AC Sizing for Alabama Homes: The Humidity Factor Most Contractors Miss

By Chad Wiswall, Owner & Lead HVAC Technician, Alabama HVAC License #92244

The most common HVAC mistake I see in Central Alabama homes is not a brand choice and it is not a maintenance gap. It is sizing. A system that is one ton too big runs your bill up, leaves your house clammy, and dies five years early. A system that is one ton too small runs nonstop in July, burns out the compressor, and never gets you below 78 degrees on the worst days. Either way you lose. And the reason this happens so often in Alabama is that most contractors size off square footage and a rule of thumb, when Alabama's real sizing variable is humidity. I have been installing systems in Montgomery, Pike Road, Auburn, Dadeville, Wetumpka, and the rest of the River Region since 1993. This guide explains how AC sizing actually works in a humid climate, why the standard charts get Alabama wrong, and what we do differently. This guide is part of our complete guide to HVAC in Central Alabama.

Why standard tonnage charts get Alabama wrong

Open any contractor's flyer or any home improvement website and you will see the same chart: 1 ton of cooling per 400 to 600 square feet. So a 2,000 square foot house "needs" a 3.5 to 4 ton system. Pick a number, install the equipment, collect the check.

That math is wrong everywhere, but it is especially wrong in Alabama.

The chart assumes a "typical" home in a "typical" climate. Alabama is not typical. Our summer dew points run 70 to 75 degrees for weeks at a time, which means the air outside your house is carrying massive amounts of moisture, and every cubic foot that leaks into your house through windows, doors, attic penetrations, can lights, and gaps in the building envelope brings that moisture with it. An AC system in Alabama has two jobs: cool the air (sensible cooling) and remove moisture (latent cooling). The rule of thumb charts only account for the first job.

Sizing off square footage gives you a system that can drop the temperature but cannot dehumidify. The result is a house at 72 degrees that feels like a cave at 65 percent indoor relative humidity. Sticky walls, musty smells, mildew on the inside of closets, sinuses inflamed all summer, and a system that short-cycles itself to death because it satisfies the thermostat before it has been running long enough to pull moisture out of the air.

The Manual J load calculation (more on this below) accounts for both sensible and latent load. It is the standard, it is what the industry has agreed on for decades, and it is what you should expect from any contractor giving you a quote in Alabama. If a contractor gives you a tonnage number without measuring your home and running the calc, walk away. They are guessing with your money.

What proper AC sizing actually looks like (Manual J overview, simplified)

Manual J is a load calculation method developed by the Air Conditioning Contractors of America (ACCA). It is the industry-standard process for figuring out exactly how much cooling and heating a specific home needs. It accounts for:

Square footage and ceiling height (cubic feet of air to condition)
Window area, orientation, and glazing type (huge heat gain factor)
Insulation R-values for walls, ceiling, and floor
Air infiltration rate (how leaky the building envelope is)
Number of occupants (each person contributes about 400 BTU/hr of body heat)
Major appliances (refrigerator, stove, dryer, all add heat)
Lighting load
Local design temperatures (the 1 percent cooling design temp tells us the hottest realistic day to size for)
Latent load (moisture removal requirement, calculated separately)

For Montgomery, the 1 percent cooling design temperature is 95 degrees dry bulb / 76 degrees wet bulb. That wet bulb number is the humidity flag. A 76 degree wet bulb means heavy moisture load, and the load calc weights latent capacity heavily for our climate.

A proper Manual J calc takes 30 to 60 minutes per home to do right. You measure rooms, you log window areas and orientations, you note insulation type and depth, you check the ductwork for sizing and leakage, and you run it through software (we use a current version of Wrightsoft or equivalent) that outputs total BTU/hr required, broken into sensible and latent.

That output translates into a system size. Residential AC sizes are measured in tons, where 1 ton = 12,000 BTU/hr of cooling capacity. Common residential sizes are 1.5 ton, 2 ton, 2.5 ton, 3 ton, 3.5 ton, 4 ton, and 5 ton.

Here is what most homeowners do not realize: the calculated load almost never lands exactly on a clean tonnage number. A home might calculate at 36,500 BTU/hr, which is just over 3 tons but well under 3.5. In Alabama, the right move is almost always to round DOWN, not up, and then verify the latent capacity is sufficient. A 3 ton system running longer per cycle will pull more moisture out of the air than a 3.5 ton system that satisfies the thermostat faster and shuts off before it has done the dehumidification work.

This is exactly the opposite of how most contractors size jobs. Most contractors round up "to be safe." In Alabama, rounding up makes you LESS comfortable, not more.

The oversizing trap

Oversizing is the single most common AC mistake I encounter when I am called to a home for a comfort complaint. The symptoms are predictable:

Thermostat says 72 but the house feels warm, sticky, and oppressive
Indoor humidity reads 60 to 70 percent on a hygrometer (it should be 45 to 55 percent in Alabama)
AC kicks on for 6 to 8 minutes, satisfies the thermostat, shuts off, and repeats every 15 to 20 minutes
Allergy and sinus symptoms worse in summer than winter
Mold or mildew forming around vents, in closets, or behind furniture against exterior walls
Wood floors cupping, doors swelling and sticking
Electric bill higher than a neighbor's with a similar-sized home

Here is the mechanism. Air conditioners remove moisture from the air when refrigerant lines cool the indoor coil below the dew point of the room air. Moisture condenses on the coil, drips into the condensate pan, and drains out of the house. That moisture removal takes time. The coil needs to be cold and the system needs to run continuously for that process to actually pull humidity down.

An oversized system cools the room temperature very fast (which is the sensible job) but shuts off before the coil has been cold long enough to remove meaningful moisture (the latent job). The thermostat is satisfied, the compressor stops, and the moisture stays in the air. Worse, when the system shuts off, any moisture sitting on the coil evaporates back into the indoor air on the next blower cycle. The system actually adds humidity back during the off cycle.

The result: cold, damp, miserable.

Oversizing also kills equipment. Every time a compressor starts, it draws 4 to 7 times its running current for a few seconds. Short-cycling means more starts per hour, more wear on the contactor, more wear on the start capacitor, more wear on the compressor bearings. An oversized system in Alabama often dies at year 8 to 10 instead of year 14 to 16.

The undersizing trap

Undersizing is less common than oversizing but still happens, usually when a contractor sized a system 15 years ago without considering future additions or insulation degradation. Symptoms:

AC runs constantly in July and August
Thermostat set to 72 but indoor temp drifts to 76 to 78 on the hottest days
Bedrooms farthest from the air handler stay 4 to 6 degrees warmer than the room with the thermostat
Electric bill very high because the system never gets to cycle off
Compressor running 18 to 22 hours a day for weeks at a time
System dies young from continuous operation

An undersized system never satisfies the load on design days, never gets a rest, runs the compressor into the ground, and never feels comfortable because the worst rooms in the house cannot keep up.

The fix is rarely "buy a bigger system." More often it is fixing the underlying problem: bad ductwork, missing insulation, leaky envelope, or a load that has changed since the system was installed (new addition, finished basement, replaced single-pane windows with the same R-value double-pane). A good contractor diagnoses the root cause before recommending a larger replacement.

How Chad's measures your home for the right size

When we come out to quote a system replacement, this is the process:

Step 1: Walk-through and measure. We measure every conditioned room, note ceiling heights, log window count, orientation, area, and glazing type. We check insulation depth in the attic and the wall cavity construction era. We look at the building envelope for obvious leaks (gaps at top plates, recessed lighting, around plumbing penetrations, attic hatch sealing).

Step 2: Duct inspection. We pull a few register grilles, look at trunk and branch sizing, check for obvious leakage at joints, and assess insulation quality on any ducts running through unconditioned space (attic, crawlspace). If we can, we run a basic duct leakage test.

Step 3: Equipment inspection. We look at your current condenser, air handler, and coil. Model numbers, age, refrigerant type, capacity. We ask what is working and what is not, where you are uncomfortable, what your electric bills look like in July, and how the system has performed historically.

Step 4: Manual J load calc. We input everything into the load calc software. Output is total BTU/hr required, broken into sensible and latent capacity needs.

Step 5: Equipment selection. We match available equipment to the calculated load, with attention to latent capacity ratings for our humid climate. Variable-speed inverter systems usually win for Alabama because they can run at part load for long stretches, pulling humidity continuously instead of slamming on at full capacity and shutting back off.

Step 6: Quote with explanation. You get a written quote with the recommended tonnage, the equipment specs, the reasoning, and the alternatives. No high-pressure pitch. No "today only" discount theater. Just the work and the price.

Sample AC tonnage ranges by Alabama home type

Treat these as rough starting points only. The real number depends on insulation, windows, ducts, and shade, and the only honest answer is a Manual J load calc. But for general planning:

1,200 to 1,500 sq ft single-story brick ranch (typical 1960s-1980s Montgomery build): 2 to 2.5 tons. Older brick ranches usually have decent thermal mass but mediocre attic insulation and original windows. Often the windows are the leading load factor.

1,500 to 1,800 sq ft single-story (older home, original windows): 2.5 to 3 tons. Window upgrade and attic insulation top-off can sometimes drop this by half a ton.

1,800 to 2,200 sq ft single-story (newer construction, 2000-2015): 2.5 to 3.5 tons. Better envelope, better windows, lower load per square foot.

2,200 to 2,800 sq ft single-story (newer, energy-code build): 3 to 4 tons. Modern code-built homes can run surprisingly low loads.

1,800 to 2,400 sq ft two-story (older): 3 to 4 tons, often split into two zones or two systems (upstairs/downstairs).

2,400 to 3,500 sq ft two-story (newer): 3.5 to 5 tons, almost always two zones or two systems.

Lake Martin or rural home with cathedral ceilings and lots of glass: Loads are unpredictable. Could be 3 tons for a small open-concept home, could be 6 tons for a large one with multiple southern exposures. Manual J is mandatory.

Again, these are starting ranges. We have quoted 1,800 sq ft homes that needed 2 tons and 1,800 sq ft homes that needed 3.5 tons. Same square footage. Different envelope, different windows, different load.

Insulation and ductwork matter more than people think

A new properly sized HVAC system connected to bad ducts in a leaky envelope will underperform a 15 year old system connected to good ducts in a tight envelope. The equipment is half the equation. The delivery system and the building envelope are the other half.

The single highest ROI envelope upgrade for most Central Alabama homes is attic insulation. If you have less than R-30 (about 10 inches of blown cellulose or fiberglass) in your attic, you are leaving comfort and dollars on the table. Code minimum for our climate zone is R-38, and R-49 to R-60 is achievable and often pays back inside 5 years on energy savings alone.

The single highest ROI ductwork upgrade is sealing. Most homes built before 2010 in Alabama have ducts joined with metal screws and tape that has long since failed. Aerosol duct sealing (Aeroseal and similar products) can take a system from 25 to 35 percent duct leakage down to 3 to 5 percent in one afternoon. That is the equivalent of upsizing your AC by a half ton without replacing the equipment.

When we quote a system replacement, we tell you honestly whether your ducts and envelope are good enough to install new equipment as-is, or whether the system will underperform unless you address the underlying issues. We do not sell you equipment that is going to disappoint you.

Frequently asked questions

Q: How do I know if my current AC is the wrong size?

A: The biggest signs are short-cycling (system kicks on for less than 10 minutes at a time), high indoor humidity (over 55 percent in summer), uneven cooling room to room, or running constantly without satisfying the thermostat. Any of those usually points to a sizing problem, a ductwork problem, or an envelope problem. We can diagnose which.

Q: What size AC do I need for a 2,000 sq ft house in Montgomery?

A: There is no honest answer to that without a Manual J load calc. Probably 2.5 to 3.5 tons depending on insulation, windows, ductwork, and orientation, but I have quoted same-square-footage homes that needed wildly different tonnage based on the building envelope.

Q: Is it better to oversize or undersize an AC in Alabama?

A: Neither. Right-sized is the only correct answer. If forced to err in one direction, slight undersizing is less bad than slight oversizing in Alabama, because a slightly undersized system runs longer per cycle and pulls more humidity. But the goal is to size it correctly, not to err in either direction.

Q: What is the difference between SEER2 and tonnage?

A: Tonnage is capacity (how much heat the system can move per hour). SEER2 is efficiency (how much electricity it takes to move that heat). A 3 ton, 18 SEER2 system has the same cooling capacity as a 3 ton, 14 SEER2 system, but uses less electricity to deliver it. You need the right tonnage AND the right efficiency. They are independent specs.

Q: How much does a Manual J load calc cost?

A: We include it free with every system replacement quote. Standalone load calcs for builders or remodels start at $250 to $450 depending on home size and complexity.

Q: Can I just match the size of my old AC when I replace it?

A: Only if the old AC was sized correctly to begin with, which it often was not. Many homes are running oversized systems installed 15 to 25 years ago by contractors who guessed off square footage. A replacement is the right time to run the calc and right-size.

Q: Does Chad's serve Auburn and Lake Martin for AC sizing?

A: Yes. Our Dadeville location at 360 Windflower Dr (334-478-1438) covers Auburn, Dadeville, Tallassee, Eclectic, Blue Ridge, and the Lake Martin area. Montgomery location at 2546 Bell Rd (334-264-6464) covers Montgomery, Pike Road, Prattville, Wetumpka, Millbrook, Coosada, Deatsville, Mathews, Elmore, Tuskegee, and Old Cloverdale.

Q: Do you install variable-speed inverter systems?

A: Yes. We carry variable-speed lineups from Trane, Bryant, Daikin, and Mitsubishi. Variable-speed systems usually outperform single-stage equipment in Alabama because they run at part-load for long stretches, pulling humidity continuously. They are not always the right choice (budget, home size, ductwork condition all factor), but when they fit, they are a comfort upgrade you can feel.

Get a Manual J load calc from Chad's

If you are quoting a new system, building a new home, finishing a basement, putting on an addition, or just wondering why your current AC cannot keep up, we will run the load calc and tell you what your home actually needs. No high-pressure pitch. No guessing. We have been doing this in Central Alabama since 1993, we are family-owned and family-operated, we carry an A+ BBB accreditation, Alabama HVAC License #92244, and our customers have left 1,247 reviews at 4.9 stars (as of May 2026).