How HVAC control works (electrical view)
A thermostat/controller calls for heating or cooling; the electrical side energises the unit — refrigerant is separate.
The electrical picture
From an electrical fault-finding view, HVAC is a controller (thermostat) calling for heating or cooling, a control supply that powers that controller, and the unit's electrical side — contactors, fans, and the compressor circuit — that the call energises.
How it sequences
When the controller calls for cooling, it energises the outdoor unit's contactor and fan; the compressor runs and the refrigeration cycle moves heat. A condensate system removes the water that condensing produces, with a float/safety switch that shuts the unit down if it can't drain — preventing overflow.
The electrical job is to confirm the unit is powered and being correctly commanded. The refrigeration cycle itself — gas, pressures, the sealed system — is restricted to licensed refrigeration technicians.
Where the line is
Electrically you check: dedicated supply/isolator, controller power and call, contactors/capacitors/fans, and the condensate safety. If the electrical side is healthy and commanded but it still won't cool/heat, that's refrigeration-technician territory.
Safety first
Isolate before electrical work. Refrigeration/gas work is restricted to licensed refrigeration technicians — stick to the electrical side.
Isolate, lock out / tag out, and prove dead before working unless a live test is specifically required, authorised, and carried out under proper supervision. Always follow local regulations, your site procedures, and the equipment manufacturer's documentation.
Related faults
Split-system air-con not running at all
A split-system (head unit + outdoor condenser) is completely dead — no response from the remote/controller — pointing at supply, isolator, controller, or the indoor PCB.
Air-con runs but isn't cooling (electrical checks)
The unit runs (fan blows) but doesn't cool — from an electrical standpoint, checking whether the compressor/outdoor unit is actually being commanded and powered before handing to refrigeration.
Air-con condensate pump not working / unit shut down on overflow
An aircon has shut down on a condensate overflow/float switch, or its condensate pump isn't running — water isn't being removed, so the unit stops to prevent overflow.
Related definitions
Thermostat
Switches a heating or cooling load on and off to hold a temperature at a setpoint.
Contactor
An electrically-operated switch that uses a coil to make or break a load circuit, usually three-phase power.
Control vs power circuits
Low-power control logic decides what happens; the power circuit carries the load — kept separate for safety and clarity.