Honeywell T874A,D Zone Thermostats / Q674B,D,E Subbases
Installation Instructions for the Trained Service Technician.
Application
These T874A,D Zone Thermostats and Q674B,D,E Subbases provide 24 to 30 Vac temperature control in Honeywell Trol-A-Temp heating/cooling zone systems. Refer to system specifications for complete details.
Thermostat/Subbase and System Compatibility
Table 1: Lists compatibility between thermostat/subbase models and various zone systems (Mini-Zone, Mabs II, Mabs II-L, Mabs XX) for single-stage and multistage applications.
| Thermostat/Subbase Model Number a | Application | Mini-Zone | Mabs II, Mabs II-L | Mabs XX | Stages |
|---|---|---|---|---|---|
| T874A/Q674B | Zone 1; conventional heat-cool. | Zone 1; conventional heat-cool. | Zone 1; conventional heat-cool. | Any zone; conventional heat-cool. | Single-stage |
| T874A/Q674D | Zones other than zone 1; conventional heat-cool. | Zones other than zone 1; conventional heat-cool. | Not applicable | Single-stage | |
| T874A/Q674E | Not applicable | Not applicable | Any zone; conventional heat-cool. | Single-stage | |
| T874D/Q674B | Not applicable | Zone 1; conventional heat-cool. | Any zone; conventional heat-cool. | Multistage | |
| T874D/Q674D | Not applicable | Zones other than zone 1; conventional heat-cool and heat pump. | Not applicable | Multistage | |
| T874D/Q674E | Not applicable | Not applicable | Any zone; conventional heat-cool. | Multistage |
a Subbase not included with thermostat; order separately.
Thermostat/Subbase Specifications
Table 2: Details specifications including system and fan switching, color, and anticipator ranges for heating and cooling.
| Thermostat/Subbase Order Number a | System | Fan | Thermostat/Subbase Color | Anticipator Range (A) Heating (Adj.) | Anticipator Range (A) Cooling (Fixed) |
|---|---|---|---|---|---|
| T874A1465/Q674B1471 | HEAT-OFF-COOL | ON-AUTO | White/Gray | Stg. 1- 0.1 to 1.2 | Stg. 1- 0 to 1.5 |
| T874A1465/Q674D1123 | None | None | White/Gray | ||
| T874A1465/Q674E1379 | HEAT-OFF-COOL-AUTO | ON-AUTO | White/Gray | ||
| T874D1801/Q674B1463 | HEAT-OFF-COOL | ON-AUTO | Beige/Brown | Stg. 1- 0.1 to 1.2 | Stg. 1- 0 to 1.2 |
| T874D1827/Q674B1471 | HEAT-OFF-COOL | ON-AUTO | White/Gray | ||
| T874D1801/Q674D1115 | None | None | Beige/Brown | Stg. 2- 0.1 to 1.0 | Stg. 2- 0 to 1.0 |
| T874D1827/Q674D1123 | None | None | White/Gray | ||
| T874D1827/Q674E1379 | HEAT-OFF-COOL-AUTO | ON-AUTO | White/Gray |
a Subbase not included with thermostat; order separately.
Operation
On a 2-heat thermostat, stages make sequentially as temperature drops. Make refers to the mercury switch initiating a call for heat or cool. There is approximately a 2°F [1°C] interstage differential.
Recycling Notice
This control contains mercury in a sealed tube. Do not dispose of in trash. Contact local waste management for recycling and disposal instructions.
Installation Instructions
When Installing This Product...
- Read instructions carefully.
- Check ratings for suitability.
- Installer must be a trained, experienced service technician.
- Check product operation after installation.
Caution
- Disconnect power supply before installation.
- Keep wires to a minimum and run close to the subbase to prevent interference.
- Do not overtighten mounting screws.
- Do not short across terminals; this can burn out the heat anticipator.
Important: Subbase must be mounted level for accurate thermostat control.
Location
Locate the subbase about 5 ft [1.5 m] above the floor in an area with good air circulation at average temperature. Avoid locations affected by drafts, dead spots, direct sunlight, heat from appliances, concealed pipes, chimneys, or unheated/uncooled areas.
Mounting the Subbase
The subbase can be mounted on a horizontal outlet box or directly on the wall.
- Mount subbase on outlet box (Fig. 1) or mark holes for wall anchors (Fig. 2). Drill holes, insert anchors, and pull wires through.
- Secure subbase with screws, but do not fully tighten.
- Level the subbase using a spirit level (Fig. 3) and then firmly tighten mounting screws.
Important: An incorrectly leveled subbase will cause temperature control deviation.
Wiring the Subbase
Comply with local electrical codes and equipment manufacturer instructions.
- Connect wires to subbase terminals using letter codes for identification. The terminal barrier allows straight or wraparound wiring (Fig. 4).
- For systems with separate heating and cooling transformers (RC and RH terminals), jumper RC and RH terminals and wire as a single R terminal (Fig. 5).
- Firmly tighten terminal screws.
- Fit wires close to the subbase and push excess wire back into the hole.
- Plug the hole with nonflammable insulation to prevent drafts.
Fig. 6: Typical hookup for MM-2 Mini-zone Control Panel with T874A Thermostat/Q674B,D Subbase in a single-stage heating-cooling two-zone system.
Fig. 7: Typical hookup for MABS II and MABS II-L Control Panels with T874A Thermostat/Q674B,D Subbases in a single-stage heating-cooling zone system.
Fig. 8: Typical hookup for MABS II and MABS II-L Control Panels with T874D Thermostat/Q674B, D Subbases in a multistage heating-cooling zone system.
Fig. 9: Typical hookup for MABS XX Control Panel with T874A Thermostat/Q674B,E Subbases in a single-stage heating-cooling zone system.
Fig. 10: Typical hookup for MABS KX Control Panel with T874D Thermostat/Q874B, E Subbases in a multistage heating-cooling zone system.
Mounting the Thermostat
- Remove the thermostat cover by pulling the bottom edge outward until it snaps free. The cover is hinged at the top.
- Remove and discard the polystyrene packing insert.
- Check that the spring fingers on the thermostat base are not bent flat.
- Set the heat anticipator indicator(s) to 0.1 A (Fig. 11).
- Align the two tabs on the thermostat base with the slots on the top of the subbase and mount the thermostat (Fig. 12).
- Align the captive mounting screws in the thermostat base with the posts on the subbase and tighten both screws. Do not overtighten.
Settings
Subbase Setting (Q674B and E 1004.1): Refer to Tables 3 and 4 for descriptions of subbase switch functions and positions.
Table 3: Subbase System Switching Positions
| Switching Position | Mini-Zone, Mabs II, Mabs II-1, System | Mabs XX System |
|---|---|---|
| OFF | Heat: last stage; cooling: systems 100-201. If 201 heats, it acts as 'auto' for heating or cooling. | |
| HEAT | System 1 (specific heating); heating only. | Turn the thermostat to heat 201 for heating only. |
| COOL | System 2 (specific cooling); cooling only. | Turn the thermostat to cool 201 for cooling only. |
System Checkout
Before checking thermostat operation, set control panel switches for desired system operation. Adjust thermostat settings as shown in Table 5.
Table 5: System Set-Up for Thermostat Checkout
| Control Panel | Heating | Cooling | ||
|---|---|---|---|---|
| Zone 1 | Zones Other Than Zone 1 | Zone 1 | Zones Other Than Zone 1 | |
| Mini-zone, MABS II, MABS II-L | Set system switch to HEAT; fan to AUTO. Move heating set point lever 10°F [6°C] below room temp. | Move heating set point lever 10°F [6°C] below room temp. | Set system switch to COOL; fan to AUTO. Move cooling set point lever 10°F [6°C] above room temp. | Move cooling set point lever 10°F [6°C] above room temp. |
| MABS XX | On all zone thermostats, set system switch to HEAT; fan to AUTO. Move heating set point lever 10°F [6°C] below room temp. | On all zone thermostats, set system switch to COOL; fan to AUTO. Move cooling set point lever 10°F [6°C] above room temp. | ||
| NOTE: Control panel has a 3-minute time delay for fan operation. | NOTE: Control panel has a 3-minute time delay for fan operation. | |||
Heating Checkout
Note: Allow 30 seconds for damper operation.
- Set fan switch to ON (if applicable). Check for airflow at register after 30 seconds.
- Reset fan switch to AUTO. Airflow should stop after 30 seconds.
- Move heating set point lever 10°F [6°C] above room temperature to call for heat.
- Check for warm air at the register after 30 seconds (longer if plenum fan control has a delay).
- Move heating set point lever 10°F [6°C] below room temperature to end call for heat. Airflow should stop after 30 seconds (longer if plenum fan control or control panel has a delay).
- Repeat for each zone thermostat.
Cooling Checkout
Caution: Do not operate cooling below 50°F [10°C] outdoor temperature.
- Move cooling set point lever 10°F [6°C] below room temperature to call for cooling.
- Check for cool air at the register after 30 seconds.
- Move cooling set point lever 10°F [6°C] above room temperature to end call for cooling. Airflow should stop after 30 seconds (longer if control panel has a delay).
- Repeat for each zone thermostat.
Return all thermostats to desired settings for normal operation.
Thermostat Calibration
T874 thermostats are factory calibrated and do not have field calibration provisions.
Thermometer Calibration
The thermometer is factory calibrated. Adjust only if dropped or mishandled.
- Remove thermostat cover by pulling up from the bottom.
- Place cover near an accurate thermometer and allow ten minutes for stabilization. Avoid touching or breathing on the thermometer.
- If readings match, replace cover.
- If readings differ, insert a small screwdriver into the thermometer slot (Fig. 14) and adjust until readings match.
- Replace cover.
Note: Hand heat can affect readings. Wait ten minutes for stabilization after each adjustment.
