The HVAC Field Glossary

HVAC has its own dialect — AHRI, MERV, TXV, SEER2, BTU, HSPF, ECM, AFUE, CFM, EER, COP, IAQ, VRF, DOAS, ERV, HRV — and getting a definition wrong costs real money. Wrong-sized unit. Wrong filter spec. Wrong line item on the proposal. Wrong call on a warranty claim.

This is our running field glossary. Plain English. Why each term matters. What people consistently mix up. We add entries as we have time to do them justice, not faster.

Why this exists

Most HVAC glossaries you can find online are one of two things. Either they're Wikipedia-grade, technically correct but written for somebody studying the trade in a vacuum — definitions stripped of context, no sense of when the term comes up or why it matters. Or they're equipment-manufacturer marketing copy with footnotes, where every term is defined in a way that makes the manufacturer's product sound like the right answer.

Neither one helps when you're standing in a customer's mechanical room on a Friday afternoon, the house is 85°F, your tech texted you that “the TXV is hunting,” and you need to know — quickly — what that means, whether it's something you can diagnose from the office, and what parts to throw on the truck before you head over.

This glossary is the version we wished we had when we started. It's written by people who have actually been on the other end of those calls.

How HVAC vocabulary actually works

HVAC terms come from four different worlds, and once you can see which world a term comes from, the term itself feels a lot less foreign:

Acronyms that hide a unit

These are measurements with a unit baked in. BTU is energy. CFM is volume per minute. PSIG is gauge pressure. The trap is when somebody drops the unit suffix — “a 36,000 BTU unit” almost always means BTU per hour, but you'll see specs that quietly leave out the “/hr” and people get confused about what they're comparing. Read the unit on the spec sheet, not just the number.

Acronyms that name a component or technology

These point at a thing you can hold in your hand or trace on a schematic. TXV is a valve. EEV is its electronic cousin. ECM is a motor type. VRF is a system topology. DOAS, ERV, and HRV are categories of fresh-air equipment. The trap with these is using the acronym without knowing which physical part you're actually pointing at — somebody says “the EEV is hunting” on a system with a fixed-orifice metering device, and the conversation falls apart.

Acronyms that rate performance

SEER, EER, SEER2, HSPF, COP, AFUE, MERV. Every one of these is a number that ranks something — efficiency, capture rate, performance. The trap here is that the test conditions vary, and so do the rating scales. A “16 SEER” unit and a “15 SEER2” unit can be the exact same machine measured two different ways. Knowing which test produced the number is half the battle.

Acronyms that govern compliance

AHRI, ASHRAE, NEC, IECC, EPA, DOE. These name standards bodies or regulations. When a spec sheet says “AHRI matched system,” that's a compliance claim — the indoor and outdoor units have been tested as a pair and the rating you see is for that specific combination. Pairing an outdoor unit with a different indoor coil voids the AHRI rating and, often, the tax credit eligibility that depended on it.

The expensive misunderstandings we keep seeing

Every one of these is a real call we've taken or a real install we've inherited:

• Quoting a unit's BTU rating without checking input vs output. An 80,000 BTU furnace at 80% AFUE only delivers 64,000 BTU/hr of usable heat. Quote the wrong one and the system's undersized before the install starts.
• Comparing “16 SEER” against “15 SEER2” as if the higher number wins. SEER2 ≈ SEER × 0.95 for the same machine. The 2024 unit is being more honest, not less efficient.
• Dropping a MERV 13 into a system designed for MERV 8. Static pressure jumps, blower works harder, coil freezes, callbacks start. The filter is “better”; the system is worse.
• Calling all refrigerant “Freon.” Freon is a brand name for a family of refrigerants, most of which are phased out. Saying “add Freon” when you mean R-410A or R-454B is sloppy, and it bites you when the wrong refrigerant gets put in.
• Reading “AHRI matched” without checking the matched configuration. The AHRI certificate lists specific indoor units paired with specific outdoor units. Swap either side without re-checking and the rating evaporates.
• Using “ton” loosely. One ton of cooling is exactly 12,000 BTU/hr — a number that traces back to the energy it takes to melt a US ton of ice in 24 hours. “A 3.5-ton system” is 42,000 BTU/hr, not a vibe.

None of these are pedantic. Each one shows up in a real conversation with a customer, an installer, or a manufacturer rep, and getting any of them wrong costs somebody money or time or both.

How we write entries

Every entry in this glossary follows the same four-part shape so you know what to expect:

• What it stands for — the literal definition, in one sentence.
• What it actually measures or does — in plain English, with whatever physics or units matter.
• Why it matters on the job — the practical context. What conversation does this term come up in? What decision does it inform?
• What people get wrong — the specific gotchas that keep biting people. Real misunderstandings, not theoretical ones.

How to use this glossary

Depending on who you are, the entries work different ways:

If you're a tech on a callback, these entries are quick reference. You know what a TXV is — you just want a refresher on the four failure modes before you head out, or a sanity check on superheat numbers before you condemn a part. Skim the “what people get wrong” section first.

If you're a contractor specing a job, these entries are arguments you can hand to a customer who's been Googling. When the homeowner asks “why aren't you putting in a 20 SEER2 unit,” the SEER vs SEER2 entry is the explanation. Send the link. Save the phone call.

If you're a homeowner trying to make sense of a quote, read the entry for whatever term is on the proposal that you don't recognize. Then ask the contractor a question from the “what people get wrong” section. If they can answer it cleanly, they know what they're talking about.

What's in here so far

Ratings & measurements

• BTU — the energy unit everything else in HVAC is built on
• SEER vs SEER2 — why the federal rating changed in 2023 and what that means for tax credits

Air & filtration

• MERV — what filter numbers actually mean, and why higher isn't automatically better

Components & circuits

• TXV — the metering valve that controls everything downstream of the liquid line

More entries on the way — HSPF, COP, AFUE, HEPA, ECM, EEV, AHRI, and the rest of the alphabet. We publish them when they're ready, not before. If a term you need isn't here yet, email us and we'll move it up the list.

One more thing

This glossary is alive. If a definition here is wrong, unclear, or missing the gotcha that bit you on a job last week, that's our job to fix. Send a note to [email protected] with the term and what we got wrong, and we'll update the entry with credit.

The goal is the version of this reference we wished we had when we started. The only way to get there is to keep editing it.