How your system works

Watch a cycle, step by step

The same house picture you see on your dashboard, playing out a full cooling or heating run. Tap the house to pause.

It doesn't make cold. It moves heat.

An air conditioner works like your fridge: it picks up heat in one place and drops it somewhere else. In cooling it carries heat from inside your house to outside. A heat pump is the same machine that can also run backwards in winter, pulling heat out of the outdoor air (there is useful heat even at 30°F) and bringing it inside.

The heat rides in refrigerant (a fluid that boils at very low temperatures), which flows round a sealed loop of copper pipe. Where it boils it soaks up heat, like water boiling away on a stove. Where the compressor squeezes it and it turns back to liquid, it gives that heat off. Nothing is used up: the same refrigerant goes round and round.

The thermostat: the brain and the switch

The thermostat senses the room temperature and compares it to your setpoint (the temperature you asked for). When the room drifts too far, it sends a call: it connects 24-volt power onto one or more thin wires. All of them run to the indoor unit’s terminal board; the calls the outdoor unit needs are passed on from there. Each wire means one thing:

• R24 V power: the live side of the low-voltage control circuit.
• CCommon: the return path, so the thermostat itself has steady power.
• YCompressor on: calls for cooling or heat-pump heat; the air handler passes it on to the outdoor unit’s contactor.
• GFan on: runs the indoor blower.
• O/BReversing valve: picks cooling vs heating on a heat pump.
• WAuxiliary / emergency heat (AUX / E): turns on electric heat strips.

The calls

• Cooling: Y + G (and O on most heat pumps): compressor and fan on.
• Heat-pump heating: Y + G with the valve in its heating position. If the room falls about 2°F or more below the set point, or the heat pump can't keep up, the thermostat adds W automatically for auxiliary heat (“AUX”): strips plus the still-running heat pump. Emergency heat (“E”) is chosen by hand, e.g. when the outdoor unit fails; the strips run alone with the compressor and outdoor fan off, which costs the most.
• Fan only: G on its own: air moves, nothing is heated or cooled.

Fan “Auto” vs “On”

Auto runs the blower only while heating or cooling. On runs it all the time: steadier temperatures and more filtering, but more electricity, and in summer it can blow moisture off the wet coil back into the house.

Deadband and short-cycle protection

The thermostat waits until the room is a degree or so off before calling (the deadband), and most wait a few minutes after the compressor stops before allowing it to restart. Both stop the system short cycling (switching on and off every few minutes, which wears parts and removes little humidity).

Why it matters for the monitor: we read these calls, so we know what the system was asked to do and can compare it with what it actually did. Y on but no compressor current? Something between the thermostat and the outdoor unit isn't working.

The indoor unit (air handler)

Usually in the attic, a closet, or the basement. House air comes in through the return grille, passes through it, and goes back out through the supply ducts.

• Blower: the big fan that pushes air through the house.
• Filter: catches dust before it reaches the coil. A clogged filter chokes the airflow, the most common cause of trouble.
• Evaporator coil: a radiator-like block of fins. In cooling, refrigerant boils inside it, soaking up heat from the air blowing across it. It runs at about 40°F, so it also pulls water out of the air (that's dehumidifying).
• Metering device (TXV or piston): a tiny restriction where the liquid line enters the coil. Squeezing through it drops the pressure so the refrigerant can boil cold. A TXV (thermostatic expansion valve) adjusts itself; a piston is a fixed hole.
• Condensate drain: carries the water dripping off the coil away. If it clogs, a safety float switch usually shuts the system off.
• Heat strips: electric heating elements, like a toaster, for auxiliary heat (alongside the heat pump) and emergency heat (strips alone). They work in any weather but cost about two to three times as much to run as the heat pump.

The outdoor unit (condenser / heat pump)

• Compressor: the heart. It pumps refrigerant round the loop and squeezes it, which makes it much hotter than the outdoor air, hot enough to give heat away even on a 100°F day.
• Condenser coil and fan: the fins around the sides and the fan on top. The fan pulls outside air through the coil and out the top, carrying the heat away. In cooling the air coming out the top feels hot. Keep the coil clean and clear of plants.
• Reversing valve (heat pumps only): a valve that flips the direction the refrigerant flows, switching between cooling and heating. Controlled by the O/B call, passed on from the air handler.
• Contactor: a heavy-duty switch. When the Y call arrives (via the air handler’s terminal board), it clicks in and sends full 240 V power to the compressor and fan.
• Capacitor: a can that gives the motors a kick to start and keep turning efficiently. One of the most common parts to fail, especially after heat waves.
• Defrost (heat pumps in winter): pulling heat from cold, damp air can frost the outdoor coil. Every so often the unit briefly switches to cooling, with its fan off, to melt the ice. Steam rising from the unit is normal; the backup strips may come on so you don't feel cold air.

The two copper lines

Two pipes connect the indoor and outdoor units:

• Suction line, the big insulated one. In cooling it carries cool vapor back to the compressor and feels cold and often sweaty. In heat-pump heating the same pipe carries hot gas indoors and is called the vapor line.
• Liquid line, the small one, usually bare. It carries warm liquid refrigerant (a little warmer than outdoors in cooling).

Their temperatures are a window into the refrigerant without opening the system.

Step by step

Cooling

1. The thermostat calls (Y + G). The blower starts, and the Y call passes through the air handler’s board to the outdoor unit, which kicks on.
2. The compressor squeezes refrigerant vapor into a hot, high-pressure gas.
3. The outdoor coil and fan cool it until it turns into warm liquid, dumping your house's heat outside.
4. The liquid travels indoors in the small line, then squeezes through the metering device and drops in pressure.
5. In the indoor coil it boils at about 40°F, soaking up heat (and water) from the house air blowing across it.
6. The cool vapor returns outside in the big line, and the loop repeats.

Heat-pump heating

1. The thermostat calls (Y + G, valve set to heat).
2. The compressor sends hot gas indoors through the big line.
3. The indoor coil now acts as the hot radiator: the blower pushes house air across it, and the refrigerant gives off its heat and turns to liquid.
4. The liquid goes outside in the small line and through the outdoor metering device, where it drops in pressure and gets very cold.
5. Colder than the outdoor air, it boils in the outdoor coil, picking up heat from outside.
6. Back to the compressor. If it can't keep up, W brings in the strips as auxiliary heat while the heat pump keeps running.

What our sensors measure, and why

This guide is general; every system is a bit different. Anything involving refrigerant or 240 V belongs to a licensed technician. Back to your systems