24V Grid Logic: HVAC Controller Retrofit

A standard $10 Arduino relay handles simple DC loads. The HVAC Controller Retrofit replaces a $200 Nest or Honeywell thermostat on the living room wall. It requires completely mastering the legacy 24V AC Control Infrastructure built inside 99% of modern residential heaters. The programmer must seamlessly switch these 24V high-loads using specific, isolated relay cascades while crunching non-stop temperature algorithms to keep a physical 2,000-square-foot house perfectly at 70°F!

The Standard HVAC Wire Standards (Rh, G, W, Y)

You cannot plug a furnace directly into an Arduino. The wall has 4 or 5 specific copper wires!

1. Rh (Red): The massive 24 Volt AC Transformer Hot source directly from the basement unit!
2. G (Green): Activates the intense physical Fan Blower.
3. W (White): Violently engages the Heating stage (Ignites the Fire/Gas valve!).
4. Y (Yellow): Triggers massive outdoor Air Conditioning compressors!
5. The Execution Trap: To turn on the heat, the system must establish an exclusive physical connection bridging Rh (24V) directly into W and G concurrently!

24V AC Relay Matrix Isolation

The Arduino cannot output 24V AC, it outputs 5V DC.

• You MUST utilize an enormous 4-Channel 5V Relay Module.
• Relay 1 (FAN) hooks Common to Red, and NO to Green.
• Relay 2 (HEAT) hooks Common to Red, and NO to White.
• The C++ Software Safety Dead-Bands:

// If the House Temp is 68, and the Goal is 70!
if (currentTemp < (targetTemp - 1.0)) { // Minus 1 to prevent violent AC rapid-cycling!
  digitalWrite(RelayFan, HIGH);
  delay(5000); // 5 SEOND DELAY RULE! Never ignite gas before the fan spools up!
  digitalWrite(RelayHeat, HIGH); // Ignite furnace!
} 

• If the system ignores "Rapid Cycling" protection, it will violently turn the compressor ON/OFF 30 times a minute as the temperature fluctuates 0.1 degree, permanently destroying a standard $5000 Air Conditioning unit within an hour!

Industrial Thermodynamic Loadout

• ESP32 or NodeMCU (Brings the thermostat explicitly into HomeAssistant or the IoT Cloud Web UI!).
• DHT22 or BME280 Precision I2C Sensor (Must track decimal temperatures precisely for the Dead-Band loop).
• Absolute Multi-Channel 5V Optoisolated Relay Board.
• (DANGER: This directly interfaces with expensive Home Infrastructure! The 24V AC transformer can easily start a fire or destroy control boards if wired incorrectly to ground. Ensure the logic completely separates 5V DC and 24V AC systems!).