Glossary
HVAC & plumbing terms, in plain English
50 terms you’ll hear from a technician, on a quote, or in a rebate form — each with what it means and why it matters in a Lowcountry home.
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- Heating & Heat Pumps AFUE AFUE (Annual Fuel Utilization Efficiency) is the efficiency rating for gas and oil furnaces and boilers, expressed as the percentage of fuel energy converted into usable heat over a heating season. An 80% AFUE furnace turns 80 of every 100 units of fuel energy into heat, with the remainder lost mainly through exhaust venting. What it means for you →
- Money, Code & Permits AHJ (Authority Having Jurisdiction) The Authority Having Jurisdiction (AHJ) is the government office legally responsible for permitting and inspecting work at a specific address — usually a city, town, or county building department. It follows the legal boundary, not the mailing address or neighborhood name, and it decides which permits are required, what they cost, and whether completed work passes inspection. What it means for you →
- Air Quality & Ductwork Air Duct Cleaning Air duct cleaning is the removal of dust, debris, mold, or pest contamination from supply and return ducts, registers, and air handler components. Proper source-removal cleaning uses powerful negative-pressure vacuum equipment with mechanical agitation brushes. The EPA does not recommend routine duct cleaning; it is justified when a specific, verifiable problem exists inside ductwork. What it means for you →
- Plumbing & Water Heating Anode Rod An anode rod is a sacrificial metal rod, usually magnesium or aluminum, installed inside a tank water heater so that corrosive water attacks it instead of the steel tank. Once the rod is consumed, the tank lining begins corroding, which is why periodic anode inspection and replacement extends a tank water heater's service life. What it means for you →
- Air Quality & Ductwork Attic Ventilation Attic ventilation is the controlled flow of outdoor air through a vented attic, usually entering at low soffit intake vents and exiting through high ridge or roof exhaust vents. Balanced intake and exhaust remove built-up heat and moisture, protecting roof decking and insulation and reducing the heat load radiating into the rooms below. What it means for you →
- Heating & Heat Pumps Auxiliary (Emergency) Heat Strips Auxiliary heat strips are electric resistance heating elements installed in a heat pump's indoor air handler. They supplement the heat pump automatically during very cold weather, defrost cycles, or large thermostat increases. Emergency heat is the same strips run alone, manually selected when the heat pump itself is not working; resistance heat costs considerably more to run. What it means for you →
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- Cooling Condenser The condenser is the outdoor half of a split air conditioning system. It houses the compressor, a large coil, and a fan that together release the heat pulled from inside the house into outdoor air. Refrigerant arrives as a hot vapor, gives up its heat across the coil, and returns indoors as a liquid. What it means for you →
- Lowcountry Home & Climate Crawlspace Moisture Crawlspace moisture is excess humidity or standing water in the space beneath a raised floor, driven by damp soil, flooding, and humid outdoor air entering through vents. It causes condensation on cool ductwork and framing, mold, musty odors, and cupping floors. Fixes include drainage, a vapor barrier, and full encapsulation with dehumidification. What it means for you →
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- Heating & Heat Pumps Defrost Cycle A defrost cycle is the automatic routine a heat pump runs in heating mode to clear frost from its outdoor coil. The system briefly reverses into cooling mode, warming the outdoor coil to melt the ice, while auxiliary heat typically keeps indoor air from feeling cold. Cycles usually last a few minutes and repeat as conditions require. What it means for you →
- Heating & Heat Pumps Dual-Fuel System A dual-fuel system pairs an electric heat pump with a gas furnace in one ducted system. The heat pump handles cooling and most heating, and a controller switches to the gas furnace when outdoor temperatures drop below a set balance point, using whichever fuel is more efficient or economical for the conditions at the time. What it means for you →
- Air Quality & Ductwork Duct Leakage & Duct Sealing Duct leakage is conditioned or return air escaping through gaps, loose joints, and torn connections in ductwork. Duct sealing closes those openings with mastic, approved foil tape, or aerosol sealant, and is verified with a duct leakage test that pressurizes the system and measures the airflow lost. Sealing improves delivered airflow, efficiency, and humidity control. What it means for you →
- Ductless & Geothermal Ductless Mini-Split A ductless mini-split is a heat pump or air conditioner that connects an outdoor compressor to one or more wall, ceiling, or floor-mounted indoor air handlers through a small refrigerant line set, with no ductwork. Each indoor head has its own thermostat, so every room or zone is controlled independently, typically by an inverter-driven, variable-speed compressor. What it means for you →
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- Money, Code & Permits Federal 25C Tax Credit (Ended) The federal 25C Energy Efficient Home Improvement Credit was a tax credit covering 30% of the cost of qualifying heat pumps, air conditioners, furnaces, and heat pump water heaters, up to annual caps. Federal legislation signed in July 2025 terminated it, and it does not apply to equipment placed in service after December 31, 2025. What it means for you →
- Lowcountry Home & Climate Flood Zone & Equipment Elevation A flood zone is a FEMA-mapped area of flood risk, such as an AE zone with a set base flood elevation, a VE coastal high-hazard zone with wave action, or lower-risk Zone X. Equipment elevation means mounting HVAC equipment on platforms or stands at or above the elevation that local floodplain rules require. What it means for you →
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- Plumbing & Water Heating Hard Water & Water Softener Hard water carries dissolved calcium and magnesium that leave scale on fixtures, glassware, heating elements, and inside water heaters. A water softener removes those minerals through ion exchange, swapping them for sodium or potassium as water passes through a resin bed. Hardness is measured in grains per gallon, and testing determines whether softening is needed. What it means for you →
- Heating & Heat Pumps Heat Exchanger A heat exchanger is the sealed metal chamber in a gas furnace that transfers heat from burning fuel to the household air blown across it, while keeping combustion gases separate and directing them out the vent. Condensing furnaces add a secondary heat exchanger that captures additional heat from exhaust. A cracked heat exchanger can leak carbon monoxide. What it means for you →
- Heating & Heat Pumps Heat Pump A heat pump is an electric heating and cooling system that moves heat rather than generating it. In summer it works like an air conditioner, carrying indoor heat outside; in winter a reversing valve flips the refrigerant flow to pull heat from outdoor air and release it indoors, delivering more heat energy than the electricity it uses. What it means for you →
- Plumbing & Water Heating Heat Pump Water Heater A heat pump water heater is an electric tank water heater that uses a small refrigerant heat pump to pull heat from surrounding air into the stored water, rather than generating heat with resistance elements alone. It uses considerably less electricity than a standard electric tank, cools and dries the room it occupies, and requires a condensate drain. What it means for you →
- Lowcountry Home & Climate Historic District Review Historic district review is the approval process the City of Charleston's Board of Architectural Review (BAR) applies to exterior work visible from the public right-of-way in its historic districts. For mechanical equipment, the BAR's policy is to minimize visibility: rear-of-property placement where possible, screening for anything still visible, and avoiding penetrations of historic masonry. What it means for you →
- Heating & Heat Pumps HSPF2 HSPF2 (Heating Seasonal Performance Factor 2) is the U.S. Department of Energy rating for heat pump heating efficiency, in effect since January 2023. It divides total seasonal heating output in BTUs by the watt-hours of electricity used, including supplemental heat. New split-system heat pumps must rate at least 7.5 HSPF2; higher numbers mean cheaper heating. What it means for you →
- Lowcountry Home & Climate Hurricane Prep (HVAC & Plumbing) Hurricane prep for HVAC and plumbing is the set of steps that protect mechanical systems from wind-blown debris, power surges, and flooding during the Atlantic hurricane season, June 1 through November 30. It includes pre-cooling the house, shutting equipment off at the disconnect, clearing loose objects, surge protection, and inspecting before restarting. What it means for you →
- Money, Code & Permits HVAC Financing HVAC financing is a loan that spreads the cost of new heating, cooling, or water heating equipment into monthly payments instead of one upfront payment. Financing is typically arranged through a third-party lender at the time of the quote, subject to credit approval, with the lender setting the actual rate, term, and monthly payment for each borrower. What it means for you →
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- Money, Code & Permits Maintenance Plan A maintenance plan is a paid membership in which an HVAC contractor performs scheduled tune-ups on your heating and cooling system, usually in exchange for a monthly or annual fee, and typically adds member benefits such as repair discounts, reduced service fees, and priority scheduling. Its purpose is preserving efficiency and catching failing parts before breakdowns. What it means for you →
- Air Quality & Ductwork Manual J Load Calculation Manual J is the Air Conditioning Contractors of America (ACCA) method for calculating a home's heating and cooling loads room by room. It accounts for insulation, windows, orientation, air leakage, duct location, occupants, and local design temperatures, producing sensible and latent loads that guide equipment selection under Manual S and duct design under Manual D. What it means for you →
- Money, Code & Permits Mechanical Permit A mechanical permit is official permission from a local building department to install, replace, or significantly alter heating, cooling, gas, or duct systems. In South Carolina, system replacements and new installations generally require one, while minor repairs generally do not. The licensed contractor pulls it, and a final inspection confirms the work meets code. What it means for you →
- Air Quality & Ductwork MERV Rating MERV, or Minimum Efficiency Reporting Value, is a 1-to-16 scale from ASHRAE Standard 52.2 that rates how well an air filter captures particles between 0.3 and 10 microns. Higher numbers capture smaller particles such as fine dust, smoke, and bacteria, but denser media also adds airflow resistance the system's blower must overcome. What it means for you →
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- Cooling R-454B (A2L Refrigerant) R-454B is a lower-global-warming-potential refrigerant replacing R-410A in new residential air conditioners and heat pumps under the EPA's HFC phase-down. It is classified A2L, meaning low toxicity and mild flammability, so equipment and installations follow updated safety standards. R-32 is the other common A2L refrigerant in new residential equipment. What it means for you →
- Cooling Refrigerant Leak A refrigerant leak is a loss of refrigerant from the sealed circuit of an air conditioner or heat pump, usually through corroded coils, worn joints, vibration-cracked tubing, or faulty valves. Refrigerant is not consumed in normal operation, so a low charge always means a leak, which reduces cooling capacity and efficiency and can freeze the evaporator coil. What it means for you →
- Air Quality & Ductwork Relative Humidity & Dew Point Relative humidity is the amount of water vapor in air expressed as a percentage of the maximum that air can hold at its current temperature. Dew point is the temperature at which that air becomes saturated and moisture condenses. Dew point measures absolute moisture, while relative humidity changes whenever the air warms or cools. What it means for you →
- Money, Code & Permits Repair vs. Replace Repair vs. replace is the decision whether to fix a failing HVAC system or install a new one. It weighs system age, the size and frequency of repairs, refrigerant type, efficiency, and comfort. Repair usually suits a younger system with a first, well-understood failure; replacement deserves consideration for aging equipment needing a major component or repeated repairs. What it means for you →
- Plumbing & Water Heating Repiping Repiping is the replacement of a home's water supply lines, either whole-house or by section, typically when original piping is failing or problem-prone. Common triggers include aging galvanized steel that corrodes and restricts flow internally, gray polybutylene supply lines known for leaks, repeated pinhole leaks, discolored water, and persistently low pressure. Replacement usually uses PEX or copper. What it means for you →
- Heating & Heat Pumps Reversing Valve A reversing valve is the electrically controlled valve in a heat pump that switches the direction of refrigerant flow, letting the same system either cool or heat. In cooling mode the outdoor coil releases heat; when the valve shifts, the indoor coil releases heat instead. The valve also reverses briefly during defrost cycles. What it means for you →
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- Lowcountry Home & Climate Salt-Air Corrosion & Coil Coatings Salt-air corrosion is the deterioration of outdoor HVAC and plumbing equipment caused by airborne salt that settles on metal, absorbs humidity, and forms a corrosive film. It destroys aluminum condenser coil fins first, then cabinets, fasteners, and electrical terminals. Factory or professionally applied coil coatings add a protective barrier that resists this attack. What it means for you →
- Money, Code & Permits SC Mechanical Code The SC mechanical code is the set of International Code Council rules, including the International Mechanical Code, that South Carolina's Building Codes Council adopts statewide for heating, cooling, ventilation, and ductwork. Local building departments enforce the currently adopted editions through permits and inspections, covering clearances, electrical disconnects, condensate drainage, refrigerant lines, and combustion venting. What it means for you →
- Plumbing & Water Heating Sediment Flush A sediment flush is routine water heater maintenance that drains water from the tank's bottom drain valve to remove settled minerals and debris. Sediment insulates the burner or heating element from the water, lengthening heating cycles, raising energy use, causing rumbling noises, and accelerating tank wear, so annual flushing helps a tank last longer. What it means for you →
- Cooling SEER2 SEER2 (Seasonal Energy Efficiency Ratio 2) is the U.S. Department of Energy rating for air conditioner and heat pump cooling efficiency, in effect since January 2023. It divides total seasonal cooling output in BTUs by the watt-hours of electricity used, measured under tougher test conditions than the older SEER scale. Higher SEER2 means less electricity per unit of cooling. What it means for you →
- Heating & Heat Pumps Short-Cycling Short-cycling is when an air conditioner, heat pump, or furnace turns on and off in rapid, brief cycles instead of running full cycles. Common causes include oversized equipment, a poorly located thermostat, clogged filters, low refrigerant, and overheating or safety controls tripping. It wastes energy, wears out components faster, and leaves temperatures uneven. What it means for you →
- Plumbing & Water Heating Slab Leak A slab leak is a water supply or drain line leak beneath a home's concrete foundation slab. Hot-side slab leaks often show up as warm spots on the floor; cold-side leaks as damp spots, lifting flooring, or cracks. Other signs include an unexplained jump in the water bill and a water meter moving with every fixture off. What it means for you →
- Air Quality & Ductwork Static Pressure Static pressure is the resistance to airflow that a duct system, filter, and coil place on an HVAC blower, measured in inches of water column. Technicians read total external static pressure with a manometer and compare it to the equipment's rated limit. High static pressure means restricted airflow, lower capacity, noisy registers, and extra wear on the blower motor. What it means for you →
- Plumbing & Water Heating Sump Pump A sump pump is an electric pump set in a pit, called a sump basin, at the lowest point of a crawlspace or basement. When groundwater or storm water fills the pit, a float switch starts the pump, which discharges the water through a pipe and check valve to a safe drainage point away from the foundation. What it means for you →
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- Plumbing & Water Heating Tankless Water Heater A tankless water heater heats water only when a hot tap opens, passing it through a gas burner or electric heat exchanger instead of storing a heated tank. It delivers continuous hot water within its flow-rate limit, mounts on a wall, and needs periodic descaling, especially where water is hard or carries sediment. What it means for you →
- Cooling Tonnage (Cooling Capacity) Tonnage is the cooling capacity of an air conditioner or heat pump. One ton equals 12,000 BTUs of heat removed per hour, the cooling effect of melting one ton of ice over a day. Residential systems typically range from about 1.5 to 5 tons, and correct tonnage comes from a room-by-room load calculation rather than square footage alone. What it means for you →
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