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HVAC Industry Standards
In order to read and understandan electrical diagram you need toknow how different controls andloads are drawn.
22
Relays
Normally Closed Contact
Normally Open Contact
Relay Coil
Relays or Contactors
Contactors
R
R
C
C
Contactor Coil29
SPST Opens on Rise(Safety Control)
SPST Opens on Drop(Safety Control)
High-PressureSwitch
Low-PressureSwitch
Pressure Switches
33
SPST Closes on Rise(Operating Control)
Air PressureSwitch
Fan CyclingPressure Switch
SPST Closes on Drop(Operating Control)
Pressure Switches
34
SPST Opens on Rise(Differential)
SPST Closes on Rise(Differential)
DifferentialAir Pressure Switch
DifferentialRefrigerantPressureSwitch
Pressure Switches
35
Pressure SwitchesTHE CONTACTS OF A SAFETY CONTROL ARE DRAWN IN THE NORMAL RUNPOSITION, USUALLY CLOSED
36
SPST Closes on Drop(Operating Control)
HeatingThermostat
Defrost Thermostat
Return WaterTemperature ControllerCooling Thermostat
SPST Closes on Rise(Operating Control)
Temperature Switches
37
SPST Opens on Drop(Safety Control)
SPST Opens on Rise(Safety Control) Discharge Gas
ThermostatOvertemperature
Limit Switch
Freeze Protection Thermostat
Temperature Switches
38
SPDT Temperature(Safety or Operating)
Bi-Metal Temperature(Safety)
Low-Water Temperature Cutout Control
Klixonâ
OvertemperatureCutout Control
Temperature Switches
39
THE CONTACTS OF AN OPERATING CONTROL ARE DRAWN OPEN, WITH NO CALLFOR COOLING, HEATING
Temperature Switches
40
SPST De-HumidistatCloses on Rise
(Operating Control)
SPST HumidistatCloses on Drop
(Operating Control)
Humidity Control
41
Time Delay RelayDelays Closing
(Mechanical/Thermal)(Electronic)
Time Delay RelayDelays Opening
(Mechanical/Thermal)(Electronic)
Time Delay
Time Delay Relay(Mechanical)
Time Delay Relay(Thermal)
Time Delay Relay(Electronic)
42
Time DelayTHE CONTACTS OF A TIMER ARE DRAWN IN THE
READY-TO-START POSITION OR AT TIME ZEROON A BAR CYCLE DIAGRAM
NOTE: BLACK DENOTES CLOSED CIRCUITS
43
Manual Switches
Momentary Push Button
SPST Normally OpenManually Operated
Push Button
SPST Normally ClosedManually Operated
Push Button
SPDT Switch Single-PoleDouble-Throw
Switch44
Flow Switches
SPST Flow SwitchCloses on Rise
(Safety and Operating)
SPST Flow SwitchOpens on Rise
(Safety Control)
Flow Switch
Sail Switch
Thermal DispersionFlow Switches 45
Enthalpy Switches
ElectronicEnthalpy Control
SPDT Enthalpy Control(Operating Control)
MechanicalEnthalpy Control
46
Run Capacitor(Marked Terminal)
Start Capacitor
Capacitors
Dual RunCapacitor
StartCapacitor
Run Capacitor
47
NF F
230 VAC
24 VAC
Fused & Non FusedSecondary
Multi Tap Primary
COMM
C575 480 380 230 200
24 VAC
50