Residential HVAC Controls) Electronic Thermostats

 Introduction to HVAC Controls:

• HVAC controls are essential for managing heating and cooling systems due to varying heating and cooling loads.
• They sense changes in space temperature and adjust heating or cooling equipment accordingly.
• The most common control device is the electronic thermostat.

Electronic Thermostats:

• Primary component responsible for starting/stopping air conditioning and heating operations based on space temperature.
• Can be programmed for comfort and occupancy schedules.
• Often required by energy codes in some jurisdictions.

Components of Electronic Thermostats:

• Main Body: Contains temperature-activated switches.
• Sub Base: Contains function switches for heating, cooling, and fan settings.
• Modern thermostats predict heating/cooling needs and use sensors like thermistors for temperature sensing.

Thermistors:

• Electrical resistors that change resistance with temperature.
• Types:
  • PTC (Positive Temperature Coefficient): Resistance increases with temperature.
  • NTC (Negative Temperature Coefficient): Resistance decreases with temperature, commonly used in HVAC.

Low Voltage Operation:

• Operate at 24 volts for safety.
• Use relays/electronic switches for control.
• Thermostat control wires have multiple conductors, each color-coded for different functions.

Thermostat Wiring:

• Red Wire (R Terminal): Provides 24-volt power.
• White Wire (W Terminal): Controls heating.
• Yellow Wire (Y Terminal): Controls compressor contactor.
• Green Wire (G Terminal): Controls indoor blower relay.
• Blue Wire (C Terminal): Common wire for current flow.

Operation Modes:

• Heating Mode: Thermostat closes heating contacts when temperature drops below set point.
• Cooling Mode: Thermostat closes cooling contacts when temperature rises above set point, also powers the fan relay.

Selection Criteria for Electronic Thermostats:

• Temperature Set Point Changes (Setbacks): Automatically adjust based on occupancy and comfort needs to save energy.
• Remote Communication: Can be controlled via Wi-Fi/Bluetooth using smart devices, allowing remote access and control.

Electromechanical Thermostats:

• Non-programmable devices using a bimetal coil/strip that moves with temperature changes.
• May contain mercury, which is hazardous and no longer used in new devices.
• Use manual switches for heating/cooling modes and fan settings.