Reference

Ductless mini-split glossary

Plain-English definitions for the terms that show up in mini-split quotes, spec sheets, and install scopes.

33 entries with cross-references and entity links.

Reference glossary for ductless mini-split and air-source heat-pump terminology — sizing terms like Manual J and load calculation, efficiency ratings like SEER2 and HSPF2, cold-climate and refrigeration vocabulary, line set and condensate, plus New Hampshire regulatory terms, with links to authoritative sources where applicable. Useful when comparing mini-split quotes that use unfamiliar technical language, or when reading a spec sheet before you sign.

Air-source heat pump: A heating and cooling device that moves heat between the outdoor air and a building using a refrigeration cycle, rather than generating heat by burning fuel. In heating mode it extracts heat from outdoor air even when it is cold; in cooling mode it reverses to remove heat from inside. A ductless mini-split is one form of air-source heat pump.

Backup / supplemental heat: A secondary heat source — an existing furnace or boiler, or electric resistance strip heat — that covers the home on the coldest nights when temperatures fall below the heat pump’s effective range, and briefly during defrost. Planning the backup in is part of an honest cold-climate install, not an admission the heat pump failed.
BTU / ton: A British Thermal Unit is the standard unit of heating and cooling capacity; a "ton" of cooling equals 12,000 BTU per hour. Mini-split heads and condensers are rated in BTU per hour. Capacity should be matched to the Manual-J load, not chosen by a rule of thumb.

Cold-climate heat pump: A heat pump, often marketed as hyper-heat or low-ambient, engineered to keep delivering useful — often rated — heating capacity at low outdoor temperatures, typically down to roughly -5°F to -15°F depending on the model. In a New Hampshire winter this rating, not the brand name, determines whether the unit performs.
Capacity derating: The reduction in a heat pump’s heating output as the outdoor temperature falls. A unit rated at a nameplate BTU at 47°F produces less at 5°F. Honest cold-climate sizing checks capacity at the cold design temperature rather than at a mild rating point, so the system still meets the load on the coldest days.
Condensate: The water that forms on the indoor coil when the system removes humidity in cooling mode, and on the outdoor unit during heating-mode defrost. Indoor condensate must drain to a proper location by gravity or a condensate pump. Poor condensate drainage is a frequent callback — drips, stains, or a backed-up line.
Condensate pump: A small pump used to move condensate from an indoor head to a drain when gravity drainage is not possible, such as a low-wall or basement install. It is a common and reliable part of many mini-split installs, but it adds a component that needs to be installed and maintained correctly.
Coefficient of performance (COP): The ratio of heat energy delivered to electrical energy consumed at a given operating condition. A COP above 1 means the heat pump moves more heat than the electricity it uses — for example a COP of 3 delivers three units of heat per unit of electricity. COP falls as the outdoor temperature drops.
Ceiling cassette: A type of indoor head recessed into a ceiling, distributing air through multiple sides rather than from a wall-mounted unit. Cassettes suit rooms where a wall unit is awkward or where a more discreet, central-feeling distribution is wanted. They still connect to the same outdoor condenser by a line set.
Commissioning: The final step of an install in which the system is started up and verified: the line set is evacuated and the charge confirmed, condensate drainage is tested, and operation is checked in both heating and cooling. Proper commissioning is what separates a system that performs as designed from one that quietly underperforms.

Ductless mini-split: An air-source heat pump system made of an outdoor condenser connected by an insulated refrigerant line set to one or more indoor heads, delivering heating and cooling to a space without ductwork. "Mini-split" refers to the split between the outdoor and indoor units; "ductless" distinguishes it from central ducted systems.
Design temperature: The outdoor temperature a heating or cooling system is sized to handle, usually a statistically cold (or hot) value for the location rather than the all-time extreme. Sizing at the winter design temperature ensures the system meets the load on nearly all cold days; the few colder nights are covered by backup heat.
Defrost cycle: A periodic operation in which the heat pump briefly reverses to melt frost off the outdoor coil that builds up in cold, humid conditions. The visible steam from the outdoor unit during defrost is normal operation. Without defrost, frost would block airflow and cut heating capacity.

Evacuation / vacuum: Pulling a deep vacuum on the refrigerant lines before charging, to remove air and moisture that would otherwise degrade performance and form acids inside the system. A proper evacuation is verified with a micron gauge. Skipping or rushing the vacuum is one of the most common hidden install defects.
EPA Section 608: The federal certification required under the Clean Air Act to handle refrigerant. It is illegal to knowingly vent refrigerant, and charging or recovering refrigerant on a mini-split is 608 work. A 608-certified installer is the baseline for any install or service that touches the refrigerant charge.

Heat pump: A device that transfers heat from one place to another using a vapor-compression refrigeration cycle. Because it moves heat instead of creating it, a heat pump can deliver more heat energy than the electrical energy it consumes, which is why its heating efficiency can exceed 100 percent.
HSPF2 (heating efficiency): The Heating Seasonal Performance Factor 2, the current US metric for heating-season efficiency of a heat pump, replacing the older HSPF under updated test procedures. A higher HSPF2 means more heating output per unit of electricity over a season. For a New Hampshire home, heating-season efficiency is the number to weigh most.

Indoor head: The indoor unit of a mini-split that delivers conditioned air to a room. Common types are wall-mounted, ceiling cassette, and low-wall or floor-mounted console units. Floor consoles are often favored in cold climates because warm air starts low. Head type and placement are chosen for how the room is used.
Inverter compressor: A variable-speed compressor that ramps its output up and down to match the load, instead of cycling fully on and off like a single-stage compressor. Inverter operation is the main reason mini-splits hold steady temperatures, run efficiently at part load, and can sustain heating capacity in cold weather.

Load calculation: The engineering process of determining a building’s heating and cooling loads in BTU per hour at the design conditions. The load calc is what sizing should be based on; getting it right in both directions prevents an undersized system that cannot keep up and an oversized one that short-cycles.
Line set: The pair of insulated copper refrigerant lines connecting the outdoor condenser to each indoor head. Runs have manufacturer length and vertical-lift limits, and the connection must be properly evacuated and leak-checked before the system is charged. A too-long, poorly insulated, or poorly commissioned line set robs capacity and efficiency.

Manual J: The ACCA residential load calculation standard used to determine how much heating and cooling capacity a home actually needs. A Manual-J calc accounts for square footage, insulation, windows, orientation, and air leakage to produce a load in BTU per hour, and is the correct basis for sizing a mini-split — not a rule-of-thumb BTU-per-square-foot estimate.

NHSaves: The utility-funded energy-efficiency program in New Hampshire, run jointly by Eversource, Liberty, Unitil, and NH Electric Co-op, that offers rebates and incentives for qualifying high-efficiency equipment including heat pumps and ductless mini-splits. The specific rebate depends on the equipment and program terms in effect.
New Hampshire contractor licensing: New Hampshire has no statewide general contractor license. HVAC and refrigerant work is governed instead by federal EPA Section 608 certification and by local building codes — in Keene, enforced by the City. A proper installer is EPA 608-certified and pulls the permits the work requires.

Outdoor condenser / compressor: The outdoor unit of a mini-split, containing the compressor and the outdoor coil. It releases or absorbs heat to or from the outside air through the refrigeration cycle, and connects to the indoor heads via the line set. On multi-zone systems one outdoor condenser serves several indoor heads.
Oversizing: Installing more heating or cooling capacity than the load requires. Counterintuitively, oversizing is a defect rather than a safety margin: an oversized mini-split short-cycles, dehumidifies poorly, and runs less efficiently. Correct sizing matters in both directions, which is why the load calc comes first.

Reversing valve: The component that switches the direction of refrigerant flow so a heat pump can both heat and cool, and so it can run a defrost cycle. Reversing the cycle moves the system between extracting heat from outside (heating) and rejecting heat outside (cooling).
Refrigerant: The working fluid that absorbs and releases heat as it changes between liquid and gas through the refrigeration cycle. Handling refrigerant requires EPA Section 608 certification, and it is illegal to knowingly vent it. Common modern mini-split refrigerants include R-410A and lower-GWP alternatives such as R-32.
Refrigerant charge: The amount of refrigerant in the system, which must match the manufacturer’s specification for the equipment and the line-set length. An undercharged or overcharged system loses capacity and efficiency and can damage the compressor over time. Verifying the charge is part of both a proper install and routine service, and is EPA 608 work.

Single-zone vs multi-zone: Single-zone is one outdoor condenser connected to a single indoor head, used for one room, an addition, or one problem space. Multi-zone is one outdoor condenser feeding several indoor heads, each on its own thermostat, used for whole-house or multi-room comfort. The choice follows the floor plan and the load, not the lowest sticker price.
SEER2 (cooling efficiency): The Seasonal Energy Efficiency Ratio 2, the current US metric for cooling-season efficiency, replacing the older SEER under updated test procedures. A higher SEER2 means more cooling output per unit of electricity over a season. It is one of the two ratings that actually matter when comparing units.
Short-cycling: When a system turns on and off too frequently because it is oversized for the load. Short-cycling wastes energy, controls humidity poorly, wears components, and leaves the space less comfortable. It is the classic symptom of a mini-split sized off square footage instead of a load calc.

Vapor-compression refrigeration: The thermodynamic cycle that mini-splits and heat pumps use to move heat: a refrigerant is compressed, condensed, expanded, and evaporated in a loop, absorbing heat at the evaporator and releasing it at the condenser. Reversing the cycle is what lets one unit both heat and cool.

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