Short answerCentral air conditioners are sized in tons, each ton being 12,000 BTU per hour of cooling. In Southeast Michigan most homes need roughly one ton per 600 to 800 square feet, so a 1,500-square-foot ranch is usually a 2- to 2.5-ton system and a 2,400-square-foot colonial a 3- to 3.5-ton. The right number comes from a Manual J load calculation, not square footage, and oversizing is the most common mistake.
Tons, BTUs and what they mean
Air conditioner capacity is rated in tons, a leftover from the days of ice delivery: one ton of cooling is the heat needed to melt a ton of ice in a day, which works out to 12,000 BTU per hour. Residential split systems come in half-ton steps from 1.5 to 5 tons. A 3-ton condenser removes about 36,000 BTU per hour of heat from the house on a design day. The outdoor unit's model number usually encodes the size: a 36 in the string means 36,000 BTU, 3 tons; a 24 is 2 tons; a 42 is 3.5 tons.
Two caveats. The nameplate is nominal; actual capacity depends on the outdoor temperature, the indoor coil match and the blower airflow, and a 3-ton system might deliver 33,000 BTU on a 95-degree day. And a portion of that capacity goes to removing moisture (latent load) rather than lowering temperature (sensible load), which in our climate is a big portion.
Why Michigan humidity changes the math
Southeast Michigan's design conditions for cooling are around 89 to 91°F dry bulb with a coincident wet bulb in the mid-70s. The dew points in July regularly sit between 65 and 72. That means a large share of what an air conditioner does here is wringing water out of the air, and that only happens when the system runs long enough for the evaporator coil to get cold and stay cold.
An oversized system in this climate is the classic mistake. A 4-ton unit on a house that needs 2.5 tons will drop the temperature to setpoint in ten minutes and shut off before the coil has pulled much moisture. The thermostat reads 74, the house feels clammy, the basement smells musty, and the system cycles on and off all afternoon. Homeowners then set the thermostat to 70 to feel comfortable, which uses more electricity than a right-sized unit at 75. We see this constantly in 1990s subdivisions in Canton and Van Buren Township where the builder installed one size for every floor plan. A correctly sized system runs 15- to 30-minute cycles on a hot day and leaves the air dry enough that 76 feels fine.
Undersizing is a smaller risk here. A slightly small system runs continuously on the three or four hottest days of the year and might fall a degree or two behind in the late afternoon, but it dehumidifies beautifully the rest of the summer. Given the choice, round down.
Rules of thumb for local house types
Square footage gets you in the neighborhood. For Southeast Michigan, with its moderate design temperature and mostly basement-and-two-story housing, the working range is 600 to 800 square feet per ton of conditioned space above grade, closer to 600 for older, leakier, sun-exposed houses and closer to 800 for tight newer construction with good shading.
- 1,100-square-foot 1950s ranch in Ypsilanti Township or Garden City, original windows, modest attic insulation: 2 tons.
- 1,500-square-foot 1960s ranch in Westland or Livonia with replacement windows and blown attic insulation: 2 to 2.5 tons.
- 1,800-square-foot 1970s colonial in Wayne or Plymouth: 2.5 to 3 tons, with a second floor that may need duct balancing or a zone.
- 2,400-square-foot 2000s two-story in Canton, Saline or Pittsfield with 2x6 walls and low-E glass: 3 to 3.5 tons, despite the size, because the envelope is tight.
- 1,900-square-foot 1920s Ann Arbor two-story with plaster walls, single-pane windows and a west-facing sunroom: 3 tons, maybe 3.5, and the ductwork is usually the harder problem.
Finished basements add very little cooling load because they are below grade and stay in the 60s on their own. Do not count them. Attic bonus rooms over garages in newer Northville and Plymouth Township builds add a lot because they are surrounded by hot attic on three sides.
What a Manual J actually looks at
Manual J is the ACCA load calculation standard, and it is what should decide the size. It takes the house room by room and accounts for:
- Orientation and the square footage of glass on each face, with the glass type and shading.
- Wall and ceiling insulation levels, and the age of the construction as a proxy for air leakage.
- Ceiling height and total conditioned volume.
- Number of occupants and the heat from cooking, lighting and appliances.
- Duct location; ducts in a hot attic lose capacity before the air reaches the room.
- Local design temperatures, which for Washtenaw and Wayne County are specific and published.
The output is a sensible and latent load in BTU per hour for the whole house and each room. A good contractor then picks equipment whose rated capacity at design conditions matches that load, within a few percent, using the manufacturer's expanded performance data rather than the nominal tonnage. The room-by-room numbers also show whether the existing ductwork can deliver the right airflow, which it often cannot in a 1950s ranch that had central air added in 1988 to a furnace duct system never designed for it.
If a contractor sizes your replacement by reading the old unit's label, or by square feet alone, ask for the load calculation. Replacing a 4-ton with a 4-ton because that is what was there repeats whatever mistake the original installer made.
Sizing with a new furnace or a heat pump
The indoor blower has to move about 400 cubic feet per minute per ton of cooling, so a 3-ton AC needs a furnace or air handler capable of 1,200 CFM at the static pressure your ducts present. A 3.5-ton AC on a small 80% furnace with a weak PSC blower will freeze its coil every July. If you are replacing both, they are sized together. If you are replacing only the AC, the furnace blower sets an upper limit.
A heat pump is sized the same way for cooling, but in Michigan the heating load usually governs, and a cold-climate heat pump sized for heating may be a half-ton larger than the cooling load wants. That is a tradeoff worth understanding before you choose; heat pump vs furnace in Michigan and cold-climate heat pumps in Southeast Michigan cover it.
Efficiency and staging interact with size
A two-stage or variable-speed condenser softens the sizing problem because it can run at 60 to 70 percent capacity on a normal day and reach full capacity only on the hottest afternoons. That lets a slightly larger nominal size behave like a smaller one most of the summer while keeping reserve. It does not excuse gross oversizing, but it does make a 3-ton variable-speed unit a reasonable choice where a 2.5-ton single-stage would be exactly right. Single-stage vs two-stage vs variable-speed goes through when that premium is worth it.
Getting the number for your house
A load calculation takes about an hour of measuring and a bit of software time, and it is part of every AC installation estimate we do. If the answer is that your current unit is oversized, we will say so, and the replacement will likely be smaller, quieter and more comfortable than what you had. If the ductwork cannot support the right size, we will show you where and talk about ductwork repair before equipment, because the best condenser in the world cannot fix a return that is half the size it should be.