How Air Conditioning Actually Works — A Homeowner's Field Guide

Air conditioning is the controlled movement of heat from inside your house to outside it, using a refrigerant that boils and condenses on demand. There are four mechanical players in the loop: the compressor (the pump), the condenser coil (outside, releases heat), the metering device — a TXV or EEV (the throttle), and the evaporator coil (inside, absorbs heat). Refrigerant runs in a closed loop between them, switching from low-pressure vapor to high-pressure vapor to high-pressure liquid and back to low-pressure liquid in a precise sequence.

The textbook image: the indoor coil is cold because liquid refrigerant flashing to vapor inside it pulls heat out of the air rushing across the fins. That warmed refrigerant vapor travels outside, where the compressor squeezes it hot — hotter than the outdoor air — and the condenser coil sheds that heat to the atmosphere. The now-cooler high-pressure liquid heads back inside through the TXV, which meters how fast it can boil, and the cycle repeats. SEER2 is just a measure of how many BTU of heat the system can move per watt-hour of electricity it burns to move it.

That's it. Everything else is variations on this loop.

This understanding matters the first time a contractor tells you something doesn't add up. Three real scenarios from this past summer:

A homeowner in Harahan called us because their newer system "couldn't keep up" past 3 PM. The previous contractor recommended a full replacement at $11,400. Knowing the cycle, we asked one question: was the outdoor condenser coil clean? It was packed solid with cottonwood fluff. Once the condenser couldn't shed heat, the high-side refrigerant pressure spiked, the compressor had to work harder against that pressure, capacity dropped, and the house drifted. A $189 coil cleaning restored full capacity.

A Mandeville client was sold a "low refrigerant" recharge every spring for four years. Refrigerant doesn't get used up — the system is sealed. If it's low, there's a leak. Knowing that one fact would have saved them roughly $1,800 in recharge bills and pushed them toward the actual fix.

A Slidell rental had a brand-new condenser but the inside still felt sticky at 74°F. The indoor coil was sized for an old single-stage unit; with the new high-efficiency outdoor unit, air moved too fast across the smaller coil to wring out moisture. The textbook would have said "it's cooling, you're fine." The cycle understanding said "your latent capacity is wrong."

Each of these required reading the cycle, not the symptom.

Three places, almost every time.

Latent vs. sensible load. Air conditioning has two jobs: lowering temperature (sensible) and removing moisture (latent). Outside of the Deep South, most systems are sized for the sensible load and latent comes along for the ride. On the Gulf Coast, where outdoor dewpoints sit in the mid-70s for months at a time, the latent load can be 30-45% of total cooling load. An oversized system shuts off before it has time to wring water out of the air, so the house feels cold and clammy at the same time. The textbook says bigger is better. In our climate, bigger is worse — a problem we go deeper on in why Louisiana homes hide humidity problems.

Static pressure. The cycle calculus assumes air moves freely across the indoor evaporator coil. In our market, where retrofitted ductwork from the 1970s-1990s is everywhere, static pressure is often double what the equipment was designed for. The compressor sees normal pressures, but the coil never gets the airflow it needs, so it ices up, capacity tanks, and a homeowner blames the unit.

Salt-laden air. Within a few miles of Lake Pontchartrain and the Gulf, condenser coils corrode faster than the textbook curves predict. Aluminum fins pit, copper develops formicary corrosion, and the system loses heat-rejection capacity years before its rated life. The cycle still works — it just works harder for less output, especially in older slab homes around our Metairie service area where outdoor units sit close to the pavement.

These regional realities don't break the physics. They break the assumptions PE-consolidator quoting tools were built on.

You don't need lab gear to read a cooling cycle. We use four tools on almost every call, and any homeowner can spot the basics with a $25 hygrometer.

A digital manifold gauge set reads the suction-line and liquid-line pressures and shows whether the system is properly charged. From those pressures and the line temperatures, we calculate superheat and subcooling — the two numbers that tell us whether the metering device is doing its job and whether the charge is right. Wrong subcooling almost always means a leak, an overcharge, or a restricted TXV. Wrong superheat usually means low airflow across the indoor coil.

A clamp-on ammeter on the compressor reads running current. Compare it to the rating plate; a current 20% over rated is doing extra work — usually fighting a dirty condenser coil or high head pressure.

An infrared camera shows uneven coil temperature. A healthy evaporator coil shows a near-uniform cold field across its face. Hot spots mean the refrigerant isn't distributing evenly — a sign of a bent TXV sensing bulb, a partial restriction, or air bypassing sections of the coil.

A hygrometer (yours) tells you what your home actually feels like. In Louisiana, indoor relative humidity above 60% means the latent side of the cycle is losing — even if the thermostat is satisfied.

If a contractor diagnoses without touching at least the first three, ask why. That's the diagnostic floor for a real cooling-cycle conversation, and it's why we cover the difference between a tune-up, an inspection, and an actual diagnostic separately.

Use it to ask better questions. Before you sign anything, pose three: What did you measure? What were the values? What do they tell you about which part of the cycle is failing? Those three questions filter for a real diagnosis.

If the conversation drifts toward replacement without clear answers, that's the moment for an honest second look. Our 7-question repair-vs-replace framework is the next read; it walks through the financial and lifecycle math without assuming the answer.

If you're already at the point of needing eyes on the system, the second-opinion diagnostic walkthrough lays out what we measure and what we'll show you on the truck's tablet before we quote anything. The point of this article isn't to make you a technician — it's to make sure you're never the only person in the room who doesn't speak the language.

SEER2 is the federal efficiency rating that took effect in January 2023 and replaced the older SEER number. The change wasn't cosmetic. The testing protocol now uses a 0.5 in.w.c. external static-pressure assumption (up from 0.1), which is closer to the real ductwork most homes have. A 15 SEER unit and a 14.3 SEER2 unit are roughly equivalent. Two things SEER2 still doesn't tell you are worth knowing.

First, SEER2 averages efficiency across a full cooling season at one set of part-load conditions. South Louisiana's cooling season is longer and more humid than the national average, so a SEER2 rating slightly understates what a properly sized system actually does for our annual bill.

Second, SEER2 is a sensible-cooling number. It says nothing about how well the system dehumidifies. A 16 SEER2 single-stage unit and a 16 SEER2 two-stage variable-speed unit will look identical on the label, but the variable-speed will run longer at lower capacity, pulling far more moisture out of the air. In our market, that dehumidification difference often matters more than the headline efficiency number.

When you compare quotes, ask for two numbers: SEER2 and the system's sensible heat ratio (SHR) at design conditions. The lower the SHR, the more dehumidification you get. That's the question that separates a sales-driven proposal from a load-driven one. The same homework matters whether you're comparing central systems or weighing a ductless mini-split against central air.