HVAC TstatsUnderstanding Thermostats

HVAC Tstats

Thermostats are the central

control forHVACsystems

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HVAC Technicians must know:

• How to Thermostats operate

• How to wire them• How to

troubleshoot them

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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 TstatsBasic 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

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Basic heat/cool thermostat

• Some thermostats will have “off” as a mode

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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 Tstats2 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

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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 TstatsHeat 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 TstatsHeat 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

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Basic heat/cool thermostat wired to oil primary control

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Basic heat/cool thermostat wired to oil primary control with A/C option

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Basic heat/cool thermostat wired to Gas furnace control

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Basic cooling thermostat wired to A/C controls

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• Basic heating/cooling thermostat with isolating sub-base wired to oil primary and A/C controls

• Note both systems have their own transformers

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• 2 Stage cooling 1 Stage heat thermostat wired to Heating and A/C controls

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• 2 Stage cooling 2 Stage heat thermostat wired to Heating and A/C controls

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• 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 !!!!!!!