The Plain-English HVAC Glossary for Homeowners

Why These Words Are Worth Knowing

A quote for a new system, or a technician standing at the air handler explaining what they found, comes with its own vocabulary built in. Tonnage, SEER2, static pressure, superheat. None of it is complicated once someone translates it, but all of it can sound like a wall of jargon the first time it gets said out loud in your kitchen.

Knowing these words doesn't turn a homeowner into a technician, and it isn't supposed to. It does mean a quote or a diagnosis stops sounding like a foreign language, and it makes it easier to ask a sharper question when a number doesn't add up. This guide runs through the terms that come up most, roughly in the order a homeowner actually hears them: the ones printed on a spec sheet first, then the ones a technician says out loud mid-repair.

Sizing and Output: Tons and BTUs

Tonnage is the unit that describes how much heat a system can pull out of a house in an hour. It has nothing to do with weight. One ton equals 12,000 BTUs, short for British thermal units, of heat removed every hour, a figure that traces back to how much heat a melting block of ice could absorb in a day, from back when actual ice was how buildings got cooled. Homes around Southwest Florida typically fall somewhere in the 2-to-5-ton range, and a 5-ton system simply carries away more heat per hour than a 2-ton one, not more comfort by default.

That number doesn't come from a guess or a square-footage chart. It's decided by measuring the actual house, room by room, the same way any proper AC installation estimate is built, rather than carried over from whatever size the old equipment happened to be. Getting that math right matters enough that it has its own extended guide on this site. Here, the short version is enough: BTUs measure heat, a ton is just a bundle of 12,000 of them per hour, and the right number of tons is a conclusion, not a starting assumption.

You might also see tonnage written a different way on a permit or a spec sheet, something like "2.5 ton" next to a line that reads "30,000 BTU/hr." Both are describing the exact same equipment, just in different units, the way a thermostat could just as easily be marked in Celsius instead of Fahrenheit without changing anything about the temperature in the room. If the two numbers on a piece of paperwork ever seem to disagree, that's worth asking about directly rather than assuming one of them is a typo.

The Efficiency Label: SEER2

SEER2 is the efficiency rating printed on every new system's spec sheet, and it answers one question: how much cooling comes out for the energy that goes in, averaged across a full season rather than any single afternoon. It replaced the older SEER rating with a tougher, more realistic test, so a SEER2 number and an old SEER number for the same equipment aren't directly comparable side by side.

A higher SEER2 number generally means lower running costs, but how much that difference actually shows up on a power bill depends on how many hours a year the system runs, and Southwest Florida systems log more of those hours than equipment in most of the country. The rating also assumes correct installation and sizing behind it. A high number connected to a system that's the wrong size for the house won't perform the way the sticker suggests.

Every new system sold today also has to clear a required minimum on this scale before it can legally be installed, so even the most basic option on a quote still carries a real efficiency floor. That's worth knowing mainly so a number on a quote doesn't sound more impressive, or more alarming, than it actually is. SEER2 is one useful word among several here, not the single number that decides whether a quote is a good one.

Airflow Words: CFM and Static Pressure

CFM stands for cubic feet per minute, and it measures something completely different from tonnage: not how much heat a system removes, but how much air its blower actually moves to carry that cooling into the rooms. As a rough rule of thumb, installers generally want somewhere in the neighborhood of 350 to 400 CFM moving for every ton of cooling capacity, since a system can have plenty of capacity on paper and still cool poorly if the air carrying that capacity into the house is too thin.

Static pressure is the resistance air runs into on its way through the ductwork, filters, and vents, measured in a unit small enough that even a minor restriction moves the needle. A dirty filter, undersized ducts, sharp bends, or too many closed vents all add to that resistance, and a blower fighting high static pressure has to work harder without the equipment itself being any less capable. It's a number a technician measures with a gauge, not something you can judge by ear or by holding a hand up to a vent.

The Two Kinds of Cooling: Sensible Heat, Latent Heat, and Delta-T

An air conditioner is quietly doing two jobs every time it runs, and HVAC terminology has a word for each one. Sensible heat is the heat a thermometer measures, the part that makes a room feel hotter or cooler. Latent heat is the energy tied up in moisture, and removing it lowers humidity without necessarily moving the number on the thermostat. A cold coil handles both at once, but in a humid climate, the latent side of that job is doing a lot more work than it would somewhere dry.

Delta-T is a related number a technician checks on almost every visit: the difference between the temperature of the air returning to the system and the air coming back out of the supply vents. A bigger gap generally means the coil is pulling more heat out of the air passing over it; too small a gap, or too large one, can point toward an airflow or refrigerant problem before it becomes an obvious complaint. It's one of the fastest health checks a technician has, precisely because it's just two thermometer readings and simple subtraction.

This is also the reason a Florida-specific conversation about comfort spends so much time on humidity instead of temperature alone. The latent side of the job, the moisture removal, is doing real work here for most of the year in a way it simply isn't in a drier climate, and a house can hit its target temperature and still feel off if that side of the equation isn't keeping up.

Refrigerant Words: TXV and Superheat

A TXV, short for thermostatic expansion valve, is the small component that meters exactly how much refrigerant enters the indoor coil at any given moment. It's not a fixed opening; it adjusts itself on the fly, responding to conditions at the coil rather than holding one setting no matter what the system is doing outside. Older or simpler systems sometimes use a fixed orifice instead, a set restriction that doesn't adjust the way a TXV does.

Superheat is the number that tells a technician whether that valve is doing its job correctly. It's the extra warmth refrigerant gains after it has already finished turning from liquid to gas inside the coil, measured with gauges and a temperature probe rather than guessed at. Too little superheat risks liquid refrigerant reaching parts of the system built only to compress gas; too much means the coil isn't being used as efficiently as it could be. It's a number that matters far more to a technician mid-diagnosis than to a homeowner day to day, but it explains why a repair visit involves gauges and math instead of a quick look.

The Filter Word: MERV, and What a Glossary Can't Do

MERV stands for Minimum Efficiency Reporting Value, the standard scale printed on nearly every air filter box, usually somewhere between 1 and the low teens for the filters most homes actually use. A higher number means the filter catches smaller particles: pollen, mold spores, fine dust. It doesn't automatically mean better, though, because a denser filter that catches more also resists airflow more, and a system built around a looser filter can struggle to pull enough air through one rated well above what it was designed for.

That trade-off is the whole reason MERV is worth knowing rather than just picking the highest number on the shelf. The same idea runs through most of the words in this guide: a number only means something once it's read against the specific system it's describing, not in isolation. Learning the vocabulary gets you most of the way to following a conversation about your own system. It doesn't replace an actual technician looking at the actual equipment, with gauges and a manometer instead of a glossary.