HVAC Tstats
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Transcript of HVAC Tstats
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HVAC TstatsUnderstanding Thermostats
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HVAC Tstats
Thermostats are the central
control forHVACsystems
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HVAC Tstats
HVAC Technicians must know:
• How to Thermostats operate
• How to wire them• How to
troubleshoot them
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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 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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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 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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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 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
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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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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
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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
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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 !!!!!!!