Central Air Conditioner Sizing

Central air conditioner sizing matters more than most homeowners realize. An AC that is too small cannot keep up on hot days. An AC that is too large is worse: it short-cycles, removes humidity poorly, costs more to operate, and wears out faster. For southern New Hampshire homes, where summer humidity is the main comfort challenge, correct sizing is the difference between a cool, dry, comfortable home and a chilly, clammy one.

At A.J. LeBlanc Heating, we size every central AC installation against the home's actual heat gain, not square footage alone. Here is what that means and why it matters.

How AC capacity is measured

Cooling capacity is measured in tons. A ton of refrigeration is the cooling power needed to melt one short ton (2,000 pounds) of ice in 24 hours. Translated to BTUs:

1 ton = 12,000 BTU per hour

Residential central AC systems for NH homes are typically sized between 1.5 and 5 tons:

1.5 ton: small condos and townhomes
2 ton: 1,200 to 1,700 square foot single-family
2.5 ton: 1,700 to 2,200 square foot single-family
3 ton: 2,200 to 2,800 square foot single-family
4 ton: 2,800 to 3,800 square foot single-family
5 ton: larger homes or homes with significant solar gain

These are very rough ranges only. The actual right size depends on far more than square footage.

Why square footage alone is not enough

Two homes of identical square footage can have very different cooling loads. Variables that affect the actual load:

Ceiling height: taller ceilings mean more volume to cool.
Insulation: R-values in walls, attic, and basement directly affect heat gain.
Windows: number, size, orientation (south-facing windows gain significant solar heat), and glazing type (single, double, low-e).
Air sealing: a tighter home gains less heat from outdoor air infiltration.
Duct design and leakage: leaky ducts in a hot attic waste cooling capacity before it reaches the rooms.
Building orientation: a south-facing home with full sun gets more solar gain than a north-facing home.
Internal heat gains: kitchen, laundry, electronics, occupant count.
Shading: mature trees, overhangs, and neighboring buildings reduce solar gain.
Climate zone: southern NH design conditions are different from northern NH.

The Manual J load calculation

A Manual J load calculation is the industry-standard method for sizing residential cooling and heating equipment. It accounts for all of the variables above and produces a calculated peak heat gain (and heat loss) for the home.

For a proper Manual J:

Each room is measured and assessed individually.
Wall, ceiling, and floor R-values are documented or inferred from construction era.
Window sizes and orientations are recorded.
Local design temperatures (the NH ASHRAE 1 percent cooling design point is around 87 to 89°F) are used.
The result is total system load and (importantly) room-by-room load, which informs duct sizing.

Modern Manual J software makes this calculation straightforward for a trained contractor. The "rule of thumb" 400 to 600 square feet per ton method is what Manual J replaces.

What an oversized AC does

An AC sized larger than the calculated load (a common mistake when contractors use rules of thumb or upsize "to be safe") creates several problems:

Short cycling: the system reaches the thermostat setpoint quickly, shuts off, then restarts a short time later. Each start uses extra electricity and stresses the compressor.
Poor humidity removal: AC removes humidity primarily during sustained run time. A short cycle drops temperature but does not have time to wring moisture out of the air. The result is a cold, clammy house.
Uneven cooling: short cycles do not allow time for cool air to distribute through the duct system. Rooms farther from the air handler stay warm.
Higher operating cost: short-cycling reduces efficiency.
Shorter equipment life: the compressor and contactor wear faster from repeated start-stop cycles.

What an undersized AC does

Runs constantly on hot days without reaching setpoint.
The owner overrides the setpoint downward, which makes the problem worse.
Indoor temperature drifts up during heat waves.
The system wears out faster from continuous run time.

Undersizing is less common than oversizing, but happens when the original load calculation missed key heat gains (large additions, new south-facing windows, removed shade trees).

Mini split sizing is just as important

For ductless mini split installations, room-by-room sizing matters even more because each indoor head serves its own zone. Oversizing a mini split head in a small bedroom produces the same short-cycle and humidity issues as oversizing a central system, with the additional concern that the head sits in the room and is noticeable.

Schedule a consultation

If you are replacing central AC, adding cooling for the first time, or wondering why your existing system never seems to get the bedrooms comfortable, contact A.J. LeBlanc Heating. We size every installation against the actual heat load, not just square footage. Serving New Hampshire families since 1928.

Frequently Asked Questions

How do I know if my current AC is oversized?

Common signs: the system runs in very short cycles (less than 10 minutes at a time), the house feels chilly but humid, some rooms stay warmer than others, and indoor humidity is consistently above 55 percent in summer.

What size AC do I need for a 2,000 square foot home?

Very roughly, 2.5 to 3 tons, but the right answer depends on insulation, windows, orientation, and other factors. A Manual J calculation gives you a real answer for your specific home.

Is it better to size slightly larger to handle the hottest days?

No. Manual J already accounts for design-day peak conditions. Sizing larger than the calculated load creates short-cycling problems that hurt comfort and efficiency on every other day of the cooling season.

Can a contractor just look at my house and tell me what size I need?

An experienced contractor can make an educated guess from square footage and construction era, but a Manual J calculation is the only way to confirm the right size. Reputable installers run the calculation; less careful contractors skip it as a shortcut.

Does mini split sizing follow the same rules?

Yes, with the additional consideration that each indoor head serves its own zone. Room-by-room load calculations are essential for ductless installations.