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HVAC Tstats Understanding Thermostats HVAC Tstats Thermostats are the central control for HVAC systems HVAC Tstats HVAC Technicians must know: • How to Thermostats operate • How to wire them • How to troubleshoot them HVAC Tstats Basic heating thermostat • R- is voltage input from 24v transformer • W- is heating output to signal heating system to turn on • Heat anticipator can be adjusted and is energized when thermostat temperature sensor is calling for heat HVAC Tstats Basic Cooling thermostat • Y- is cooling output to signal cooling system to turn on • G- is fan output to signal indoor fan/blower to start • Cooling anticipator is a fixed resistor energized when thermostat temperature sensor is not calling for cooling HVAC Tstats Basic heat/cool thermostat • Some thermostats will have “off” as a mode HVAC Tstats Basic heat/cool thermostat with isolating sub-base • Some thermostats will have RH + RC terminal to isolate transformers from the heating unit and the cooling unit HVAC Tstats 2 stage cooling thermostat with single stage heat • Y1- is first stage cooling, normally 1 of 2 compressors • Y2- is second stage or second compressor • Y2 is internally warmer than Y1 by about 2 degrees F HVAC Tstats 2 stage heat/cool thermostat • W1- is first stage of heating, example 1st bank of 2 banks electric heaters or low side of 2 pressure gas valve • W2- is second stage, second electric heater or high pressure side of 2 pressure gas valve HVAC Tstats Heat pump thermostat • O- is output signal when selector switch is moved to cool • B- is output signal when selector switch is moved to heat HVAC Tstats Heat pump with emergency heat • When selector switch is moved to emergency heat Y is no longer energized as first stage • Back up heat is now first stage • Emergency heat light is on to indicate selector switch was moved and compressor is off HVAC Tstats Basic heat/cool thermostat wired to oil primary control HVAC Tstats Basic heat/cool thermostat wired to oil primary control with A/C option HVAC Tstats Basic heat/cool thermostat wired to Gas furnace control HVAC Tstats Basic cooling thermostat wired to A/C controls HVAC Tstats • Basic heating/cooling thermostat with isolating sub-base wired to oil primary and A/C controls • Note both systems have their own transformers HVAC Tstats • 2 Stage cooling 1 Stage heat thermostat wired to Heating and A/C controls HVAC Tstats • 2 Stage cooling 2 Stage heat thermostat wired to Heating and A/C controls HVAC Tstats • Basic Heat pump Thermostat with “B” and “O” terminal wired to Heat pump controls HVAC Tstats • Heat pump Thermostat with Supplemental and Emergency Heat lights wired to Heat pump controls • Supplemental light is on when ever second stage heat is on • Emergency light is on when mode switch is moved to Emergency heat HVAC Tstats Troubleshooting • With power on to both systems a volt meter should read the switches • Top meter reads 24v because the heat sensor is open HVAC Tstats Troubleshooting • With power on to both systems a volt meter should read the switches • Middle meter reads 24v because the cool sensor is open HVAC Tstats Troubleshooting • With power on to both systems a volt meter should read the switches • Bottom meter reads 0v because the fan switch is set to “ON” • Jumping out “R” to “?” should start the controlled unit or system • NEVER SHORT “R” TO GROUND HVAC Tstats • Electronic thermostats “Can” have set back programming options • Allowing different temperature for different times during the day • These thermostats save energy by raising or lowering the temperature when a building is not being use HVAC Tstats Review • Thermostats are switches that open and close by temperature • Different thermostats are need for different applications • Wiring is simple if you know the color/terminal codes • Troubleshooting is done much like checking switches • NEVER SHORT “R” TO GROUND !!!!!!!