Honeywell TH9421C1004 VisionPro IAQ Programmable Thermostat
Installation
Thermostat can be mounted on 2″ x 4″ J box or on drywall or plaster using included wall anchors (drill 3/16″ holes for drywall; 7/32″ holes for plaster). Connect 1, 2, 3 terminals on thermostat to 1, 2, 3 terminals on EIM or Zone Panel; see EIM or Zone Panel Installation Guide for wiring diagrams.
Set date/time
Installer setup
- Press SYSTEM
- Press and hold for 5 seconds
- Change settings (see p. 2-6)
Special function
Auto Discover: Setup functions set to “E” (Auto Discover) will be configured automatically, based on information sent from connected equipment. When using Auto Discover, verify that the discovered value is correct. If it is not, the equipment is not communicating the information, and you must change settings manually.
Adaptive Intelligent Recovery (Setup Function 0530): Allows the thermostat to “learn” how long the furnace and air conditioner take to reach programmed temperature settings, so the temperature is reached at the scheduled time.
Compressor Protection (Setup Function 0580): Forces the compressor to wait a few minutes before restarting, to prevent damage. During this time, the message “Wait” flashes on the display.
Southern Away Dehumidification Control (Setup Functions 0390-0394): This feature is useful for homes in southern climates. It allows humidity control when the the home is unoccupied for long periods during the hot/humid season. You can control temperature (Function 0393) and dehumidification (Function 0394) while away. You can control humidity with a dehumidifier or a cooling system (Function 0379). If a cooling system is used, humidity is controlled by cooling indoor air to the Low Limit Temperature (Function 0392). When the desired humidity is reached, the system will maintain the temperature you select in Function 0393.
Heat Pump Control — Electric Backup (Setup Functions 0350-0360): If an outdoor temperature sensor is installed, select a compressor lockout temperature (Function 0350). Below this temperature, only electric heat operates. Also select an auxiliary lockout temperature (Function 0360). Above this temperature, only the compressor operates. Between these temperatures, both heat sources operate.
Heat Pump Control — Fossil Fuel Backup (Setup Function 0345): Option 0 (Balance point only): If outdoor temperature is above balance point (Function 0350), only the compressor operates. Below this temperature, only backup heat operates.
Option 1 (Balance point + 2°F droop): As above, but backup heat is activated if room temperature drops by 2°F (compressor is deactivated).
Option 2: (Balance point/Aux Heat +2°F droop): Compressor works only above auxiliary lockout temperature, backup heat works only below balance point, 2°F droop between temperatures. If temperature is not reached in a reasonable time, set the upstage timer (Function 0346). After the designated time, the compressor will be deactivated and the system will switch to backup heat.
Ventilation Control (Setup Functions 0400-0406): If the system includes a self-powered ventilation unit (Function 0400), the home can be ventilated on demand, or ventilated automatically if high humidity is detected while in heating mode (Function 0406).
Installer system test
During installer setup, press 
repeatedly until “Test” appears
System test System status
Other messages
P Ventilation meets ASHRAE 62.2 standard (Function 0404)
F Ventilation does not meet ASHRAE 62.2 standard (Function 0404)
Err See error codes
How to Use the Installer System Test
The Installer Test is part of the Installer Setup Menu
- Enter the Installer System Test by entering Installer Setup.
- The test appears at the end of the Installer Setup Numbers.
- See above for available Installer System Tests.
- If installed with a Discharge Air Temperature Sensor (ISU365=1), press MORE to view the discharge temperature, and press CANCEL to return to the Installer test screen.
Error codes
027 Faulty thermostat (must be replaced)
035 Duplicate thermostat zone detected
036 Duplicate UV timer
037 Duplicate filter timer
038 Duplicate humidifier pad timer
039 Duplicate indoor humidity sensor
040 Compressor should be running but it is not
050 Duplicate outdoor temperature sensor
052 Faulty post-coil air discharge sensor
053 Faulty outdoor temperature sensor
054 Faulty indoor humidity sensor
089 Heating/cooling equipment is missing
090 Data interface is continuously busy
091 Thermostat is not receiving data from other equipment
118 Faulty discharge air temperature sensor
119 Faulty temperature sensor
121 Duplicate post-coil air discharge sensor
Accessories
Please contact your distributor to order replacement parts.
- Outdoor temperature sensor: Part number C7089U1006
- Remote indoor temperature sensor: Part number C7189U1005
- Discharge air temp. sensor: Part number C7835A1009
- Energy recovery ventilator: Part number ER200B2006
- Heat recovery ventilator: Part number HR200B1005
- Fresh air damper: Part number EARD6
- 2-Channel Telephone access module: Part number W8735D1009
- 4-Channel Telephone access module: Part number W8735D1017
- Bypass flow-through or steam humidifier: Part number HE265A1007
- UV air purifier: Part number UV100E1043
- Cover plate: Part number 32003796-001
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Honeywell TH9421C1004 VisionPro IAQ Programmable Thermostat -FAQs
How should I set my Honeywell thermostat?
Touch the displayed temperature to adjust the desired heating or cooling setting. Press SELECT when done and use the > icon until DONE is displayed. Touch SELECT again to save.
How do I manually use my Honeywell thermostat?
Use the “+” and “-” buttons to set the temperature. Press the “Hold” button to maintain that temperature until you change it again.
Is the Honeywell thermostat battery-operated?
Yes, Honeywell thermostats usually use AA or AAA batteries. Check your manual or the battery compartment for the correct type.
What is the mode on a Honeywell thermostat?
Modes include:
1. AUTO: Follows the programmed temperature schedule.
2. MAN (Manual): Maintains a fixed temperature without a schedule.
How do I program a thermostat?
1. Read the manual.
2. Set the time and date.
3. Use the ‘Set’ or ‘Program’ button to configure temperatures for specific times, like leave, return, or sleep.
How does a Honeywell thermostat work without batteries?
Without batteries, it uses a 24-volt AC power source connected through C and Rc wires. It may lose settings during power failures.
How do I reset my Honeywell thermostat after a power outage?
1. Turn off the thermostat.
2. Reset the circuit breaker.
3. Wait 30 minutes.
4. Turn the thermostat back on.
What does “Hold” mean on a thermostat?
“HOLD” lets you maintain a specific temperature without following a schedule, ideal for vacations or temporary comfort.
What happens if my Honeywell thermostat goes blank?
Check if it needs new batteries. If not, inspect the wiring or ensure the power source for the furnace or air handler is on.
What is thermostat mode?
Thermostat modes configure your HVAC system for different comfort settings like heating, cooling, or auto-adjusting between them.