PillarDavid Rosenfeld
The HVAC Field Glossary
A running, plain-English reference for the terms, codes, and acronyms that come up daily on the jobsite. Definitions, why they matter, and the things people consistently get wrong.
HVAC has its own dialect — AHRI, MERV, TXV, SEER2, BTU, HSPF — and getting a definition wrong costs real money. Wrong-sized unit. Wrong filter. Wrong call on a warranty claim.
This is our running field glossary. Plain English. Why each term matters. What people commonly mix up. We add entries as we have time to do them justice, not faster.
Why we built this
Most glossaries are either Wikipedia-dry or marketing copy with footnotes. Neither one tells you what to actually do with the term on a Tuesday afternoon when the customer's house is 85°F and your tech says “the TXV is hunting.” This one tries to.
Every entry follows the same shape:
- What it stands for — the literal definition, in one sentence.
- What it actually measures or does — in plain English.
- Why it matters on the job — the practical context.
- What people get wrong — the gotchas we keep seeing.
What's in here
Ratings & measurements
- BTU — the energy unit everything else builds on
- SEER vs SEER2 — why the rules changed in 2023
- HSPF — coming soon
- COP — coming soon
- AFUE — coming soon
Air & filtration
- MERV — filter ratings, decoded
- HEPA — coming soon
Components & circuits
- TXV — the metering valve that controls everything downstream
- ECM — coming soon
- TDR — coming soon
How to use it
Bookmark this page. Search-and-link from your team chat. When we publish a new entry, it shows up at the top of the list. If a term keeps tripping up your apprentices, tell us and we'll write it.
And if a definition here is wrong or could be clearer, that's our job to fix. Email us.
▾ Entries in this pillar
4 entries so far.
- 01
BTU, Explained: What One British Thermal Unit Actually Does
A British Thermal Unit is the amount of energy it takes to heat one pound of water by one degree Fahrenheit. That tiny definition is the foundation underneath almost every HVAC capacity number you'll ever read.
- 02
MERV Ratings: What Filter Numbers Actually Mean
MERV is a 1-to-20 scale that tells you what size particles a filter can catch. Higher isn't automatically better — go too high in the wrong system and you strangle the airflow. Here's what each band actually does.
- 03
SEER vs SEER2: What Changed in 2023 (and Why It Matters)
SEER got replaced by SEER2 in 2023 — new test protocol, new minimums, new tax-credit thresholds. Same machine, just measured more honestly. Here's what changed and how to read a spec sheet without getting tricked.
- 04
TXV Explained: The Thermostatic Expansion Valve and What It Does
A thermostatic expansion valve meters how much liquid refrigerant enters the evaporator. Get it wrong — stuck, hunting, miscalibrated — and the whole system runs hot, cold, or just plain bad. Here's how to read one.