An OBC Built company (opens in a new tab)
(321) 405-1806Start My Estimate
An HVAC technician checking system readings (stock photo)

Guide · OBC Air & Heat

Sizing an AC for Florida Humidity: Why Bigger Isn't Better

Why rules of thumb miss, how a Manual J load calculation sizes an AC, and how an oversized system leaves a Florida home cold but clammy.

Why square footage alone doesn't work

Two 1,500-square-foot homes in Brevard can need very different systems. One might be a 1960s block home with single-pane windows facing west. The other might be a newer, tight home with low-E glass and ducts inside the conditioned space.

ENERGY STAR tells homeowners to make sure the contractor "uses the actual characteristics of your home" and does "not just rely on a 'rule of thumb'" (ENERGY STAR: HVAC Quality Installation).

What a Manual J load calculation looks at

ACCA Manual J is the ANSI-recognized standard for calculating heating and cooling loads. It covers single-family homes, small multi-unit buildings, condos, townhouses and manufactured homes (ACCA Manual J). A load calculation considers things like:

  • Window size, type and direction (west glass adds a lot of afternoon heat).
  • Insulation in walls and ceilings.
  • Air leakage.
  • Where the ducts run, and how well they're sealed.
  • Shading, roof color and orientation.
  • How many people live there, plus internal heat sources.

Manual S then uses those loads to choose equipment whose real capacity matches them at local design conditions.

What Florida's energy code requires

The Florida Building Code, Energy Conservation (2023), Section R403.7.1, says heating and cooling equipment "shall be sized in accordance with ACCA Manual S" based on loads from "ACCA Manual J or other approved heating and cooling calculation methodologies." It also says the code "does not allow designer safety factors, provisions for future expansion or other factors that affect equipment sizing."

Section R403.7.1.1 adds a limit for cooling-only equipment. Total capacity must be at least the calculated load, but not more than 1.15 times that load (or the closest size the manufacturer makes). The equipment's moisture-removal (latent) capacity must also meet the calculated latent load (2023 FBC Energy Conservation, Residential Ch. 4).

In other words, Florida's code is built to keep systems from being oversized.

How oversizing leaves you cold but clammy

An AC does two jobs: it lowers the temperature and it removes moisture. Moisture removal takes run time. The coil has to get cold and stay cold long enough for water to condense and drain away.

  • ENERGY STAR: "Oversized equipment may cycle too frequently, resulting in less comfort and a shortened lifespan."
  • DOE: "an oversized air conditioner performs less efficiently and effectively than a smaller, properly sized unit" (DOE Energy Saver Guide).
  • Florida Solar Energy Center (FSEC) research found that oversized equipment, "which is commonly installed, will cycle on/off more often which will negatively impact dehumidification performance" (FSEC-CR-1537-05).

Signs you might be living with an oversized system:

  • The house hits the setpoint quickly, but feels sticky.
  • The system runs in short bursts and shuts off.
  • Humidity indoors stays above EPA's recommended level of below 60%. See our thermostat and humidity guide.

Fan settings and airflow can cause similar symptoms, so it's worth a proper look before blaming size.

Undersizing is a problem too

The answer isn't "go small." An undersized system may run nonstop on the hottest afternoons and still fall behind. The goal is a system matched to the calculated load, which is what Manual J and Manual S are for.

Ductwork is part of sizing

A correctly sized unit on leaky or undersized ductwork won't perform as rated. DOE notes that oversizing, improper charge and leaky ducts "result in efficiency losses, occupant discomfort, and shortened equipment life" (DOE FEMP). Efficiency ratings assume a good installation. See SEER2 Explained.

Condos, mobile homes and small businesses

Manual J covers condos and manufactured homes too. Each has its own quirks. Manufactured homes come with a Comfort Cooling Certificate showing how the original system was sized. See Mobile and Manufactured Home AC and Condo AC Replacement. For shops and offices, the load depends heavily on occupancy, lighting and equipment, so we look at the space before recommending a system. See light commercial.

The next step

We look at the home or building first, then give you a written quote.

Firefighters, law enforcement, EMS and paramedics, healthcare workers, teachers, ministry, government employees and military (active and veterans) can get 10% off the total, up to $2,500, on new units and repairs through our Local Heroes program. Family members of eligible heroes qualify too. It can't be combined with other offers.

Frequently asked questions

What size AC do I need for my Florida home?

It depends on the home, not just the square footage. Windows, orientation, insulation, air leakage and ductwork all change the load. Florida's energy code requires sizing with ACCA Manual S, based on a Manual J or other approved load calculation.

What size AC does a 1,500 sq ft house in Florida need?

There's no reliable single answer. Two homes the same size can have very different loads. A load calculation gives the real number.

Is a bigger AC better in Florida?

No. An oversized system cycles on and off often, which hurts moisture removal and comfort. ENERGY STAR says it can also shorten the system's lifespan.

Why does my house feel cold but clammy?

A common cause is an oversized system that doesn't run long enough to remove moisture. Fan settings (On instead of Auto) and airflow problems can also contribute.

What is a Manual J load calculation?

It's the ANSI-recognized ACCA standard for calculating a home's heating and cooling loads. Manual S then uses those loads to choose equipment.

Does Florida limit how oversized an AC can be?

Yes. For cooling-only equipment, Florida's energy code caps total capacity at 1.15 times the calculated load, or the closest size the manufacturer offers.

Sources

  1. ENERGY STAR: HVAC Quality Installation
  2. ACCA Manual J
  3. 2023 FBC Energy Conservation, Residential Ch. 4
  4. DOE Energy Saver Guide
  5. FSEC-CR-1537-05
  6. DOE FEMP

Written by OBC Air & Heat, Space Coast, Brevard County, FL. State Certified Air Conditioning Contractor, License No. CAC1825377. General information only; your system and home are reviewed before any recommendation.

Start My Estimate Call (321) 405-1806

Keep reading

More guides.

Aerial view of a Space Coast beach and barrier island at sunset
GuideCoastal AC Units on the Space Coast: Why Salt Air Changes the ChoiceSalt air is hard on outdoor AC coils. What coastal-rated units include, which Brevard beachside homes should consider one, and how to protect yours.
A modern Florida-style home with palm trees
GuideSEER2 Explained: What the Rating Means for Your Florida Power BillFlorida's minimum SEER2 for a new AC, how SEER2 differs from SEER, and how to estimate your own savings from a higher rating. No guesswork.
An HVAC technician inspecting an outdoor condenser (stock photo)
GuideRepair or Replace Your AC in Brevard County?Age, refrigerant, repair history, efficiency and comfort: how to decide whether to repair or replace your AC on the Space Coast, using real guidance.
A family relaxing on a sofa in a cool Florida living room
GuideBest Thermostat and Humidity Settings for Florida HomesWhat DOE suggests for summer thermostat settings, the indoor humidity target, fan Auto vs. On, and what to set when you're away from a Space Coast home.
Aerial view of a Space Coast beach and barrier island
GuideHurricane Prep and Post-Storm AC ChecksHow Florida code handles AC tie-downs, what to do with your AC before a hurricane, and safe checks after a storm, flood or outage. No DIY electrical work.
A Brevard County single-family home at sunset
GuideHeat Pumps in Brevard: Do They Make Sense on the Space Coast?A heat pump cools exactly like an AC and also heats. How it fits the Space Coast climate, what HSPF2 means, and when backup heat matters.
A ductless mini-split indoor unit on a wall
GuideMini-Splits for Additions, Garages and LanaisWhere a ductless mini-split fits in a Florida home, why an open screened lanai can't really be cooled, and the permits and condensate to plan for.
A manufactured home in Florida
GuideAC for Mobile and Manufactured Homes: What's DifferentPackage vs. split systems, belly ductwork, HUD-code rules and community coordination: what's different about AC for mobile and manufactured homes.
A beachfront condominium building on the Space Coast
GuideCondo AC Replacement: HOA Rules, Access and PlanningWho owns what, association approvals, roof and closet access, and permits. What to check in your condo documents before replacing AC on the Space Coast.
An HVAC technician inspecting an outdoor condenser (stock photo)
GuideR-22 Phase-Out: What It Means If Your System Is OlderCan an R-22 AC still be recharged? EPA facts on the R-22 phase-out, leak repair, retrofits, and the newer refrigerants for Space Coast homes and shops.
An HVAC technician replacing an air filter (stock photo)
GuideSpace Coast AC Maintenance Checklist (Plus FPL Rebate Basics)How often to service your AC in Brevard County (twice a year), what our tune-up includes, and safe checks to do yourself: filters, drains and rinsing.

Florida’s Space Coast

Ready when you are.

(321) 405-1806
Start My Estimate Schedule Service