Updated July 2026 · by Murugan Vellaichamy
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A unit of cooling rate, not weight: one ton = 12,000 BTU/hr (≈3.517 kW), historically the cooling from melting a ton of ice over a day. See the converter.
How many times a space’s full air volume leaks in and out per hour through the building envelope; higher ACH means a bigger infiltration load.
A load calculation treating the whole building as one zone. It sizes the equipment but says nothing about how much air each room needs.
A British Thermal Unit is the heat needed to raise one pound of water by 1°F. Cooling capacity is quoted in BTU per hour.
The physical boundary between conditioned and unconditioned space: roof, exterior walls, windows, floor. Cooling load tracks envelope area, which is why BTU per square foot falls as buildings get larger.
An adjustment for rooms taller than the standard assumed in a sizing chart, since load scales with volume rather than floor area. Indian rooms commonly need it because ceilings run higher than the US norm.
A running total of how far and how long outdoor temperature sits above a base temperature over a season. Useful for estimating annual running cost, not for sizing.
The temperature drop across the indoor coil between return and supply air; about 8–11 °C (15–20 °F) is normal.
The outdoor temperature a system is sized to handle, taken from a percentile of local weather records rather than the record extreme. Sizing to the record high is the most common cause of oversizing.
The assumption that not every room peaks at the same moment, applied when sizing a single system for many rooms. Without it, a multi-room system is sized on the sum of loads that never coincide.
Cooling lost from ducts running through unconditioned space such as an attic. It can add 10-30% to the load a system must deliver.
The indoor temperature and humidity the load calculation targets, conventionally 75°F/50% RH in the US and 24-25°C in India. Lower targets raise the load sharply.
Unconditioned outside air leaking in through gaps in the envelope. On a leaky building it can account for a quarter of the cooling load.
Heat produced inside the space by people, lighting, appliances and equipment. In a server room it dominates the load entirely.
The share of a system's output that removes moisture rather than lowering temperature. An oversized unit satisfies the thermostat before delivering its latent capacity, which is why the room ends up cold and clammy.
The energy needed to remove moisture from the air (dehumidification), as opposed to lowering temperature. See humidity & comfort.
The sensible and latent heat given off by people, roughly 250 BTU/hr sensible per seated adult. Load calculations typically assume two occupants and add an allowance for each additional person.
Installed capacity divided by calculated load. Above roughly 1.25 the system starts short-cycling and dehumidifying poorly.
The fraction of rated capacity a system is actually delivering at a given moment. Single-stage equipment cannot run below 100%, so it cycles instead.
The highest cooling load the space reaches, usually mid-afternoon on the design day. Equipment is sized to this, which is why it runs at part load almost all the time.
R-value measures insulation’s resistance to heat flow (higher is better); U-factor is its reciprocal for a whole assembly like a window (lower is better).
A load calculation done per room, needed to size ducts and registers. Manual J produces both this and the block load.
The energy needed to change air temperature, as opposed to latent (moisture) load.
How much solar heat a shading device blocks relative to unshaded glass. External shading beats internal blinds by a wide margin because it stops the heat before it enters.
The fraction of solar heat a window lets through, from 0 to 1; lower blocks more sun.
The share of a system’s cooling that goes to lowering temperature versus removing moisture (sensible ÷ total).
Heat entering through glazing as sunlight. Orientation matters more than area: a west-facing window can contribute several times the gain of a north-facing one of the same size.
A rule of thumb dividing floor area by capacity, commonly quoted as 400-600 sq ft per ton. It ignores climate, insulation, glazing and occupancy, so it is a sanity check rather than a sizing method.
Heat bypassing insulation through a conductive path such as a steel stud or concrete slab edge. It makes the effective R-value of an assembly lower than the insulation's nominal rating.
The US industry standard that defines how SEER2, EER2 and HSPF2 are measured for residential AC and heat-pump equipment.
The standard defining thermal comfort conditions, which is where indoor design temperature and humidity targets come from.
The ventilation standards setting minimum outdoor air rates for commercial and residential buildings. Ventilation air is a load the equipment must carry.
India's Energy Conservation Building Code, which sets envelope and equipment efficiency requirements for larger commercial buildings.
The US climate zone map underlying code requirements for insulation and equipment. Zone determines the design temperature a load calculation should use.
The international test standard for non-ducted air conditioner performance, the basis for capacity ratings in markets including India.
ACCA residential standards: Manual J calculates the load, Manual S selects equipment, Manual D designs the ducts. See Manual J explained.
The Indian code covering building services including ventilation and air conditioning provisions.
The US electrical code article covering air conditioning and refrigeration equipment, including how breaker and conductor sizes derive from nameplate values.
A yearly service agreement covering scheduled cleaning and sometimes parts. Comprehensive contracts include gas and components; basic ones cover labour only.
India’s Bureau of Energy Efficiency 1–5 star label for room ACs, based on ISEER. See the star rating guide.
Copper coils resist corrosion and are easier to repair; aluminium is cheaper and lighter. In coastal and polluted areas the difference in service life is substantial.
The copper piping, drain pipe, wiring and brackets supplied or bought separately when a split is installed. Standard kits assume a short pipe run; longer runs cost extra and need a charge check.
Indian Seasonal Energy Efficiency Ratio — the BEE metric behind India’s star ratings, tested across 24–43 °C.
The moisture load during the monsoon, when outdoor humidity is high but temperature is moderate. Sensible cooling drops while moisture removal matters more, which is when oversized units feel worst.
The heat load from a reinforced concrete roof under direct sun, which makes top-floor rooms markedly harder to cool than the same room on a middle floor.
Indian BEE star thresholds tighten periodically, so a unit's star rating is tied to a label period. A 5-star model from an earlier period may only meet a lower rating under current thresholds.
The convention in Indian model numbers of encoding capacity as a number, usually kW of cooling or a marketing code, rather than as BTU hundreds as in the US. This is why decoding differs by manufacturer.
A setting running the unit at maximum output to cool a room quickly. Useful briefly, but leaving it on defeats the efficiency advantage of an inverter compressor.
Adjusting dampers so each room receives the airflow its load calls for. Skipped on most residential installs, and the usual reason one room never gets cold.
Cubic feet per minute of air moved by the system — roughly 400 CFM per ton in residential cooling.
A rule of thumb of roughly 400 CFM per ton of capacity. Lower airflow shifts output toward moisture removal, higher airflow toward temperature drop.
An adjustable plate that throttles airflow in a duct, used for balancing or for zoning.
Conditioned air escaping the duct system, typically tested as a percentage of system airflow. Leakage into an attic wastes the cooling entirely.
The straight duct length that would produce the same resistance as a fitting. Elbows and takeoffs can each add the equivalent of many feet of straight duct.
The pressure the blower works against, measured across the air handler. Above the rated value, airflow drops and capacity falls with it.
Air speed at a grille or coil face. High face velocity at a supply register is what people register as a draught.
Insulated flexible duct, quick to install but with far higher resistance than rigid metal, especially when compressed or sharply bent.
The static pressure a duct design is allowed to lose per unit length, the starting input for Manual D duct sizing.
Grilles cover an opening, registers add an adjustable damper, diffusers shape the airflow pattern. The terms are often used loosely but describe different components.
A sealed chamber at the air handler where supply or return air collects before entering the duct branches.
Air drawn back to the air handler. Undersized returns are a very common cause of poor airflow and high static pressure.
When an oversized AC cools so fast it shuts off within minutes, hurting dehumidification and wearing the compressor. See the oversizing guide.
Total External Static Pressure — the resistance the blower pushes against (ducts, filter, coil); high values choke airflow.
Conditioned air delivered into the space. Volume is set by the room-by-room load, not divided evenly between rooms.
How far a diffuser projects air into the room before the stream slows. Too short and the far side of the room stays warm.
A duct layout with a main trunk feeding smaller branches to each register. The most common residential arrangement.
A reservoir on the suction line that prevents liquid refrigerant reaching the compressor. Standard on heat pumps because reversal can return liquid.
The indoor cabinet containing the blower, evaporator coil and filter in a ducted system. It moves and conditions air but produces no cooling of its own.
A protective coating on the outdoor coil, sold as blue fin or gold fin. Worth having in coastal and industrial areas where salt or pollutants attack bare aluminium.
Electric resistance or gas heat that supplements a heat pump below its balance point. Far more expensive to run than the heat pump itself.
The outdoor temperature at which a heat pump's output exactly matches the building's heat loss. Below it, supplementary heat is needed.
An indoor unit recessed into a false ceiling, blowing from multiple sides. Common in Indian commercial spaces where a wall-mounted head would not throw far enough.
The pump that compresses refrigerant gas and drives the refrigeration cycle; the most expensive component to replace.
A small pump lifting condensate to a drain when gravity drainage is not available. A common failure point, and the usual cause of water appearing under an indoor unit.
The outdoor coil that rejects heat from the refrigerant to outside air.
The electrically operated switch that starts the compressor when the thermostat calls for cooling. Pitted contacts are a frequent cause of a unit that hums but will not start.
A feature on many Indian split units letting the user cap the compressor at a fraction of rated capacity, marketed as convertible tonnage. It reduces output, not the unit's actual rating.
A small heater keeping compressor oil warm so refrigerant does not dissolve into it while off. Why manufacturers ask for power to be on some hours before first start.
A periodic reversal that melts frost off the outdoor coil in heating mode. Brief, normal, and the reason a heat pump sometimes blows cool air in winter.
The tray under the evaporator coil that catches condensate. A blocked pan or drain is the first thing to check on a leaking indoor unit.
A heat pump paired with a gas furnace, switching to gas when that becomes cheaper than electric heat. Requires a controller aware of both fuel prices.
A split system feeding concealed ducts, giving a single system for multiple rooms without visible indoor units. Usual choice for larger Indian homes above the practical limit for per-room splits.
A split system where the indoor unit blows directly into the room instead of into ducts. Efficient because it avoids duct losses, and the usual choice for adding cooling to one room.
The indoor coil where cold refrigerant absorbs heat from room air.
A component in the liquid line that traps moisture and debris. Replaced whenever the refrigerant circuit is opened.
An air conditioner that can reverse, providing heating as well as cooling. Sizing must satisfy both loads, and the cooling load usually governs in warm climates.
The wall-mounted indoor head familiar from Indian residential installations, mounted high on the wall so cool air falls across the room.
A compressor that modulates its speed to match the load instead of cycling fully on and off; steadier temperatures and better efficiency. See inverter vs non-inverter.
The insulated pair of copper refrigerant lines connecting indoor and outdoor units. Length affects performance, and long runs need a charge adjustment.
One outdoor unit serving several indoor heads on a shared refrigerant circuit. Cheaper on outdoor space than separate systems, but the heads cannot heat and cool simultaneously.
A single cabinet holding compressor, condenser, coil and blower, usually mounted on a roof or slab. Common in commercial buildings and in some US regions for homes.
The valve that lets a heat pump swap refrigerant flow to heat in winter and cool in summer.
A component supplying the phase shift a single-phase motor needs to keep running. A failed capacitor typically presents as a fan that will not spin or a compressor that hums and trips.
Seasonally Adjusted Cooling Capacity — the honest rating for a portable AC, typically 50–60% of the box BTU. See the portable calculator.
A valve on the outdoor unit allowing gauges to be connected without opening the circuit. The Schrader type is the small spring-loaded port under the cap.
The standard configuration with a compressor and condenser outdoors and an evaporator indoors, joined by refrigerant lines. Nearly all residential air conditioning is a split system of some kind.
Variable Refrigerant Flow, a commercial system modulating refrigerant to many indoor units independently. The standard approach for large Indian homes and offices where per-room splits become impractical.
Using motorised dampers and multiple thermostats to direct cooling only to the areas that need it.
The identifier tying an installed indoor and outdoor unit combination to its tested performance. Efficiency ratings apply to the matched pair, not to either component alone.
The drop in delivered cooling as outdoor temperature rises above rating conditions. A unit rated at 95°F can lose 10-15% of capacity at 115°F, which matters for sizing in hot climates.
Combined Energy Efficiency Ratio, the US metric for window and portable units. It includes standby power, so it runs slightly below the older EER for the same machine.
The ratio of useful heat moved to electrical energy consumed; a heat pump with COP 3 delivers 3 units of heat per unit of electricity.
The proportion of an hour a compressor actually runs. A correctly sized unit on a design day approaches 100%; an oversized one may sit near 30% and cycle repeatedly.
Efficiency measured at a single hot operating point (95°F outdoor), reflecting peak-day performance.
A US voluntary label awarded to equipment exceeding the federal minimum efficiency by a set margin. It is a threshold, not a ranking, so two labelled units can differ substantially.
The yellow US label showing estimated annual operating cost and where the model sits in its efficiency range. The cost figure assumes a national average electricity rate.
Heating Seasonal Performance Factor (2023 test) — the seasonal heating efficiency of a heat pump.
Integrated Energy Efficiency Ratio, a part-load weighted efficiency metric used for commercial equipment above the residential SEER2 range.
Purchase price plus energy, maintenance and repair over the equipment's life. On air conditioning, energy usually dominates purchase price several times over.
The rounded capacity a unit is sold as, such as 3 ton. Actual tested capacity at rating conditions is usually a few percent either side.
The time for efficiency savings to repay the extra purchase cost of a higher-rated unit. It shortens with electricity tariff and running hours, which is why the calculation differs sharply between markets.
Seasonal Energy Efficiency Ratio — average cooling efficiency over a season; SEER2 is the tougher post-2023 US test. See SEER vs SEER2.
Power drawn while the unit is off but plugged in, for the remote receiver and control board. Small per hour, but continuous across a year.
Total capacity is all the heat removed; sensible capacity is only the part that lowers air temperature. The gap is the moisture removal, and it shrinks as a unit is oversized.
A circuit serving only the air conditioner. Required for most units above the smallest window sizes, since compressor inrush would trip a shared circuit.
The brief current surge at start-up. Inverter units have far lower inrush because they ramp the compressor up rather than switching it on.
The current a compressor draws at the instant of starting, several times its running current. It sets the breaker and wire sizing, not the running load.
Minimum Circuit Ampacity and Maximum Overcurrent Protection — the nameplate figures that set wire and breaker sizing for an AC.
Miniature Circuit Breaker, the standard overcurrent device in Indian and European installations. Sized from the unit's nameplate rather than from its running current.
The ratio of useful power to apparent power drawn. Poor power factor raises current for the same work, which matters for cable and breaker sizing on larger installations.
Residual Current Circuit Breaker, which trips on earth leakage to protect people rather than equipment. Distinct from the MCB and does not replace it.
The current the compressor draws in normal operation at rating conditions. Printed on the nameplate alongside LRA.
The maximum load an Indian electricity connection is contracted to supply, in kW. Adding air conditioning can exceed it, requiring an application to raise the sanction rather than just a new breaker.
A supply with three live conductors, used for larger equipment. Above roughly 3 tons in India and on most commercial equipment, three-phase is the normal connection.
The loss along a cable run under load. Long runs to an outdoor unit need a larger conductor than the current alone would suggest.
A device correcting supply voltage swings before they reach the unit. Widely used in India where grid voltage varies; many modern inverter units have a built-in range and do not need one.
Conductor size, given in AWG in the US and square millimetres in India. Undersized wire on a long run causes voltage drop that shortens compressor life.
A classification for refrigerants that are mildly flammable but low in toxicity, including R-32 and R-454B. Installation requires charge limits by room volume and A2L-rated tools.
Pulling the system to a deep vacuum before charging, to remove air and moisture. Measured with a micron gauge, since a compound gauge cannot resolve the range that matters.
A mechanical joint formed by flaring the copper tube against a fitting, standard on mini-splits and Indian splits. Torque matters: over-tightening cracks the flare and causes slow leaks.
Adding refrigerant to a system that has lost charge. A system that needs regular top-ups has a leak, and repeated topping up without leak repair is a maintenance failure rather than a service.
How much a refrigerant contributes to warming relative to carbon dioxide over 100 years. Regulation is progressively pushing the industry toward lower-GWP options.
A charge adjustment for refrigerant lines longer than the factory-charged length. Skipping it leaves a long-run installation undercharged.
Flowing dry nitrogen through lines while brazing so the copper interior does not oxidise. Omitting it leaves scale that later blocks the metering device.
Refrigerants: R-410A is being phased out for lower global-warming R-32 and R-454B in new US equipment from 2025 (India already uses R-32).
The mass of refrigerant in the system. Both overcharge and undercharge cut capacity and efficiency, and charge is the single most common installation fault.
The temperature spread across evaporation or condensation in a blended refrigerant. Blends with significant glide need superheat and subcooling measured against the correct dew and bubble points.
Changing a system to a different refrigerant, which may require new oil, metering device or components. Rarely economic on residential equipment.
How far liquid refrigerant is cooled below its condensing temperature; a key charging target on TXV systems.
How far refrigerant gas is warmed above its boiling point leaving the coil; monitored to protect the compressor.
A metering device that varies refrigerant flow to hold correct superheat across changing conditions.
A refrigerant mixture whose components evaporate at different temperatures, producing glide. Must be charged as a liquid so the composition stays correct.