HVAC Tstats

1
HVAC Tstats

• Understanding Thermostats

2
HVAC Tstats

• Thermostats
• are the central
• control for
• HVAC
• systems

3
HVAC Tstats

• HVAC Technicians must know
• How to Thermostats operate
• How to wire them
• How to troubleshoot them

4
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

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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

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HVAC Tstats

• Basic heat/cool thermostat
• Some thermostats will have off as a mode

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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

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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

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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

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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

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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

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HVAC Tstats

• Basic heat/cool thermostat wired to oil primary
  control

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HVAC Tstats

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

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HVAC Tstats

• Basic heat/cool thermostat wired to Gas furnace
  control

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HVAC Tstats

• Basic cooling thermostat wired to A/C controls

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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

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HVAC Tstats

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

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HVAC Tstats

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

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HVAC Tstats

• Basic Heat pump Thermostat with B and O
  terminal wired to Heat pump controls

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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

21
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

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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

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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

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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

25
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 !!!!!!!