Quick answer
The right AC size for a New Orleans house comes from a load calculation, usually ACCA Manual J, which accounts for the house's size, windows, insulation, orientation, air leaks and ducts. Rules of thumb based on square footage, or the old unit's label, often oversize the system. In our humidity, a correctly sized system that runs longer usually keeps the house drier and more comfortable.
Why bigger is not better here
An air conditioner does two jobs: it cools the air and pulls water out of it. Moisture removal happens while the coil is cold, so it needs reasonably long run cycles. In the worst weeks of summer, dew points here sit in the upper 70s and average summer humidity is around 76 percent, so there is a lot of water to remove.
An oversized system cools the house quickly, satisfies the thermostat and shuts off before it has pulled much moisture out. The house ends up cold and clammy, the system short cycles, and parts wear faster. That is why a system sized for our humidity often feels better than a bigger one.
Why the old label is a poor guide
The old unit's size tells you what someone installed, not what the house needs. Houses change: new windows, attic insulation, sealed ducts, a closed-in porch or an addition all change the load. The original installer may also have sized by habit rather than calculation.
Square-footage rules of thumb share the same weakness. Two houses with the same floor area can need very different systems if one is a raised shotgun with tall ceilings and the other is a slab ranch with a hot attic and west-facing glass.
What a load calculation looks at
A proper load calculation, usually ACCA Manual J, works room by room. It looks at:
- Floor area, ceiling height and the number of floors
- Window size, type and which way each window faces
- Wall, attic and floor insulation
- Air leakage, and whether the house is raised or on a slab
- Where the ducts run: attic, crawlspace or conditioned space
- How many people live there and what heat the kitchen adds
- The moisture load as well as the heat load
The result is a heating and cooling load for each room, which then guides equipment selection, often with Manual S, and duct design, with Manual D.
Choosing equipment for our climate
Once the load is known, the type of equipment matters as much as its size. Single-stage systems run at full output every time. Two-stage and variable-speed systems can run long, low cycles that keep the air drier and the temperature steadier, which suits our humid spring and fall especially well.
Any new central AC installed in Louisiana must meet at least 14.3 SEER2, so efficiency starts from a reasonable floor, and higher ratings mostly pay off through lower bills in a climate where the system runs much of the year.
The ducts have to match
A perfectly sized system on undersized or leaky ducts still performs poorly. Before a replacement, the ducts should be checked: static pressure measured, sizes compared with the new system's airflow, and leaks sealed. Sometimes only the return needs enlarging. Our ductwork replacement page explains when the ducts need more than sealing.
Questions to ask before you buy
- How was the size chosen?Ask for the load calculation, not a rule of thumb.
- What happens to humidity?Ask how the system and its settings will handle moisture.
- Will the ducts carry the airflow?Ask whether static pressure was measured.
- Is the coil matched?Ask for the matched outdoor unit and indoor coil.
- How is it charged?Ask for a deep vacuum and a charge by weight.
Sources
Figures in this guide come from these sources. Programs and prices change, so check current terms.
About this guide. Ward Teague writes the HVAC guides for Byron Vegas. Each guide starts from questions people ask when they call, and the figures in it come from the sources listed at the end of the article. More about the writer.