HVAC glossary: every term on your invoice, plain-English

R-22 (Freon)

The old-school refrigerant used in residential AC until roughly 2010. Ozone-depleting, phased out of new equipment and now expensive-to-impossible to source for repairs. If a system on your property still uses R-22, replacement is the honest answer.

R-410A (Puron)

The refrigerant used in nearly all residential ACs installed between 2010 and 2024. Non-ozone-depleting but with a high global-warming potential. Phased out for new equipment starting January 2025.

R-454B (Opteon XL41 / Puron Advance)

The current-generation refrigerant for new residential AC and heat pumps as of 2025. An A2L (mildly flammable) refrigerant with ~78% lower GWP than R-410A. Full context in <a href="/learning-center/heating-cooling/r-454b-vs-r-410a">R-454B vs. R-410A</a>.

A2L

ASHRAE safety classification for refrigerants that are mildly flammable but have low toxicity. R-454B is A2L. New code requires specific leak-detection sensors and shutoff hardware when using A2L refrigerants — part of why 2025+ installs cost slightly more than 2024 installs.

GWP (Global Warming Potential)

A number expressing how much heat a refrigerant traps compared to CO2 (which is 1). R-410A's GWP is ~2,088; R-454B's is ~466. Lower GWP is why the industry moved.

Refrigerant leak

Any loss of refrigerant from the sealed system. Because the system is designed to be sealed, "topping off" refrigerant without finding and fixing the leak is a repair tell — the leak will come back, and the released refrigerant is an EPA violation.

Subcooling

The temperature difference between the refrigerant leaving the condenser and its saturation temperature at that pressure. A key measurement during commissioning to verify the system is correctly charged.

Superheat

The temperature difference between the refrigerant leaving the evaporator and its saturation temperature. Also used to verify correct charge; different diagnostic significance than subcooling.

Evacuation

Pulling a deep vacuum on the refrigerant lines before charging, to remove air and moisture. Cutting this step short is the single most common cause of premature compressor failure on otherwise good equipment.

EPA 608

Federal certification required to handle refrigerants. Any technician purchasing or reclaiming refrigerant must hold it. If a bid doesn't mention EPA 608 reclamation of the old refrigerant during a replacement, ask.

SEER

Seasonal Energy Efficiency Ratio — the old (pre-2023) measure of AC efficiency. Higher = more efficient. Replaced by SEER2, so don't compare a SEER number on a 2019 unit to a SEER2 number on a 2026 unit directly.

SEER2

The 2023+ replacement for SEER. Same idea, harder test conditions, so numbers look ~4-5% lower for the same physical equipment. Deep dive in <a href="/learning-center/equipment-brands/what-is-seer2">what is SEER2</a>.

EER / EER2

Energy Efficiency Ratio at a single design condition (95°F outdoor). Better proxy than SEER for peak-summer efficiency in hot climates like ours. EER2 replaced EER in the 2023 test protocol.

HSPF

Heating Seasonal Performance Factor — the heating equivalent of SEER for heat pumps. Higher = more heat delivered per unit of electricity.

HSPF2

The 2023+ replacement for HSPF, aligned with SEER2's harder test conditions. Federal 25C credit for heat pumps typically requires HSPF2 ≥ 7.8 in our climate.

AFUE

Annual Fuel Utilization Efficiency — the percentage of the energy in the natural gas that ends up as usable heat in the house. 80% AFUE = 80% of the gas becomes heat, 20% goes up the flue. 96% AFUE = 96% becomes heat. Detailed comparison in our <a href="/learning-center/cost-guides/furnace-replacement-cost">furnace replacement cost</a> guide.

COP (Coefficient of Performance)

A dimensionless ratio of heat energy output to electrical energy input. A heat pump with a COP of 3 delivers three units of heat for every one unit of electricity — the reason heat pumps aren't magic, just efficient.

Tonnage

AC capacity measurement — 1 ton = 12,000 BTU/hr of cooling. A typical Van Buren home runs 2–4 tons. Right-sizing this number is where most comfort problems are born or solved (see <a href="/learning-center/cost-guides/ac-replacement-cost-arkansas">AC replacement cost</a>).

BTU (British Thermal Unit)

The base unit of heating and cooling capacity. 12,000 BTU/hr = one ton of AC. Furnaces are rated in BTU/hr input (the gas burn rate). A typical Van Buren home furnace is 40,000–80,000 BTU/hr.

CFM (Cubic Feet per Minute)

The volume of air the blower moves per minute. A typical residential AC delivers 350–400 CFM per ton of cooling — measured at commissioning, not assumed.

Static pressure

The resistance the blower has to push against, measured in inches of water column. High static = restricted ducts, dirty filter, or oversized coil for the blower. Chronic high static is a killer of blower motors.

MERV

Minimum Efficiency Reporting Value — how much a filter catches, on a 1–20 scale. Higher = catches more, restricts more airflow. Full context in <a href="/learning-center/air-quality/air-filters-and-merv-ratings">air filters and MERV ratings</a>.

ECM motor

Electronically Commutated Motor — a variable-speed DC blower motor that ramps smoothly, holds low speeds for continuous circulation, and uses less electricity than a PSC. The blower behind almost every high-tier furnace or air handler sold today.

PSC motor

Permanent Split Capacitor — the older single-speed AC blower motor. Simple and reliable, but on-or-off at 100% — no low-speed continuous circulation and no comfort staging benefit.

Blower door

A diagnostic tool that pressurizes or depressurizes a home to measure how leaky the envelope is. Reported in ACH50 or CFM50. Occasionally part of a Manual J load calc; more common in new construction.

Return

The ducts and grilles that pull warm/humid room air <em>back</em> to the air handler for reconditioning. Undersized returns are the most common home-airflow problem in the River Valley.

Supply

The ducts and registers that deliver conditioned air <em>out</em> to each room.

Damper

An adjustable plate inside a duct that restricts or opens airflow to balance one branch against another. Zone-controlled systems use motorized dampers driven by their thermostats.

Manual D

The ACCA duct-design methodology. When someone says "the ducts were sized to Manual D," they mean the duct diameters and lengths were calculated for the airflow the equipment actually moves — not eyeballed.

Manual J

The ACCA residential load-calculation methodology. The right way to determine what size AC, heat pump, or furnace a specific home needs. Any bid that skips Manual J is guessing at tonnage.

Dew point

The temperature at which water vapor in the air condenses to liquid. Van Buren summer dew points sit near 71°F — the single most useful number for understanding indoor humidity. See <a href="/learning-center/air-quality/reduce-home-humidity">reduce home humidity</a>.

RH (Relative Humidity)

How much moisture the air is holding compared to what it could hold at that temperature. 45–55% RH is the summer comfort target for River Valley homes.

ACH (Air Changes per Hour)

How many times the total volume of air in a home is replaced with outdoor air per hour, from either intentional ventilation or leakage.

Sensible vs. latent load

Sensible = heat that changes air temperature (what a thermometer reads). Latent = heat carried by water vapor (what a dew-point meter reads). An AC handles both. In humid Van Buren summers, latent load is often a bigger comfort story than sensible.

Temperature drop

The difference between the air temperature entering the return and the air temperature leaving the supply, measured at the air handler. A healthy summer temperature drop is 17–22°F. Higher usually means low airflow; lower usually means low charge or high humidity.

Balance point

For a heat pump, the outdoor temperature below which supplemental heat (electric strips or a gas furnace) is needed to maintain the setpoint. In Van Buren, cold-climate heat pumps typically balance below 20°F — meaning the strips rarely engage.

Defrost cycle

In heating mode, a heat pump briefly reverses to melt frost off the outdoor coil. Normal, brief, and expected — a small amount of icing at the base of the unit in cold weather is not a problem. Persistent thick ice is.

Cold-climate rating

A manufacturer designation (often tied to ENERGY STAR Cold Climate Heat Pump certification) confirming the heat pump maintains capacity down to 5°F or below. Not required in our 3A climate — a standard heat pump handles Van Buren winters — but a cold-climate model has more headroom.

IECC climate zone

The International Energy Conservation Code climate zoning that determines minimum efficiency requirements. Van Buren is in zone 3A: hot-humid.

Air handler

The indoor unit that houses the blower and evaporator coil, in electric-heat and heat-pump systems. In gas-furnace systems the furnace assembly serves the same role.

Condenser

The outdoor unit on a split-system AC or heat pump. Houses the compressor, condenser coil, and outdoor fan.

Evaporator

The indoor coil that absorbs heat from house air. Sits in the plenum above (or beside) the furnace/air handler.

Compressor

The heart of the outdoor unit — pumps refrigerant around the loop. Single-stage compressors run at one speed; two-stage have high and low modes; variable-speed (inverter) compressors ramp smoothly across a wide range.

TXV (Thermostatic Expansion Valve)

A metering device that regulates refrigerant flow into the evaporator based on the load. Better than the older fixed-orifice metering — most modern systems have a TXV.

Contactor

An electrical relay that turns the outdoor unit on and off in response to the thermostat. A common wear-out part — replacement is inexpensive but not "trivial DIY."

Capacitor

A component that provides the starting boost to compressor and fan motors. The single most common cause of "AC won't start" summer service calls in the River Valley.

Condensate pan

The tray under the indoor coil that catches the water condensed out of the air. Drains via the condensate line — clogs here cause water damage.

Safety switch

A float switch on the condensate pan (or in the drain line) that shuts the AC off if water backs up. On homes with attic air handlers this switch is the single most important piece of $10 hardware in the whole system.

Heat exchanger

In a gas furnace, the metal chamber where combustion gases transfer heat to the house air without the two streams mixing. A cracked heat exchanger is a red-tag safety issue — CO can leak into the home.

Flue

The exhaust path for combustion gases from a gas furnace or water heater — metal on 80% AFUE furnaces, PVC on 96% AFUE.

Burner

The component in a gas furnace that mixes gas and air and combusts it — a series of individual burners across the base of the heat exchanger.

Ignitor

Modern furnaces use hot-surface or spark ignitors instead of standing pilot lights. A common furnace repair — usually inexpensive if diagnosed promptly.

Flame sensor

A metal rod inside the burner flame that confirms the flame is present. Dirty flame sensors cause intermittent no-heat calls; a five-minute cleaning fixes it.

25C credit

The Federal Energy Efficient Home Improvement Credit — 30% of qualifying HVAC equipment cost, capped by category. Details in <a href="/learning-center/cost-guides/swepco-rebates-and-federal-25c-credit">SWEPCO & Federal 25C credit</a>.

IRA (Inflation Reduction Act)

The 2022 federal legislation that expanded the 25C credit and created the (income-based) HEEHRA rebate program. The 25C credit is what most Van Buren homeowners use.

AHRI certificate

A document from the Air-Conditioning, Heating, and Refrigeration Institute certifying the performance of a specific matched indoor + outdoor combination. The IRS-facing evidence for the 25C credit.

ENERGY STAR

A federal label for equipment meeting certain efficiency thresholds. "ENERGY STAR" alone doesn't guarantee 25C eligibility — the CEE tier does — but nearly all 25C-qualifying equipment is also ENERGY STAR.

CEE tier

The Consortium for Energy Efficiency's tiering system, used by the IRS to define which equipment qualifies for 25C. Higher tier = more stringent efficiency threshold.

Load calculation

The BTU-per-hour calculation for a specific home under design conditions. Manual J (see above) is the residential methodology. Skipping this step is how homes end up with oversized equipment.

Manufacturer warranty

Coverage on equipment components — typically 10 years parts, 20+ years or lifetime on furnace heat exchanger, provided the equipment is registered on time.

Contractor labor warranty

Coverage on the labor to install or repair equipment — varies by installer. A strong parts warranty is only useful if the labor to install those parts is also covered. Full picture in <a href="/learning-center/equipment-brands/hvac-warranty-explained">HVAC warranty explained</a>.

Gas valve

The electromechanical valve that admits gas to the burner when the thermostat calls for heat. Modern gas valves are two- or three-stage (matching two-stage or modulating furnaces).

High-limit switch

A safety switch that shuts the furnace off if the heat exchanger overheats — usually a sign of low airflow (dirty filter, restricted return).

Thermocouple

The old-style flame-sensing device on standing-pilot furnaces. Modern electronic-ignition furnaces don't use one — they use a flame sensor instead.

Primary heat exchanger

The main combustion chamber in a furnace. Standard on both 80% and 96% units.

Secondary heat exchanger

An additional condensing heat exchanger unique to 96%+ AFUE furnaces — extracts additional heat from the exhaust by condensing water vapor out of it. The reason 96% furnaces vent through PVC.

Condensate line (condensing furnaces)

The PVC drain line that carries acidic condensed water away from a 96% furnace. Needs a neutralizer or an appropriate drain path — plumbing code doesn't allow raw acidic condensate to enter a cast-iron drain.