How a split system works
Open up a typical New Hampshire split system, follow the refrigerant around the loop, switch it to heating, then break it the ways it breaks in the field. Owners, dispatchers and techs can all use the same picture.
- Hot gas
- Warm liquid
- Cold liquid and vapor
- Cool vapor
Right now
- Compressor
- Running
- Outdoor fan
- Spinning
- Air at the vents
- Cold, about 55°F
- Big copper line
- Cold and sweating
- Hot-side pressure
- Normal
One system in two boxes.
A rooftop packaged unit keeps the compressor, both coils and the blower in one cabinet. A split system puts them in two: a condensing unit outside and an air handler or wall-mounted head inside, joined by a sealed pair of copper lines and a control wire.
The system moves heat from one place to another, and the refrigerant carries it. Once you can see where the refrigerant is hot, cold, liquid or vapor, most service calls make sense.
Four stations, the same in every split system.
- 1
Compressor
Outdoor unit
Takes in low-pressure refrigerant vapor and squeezes it. Pressure and temperature jump. It is the pump for the whole loop, and usually the first part to fail when a system is short on charge, dirty or short-cycling.
- 2
Outdoor coil and fan
Outdoor unit
Hot gas runs through the outdoor fins and the fan carries its heat away. The gas condenses into a warm, high-pressure liquid. On a heat pump in heating mode this coil does the opposite job.
- 3
Metering device
Indoor coil, or the outdoor unit on a mini-split
A TXV, an electronic valve or a fixed orifice. It is a narrow restriction, so the pressure drops all at once, some of the liquid flashes to vapor and the rest gets very cold.
- 4
Indoor coil and blower
Air handler or wall head
Room air is pulled across the cold coil. The refrigerant boils, taking heat and moisture out of the air, and cooler, drier air goes back to the rooms. The vapor heads back to the compressor.
One extra valve runs the loop backward.
A cooling-only air conditioner runs its loop one way. A heat pump adds a reversing valve, so in winter the outdoor coil becomes the cold side and pulls heat out of the outdoor air, and the indoor coil becomes the hot side. That is how a heat pump heats a New Hampshire house without burning oil.
The colder it gets outside, the less heat the heat pump can gather and the more the house loses. Below the balance point, backup heat covers the gap: electric strips in the air handler, or the furnace or boiler the house already has. Cold-climate models push that point well below zero.
The difference is how the air gets delivered.
| Ducted split | Mini-split | |
|---|---|---|
| Indoor side | Air handler or furnace with a coil, hidden away, with air sent through ducts | A head on the wall or ceiling of the room, with no ducts |
| Control | Usually one thermostat for the house | A setpoint for each room or zone |
| Best fit | Whole-house comfort where the ductwork is decent | Additions, older New Hampshire houses, and rooms that never heat or cool evenly |
| Inside the box | Compressor, two coils, metering device and the refrigerant loop | The same loop, usually with an inverter compressor that changes speed |
The loop keeps running. It just runs badly.
Most service calls are one of a handful of faults, and each one shows up in a place you can point to on the diagram. Pick one to see what changes and what to ask on the phone.
Dirty outdoor coil
"It runs all day and the house barely keeps up on hot afternoons."
Cottonwood fluff, grass clippings and dryer lint mat the fins. Outdoor air can't carry the heat away, so pressure on the hot side climbs and the compressor runs harder and hotter.
Clogged filter, iced coil
"It's running but hardly any air comes out, and there's ice on the pipes outside."
Too little air crosses the indoor coil, so the coil drops below freezing and ices over. The ice blocks more air. When the system shuts off, the ice melts and can overflow the pan.
Low charge (leak)
"It runs and runs and never gets cold. There's frost on the copper by the indoor unit."
A leak leaves too little refrigerant in the loop. It boils off early in the indoor coil, so most of the coil does no work. The vapor comes back warm, and the compressor, which that vapor is supposed to cool, runs hot.
Failed capacitor
"The fan inside is blowing but the air isn't cold. The outside unit is humming."
The capacitor gives the compressor and outdoor fan motors the kick they need to start. When it fails, the contactor closes and the motors hum without turning. Heat and age wear them out, and they often go on the first hot day of the year.
Clogged condensate drain
Summer"It just stopped cooling. The thermostat looks normal."
The indoor coil pulls water out of the air all summer, and it runs to a pan and a drain line. When algae plugs the line, the pan fills and a float switch shuts the system off before the water reaches the ceiling. Without a float switch, the ceiling gets it.
Stuck reversing valve
Winter"It's blowing cold air and the thermostat says Heat."
The reversing valve is what lets a heat pump heat. If it sticks in the cooling position, the system runs its summer loop in January: the indoor coil gets cold and the heat is thrown outside.
Where AI helps the office and the trucks.
A shop lives in dispatch, parts, callbacks and "the house is 58 degrees." When the office and the techs share the picture above, it gets much easier to pick out the calls software can triage.
Intake that asks the right questions
An after-hours agent answers the call and asks what a good dispatcher would: heat or cool, is the outdoor fan spinning, is there ice on the lines, when was the filter changed, how old is the outdoor unit. The tech reads the answers before the truck rolls.
Custom AI agentsWork orders the tech signs off on
The diagnosis stays with the tech. AI drafts the work order from their notes and photos, and asks for what's missing, like the nameplate photo or a picture of the iced coil, before the job closes.
AI Cowork trainingCallbacks and parts, handled
Same address failing twice in 30 days goes to a callback queue. The model number on the nameplate pulls the likely parts list. We map how your office runs this today, then decide which steps a person keeps and which ones software can take.
Workmap
This page shows the standard vapor-compression cycle used in residential split systems, mini-splits and heat pumps, simplified for learning. Temperatures are typical values, not specifications. It is not a service manual, and refrigerant work needs an EPA-certified technician.
