HVAC glossary

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.

How to choose

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.

Ask which refrigerant the system uses

R-410A and newer low-GWP refrigerants are not interchangeable, per Lennox. It affects future service.

Static pressure is a diagnostic, not jargon

If a contractor measured it, they investigated your ducts. If nobody mentions it, nobody looked.