Learning Center · Glossary
Every HVAC term we use in the On Time Learning Center, defined in plain English by Shane Herro and the On Time team. No jargon, no upsell, no brand spin.
Annual Fuel Utilization Efficiency — the percent of fuel a furnace converts into actual home heat over a full heating season. A 96% AFUE furnace converts 96% of the gas it burns into heat that reaches your living space; the remaining 4% leaves through the flue. The U.S. minimum for new gas furnaces is 95% as of 2026, so older 80% units are leaking real money every winter month in Wisconsin.
Seasonal Energy Efficiency Ratio, version 2. The cooling efficiency rating in use since 2023 (replacing the older SEER). Higher SEER2 = lower summer electric bills for the same cooling output. The current federal minimum for the north region is SEER2 14.3 (about SEER 15 under the old scale). 'High-efficiency' typically starts around SEER2 16–17.
Heating Seasonal Performance Factor, version 2 — heat pumps' heating-side efficiency rating, updated alongside SEER2. The higher the HSPF2, the more heat the pump pulls per kWh of electricity. Cold-climate heat pumps spec'd for Wisconsin typically run HSPF2 8.0 or higher.
British Thermal Unit (and BTUs per hour). The basic unit of HVAC capacity. A 60,000 BTUh furnace can put out 60,000 BTU of heat per hour at full fire; a 3-ton AC moves about 36,000 BTUh of heat out of the house. Bigger is not better — an oversized system short-cycles, dehumidifies poorly, and wears out early. Sizing comes from a Manual J load calculation, not a thumb rule.
The ACCA-published room-by-room heat-loss / heat-gain calculation used to correctly size an HVAC system for a specific house. Honest contractors do Manual J (or an equivalent room-by-room calc) before quoting a replacement system. We do.
An AC or heat-pump capacity term. 1 ton = 12,000 BTUh of heat moved. Typical SE Wisconsin homes run 2–4 tons of cooling. The right tonnage is determined by Manual J, not by what was in the house before.
A blower motor or compressor that ramps smoothly through many speeds instead of clicking on and off at full blast (single-stage) or two speeds (two-stage). Variable-speed equipment runs quieter, dehumidifies better, holds temperature steadier, and costs more up front.
Minimum Efficiency Reporting Value — the filtration rating of an HVAC air filter, from MERV 1 (catches lint) up to MERV 16 (hospital-grade). Most residential systems are happy at MERV 11–13. Going too high on the wrong system starves airflow and damages the blower — match the filter to the system, not to a marketing claim.
High-Efficiency Particulate Air — a filter standard that captures 99.97% of particles 0.3 microns and larger. True HEPA usually needs a bypass setup or a standalone air purifier, not a slot in a normal furnace filter cabinet.
Energy / Heat Recovery Ventilator. Brings fresh outside air into a tight house while recovering most of the conditioning of the stale exhaust air. In Wisconsin, ERVs/HRVs are how tight new-construction and well-sealed older homes get enough fresh air without dumping winter heat or summer cool to the outside.
The new class of mildly flammable, lower-global-warming refrigerants replacing R-410A as of 2025–2026 under the U.S. EPA AIM Act. Your new AC or heat pump will use an A2L refrigerant; old systems running R-410A keep running with R-410A until they need a major repair.
The exact amount of refrigerant in a sealed AC or heat-pump loop, measured by weight. Under- or over-charge kills efficiency, freezes coils, and shortens compressor life. A clean install (and a clean repair) verifies subcooling/superheat, not just pressure.
A piece of equipment that moves heat instead of generating it. In summer it pulls heat from the house and dumps it outside (like an AC); in winter it pulls heat from outside air (or the ground) and pumps it inside. Cold-climate heat pumps are now real options for SE Wisconsin homes, especially in dual-fuel configurations.
A heat pump paired with a gas furnace. The heat pump handles mild and shoulder-season heating efficiently; the furnace kicks in when outdoor temperatures fall below the heat pump's economic crossover point (often around 25–35 °F). Common, sensible, and often the lowest-operating-cost design in Waukesha County.
A heat pump with an outdoor condenser feeding one or more wall- or ceiling-mounted indoor heads connected by small refrigerant lines instead of ducts. Great for additions, bonus rooms over the garage, sunrooms, and houses without ducts.
When a furnace or AC fires up, runs briefly, shuts off, and repeats. Usually a sign of oversizing, a dirty filter, a bad thermostat location, or a deeper control problem. Hard on equipment, bad for comfort, and worth a diagnostic visit.
The metal box inside a furnace that separates combustion gas from the air you breathe. A cracked heat exchanger can leak carbon monoxide; this is one of the few HVAC problems we treat as a 'shut the system down today' issue.
The fresh outside air a gas furnace or boiler needs to burn its fuel safely. Tight modern homes often need a dedicated combustion-air or makeup-air provision; older homes usually get it through normal infiltration. Code rules apply.
The two refrigerant readings a competent technician uses to confirm an AC or heat pump has the right charge for the conditions. We always check both during a tune-up or after a repair.
The pressure the blower has to fight to push air through the ductwork and filter. Like blood pressure for an HVAC system: too high and the blower works too hard, airflow drops, the coil freezes, the system fails early. Measuring static pressure is part of an honest install commissioning.
A mechanical permit pulled with the local Authority Having Jurisdiction (AHJ) before installing, replacing, or significantly modifying HVAC equipment. The inspector verifies that the work meets the adopted Wisconsin Mechanical Code, that combustion venting is safe, and that gas/electrical/refrigerant connections meet code. We pull the permit; the inspector closes it out. This protects resale value, homeowner's insurance, and your manufacturer warranty.
Wisconsin's statewide energy-efficiency program. It offers rebates for qualifying high-efficiency furnaces, AC, heat pumps, and certain IAQ upgrades. We'll tell you which rebates currently apply to your project and how to claim them — never inflated, never invented.
Our membership program. $25/month + $25 activation, 6-month minimum. Includes two seasonal tune-ups, 20% off repairs, a $0 service fee at any hour (no overtime), priority service, $1,000 install credit, owner direct line, lifetime repair guarantee while enrolled, price protection, and an annual duct video check.
Missing a term? Email [email protected] and Shane will add it. This glossary is reviewed at least every 12 months and whenever a material code or refrigerant change goes into effect.