A whole-home system distributing cooled air through ducts.
also: central AC, ducted air conditioning
The basics
What to know about air conditioners
It moves heat, it does not make cold
An air conditioner does not make cold. Moving heat is the whole trick. A compressor squeezes refrigerant until it turns hot, the condenser coil dumps that heat outdoors, the refrigerant then expands and turns very cold inside the evaporator coil, and room air blows across it. Capacity is measured in BTU per hour, a BTU being the heat needed to warm one pound of water by one degree Fahrenheit, and 12,000 BTU per hour equals one ton or about 3.5 kilowatts. Dehumidifying comes along for free, because the evaporator sits below the dew point and water condenses onto it. That water has to go somewhere, which is why every one of these machines carries a drain, a tank or a hose.
Where the hot half sits
Where the hot half sits defines the type, and the compressor decides the comfort. Single speed compressors run flat out or not at all, overshoot the set point, then restart minutes later. Variable speed, also sold as inverter, compressors modulate between roughly 20 and 100 percent, hold a room within about one degree, and run longer at low output. This is exactly what wrings moisture out of the air. Sizing controls the rest. An oversized unit satisfies the thermostat in six minutes, shuts off before it has removed any humidity, and leaves the room cold and clammy. Use 20 BTU per hour per square foot as a sanity check, never as a design, since sun exposure, ceiling height, insulation, window area and how many people sit in the room all move the number. Efficiency labels are really confusing and worth ten minutes. EER divides cooling output by watts drawn at one fixed condition. SEER averages across a season. SEER2 applies a tougher test with higher duct pressure, so identical equipment scores about 4.5 percent lower than it did under SEER, and Europe and Australia publish their own seasonal ratings. Compare two machines only when both carry the same local rating.
Match it to the building
Match the format to the building, not to the budget alone. A Window Air Conditioner at 5,000 to 12,000 BTU cools one closed room, goes in during an afternoon, and needs a window that opens vertically above a sill that can carry 50 to 120 pounds. Reach for a Portable Air Conditioner only when the window refuses a chassis, because the compressor stays in the room with you and a single hose model must replace every cubic foot it blows outside by pulling warm air in through gaps in the building. Quiet belongs to the Ductless Mini-Split, which delivers 9,000 to 36,000 BTU per zone at 19 to 25 decibels indoors and reverses to heat in winter. Central Air Conditioning at 2 to 5 tons fits a house that already has sound ducts and many rooms occupied daily.
Cost, comfort and permanence
Cost, comfort and permanence pull against each other. Window and portable units are cheap to buy, easy to install and loud in the room you sleep in. Mini splits and central systems are quiet and efficient and arrive with an installation bill of several thousand per zone. Running cost deserves five minutes of arithmetic before you commit: a 12,000 BTU window unit at EER 11 pulls about 1,090 watts. Eight hours a day across a 90 day summer is near 780 kilowatt hours. Ducted cooling spreads that load across the whole house and then leaks 20 to 30 percent of it into attics and crawl spaces when the ductwork is poor. A new outdoor unit bolted to old ducts disappoints so reliably. Permanent equipment also brings refrigerant, high voltage and condensate into your walls, and all three belong to a licensed installer.
Read the data sheet
Read the data sheet rather than the box. Find capacity at the 95 degree Fahrenheit outdoor design condition instead of the nominal headline, because output falls as it gets hotter outside and that is precisely when you need it. Check the sound figure at low fan speed, since an averaged number hides the roar at full output. Choose variable speed if you live anywhere humid. Ask which refrigerant is inside. New equipment in the United States has been moving from R-410A to lower warming blends such as R-32 and R-454B since 2025, timing differs by country, and you want local technicians trained and stocked for whatever you buy. On any permanent system, ask for a room by room load calculation before purchase and written commissioning readings afterward covering airflow, refrigerant charge and temperature split. A contractor who declines both has told you something useful.
Airflow comes first
Maintenance is short here, and skipping it gets expensive fast. Airflow comes first, since a filter left for three months restricts it enough to drop the evaporator below freezing. An iced coil stops cooling then floods the pan when it thaws. Dirt is the second, because cottonwood and dust packed into outdoor fins force the compressor to work against higher pressure and draw more current every hour it runs. Condensate is the third. Algae blocks the drain line, a float switch catches that and shuts the system down, and equipment without a float switch sends the water into the ceiling below. Annual professional attention covers what you cannot inspect: refrigerant charge, capacitor health and electrical connections. One rule outranks the rest, though. Refrigerant is never consumed, so a system that needs topping up has a leak, and adding gas without finding it is money burned.
Buyer checklist
Before you buy air conditioners
A major appliance must fit the space, the utilities and the service network. Running cost and repair access belong in the first comparison.
01
Measure the full installation
Include doors, handles, hoses, vents, clearance, service access and the delivery path, not only the cabinet opening.
02
Confirm every utility
Check voltage, circuit, plug, water, drain, gas and ventilation requirements against the exact model and local rules.
03
Compare ongoing use
Use the local energy or water label and include filters, detergent, bags and other consumables.
04
Check parts and service
Look for local technicians, realistic warranty coverage and the availability and price of common replacement parts.
Start sizing at 20 BTU per hour per square foot, so a 300 square foot room lands near 6,000 BTU. Add 10 percent for full afternoon sun, subtract 10 percent for permanent shade, add 600 BTU for each person over two and 4,000 BTU for a kitchen. Treat the total as a check, not a design.
Measure the window sash before anything else. Most window units need an opening 23 to 36 inches wide with at least 13 to 15 inches of sash height. They fit only windows that slide up. Casement and sliding windows need a purpose built chassis or a portable machine.
Weigh the installation literally. Window units run 50 to 120 pounds. Many buildings and codes require an external support bracket rather than resting the load on the sash. Check that rule before purchase, because it is a condition rather than an upgrade.
A portable unit's exhaust hose is 4 to 7 feet and must not be extended. So the cabinet has to stand within that distance of the window with roughly 12 inches of clearance behind it for intake air.
Mini split clearances are not negotiable: allow 6 to 8 inches above and 4 to 6 inches each side of the indoor head, 24 inches on the outdoor unit service side and 5 feet of open space overhead. Condensate must fall at least a quarter inch per foot to a legal drain, or the design needs a pump.
Circuit capacity comes next: units up to about 12,000 BTU usually run on a standard 115 volt 15 amp supply drawing 4 to 12 amps. Larger equipment needs a dedicated 230 volt circuit. Any new circuit is licensed electrician work, and voltage and plug rules vary by country.
Care
Caring for air conditioners
Wash or replace the filter every 30 days during the cooling season, because restricted airflow chills the evaporator below freezing until it ices over, stops cooling and overflows the drain pan as it melts.
Rinse the outdoor condenser coil each spring with a garden hose from the inside outward at low pressure, since packed cottonwood and dust force the compressor to work against higher head pressure and draw more current.
Flush the condensate drain line with a cup of vinegar every two or three months, as algae blocks it, the float switch then shuts the system down. A missing float switch means the water finds the ceiling instead.
Keep 24 inches clear on every side of the outdoor unit and 5 feet overhead, because a fence or a hedge recirculates hot discharge air straight back into the coil and quietly cuts capacity.
Update connected thermostat firmware, then check the schedule afterward, since an update can restore factory settings that cool an empty house all day.
Book a qualified technician annually to check refrigerant charge, capacitor health and electrical connections, and never allow a top up without a leak search, because refrigerant is not consumed and a low charge always means it escaped somewhere.
Questions
Common questions
What size air conditioner do I need for a 300 square foot bedroom?
About 6,000 to 7,000 BTU per hour. Take the higher figure if the room faces afternoon sun, has ceilings above 9 feet or opens into a hallway; take the lower one for a shaded room with good insulation. For anything permanently installed, pay for a proper load calculation, because floor area alone ignores windows, air leakage and orientation.
Is a bigger unit safer than one that is slightly too small?
No, and this is the most common expensive mistake. Oversized equipment cools the air in five or six minutes, satisfies the thermostat and shuts off before it has removed humidity. You get a cold clammy room, more compressor starts and shorter equipment life. Slightly undersized runs longer, dehumidifies better and only struggles on the hottest few days.
Are single hose portable units really that much worse?
Yes. Everything a single hose blows outside must be replaced by air pulled in through gaps around doors and windows, and that replacement air is hot. Real world efficiency can land 25 to 40 percent below the rated figure. Dual hose designs draw condenser air from outside instead and avoid depressurizing the room, so pick one when the choice exists.
Why do this year's efficiency numbers look lower than last year's?
The test changed, not the equipment. SEER2 replaced SEER using a higher external static pressure that better reflects real ductwork, and the same machine usually scores about 4.5 percent lower under it. Comparing a SEER 16 model against a SEER2 16 model flatters the older one. Only compare figures measured under the same standard.
How much does it cost to run one?
Divide BTU by the EER to get watts. Ten thousand BTU at EER 12 works out near 830 watts. That means eight hours daily across a 90 day summer reaches roughly 600 kilowatt hours. At 15 cents per kilowatt hour that is about 90 dollars for the season. Rates and climates vary widely, so redo the sum with your own bill.
Can I install a mini split myself?
Precharged line set kits exist and some people succeed with them. Understand what you are taking on: flaring copper, evacuating the lines to 500 microns, a dedicated circuit and correct condensate fall. Most countries require certification to handle refrigerant, self installation often voids the warranty, and a poor vacuum kills a compressor within a couple of seasons.
The unit runs all day but the room still feels damp. What is wrong?
Usually oversizing. Short cycles cool the air without giving the coil time to condense moisture out of it. Check whether cycles last under 10 minutes. A dry or dehumidify mode, a lower fan speed or a variable speed unit all help. Any of them beat lowering the set point, which just makes a damp room cold.
Should I switch it off when I leave the house?
A single room unit, yes, if you will be out more than two hours. Whole home systems are different: set the thermostat back 4 to 7 degrees Fahrenheit rather than shutting it off, because a house that soaks up heat all day takes hours to pull back down and the humidity returns with it, which costs more than the setback ever saved.
