Electronic Control Manual216Sensors and Stats SectionSProduct/Technical BulletinT-7900Issue Date0695©1995 Johnson Controls, Inc.1Code No. LIT-216280T-7900 Strap-On Line Voltage Thermostat forHeating-Cooling ChangeoverThe T-7900 Thermostat is designed for wiring inseries with a space thermostat to provideautomatic heating-cooling changeover on terminalair conditioning units. It consists of a single-pole,double-throw 3-wire switch and temperature sensorenclosed in a housing which protects the unitagainst damage from moisture or dirt. The unit isfurnished with a mounting clip that facilitatesstrapping it onto a water pipe.OperationThe T-7900 has fixed temperature switching pointsat 65°F (18°C) and 85°F (30°C). In a typicalapplication, when the circulated water temperaturein a terminal unit reaches 85°F, the T-7900completes the circuit to the heating contact of thespace thermostat. This allows the space thermostatto function only as a heating thermostat regardlessof the setting chosen by the occupant. In a similarmanner, the T-7900 completes the circuit to thecooling contact of the space thermostat when thecirculated water temperature reaches 65°F. Thespace thermostat will then function only as a coolingthermostat.MountingThe T-7900 may be mounted in any position on thewater pipe with the stainless steel spring clipprovided. To assure accurate heat transfer, thesurface of the pipe should be clean so that theT-7900 sensing element makes a good directcontact with the surface of the pipe.T-7900 Strap-On ThermostatWiringThe T-7900 is furnished with 24 in. color-codedinsulated 16 AWG leads. The color-coding relates tothe switch contacts -- black is common, red openson a temperature rise, and brown closes on atemperature rise. All wiring must be in accordancewith applicable electrical code requirements.Table 1Maximum Motor RatingsLeadsContacts120 Volts240 VoltsBlack & RedOpen on1310AC Full LoadTemp RiseAmperesBlack & BrownClose on5.82.9Temp RiseBlack & RedOpen on6045AC Locked RotorTemp RiseAmperesBlack & BrownClose on34.817.4Temp Rise2 T-7900 Product/Technical Bulletint7900dm1/2 inch(13 mm)24 inch(610 mm)24 inch(610 mm)1-9/32 inch(33 mm)A1-9/32 inch(33 mm)Size (in.)NOMINALOD3/81/21/25/8AInchmm1-11/16431-13/1646DimensionsBRt7_1hBLRGHT-7900ChangeoverThermostatLine VoltageFanMotorRoomThermostatHeatCoolTypical ApplicationControls Group507 E. Michigan StreetP.O. Box 423Printed in U.S.A.Milwaukee, WI 53202Next >