Honeywell T874R Thermostat Instruction Manual
Honeywell T874R Thermostat
Honeywell T874R Thermostat
Honeywell T874R Thermostat Instructions
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Honeywell T874R Thermostat Specifications
| Feature | Specification |
|---|---|
| Model | Honeywell T874R Multistage Thermostat (heat pump model) |
| Replaces | T872R |
| Application | Heat pump systems with manual changeover |
| Heating Stages | 2 |
| Cooling Stages | 1 |
| Changeover | Manual changeover in heat or cool mode |
| Compatible Subbases | Q674B or Q674L |
| Thermostat/Subbase Package | Y594R (T874R with Q674L) |
| Stage 1 Heating Anticipation | 0 to 1.5 A, fixed voltage type |
| Stage 2 Heating Anticipation | 0.1 to 1.2 A, adjustable |
| Stage 1 Cooling Anticipation | 0 to 1.5 A, fixed |
| Electrical Rating | 24 to 30 Vac |
| Switching | Mercury switches operated by coiled bimetal elements |
| Temperature Adjustment | Single common lever for heating and cooling |
| Temperature Scale Range | 42°F to 88°F (6°C to 31°C) |
| Thermometer Range | 42°F to 88°F (6°C to 31°C) |
| Mechanical Interstage Differential | 1°F (0.6°C) |
| Operating Interstage Differential | Approximately 1.9°F (1°C) |
| Finish | Beige or Premier White |
| Q674L System Switching | EM. HT.-HEAT-OFF-COOL |
| Q674L Fan Switching | AUTO-ON |
| Subbase Switch Contact Rating | 2.5 A at 30 Vac (7.5 A inrush) |
| Subbase Dimensions | 3-1/2 in. (89 mm) H x 5-5/8 in. (143 mm) W x 5/16 in. (8 mm) D |
| Mounting | On Q674 subbase; horizontal outlet box or wall; vertical outlet box with 193121A Adapter Plate |
| Recommended Wire | 18 gauge solid conductor (maximum 10 wires if stranded) |
| Field Calibration | Not available (factory calibrated) |
Honeywell T874R Thermostat Details
The Honeywell T874R is a low-voltage multistage thermostat built for heat pumps. It must be paired with a Q674 subbase, which supplies the system and fan switches, wiring terminals and the mounting base. Its typical partner is the Q674L, and the two are sold together as the Y594R package. In this setup the first heating stage runs the compressor and the second stage runs the auxiliary electric heat. As a result, the efficient heat pump runs whenever it can, and the more expensive strip heat comes on only when needed.
Changeover is manual. To reverse the system, you move the subbase system switch to HEAT or COOL. In the typical Q674L hookup, the changeover valve energizes on cooling. The EM. HT. position turns off the compressor and puts the emergency heat relay under thermostat control. A red EM. HT. LED lights whenever that position is selected. On some versions, a green AUX. HT. LED shows when the second-stage auxiliary heat is running.
The subbase can carry up to four LEDs, such as FILTER, CHECK, SERVICE, LOCKOUT and SUPL. HT. A system monitor relay wired to the L terminal can flag a compressor fault by lighting the emergency heat indicator. The LEDs can't be replaced or added in the field. On TRADELINE models, preprinted inserts are placed under the setpoint scale during installation so that the LED labels read correctly.
Installation calls for a trained technician. Mount the unit about 5 ft (1.5 m) above the floor, away from drafts, ducts, sunlight and outside walls. The subbase must be level, because a tilted unit controls away from setpoint. Remove the polystyrene packing insert that protects the mercury switches during shipping. Never short across a relay coil, since that can burn out the heat anticipator. Plug the wall opening with nonflammable insulation so drafts don't affect readings.
For heat pumps, set the adjustable second-stage anticipator to 140 percent of the measured current draw. Most heat pumps should cycle 2-1/2 to 3 times per hour. Measure the current with an AC ammeter between W2 and R, with the thermostat removed. If the equipment cycles too fast, raise the setting no more than half a division at a time. Some models use an optional C815A outdoor thermistor for outdoor reset, which reduces droop under heavy heating loads.
To check heating, set the lever about 10°F above room temperature. To check cooling, set it about 10°F below. Don't run cooling when it is below 50°F (10°C) outdoors. A minimum-off timer may delay a compressor restart by five minutes. The cover thermometer can be adjusted with a small screwdriver if it was knocked out of calibration. Because the unit contains mercury, it must be recycled rather than thrown in the trash.
Frequently Asked Questions (FAQs)
Can't find the answer you need? Here are some frequently asked questions about the Honeywell T874R Thermostat, and we answer them below.
