AC and Furnace Sizing Calculator

Tons, BTU and the Rough Number

Air conditioner capacity is measured in tons, and 1 ton of cooling equals 12,000 BTU per hour. The term is historical, from the cooling effect of melting a ton of ice in a day, and has nothing to do with the weight of the unit.

Residential systems are sold in half ton steps: 1.5, 2, 2.5, 3, 3.5, 4 and 5 tons. There is no 2.75 ton unit, so the rough figure always lands between two real products, and choosing between them is exactly where sizing decisions go wrong.

Oversizing Is the Common and Costly Mistake

The instinct with heating and cooling equipment is to add margin. With air conditioning that instinct is actively harmful, and oversizing is the single most frequent sizing error in residential HVAC.

Short cycling

An oversized unit cools the air to the thermostat setpoint quickly, then shuts off. A few minutes later the temperature drifts back up and it starts again. Instead of long steady runs, the compressor performs many short bursts. Every start draws high current and puts mechanical stress on the compressor and the contactor, so an oversized system wears out measurably sooner than a correctly sized one.

No humidity removal

This is the consequence people actually feel. An air conditioner removes moisture by condensing it on a cold evaporator coil, and that takes time. The coil has to get and stay cold, and air has to keep passing over it. A unit that runs for six minutes and stops never reaches the point of removing meaningful moisture.

The result is a house that is cold and clammy at the same time. The thermostat reads 72 degrees but the air feels damp and heavy, occupants feel uncomfortable and turn the temperature down further, which makes the short cycling worse. Persistent high indoor humidity also encourages mould and dust mites. A correctly sized unit that runs long and steady on a hot day is doing its job properly, even though it looks like it is working harder.

Cost and comfort

Oversized equipment costs more to buy, more to install, more in electrical work, and more to run because repeated starts are the least efficient part of a cycle. It also produces uneven temperatures, since short runs do not move enough air to mix the house.

Approximate Tonnage by Home Size and Climate

These figures assume average insulation and standard 8 foot ceilings. They are a starting point for a conversation with a contractor, not a specification.

A well insulated, tight, well shaded home can sit a full half ton or more below the table. A poorly insulated home with large west facing glazing can sit above it. That spread is the whole reason a real load calculation exists.

Manual J Is the Correct Method

Manual J is the residential load calculation procedure published by ACCA. It works room by room and accounts for wall, ceiling and floor construction, insulation R values, window area, glazing type and orientation, shading, air infiltration, internal gains from people and appliances, duct location and leakage, and local design temperatures.

The output is a separate heating load and cooling load in BTU per hour, along with a room by room breakdown that also drives duct and register sizing. It typically lands lower than a square footage estimate, often noticeably lower on a modern or recently insulated home.

Any competent contractor will run one, and will provide the results on request. A quote that arrives after a walk around and a glance at the old unit's nameplate is a quote based on nothing. Replacing like for like repeats whatever sizing error was made originally, and if the house has since been reinsulated, had windows replaced or been air sealed, the correct size is now smaller.

Related procedures

• Manual S selects specific equipment to match the Manual J loads, including how the unit performs at local design conditions rather than at rating conditions.
• Manual D designs the duct system to deliver the required airflow to each room.
• Manual T covers register and grille selection and placement.

Ductwork Matters as Much as Unit Size

A correctly sized unit connected to inadequate ducts will not deliver its rated capacity. Airflow is the constraint, and cooling systems generally need around 350 to 400 CFM per ton. A 3 ton system therefore needs roughly 1,050 to 1,200 CFM through the duct system.

• Undersized returns. The most common duct fault. Restricted return air starves the coil, reduces capacity and can freeze the evaporator.
• Leaky ducts. Leakage of 20 to 30 percent is common in older homes. Ducts in an unconditioned attic can lose a large fraction of the cooling before it reaches a room.
• Uninsulated ducts in unconditioned space. An attic at 130 degrees will heat the supply air on its way through.
• Too few or badly placed registers. Even a correct total airflow leaves rooms uncomfortable if the distribution is wrong.

Upgrading equipment while leaving bad ducts in place is a common and disappointing outcome. Where budget is limited, sealing and insulating ducts often returns more comfort per dollar than a higher efficiency unit.

SEER2 and Efficiency Ratings

SEER2 is the current seasonal energy efficiency ratio for cooling equipment, replacing the older SEER metric. It is measured under a higher external static pressure that better represents real duct systems, so a SEER2 number is typically a little lower than the SEER number for the same equipment. Do not compare a SEER rating directly against a SEER2 rating.

Minimum SEER2 levels are set by federal regulation and are higher in the southern and southwestern regions than in the north. Higher SEER2 equipment costs more up front and saves more per cooling hour, so the payback depends on how many hours the system actually runs. In a hot climate with a long season the case is strong. In a mild climate with a short season it is weaker.

Efficiency does not compensate for wrong sizing. A high SEER2 unit that is a ton too large still short cycles and still fails to dehumidify. Get the size right first, then choose the efficiency level.

Furnace Sizing: Input, Output and AFUE

Furnaces are rated in BTU per hour of input, meaning the fuel energy consumed. What heats the house is the output, and the difference is efficiency.

AFUE, the annual fuel utilisation efficiency, is the seasonal average share of fuel energy that becomes useful heat. Furnaces around 80 percent AFUE vent through a conventional flue. Condensing furnaces of 90 percent and up recover heat from the exhaust, vent through PVC pipe and produce condensate that needs a drain.

Size the furnace against the Manual J heating load using output, not input. Two furnaces with the same input badge deliver very different heat, so comparing input figures across efficiency levels is meaningless. Oversized furnaces short cycle for the same reasons oversized air conditioners do, producing temperature swings and uneven heat, though the humidity consequence does not apply.

Note that heating and cooling loads are independent. A house can need a 2.5 ton air conditioner and an 80,000 BTU furnace, and the two numbers have no fixed relationship. On a shared air handler the blower has to suit both, which is one more reason the whole system should be designed together rather than assembled from separately chosen parts.