HVAC Actuation: The Climate Controller

A standard temperature sensor just tells you it's hot. The Climate Controller is an active intercession device. It acts as a massive digital "Thermostat Brain," dynamically turning on 120V AC wall heaters or massive 6-Amp dehumidifiers in real-time, requiring programmers to deeply understand "Hysteresis Math" to prevent destroying the electrical relays.

The Hysteresis Thermal Buffer

If you simply tell the Arduino: Turn heater ON if Temp is below 70. Turn OFF if Temp is 70. You will cause a catastrophic failure.

1. The Relay Hammering Crisis: When the room hits 69.9F, the heater clicks ON. It blows warm air for 2 seconds. The sensor hits 70.0F and the heater clicks OFF.
2. The room instantly cools to 69.9F. The heater violently clicks ON.
3. This rapid, terrifying bouncing will literally fuse the metal contacts of the relay and burn your house down!
4. The Hysteresis Buffer Solution:

if (Temp < 68) { 
  digitalWrite(HeaterRelay, HIGH); // Wait until it's actually cold!
} else if (Temp >= 72) {
  digitalWrite(HeaterRelay, LOW);  // Don't turn off until it's genuinely warm!
}

5. This massive 4-degree buffer completely eliminates relay hammering!

Driving the 120V Output (SSR Relays)

Standard blue mechanical relays have a physical metal lever inside. They are notoriously bad for switching heavy inductive loads like massive AC Compressors or heater coils.

• The project mandates the use of a Solid State Relay (SSR) - usually a Fotek 40DA.
• The SSR has no moving parts. It uses a tiny internal LED and an optically-isolated TRIAC to violently switch 120V / 40Amps of wall power directly from the tiny 5V logic signal outputted by the Arduino Digital Pin!

Necessary Central HVAC Controls

• Arduino Uno/Mega + 16x2 I2C LCD Display.
• BME280 / DHT22 Precision Environmental Sensor Array.
• Heavy-Duty Solid State Relay (SSR) mounted to a massive aluminum block heat-sink. (An SSR switching 20 Amps of heater current will melt through plastic if not cooled!).
• Rotary Encoder to change the 'Target Temperature' seamlessly.