Full AC Operating Cycle

For homeowners

This diagram walks through everything that happens between you turning the thermostat down and cool air arriving at the registers. Seven steps, in order, every time:

1. Thermostat senses warmth — room temperature is above setpoint, so the thermostat closes its internal contacts to send a “cool” signal.
2. 24V signal travels through low-voltage wiring from the thermostat to the air handler.
3. Safety chain checked — a series of safety switches (float switch on the drain pan, pressure switches in the refrigerant lines, others) all have to be in their normal “closed” position. Any one of them open and the sequence stops.
4. Contactor and fan relay pull in — small electromagnetic switches close, connecting line voltage to the big loads.
5. Motors spin up — compressor, outdoor condenser fan, and indoor blower all start within about a second of each other.
6. Refrigerant cycles — hot gas heads outside to dump heat, cold gas comes back from the evaporator after absorbing heat from indoor air.
7. Cool air delivered — when room temperature reaches setpoint, the thermostat opens its contacts, 24V drops, everything de-energizes, system coasts to stop.

If your system doesn’t come on, the technician traces this sequence in order to find where it stopped.

---

For technicians

Step 1 — Thermostat senses. Modern thermostats use a thermistor (or sometimes a thermocouple in commercial equipment) to read room temperature. When the reading exceeds setpoint by the deadband — usually 0.5 to 1°F — the internal logic closes the R-to-Y output. On a heat pump, R-to-Y is the compressor call and R-to-O is the reversing valve. On a straight cool, just R-to-Y. R-to-G (fan) is closed concurrently in most thermostats so the blower runs whenever the compressor runs.

Step 2 — 24V travels. Through the thermostat cable — typically 18 AWG 5-conductor for a heat pump, less for a straight cool — from the wall thermostat down to the air handler’s control board. The R wire is the source of 24V, the other wires are returns through different functions. Wire colors are not standardized by code, only by convention (R = red, Y = yellow, G = green, W = white, O = orange or B = brown for reversing valve). Always verify with a meter, never assume by color.

Step 3 — Safety chain. This is the series of normally-closed switches that all must be intact for the signal to reach the contactor. Standard residential chain:

• Float switch — primary pan, secondary pan, sometimes drain line trap
• High-pressure switch (in some systems) — opens if refrigerant head pressure exceeds about 600 PSI on R-410A
• Low-pressure switch (in some systems) — opens if suction pressure drops below about 30 PSI, indicating low charge
• Door interlock switch (rare on residential) — opens if the cabinet access panel is removed

Any one of these opens, the Y signal can’t reach the contactor coil, the compressor doesn’t run. This is by design — these are protective devices that take the system off-line before damage occurs.

Step 4 — Coils pull in. The Y signal energizes the contactor coil in the outdoor unit (24V across the coil, magnetic field pulls the armature, line voltage contacts close). The G signal energizes the indoor fan relay (or routes directly to the ECM blower control). Both events happen within a few hundred milliseconds.

Step 5 — Motors start. Three motors energize:

• Compressor — single-phase induction motor, starts via the run capacitor providing phase shift. Locked-rotor current draws 5–8x nominal for the first half-second, then drops to running current as the motor reaches speed.
• Condenser fan — small PSC motor, starts via the fan section of the dual run cap. Comes up to speed in about a second.
• Indoor blower — ECM or PSC, ramps up to commanded speed.

The compressor is the loudest event. Customer hears the click of the contactor closing, then the inrush hum of the compressor starting, then settling to running noise.

Step 6 — Refrigerant cycles. Within about thirty seconds of the compressor starting, the refrigerant has made several complete loops through the system. High side (compressor discharge through outdoor coil to TXV) climbs to operating pressure — typically 350–450 PSI on R-410A. Low side (TXV through evaporator back to suction) settles to 110–140 PSI. Coil temperatures stabilize: outdoor coil runs about 20–30°F above outdoor air temperature, indoor coil runs about 35–45°F. Air across the evaporator cools and drops humidity. Supply register starts delivering air about 18–22°F colder than the return.

Step 7 — Satisfy and shutdown. When room temperature reaches setpoint, the thermostat opens its R-to-Y output. 24V to the contactor coil drops to zero. The contactor’s magnetic field collapses, the return springs push the armature back up, contacts open. Same on the fan relay. Compressor, fans, blower all coast to stop. A typical compressor stops in 5–10 seconds; fans coast longer because they have less load.

Heat pump systems usually have a minimum runtime and minimum off-time (3–5 minutes each) programmed into the control board to prevent short-cycling. The thermostat may call for cool again immediately, but the board waits before re-energizing the compressor. This protects against the rapid on-off cycling that destroys compressors.

Diagnostic uses for this sequence. When a customer says “the system doesn’t come on,” the tech traces the sequence step by step:

• Thermostat reads correctly? Set it to cool, turn it down to its lowest setting. Verify it’s actually trying.
• Check for 24V across R and C at the thermostat. If yes, control voltage is reaching the wall. If no, the transformer or wiring is the problem.
• At the air handler, verify 24V is reaching the control board, then check Y signal is being passed through the safety chain. Any open switch reveals itself here.
• At the outdoor unit, verify 24V at the contactor coil. If yes, the coil should pull in — if not, contactor is bad. If no 24V at the coil, the chain broke somewhere upstream.
• If contactor pulls in but compressor doesn’t start, check line voltage on the load side of the contactor. If line voltage is present and the motor doesn’t run, the capacitor or motor is the problem.

Tracing in order makes most “system won’t run” calls into 15-minute diagnostics.