How does a thermostat work in HVAC systems?
Thermostat in an HVAC (Heating, Ventilation, and Air Conditioning) system works as a temperature control device. It monitors the room temperature and compares it with the desired setpoint. When the temperature goes above or below this setpoint, the thermostat sends signals to the HVAC system to turn on heating or cooling.
1. Temperature Sensing
The thermostat has a temperature sensor (often a thermistor or bimetallic strip) that detects the current room temperature.
In mechanical thermostats, a bimetallic strip expands or contracts with temperature changes, triggering a switch.
In digital thermostats, an electronic sensor sends temperature data to a microprocessor.
2. Comparing Temperature to Setpoint
The user sets a desired temperature (setpoint).
The thermostat compares the actual room temperature to the setpoint.
If the room is too cold, it signals the furnace or heat pump to turn on.
If the room is too hot, it activates the air conditioner or cooling system.
3. Sending Signals to the HVAC System
Low-voltage control (24V): Most HVAC systems use a low-voltage circuit (via a transformer) to safely control relays.
The thermostat connects to the HVAC system through wires (typically labeled R, W, Y, G, C, etc.).
Heat Call (W): Activates the furnace or heating.
Cool Call (Y): Turns on the AC or heat pump in cooling mode.
Fan Control (G): Runs the blower fan independently.
Common (C): Provides continuous power for smart thermostats.
4. System Cycling (On/Off)
When the temperature deviates from the setpoint (by a small margin, called the differential), the thermostat triggers the HVAC system.
Once the setpoint is reached, the thermostat shuts off the system to prevent overheating or overcooling.
