How Your AC Works

Every standard air conditioner moves heat with four parts working in a loop: the compressor squeezes low-pressure refrigerant gas into a hot high-pressure gas; the condenser (the outdoor coil) dumps that heat outside and turns the gas into a warm liquid; the expansion valve or piston drops the pressure so the liquid turns very cold; and the evaporator (the indoor coil) lets that cold refrigerant absorb heat from your room air. Nothing is “made cold” — heat is simply pumped from inside to outside.

Refrigerant is a heat courier, not a fuel. It is chosen for a very low boiling point, so it evaporates inside the cold indoor coil and soaks up a large amount of heat as it changes from liquid to gas. Once compressed and sent outdoors, its boiling point rises, so it condenses back to a liquid and releases that heat to the outside air. The same charge circulates for the life of the system — you only add refrigerant if there is a leak.

Superheat is how many degrees the refrigerant gas has warmed above its boiling point by the time it leaves the indoor coil. A healthy superheat proves all the liquid has fully boiled into gas before reaching the compressor. If superheat is near zero, liquid can slug the compressor and destroy it; if it is very high, the system is usually undercharged or starved of refrigerant. It is one of the two numbers a good tech checks before touching the charge.

Subcooling is how many degrees the liquid refrigerant has cooled below its condensing temperature at the outdoor coil. It confirms the refrigerant is a solid column of liquid — not part gas — before it reaches the metering device. Too little subcooling usually means a low charge or a restriction; too much can mean an overcharge. On TXV systems, subcooling is the primary charging target.

The temperature split is the difference between the warm return air going into the indoor coil and the cool supply air coming out. On a correctly running system it is usually about 8–11 C (roughly 15–20 F). A very small split points to low airflow or low charge; an unusually large split often means restricted airflow, such as a dirty filter. If yours looks off, the symptom diagnostic walks through likely causes.

A reciprocating compressor uses pistons and valves like a car engine — more moving parts, more noise, more wear. A scroll compressor uses two interlocking spirals, one fixed and one orbiting, to squeeze the gas smoothly. Scrolls have fewer parts, run quieter, tolerate the odd slug of liquid better, and are now standard on most residential systems. Modern inverter units almost always use scroll compressors.

A single-stage unit is either fully on or fully off. A two-stage unit adds a lower gear (around 65–70%) for mild weather, giving longer, quieter, drier cycles. An inverter (variable-speed) unit ramps anywhere from roughly 20% to 100% to exactly match the load, so it rarely shuts off and controls humidity best. This is about how they run; for whether the extra cost pays back, see inverter vs non-inverter.

The metering device drops refrigerant pressure just before the indoor coil. A fixed orifice (piston) is a simple fixed hole — cheap, but it cannot adapt, so efficiency drops when indoor or outdoor conditions change. A thermostatic expansion valve (TXV) senses coil superheat and constantly adjusts flow, holding efficiency across a wide range of conditions and protecting the compressor. TXV or electronic expansion valves are standard on efficient and inverter systems.

A hard start kit is a start capacitor plus a relay that gives the compressor a brief jolt of extra torque at startup. Aging compressors with worn bearings, or units on long line-sets or weak voltage, sometimes struggle to spin up and may hum or trip. A hard start kit can buy time, but repeated hard starting is usually a sign the compressor is nearing the end of its life rather than a permanent fix.

The run capacitor gives the compressor and fan motors the phase shift they need to keep turning with steady torque. A weak or dead capacitor is one of the most common AC faults: the motor may hum, struggle to start, overheat, or trip the breaker. Technicians test it by measuring capacitance in microfarads against the rating printed on the can (for example 45 µF ±5%). It is a cheap, common part — but replacing it is a job for a qualified tech because capacitors store a dangerous charge.

The contactor is a heavy-duty relay. When the thermostat calls for cooling, it sends a low-voltage (24 V) signal that energizes the contactor coil, which snaps the high-voltage contacts closed and sends full power (240 V in the US, 230 V in India) to the compressor and outdoor fan. Pitted or stuck contacts are a common reason an outdoor unit either won’t start or won’t shut off.

A PSC motor runs at one fixed speed and draws a constant, fairly high current. An ECM (electronically commutated motor) is a smart brushless DC motor that varies its speed to hold steady airflow, and can use dramatically less electricity at low speed. ECM blowers pair naturally with two-stage and inverter systems and are a big part of why high-SEER2 and 5-star units sip power at partial load.

Total External Static Pressure is the resistance your blower has to push against — ducts, filter and coil combined — measured in inches of water column with a manometer. If it runs high (above about 0.5 in. wc. on a typical residential system), airflow drops, energy use climbs, vents get noisy and the blower motor can overheat. Undersized or crushed ducts, a too-restrictive filter, or a dirty coil are the usual culprits.

R-22 is an HCFC whose chlorine damages the ozone layer, so it was phased out under the Montreal Protocol and replaced by R-410A, an HFC that is ozone-safe. R-22 has not been produced or imported into the US for years; existing R-22 systems can only be topped up with expensive recycled or reclaimed gas, which is usually the point at which replacing the whole system makes more sense.

R-410A is ozone-safe but has a high global-warming potential (GWP around 2,088). Under the US AIM Act, new residential AC and heat pump equipment made or imported from 1 January 2025 must use a refrigerant with GWP under 700. The two replacements you will see on spec sheets are R-454B (GWP about 466 — Carrier, Trane, Rheem, Lennox, Bosch) and R-32 (GWP about 675 — Daikin, Mitsubishi, LG, Goodman). India standardized on R-32 years earlier, so most Indian split ACs already use it. Both are mildly flammable A2L refrigerants, which changes some installation and handling rules.

Yes. The 2025 rule stops the manufacture of new R-410A equipment; it does not outlaw the systems already in your home. R-410A will be available for repairs and top-ups for years, and R-22 systems can still be serviced with reclaimed refrigerant. The practical question is cost: as supply tightens, servicing an old, leaky, high-GWP system gets pricier, which shifts the math toward replacement. Our running-cost calculator can help compare keeping vs replacing.

A heat pump is an air conditioner that can run backwards. The reversing valve swaps the direction of refrigerant flow: in cooling it sends hot gas outdoors (outdoor coil = condenser) and cools your room at the indoor coil; in heating it flips, sending hot gas indoors to warm the room while the outdoor coil pulls heat from the cold outside air. One valve, two seasons. Sizing a heat pump is not the same as sizing an AC — see heat pump sizing basics.

The balance point is the outdoor temperature at which the heat pump’s shrinking heating output just equals the home’s heat loss. Below it, the heat pump alone can’t keep up, so the system switches on auxiliary (backup) heat — usually electric resistance strips or a gas furnace. A well-sized system spends most of the winter above its balance point and only leans on expensive backup heat during cold snaps.

A heat pump heats by harvesting heat from outdoor air, but colder air holds less heat and the refrigerant becomes less dense, so the compressor moves less energy per cycle. Capacity therefore drops exactly when demand peaks. Cold-climate (hyper-heat / inverter) heat pumps fight this with variable-speed compressors and vapor injection, holding useful output to well below freezing — which is why they are sized around the balance point, not a single rule of thumb.

In heating mode the outdoor coil runs below freezing, so frost builds on it. Periodically the unit briefly reverses into cooling mode to warm the outdoor coil and melt that frost — the visible “steam” is just that ice flashing to vapor. During defrost the indoor air can feel cooler and backup heat may kick in for a minute or two. It is normal, automatic and short. Constant or very long defrost cycles are the abnormal case worth a service call.

The filter drier sits in the liquid line and does two jobs: it traps solid debris (copper scale, flux, dirt) before it can clog the metering device, and its desiccant absorbs any moisture inside the sealed system. Moisture is the quiet killer — it reacts with refrigerant and oil to form acids that corrode the compressor windings from the inside. A new drier is standard practice whenever the refrigerant circuit is opened.

Before adding refrigerant, a tech uses a vacuum pump to evacuate the lines to around 500 microns. This does two things: it removes non-condensable gases like air and nitrogen that would otherwise raise pressures and cut capacity, and it lowers pressure so far that any trapped moisture boils off at room temperature and gets drawn out. Skipping a proper evacuation is one of the most common causes of premature compressor failure — a fair question to ask any installer.

Older coils used copper tubes, which are prone to formicary corrosion — microscopic pinhole leaks caused by everyday household acids from cleaners, paints and cosmetics attacking the copper. All-aluminum coils resist this corrosion far better, so manufacturers moved to them to cut down on premature refrigerant leaks. It is one reason a slow, unexplained loss of charge is less common on newer indoor coils.

Liquids don’t compress, so any liquid refrigerant reaching the compressor can smash valves and pistons — this is called slugging. Two parts guard against it: an accumulator, a reservoir on the suction line that catches stray liquid and lets it boil off safely; and a crankcase heater, which keeps the compressor oil warm during off cycles so refrigerant doesn’t migrate and condense into it. Both quietly extend compressor life.

They describe the same thing from opposite directions. R-value measures resistance to heat flow — higher is better insulation, and layers add up (wall + insulation + sheathing). U-factor is simply 1 divided by the total R-value; it measures how easily heat passes through a whole assembly like a window, so lower is better. Codes rate walls and ceilings by R-value and windows by U-factor. Both feed directly into a proper load calculation — see Manual J explained.