white/gray variants, listed as obsolete.
Specifications taken from the Resideo / Honeywell Home product data sheet “THX9321 Prestige 2.0 and THX9421 Prestige IAQ with EIM,” document 68-0311—10. Manufacturer specifications are subject to change without notice.
