AFUE is Annual Fuel Utilization Efficiency — the percentage of fuel a furnace or boiler converts into heat for the home over a typical year, with the rest lost to exhaust and cycling.
Full entry → HVAC · Glossary
HVAC glossary
Plain-English definitions of 6 HVAC terms you will see on quotes and inspection reports.
- 6Terms defined
- 6Letter groups
- 6Featured starters
How to use this glossary
Find the word. Read the definition. Check the bid.
SEER2 / EER2 / HSPF2. Efficiency ratings under the M1 test procedure, which raises external static pressure from 0.1 to 0.5 inches of water - five times the prior test - to better reflect real installations. Higher is more efficient. Because the test is harder, SEER2 figures read lower than legacy SEER numbers for comparable equipment.
Manual J, S and D. ACCA's standard methods: Manual J calculates your home's heating and cooling loads, Manual S selects equipment to match, Manual D designs the duct system. ACCA notes these are referenced in building codes nationwide.
Tonnage, static pressure and AHRI match. Tonnage is cooling capacity (one ton equals 12,000 BTU/h). Static pressure is the resistance the blower fights against in your duct system - high static pressure means the ducts are choking the system. An AHRI match is the specific indoor and outdoor unit combination that was tested together, which is what the efficiency rating actually applies to.
Before you sign
Three rules for every term
- If a term on your quote isn't defined, ask A contractor who can't explain a line item in plain English either doesn't know or doesn't want you to. Both are reasons to pause.
- Efficiency ratings only apply to matched systems Ask for the AHRI match reference number on the proposal.
- Tonnage should come from Manual J, not the old nameplate Copying the old system's size perpetuates a sizing error you may have lived with for two decades.
Start here
All 6 terms Common HVAC terms on quotes
High-traffic words first. Full A-Z dictionary below.
AFUE
AFUE is Annual Fuel Utilization Efficiency — the percentage of fuel a furnace or boiler converts into heat for the home over a typical ye…
HSPF2
HSPF2 is the Heating Seasonal Performance Factor 2 — the DOE seasonal heating-efficiency rating for heat pumps under the same revised M1 …
Manual J
Manual J is ACCA’s standard residential load calculation method that estimates a home’s heating and cooling loads from construction, insu…
Refrigerant
Refrigerant is the working fluid in an air conditioner or heat pump that absorbs heat indoors and releases it outdoors by repeatedly chan…
SEER2
SEER2 is the Seasonal Energy Efficiency Ratio 2 — the U.S. Department of Energy cooling-efficiency rating for central air conditioners an…
Tonnage
In HVAC, tonnage is cooling capacity: one ton equals 12,000 BTU per hour of heat removal, a unit left over from the ice-melting capacity …
6 definitions
A-Z HVAC dictionary
Search or jump by letter. Each row is a complete definition.
A
H
HSPF2 is the Heating Seasonal Performance Factor 2 — the DOE seasonal heating-efficiency rating for heat pumps under the same revised M1 test procedure used for SEER2 since January 1, 2023.
Full entry →M
Manual J is ACCA’s standard residential load calculation method that estimates a home’s heating and cooling loads from construction, insulation, windows, orientation, air leakage and internal gains — not square footage alone.
Full entry →R
Refrigerant is the working fluid in an air conditioner or heat pump that absorbs heat indoors and releases it outdoors by repeatedly changing between liquid and gas under pressure.
Full entry →S
SEER2 is the Seasonal Energy Efficiency Ratio 2 — the U.S. Department of Energy cooling-efficiency rating for central air conditioners and heat pumps under the harder M1 test procedure that took effect January 1, 2023.
Full entry →T
In HVAC, tonnage is cooling capacity: one ton equals 12,000 BTU per hour of heat removal, a unit left over from the ice-melting capacity of early mechanical cooling.
Full entry →SEER2 uses DOE's revised M1 test procedure, effective January 1, 2023, which tests at five times the previous external static pressure. Same equipment, harder test, so SEER2 numbers read lower.
ACCA's standard method for calculating a home's heating and cooling loads, used to size equipment correctly. ACCA notes it is mandated by national building codes and most states.
A specific tested pairing of indoor and outdoor units. Published efficiency ratings apply to the matched pair, not to either component alone.
A lower-GWP refrigerant that manufacturers such as Lennox are using in new whole-home systems in place of R-410A. The two are not interchangeable.
Sources
Where related figures come from
Sources last checked
- SEER2.com - About SEER2 · retrieved 2026-07-25
- ACCA - Technical Manuals · retrieved 2026-07-25
- Lennox - New HVAC Refrigerant 2025 · retrieved 2026-07-25
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