Controls, Thermostats & Customer Relations
Thermostat wiring, heat anticipators, control sequences, nameplate data, and professional customer communication.
- Identify standard thermostat wiring terminals and their functions
- Explain heat anticipator function on mechanical thermostats
- Describe the importance of matched systems for efficiency and warranty
- Apply professional communication practices with customers
- Record and use nameplate data for proper service documentation
Leçon 1
Thermostats, Wiring & Control Sequences
Standard Thermostat Wiring
The thermostat is the primary control interface between the occupant and the HVAC system. Standard thermostat terminals use a color-coded lettering system:
| Terminal | Color | Function |
|---|---|---|
| R | Red | 24 VAC power (hot) |
| C | Blue | 24 VAC common (return) |
| W | White | Heating call (gas valve/heat relay) |
| Y | Yellow | Cooling call (contactor/compressor) |
| G | Green | Fan (blower motor relay) |
| O/B | Orange | Reversing valve (heat pump) |
| W2/AUX | White | Second stage/auxiliary heat |
R, RC, and RH
Some thermostats have separate RC (cooling transformer power) and RH (heating transformer power) terminals for systems with separate transformers. A jumper between RC and RH connects them when a single transformer powers both. Modern smart thermostats typically require the C (common) wire for continuous power.
Control Sequence - Cooling Call
When the thermostat calls for cooling:
- R energizes Y - contactor pulls in, compressor and condenser fan start
- R energizes G - indoor blower motor starts
- System runs until temperature reaches setpoint
- Thermostat opens Y and G - system shuts down
Heat Anticipator
A heat anticipator on a mechanical thermostat is a small adjustable resistor that adds a tiny amount of heat to the thermostat sensor during a heating call. This causes the thermostat to shut off the furnace slightly before the room reaches the setpoint, anticipating residual heat from the furnace and ductwork.
The heat anticipator is set to match the current draw of the gas valve (typically measured with a clamp meter on the W wire during a heating call). While mechanical thermostats are increasingly rare, the NATE exam may still test this concept.
Standard thermostat terminals are R (power), W (heat), Y (cool), G (fan), and C (common). The heat anticipator on mechanical thermostats fine-tunes cycle length by adding heat to the sensor - it should be set to match the gas valve amp draw. Modern smart thermostats require the C wire for continuous 24 VAC power.