Central Air Conditioner Sizing

At A.J. LeBlanc Heating, we evaluate the home's construction, windows, insulation, air leakage, duct system, exposure, and comfort concerns before selecting central AC equipment. We perform a Manual J load calculation for every new-construction heating and air-conditioning system. For existing-home replacements and other projects, we use the sizing and load analysis appropriate to the scope. We do not choose equipment from square footage alone.

How AC capacity is measured

Cooling capacity is measured in Btu per hour, and 12,000 Btu per hour equals one ton of nominal cooling. The correct capacity cannot be selected from floor area alone. Two homes of the same size can have different loads because of windows, insulation, air leakage, orientation, ductwork, additions, and internal heat gain.

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

ACCA Manual J is the recognized residential load-calculation procedure. It can produce whole-home and room-by-room heating and cooling loads when the required building inputs are available. A.J. LeBlanc Heating performs Manual J for every new-construction heating and air-conditioning system. For existing-home replacements and other projects, we use the sizing and load analysis appropriate to the scope, then match the result with duct capacity, zoning, and the selected equipment's performance.

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 or call 603-623-0412. 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?

Square footage is not enough to answer. Windows, insulation, leakage, orientation, ductwork, occupancy, and additions can change the load substantially. We evaluate the home before recommending capacity.

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?

Ask the contractor to explain how the capacity was selected and how the existing or proposed duct system was considered.

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.