Controls & Instrumentation
Thermostats, pressure controls, PLCs, control valves, instrumentation symbols, and automatic system controls.
- Describe thermostat types and pressure control operation
- Explain expansion valve control and superheat adjustment
- Identify PLC basics, instrumentation symbols, and control loop concepts
Lección 1
Thermostats & Pressure Controls
Thermostat Function
A thermostat is a temperature-sensing switch that controls the heating or cooling system. It compares the actual space temperature to the setpoint and signals the system to turn on or off.
Mechanical Thermostat
Bimetal strip or mercury switch
Simple on/off control
Wide differential (temperature swing)
No programming capability
Electronic/Programmable Thermostat
Thermistor sensing element
Precise temperature control
Narrow differential
Schedules, setbacks, Wi-Fi
Pressure Controls
Pressure controls (also called pressostats) monitor system pressure and take action when limits are exceeded:
| Control | Function | Action |
|---|---|---|
| Low-pressure cutout (LPC) | Detects low suction pressure | Stops compressor (protects from loss of charge) |
| High-pressure cutout (HPC) | Detects high discharge pressure | Stops compressor (protects from overpressure) |
| Oil pressure safety | Monitors oil pressure differential | Stops compressor if oil pressure drops |
| Head pressure control | Maintains minimum condensing pressure | Cycles condenser fans |
Differential and Setpoint
Every pressure control has two settings:
- Cut-out - the pressure at which the switch opens (stops the system)
- Cut-in - the pressure at which the switch closes (restarts the system)
- Differential = cut-in minus cut-out
LPC vs. HPC
The low-pressure cutout protects against loss of refrigerant charge or restricted airflow. The high-pressure cutout protects against condenser failure, overcharge, or non-condensables. Both stop the compressor.
Thermostats control temperature by comparing space temperature to the setpoint. Low-pressure and high-pressure cutouts protect the compressor from dangerous operating conditions. Each has a cut-out, cut-in, and differential setting.