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Domain Setup
Nick Berardi edited this page Jul 19, 2026
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6 revisions
| COS | Yes |
| Access | R |
| Data length | 44 bytes (bytes 0–43; 16, 35, 37 reserved) |
| COS caused by | Saving thermostat settings to EEPROM from installer setup |
Thermostat installer settings configure equipment and control behavior. Some fields apply only when other settings enable them. The thermostat may return Null for unused values. Factory defaults are listed below the field table.
| Field | Byte | Values |
|---|---|---|
| Connected To (#0 on thermostat) | Byte 0 | 0: Aprilaire Cloud 1: Automation System 2-255: Reserved |
| Equipment Type (#1) | Byte 1 | 0: Heat/Cool 1: Heat Pump 2-255: Reserved |
| Temperature Scale (#2) | Byte 2 | 0: Fahrenheit 1: Celsius 2-255: Reserved |
| Reversing Valve (#3) (except 8830, 8840) | Byte 3 | 0: O/B Energized in Cooling 1: O/B Energized in Heating 2-255: Reserved Note: O/B Operation, should be ignored if Equipment Type is set to Heat/Cool. |
| Control Setup (#4) | Byte 4 | 0: Heat and Cool 1: Heat Only 2: Cool Only 3-255 Reserved |
| H/C: Cooling Stages H/P: Compressor Stages (#5) | Byte 5 | 0: One 1: Two 2: Three (except 8810, 8820) (except Heat Pump) 3-255: Reserved |
| H/C: Heating Stages H/P: Aux Heat Stages (#6) | Byte 6 | 0: One 1: Two 2: Three (except Heat Pump) 2-255: Reserved |
| H/C: Fan Control in Heating H/P: Auxiliary Equipment Type (#7) | Byte 7 | 0: Gas/Oil 1: Electric 2-255: Reserved Note: Fan control in heating should be ignored if, Control Setup is set to Cool Only and Equipment Type, is set to Heat/Cool. |
| Extended Fan – Heat (#8) | Byte 8 | 0: Disabled 1: Enabled (except 8810) 2-255: Reserved Note: Extended Fan – Heat, should be ignored Fan control in heating, is set to Gas or Oil Furnace OR if, Control Setup, is set to Cool Only and, Equipment Type, is set to Heat/Cool. |
| Extended Fan - Cool (#9) | Byte 9 | 0: Disabled 1: Enabled (except 8810) 2-255: Reserved Note: Extended Fan – Cool, should be ignored if, Control Setup, is set to Heat Only and, Equipment Type, is set to Heat/Cool. |
| Internal Temp Sensor Offset (#10) | Byte 10 | 0: -4F or -2C 1: -3F or -1.5C 2: -2F or -1C 3: -1F or -0.5C 4: 0F or 0C 5: 1F or 0.5C 6: 2F or 1C 7: 3F or 1.5C 8: 4F or 2C 9-255 Reserved |
| Internal RH Sensor Offset (#11) (except 8810) | Byte 11 | 0: -5% 1: -4% 2: -3% 3: -2% 4: -1% 5: 0% 6: 1% 7: 2% 8: 3% 9: 4% 10: 5% 11-255 Reserved |
| Auto Changeover (#12) | Byte 12 | 0: Disabled 1: Enabled 2-255: Reserved Note: Auto Changeover, should be ignored if, Control Setup is not set to Heat and Cool. |
| Deadband (#13) | Byte 13 | 0: 2F or 1C 1: 3F or 1.5C 2: 4F or 2C 3: 5F or 2.5C 4: 6F or 3C 5: 7F or 3.5C 6: 8F or 4C 7: 9F or 4.5C 8-255: Reserved Note: Deadband should be ignored if, Auto Changeover is set to disabled. |
| Wired Remote Temperature Sensor (#14) | Byte 14 | 0: Not installed 1: Installed, no fallback 2: Installed, fallback to built-in 3: Installed, average with built-in 4-255: Reserved |
| Outdoor Sensor (#15) (except 8810) | Byte 15 | 0: Not installed 1: Installed 2: Provided by Protocol (Automation) 3-255: Reserved |
| Reserved | Byte 16 | |
| Return Air Sensor (#17) | Byte 17 | 0: Not Installed 1: Installed 2-255: Reserved |
| Compressor Min Off Time (#18) (except Heat Cool configured) | Byte 18 | 0: Reserved 1: 1 Minute 2: 2 Minute 3: 3 Minute 4: 4 Minute 5: 5 Minute 6-255: Reserved |
| Heating Min Off Time (#19) | Byte 19 | 0: Reserved 1: 1 Minute 2: 2 Minute 3: 3 Minute 4: 4 Minute 5: 5 Minute 6-255: Reserved Note: Heating Minimum off Time, should be ignored if 2, Control Setup is set to Cool Only and, Equipment Type, is set to Heat/Cool. |
| Equipment Min On Time (#20) | Byte 20 | 0: Reserved 1: 1 Minute 2: 2 Minute 3: 3 Minute 4: 4 Minute 5: 5 Minute 6-255: Reserved |
| Auto Changeover Time (#21) | Byte 21 | 0: Reserved 1: 1 Minute 2: 2 Minute 3: 3 Minute 4: 4 Minute 5: 5 Minute 6-255: Reserved Note: Auto Mode Changeover Time should be ignored if, Auto Changeover is set to disabled. |
| First Stage Differential (#22) | Byte 22 | 0: 1F or 0.5C 1: 2F or 1C 2: 3F or 1.5C 3: 4F or 2C 4: 5F or 2.5C 5: 6F or 3C 6: 7F or 3.5C 7: 8F or 4C 8: 9F or 4.5C 9-255: Reserved |
| Second Stage Differential (#23) | Byte 23 | 0: 1F or 0.5C 1: 2F or 1C 2: 3F or 1.5C 3: 4F or 2C 4: 5F or 2.5C 5: 6F or 3C 6: 7F or 3.5C 7: 8F or 4C 8: 9F or 4.5C 9-255: Reserved Note: Differential 2 should be ignored if, Equipment Type is set to Heat/Cool AND, Equipment Stages, is set to 1. |
| Third Stage Differential (#24) | Byte 24 | 0: 1F or 0.5C 1: 2F or 1C 2: 3F or 1.5C 3: 4F or 2C 4: 5F or 2.5C 5: 6F or 3C 6: 7F or 3.5C 7: 8F or 4C 8: 9F or 4.5C 9-255: Reserved Note: Differential 3 should be ignored if, Equipment Type is set to Heat/Cool OR, Equipment Stages, is set to One AND, Auxiliary Heat Stages, is set to 1. |
| Forth Stage Differential (#25) | Byte 25 | 0: 1F or 0.5C 1: 2F or 1C 2: 3F or 1.5C 3: 4F or 2C 4: 5F or 2.5C 5: 6F or 3C 6: 7F or 3.5C 7: 8F or 4C 8: 9F or 4.5C 9-255: Reserved Note: Differential 4 should be ignored if, Equipment Type is set to Heat/Cool OR, Equipment Stages, is set to One OR, Auxiliary Heat Stages, is set to one. |
| Away (#26) | Byte 26 | 0: Disabled 1: Enabled 2-255: Reserved |
| Heat Blast (#27) | Byte 27 | 0: Disabled 1: Enabled 2-255: Reserved |
| Heat Blast Offset (#28) | Byte 28 | 0: 3F or 1.5C 1: 4F or 2C 2: 5F or 2.5C 3-255: Reserved |
| Stage Rate (#37) | Byte 29 | 0: Off 1: 1 2: 2 3: 3 4: 4 5: 5 6-255: Reserved |
| Progressive Recovery (#38) | Byte 30 | 0: Disabled 1: Enabled 2-255 Reserved |
| Low Balance Point (#39) | Byte 31 | 0-60: degrees F (20F default) 61: Off 62-255: Reserved Note: Low Balance Point should be ignored if, Outdoor Sensor is set to No Outdoor Sensor Attached OR, Equipment Type, is set to Heat/Cool. |
| High Balance Point (#40) | Byte 32 | 0-80: degrees F (65F default) 81: Off 82-255: Reserved Note: High Balance Point should be ignored if, Outdoor Sensor is set to No Outdoor Sensor Attached OR, Equipment Type, is set to Heat/Cool |
| Program Format (#41) | Byte 33 | 0: 7-Day Programmable 1: Non-Programmable 2-255: Reserved |
| HVAC Service Reminder (#43) (except 8810) | Byte 34 | 0: Null 1-12: Number of Months 13: Off 14-255: Reserved |
| Reserved | Byte 35 | |
| Turn Off WiFi Radio (#48) | Byte 36 | 0: No (radio enabled) 1: Yes (radio disabled) 2-255 Reserved |
| Reserved | Byte 37 | |
| Support Module Controlling Temp Sensors (#51) | Byte 38 | 0: None 1: One 2: Two 3: Three 4: Four 5: Five 6: Six 7: Seven 8: Eight 9-255 Reserved |
| Support Module Controlling RH Sensors (#52) | Byte 39 | 0: None 1: One 2: Two 3: Three 4: Four 5-255 Reserved |
| Display Monitor Support Module Sensors (#53) | Byte 40 | 0: No 1: Yes 2-255 Reserved |
| 8476 Change Air Filter Reminder, months (#54) | Byte 41 | 0: Reserved 1-12: Number of Months 13: Off 14-255: Reserved |
| 8476 Change Water Panel Reminder, months (#55) | Byte 42 | 0: Reserved 1-12: Number of Months 13: Off 14-255: Reserved |
| 8476 Humidifier Type (#56) | Byte 43 | 0: Flow through 1: Drain-less 2-255 Reserved |
Guide typo: Byte 6 (Heating / Aux Heat Stages) lists "2: Three (except Heat Pump) 2-255: Reserved" — presumed intent is 3-255 reserved (compare Byte 5). See Guide Errata.
The guide marks defaults in bold ("Note: Defaults in Bold"); that formatting was lost in earlier text extractions. Recovered from the original document:
| Byte | Default | Byte | Default |
|---|---|---|---|
| 0 | 1: Automation System | 22 | 0: 1F or 0.5C |
| 1 | 0: Heat/Cool | 23 | 0: 1F or 0.5C |
| 2 | 0: Fahrenheit | 24 | 0: 1F or 0.5C |
| 4 | 0: Heat and Cool | 25 | 0: 1F or 0.5C |
| 5 | 0: One | 26 | 0: Disabled (Away) |
| 6 | 0: One | 27 | 0: Disabled (Heat Blast) |
| 7 | 0: Gas/Oil | 28 | 0: 3F or 1.5C |
| 8 | 0: Disabled | 29 | 2: 2 (Stage Rate) |
| 9 | 0: Disabled | 30 | 1: Enabled (Progressive Recovery) |
| 10 | 4: 0F or 0C | 31 | 20 °F (Low Balance Point) |
| 11 | 5: 0% | 32 | 65 °F (High Balance Point) |
| 12 | 0: Disabled | 33 | 0: 7-Day Programmable |
| 13 | 1: 3F or 1.5C (Deadband) | 34 | 13: Off |
| 14 | 0: Not installed | 36 | 0: No (radio enabled) |
| 15 | 0: Not installed | 38 | 0: None |
| 17 | 0: Not Installed | 39 | 0: None |
| 18 | 5: 5 Minute | 40 | 0: No |
| 19 | 2: 2 Minute | 41 | 13: Off |
| 20 | 2: 2 Minute | 42 | 13: Off |
| 21 | 4: 4 Minute | 43 | 0: Flow through |
(Byte 3 Reversing Valve has no bold default in the guide.)
| COS | Yes |
| Access | R/W |
| Models | 8840 only (guide marks the attribute "8840 only") |
Null-terminated ASCII contractor fields. Each field is filled/terminated with nulls; default is all nulls.
| Field | Size | Content |
|---|---|---|
| Name | 30 chars + null (31 bytes) | Alphanumeric ASCII, nulls fill/terminate |
| Phone | 10 chars + null (11 bytes) | e.g. 1234567890\0
|
| 30 chars + null (31 bytes) | Alphanumeric ASCII | |
| Web | 30 chars + null (31 bytes) | Alphanumeric ASCII |
| COS | No |
| Access | R/W |
| Data length | 1 byte |
| Field | Byte | Values |
|---|---|---|
| User Scale | Byte 0 | 0: Fahrenheit (default) 1: Celsius 2-255: Reserved |
Does not change wire encoding — temperatures on the wire remain Celsius. See Temperature Encoding.
| COS | No |
| Access | R/W |
| Data length | 7 bytes |
Host should set the clock for schedule accuracy (at least monthly). Automation owns time base.
| Field | Byte | Values |
|---|---|---|
| Second | Byte 0 | 0-59: Second 60-255: Reserved |
| Minute | Byte 1 | 0-59: Minute 60-255: Reserved |
| Hour | Byte 2 | 0-23: Hour 24-255: Reserved |
| Date | Byte 3 | 0: Reserved 1-31: Date 32-255: Reserved |
| Day | Byte 4 | 0: Sunday 1: Monday 2: Tuesday 3: Wednesday 4: Thursday 5: Friday 6: Saturday 7-255: Reserved |
| Month | Byte 5 | 0: Reserved 1-12: Month 13-255: Reserved |
| Year | Byte 6 | 0-99: Year Note: This value is added to 2000 to determine the year. A value of 0 is assumed to be 2000. |
| COS | Yes |
| Access | R |
| Data length | 3 bytes |
| Defaults | Byte 0: 1 Yes · Byte 1: 0 No · Byte 2: 13 Off |
| Field | Byte | Values |
|---|---|---|
| Display Air Cleaning Button (except 8810) | Byte 0 | 0: No 1: Yes 2-255: Reserved Note: Display Air Cleaning Button Should be ignored if Air Cleaning Installed is set to Yes |
| Air Cleaning (#A1) (except 8810) | Byte 1 | 0: No 1: Yes 2-255: Reserved |
| Air Filter Service Reminder (#A2) | Byte 2 | 0: Reserved 1-12: Number of Months 13: Off 14-255: Reserved |
| COS | Yes |
| Access | R |
| Data length | 15 bytes |
Includes humidify/dehumidify equipment configuration and RH-related installer variables.
Factory defaults (guide bold): Byte 0: 1 Yes · 1: 0 No · 2: 0 (10%) · 3: 0 Automatic · 4: 0 Heat Only · 5: 13 Off · 6: 10 October · 7: 1 January · 8: 0 None · 9: 1 Yes · 10: 1 Yes · 11: 13 Off · 12: 2 (3F/1.5C) · 13: 0 No · 14: 5 (75F/23.5C)
| Field | Byte | Values |
|---|---|---|
| Display Humidity Button (except 8810) | Byte 0 | 0: No 1: Yes 2-255: Reserved Note: Display Humidity Button Should be ignored if Humidity Installed is set to Yes |
| Humidifier Installed (#H1) (except 8810) | Byte 1 | 0: No 1: Yes 2-255: Reserved |
| Humidity Deadband (#H2) (except 8810) | Byte 2 | 0: 10% 1: 11% 2: 12% 3: 13% 4: 14% 5: 15% 6: 16% 7: 17% 8: 18% 9: 19% 10: 20% 11-255: Reserved Note: Humidity Control Deadband, should be ignored unless, Humidifier Installed, is set to Yes and, Dehumidifier Control, is set to Air Conditioner. |
| Humidifier Mode (#H3) (except 8810) | Byte 3 | 0: Automatic 1: Manual 2-255: Reserved Note: Humidifier Mode should be ignored if, Humidifier Installed, is set to No. |
| Humidifier Operation (#H4) (except 8810) | Byte 4 | 0: Heat Only 1: Heat or Fan 2: Forces Fan 3-255: Reserved Note: Humidifier Operation should be ignored if, Humidifier Installed, is set to No. |
| Water Panel Change Reminder (#H5) | Byte 5 | 0: 1 Per Season (8810 flow through) 1: 2 Per Season (8810 drainless) 2: 300 Hours Valve Operation (except 8810) 3: 600 Hours Valve Operation (except 8810) 4-12: Reserved: 13: Off 14-255: Reserved |
| Water Panel Month 1 (#H6) (except 8810) | Byte 6 | 0: Null 1: January 2: February 3: March 4: April 5: May 6: June 7: July 8: August 9: September 10: October 11: November 12: December 13-255: Reserved |
| Water Panel Month 2 (#H7) (except 8810) | Byte 7 | 0: Null 1: January 2: February 3: March 4: April 5: May 6: June 7: July 8: August 9: September 10: October 11: November 12: December 13-255: Reserved |
| Dehumidifier Control (#H8) (except 8810) | Byte 8 | 0: None 1: Whole Home 2: Air Conditioner with IAQ output 3: Air Conditioner without IAQ output 4-255: Reserved |
| Disable Dehum with AC (#H9) (except 8810) | Byte 9 | 0: No 1: Yes 2-255: Reserved Note: Disable Dehum with AC, should be ignored unless, Dehumidifier Control is set to Whole Home. |
| Dehumidifier Forces Fan (#H10) (except 8810) | Byte 10 | 0: No 1: Yes 2-255: Reserved Note: Dehumidifier Forces Fan, should be ignored unless, Dehumidifier Control is set to Whole Home. |
| Dehumidifier Service Reminder (#H11) (except 8810) | Byte 11 | 0: Reserved 1-12: Number of Months 13: Off 14-255: Reserved |
| Air Conditioner Overcooling Limit (#H12) (except 8810) | Byte 12 | 0: 1F or 0.5C 1: 2F or 1C 2: 3F or 1.5C 3-255: Reserved |
| Dehumidify in Vacation Mode (#H13) | Byte 13 | 0: No 1: Yes 2-255: Reserved |
| Vacation Mode Dehumidify Low Temperature Limit (#H14) | Byte 14 | 0: 70F or 21C 1: 71F or 21.5C 2: 72F or 22C 3: 73F or 22.5C 4: 74F or 23C 5: 75F or 23.5C 6: 76F or 24C 7: 77F or 24.5C 8: 78F or 25C 9: 79F or 25.5C 10:80F or 26C 11:81F or 26.5C 12:82F or 27C 13:83F or 27.5C 14:84F or 28C 15:85F or 28.5C 16-255: Reserved |
| COS | Yes |
| Access | R |
| Data length | 18 bytes |
| Models | Except 8810 |
Timed vs ASHRAE setup, temperature/RH lockouts, bedrooms/occupants/home size/CFM/climate for ASHRAE, service reminder months. Many fields are ignored unless the matching setup mode is selected.
Factory defaults (guide bold): Byte 0: 1 Yes · 1: 0 No · 2: 1 Yes · 3: 0 Timed · 4: 5 (30 min) · 5: 0 No · 6: 2 (100F/38C) · 7: 0 No · 8: 4 (10F/−12C) · 9: 0 No · 10: 2 (60%) · 11: 3 bedrooms · 12: 4 occupants · 13: 25 (2500 sq ft) · 14: 12 (60 CFM) · 15: 0 Normal · 17: 13 Off
| Field | Byte | Values |
|---|---|---|
| Display Fresh Air Button (except 8810) | Byte 0 | 0: No 1: Yes 2-255: Reserved Note: Display Fresh Air Button Should be ignored if Fresh Air Installed is set to Yes |
| Fresh Air Vent Installed (#V1) | Byte 1 | 0: No 1: Yes 2-255: Reserved |
| Fresh Air Forces Fan (#V2) | Byte 2 | 0: No 1: Yes 2-255: Reserved Note: Variable V2, Fresh Air Forces Fan, should be ignored unless Variable V1, Fresh Air Vent Installed, is set to yes. |
| Fresh Air Setup (#V3) | Byte 3 | 0: Timed 1: ASHRAE 2-255: Reserved Note: Fresh Air Setup, should be ignored unless, Fresh Air Vent Installed, is set to yes. |
| Fresh Air Time (#V4) | Byte 4 | 0: 5 Minutes 1: 10 Minutes 2: 15 Minutes 3: 20 Minutes 4: 25 Minutes 5: 30 Minutes 6: 35 Minutes 7: 40 Minutes 8: 45 Minutes 9: 50 Minutes 10: 55 Minutes 11-255: Reserved Note: Fresh Air Time, should be ignored unless, Fresh Air Setup, is set to Timed. |
| Ventilation High Temp Lockout Enable (#V5) | Byte 5 | 0: No 1: Yes 2-255: Reserved Note: Ventilation High Temp Lockout Enable, should be ignored unless, Fresh Air Vent Installed, is set to Yes, Outdoor Sensor is set to Outdoor Sensor attached and, Fresh Air Setup, is set to Timed |
| Ventilation High Temp Lockout Limit (#V6) | Byte 6 | 0: 90F or 32C 1: 95F or 35C 2: 100F or 38C 3-255: Reserved Note: High Outdoor Temperature Limit, should be ignored unless, Enable Ventilation High Temperature Lockout, is set to Yes. |
| Ventilation Low Temp Lockout Enable (#V7) | Byte 7 | 0: No 1: Yes 2-255: Reserved Note: Ventilation Low Temp Lockout Enable, should be ignored unless, Fresh Air Vent Installed, is set to Yes, Outdoor Sensor is set to Outdoor Sensor attached and, Fresh Air Setup, is set to Timed |
| Ventilation Low Temp Lockout Limit (#V8) | Byte 8 | 0: -10F or -24C 1: -5F or -21C 2: 0F or -18C 3: 5F or -15C 4: 10F or -12C 5: 15F or -9C 6: 20F or -6C 7: 25F or -3C 8: 30F or 0C 9-255: Reserved Note: Ventilation Low Temperature Lockout Limit, should be ignored unless Fresh Air Vent Installed, is set to Yes, Outdoor Sensor is set to Outdoor Sensor attached and, Fresh Air Setup, is set to Timed. |
| Ventilation Indoor RH Lockout Enable (#V9) | Byte 9 | 0: No 1: Yes 2-255: Reserved Note: Enable Ventilation Indoor RH lockout should be ignored unless, Fresh Air Setup, is set to Timed. |
| Ventilation Indoor RH Lockout Limit (#V10) | Byte 10 | 0: 50% 1: 55% 2: 60% 3: 65% 4: 70% 5-255: Reserved Note: Indoor RH Limit should be ignored unless, Enable Ventilation Indoor RH Lockout, is set to Yes. |
| Bedrooms (#V11) | Byte 11 | 0: Reserved 1-6: Number of Rooms (3 is default) 7-255: Reserved Note: Bedrooms should be ignored unless, Fresh Air Setup, is set to ASHRAE. |
| Occupants (#V12) | Byte 12 | 0: Reserved 1-10: Number of Occupants (4 is default) 11-255: Reserved Note: Occupants should be ignored unless, Fresh Air Setup, is set to ASHRAE. |
| Home Size (#V13) | Byte 13 | 0-9: Reserved 10-50: 100 SQ FT Increments (10 equals 1000 SQ FT) (25 is default) 51-255: Reserved Note: Home Size should be ignored unless, Fresh Air Setup, is set to ASHRAE. |
| CFM (#V14) | Byte 14 | 0-5: Reserved 6-40: 5 CFM Increments (6 equals 30CFM) (12 is default) 41-255: Reserved Note: CFM should be ignored unless, Fresh Air Setup, is set to ASHRAE. |
| Climate (#V15) | Byte 15 | 0: Normal 1: Very Hot and Humid 2: Very Cold 3-255: Reserved Note: Climate should be ignored unless, Fresh Air Setup, is set to ASHRAE. |
| ASHRAE Time (#V16) | Byte 16 | 0: Reserved 1-60: Minutes 62-255: Reserved Note: (Result of ASHRAE Calculation/Read Only) ASHRAE Time, should be ignored unless, Fresh Air Setup, is set to ASHRAE. |
| Ventilation Service Reminder (#V17) | Byte 17 | 0: Reserved 1-12: Number of Months 13: Off 14-255: Reserved |
| COS | No |
| Access | W |
| Data length | 1 byte |
Behaves like power-cycling the thermostat. Non-volatile settings remain unchanged.
| Field | Byte | Values |
|---|---|---|
| Reset | Byte 0 | 0: Reserved 1: Reset 2-255: Reserved |
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