Transcript HVAC Wiring - SchoolRack

HVAC Tstats
Understanding Thermostats
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Thermostats
are the central
control for
HVAC
systems
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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
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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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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
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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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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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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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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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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
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• 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
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• 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 !!!!!!!