Why Sizing Matters More Than Brand
Oversized System
Short cycles, poor humidity control, uneven temperatures, higher energy bills, premature equipment failure
An oversized unit cools quickly but shuts off before removing humidity. Your house hits temperature but feels clammy. The constant on-off cycling wears out the compressor faster.
Costs 15-30% more to operate, cuts equipment life by 3-5 years
Undersized System
Runs constantly, never reaches setpoint on hot days, excessive wear, high utility bills
The unit can't keep up with the heat load. On 100°F+ days, your house stays at 78°F even when set to 72°F. The compressor runs all day, running up your electric bill.
Bills 20-40% higher than necessary, shortened equipment life
Right-Sized System
Proper dehumidification, consistent temperatures, efficient operation, maximum equipment lifespan
Runs in longer cycles that properly dehumidify your home. Maintains even temperatures. Operates at peak efficiency. Lasts its full expected lifespan of 15-20 years.
Optimal operating costs and 15-20 year equipment life
The "One Ton Per 400 Sq Ft" Myth
This outdated rule was developed decades ago for poorly insulated homes with single-pane windows. Modern homes with R-49 attic insulation, Low-E windows, and proper air sealing need significantly less tonnage. Using this rule results in systems 30-50% oversized.
Real Texas example: A 2,400 sq ft home built in 2019 was quoted 6 tons by one contractor (using the "400 sq ft rule"). Our Manual J calculation showed 3.5 tons was correct. The homeowner who went with 6 tons complains about high humidity and $400+ summer electric bills. The homeowner who went with 3.5 tons has perfect comfort and $250 summer bills.
Texas AC Sizing: Rule of Thumb vs Reality
This table shows typical sizing for modern Texas homes (built 2000+) with adequate insulation. Older homes or homes with poor insulation may need more. Always get a Manual J calculation.
| Home Size | "Rule of Thumb" | Actual Range | Notes |
|---|---|---|---|
| 800-1,000 sq ft | 2.5-3 tons | 1.5-2 tons | Newer condos with good insulation may need even less |
| 1,000-1,400 sq ft | 3-3.5 tons | 2-2.5 tons | Single-story often needs less than two-story |
| 1,400-1,800 sq ft | 3.5-4.5 tons | 2.5-3 tons | Most common size we install |
| 1,800-2,200 sq ft | 4.5-5.5 tons | 3-3.5 tons | Two-story may need zoning, not larger unit |
| 2,200-2,600 sq ft | 5.5-6.5 tons | 3.5-4 tons | Consider two smaller systems for efficiency |
| 2,600-3,200 sq ft | 6.5-8 tons | 4-5 tons | Multiple zones almost always needed |
| 3,200-4,000 sq ft | 8-10 tons | 5-6 tons | Two or more systems recommended |
*Based on modern construction with R-38+ attic insulation and Low-E windows. Older homes may need 0.5-1 ton more. Always verify with Manual J calculation.
What Manual J Considers
A proper load calculation accounts for dozens of factors, not just square footage. Here are the major inputs that affect your sizing:
Building Envelope
R-13 walls vs R-19 walls can change sizing by 0.5 tons
R-30 vs R-49 attic insulation affects cooling load significantly
Leaky homes need more capacity; well-sealed homes need less
Single-pane vs Low-E windows can change load by 20%+
Location & Orientation
West-facing windows add significant afternoon heat gain
Mature shade trees can reduce cooling load 10-15%
Shade from nearby structures affects heat gain
North Texas uses 100-102°F design temp, affects sizing significantly
Internal Loads
Each person adds ~400 BTU/hr of heat
Ovens, stoves add heat during cooking
Home offices with multiple monitors add heat
Incandescent bulbs generate more heat than LEDs
Duct System
Attic ducts in Texas add 20-30% to cooling load
R-6 vs R-8 duct insulation affects losses
Leaky ducts can waste 20-30% of cooling
Undersized ducts restrict airflow, reduce efficiency
Why Contractors Oversize (The Uncomfortable Truth)
Callback Prevention
An oversized system won't generate "not cooling enough" complaints. Contractors who size by rule-of-thumb avoid callbacks by going big.
Reality: Short-term thinking that creates humidity problems, higher bills, and premature failure—all of which generate different callbacks later.
Higher Profit Margins
Bigger systems cost more, and many contractors mark up equipment by percentage. A 5-ton system earns more profit than a 3-ton system.
Reality: A properly sized system with correct installation is more profitable long-term through referrals and maintenance contracts.
Easier Sales Pitch
"Bigger is better" sounds logical to homeowners. It's an easy sell without explaining the nuances of HVAC sizing.
Reality: Educated homeowners increasingly understand that proper sizing delivers better comfort and lower bills.
Rule-of-Thumb Shortcuts
Manual J calculations take 1-2 hours. "One ton per 400-500 sq ft" takes 30 seconds. Some contractors skip proper sizing to save time.
Reality: Professional contractors factor load calculation time into their quotes. It's part of doing the job right.
Lack of Training
Many HVAC techs were never taught proper load calculations. They size based on what the last system was or what "feels right."
Reality: NATE-certified contractors and ACCA members are trained in Manual J methodology. Ask about certifications.
Common Sizing Mistakes to Avoid
Matching the old system size
Problem: The old system may have been oversized too. Or your home's insulation, windows, or layout has changed.
Solution: Always perform a new Manual J calculation, regardless of what was there before.
Using square footage alone
Problem: A 2000 sq ft home built in 1985 with R-19 attic insulation needs very different sizing than a 2000 sq ft home built in 2020 with R-49 and spray foam.
Solution: Consider insulation, windows, orientation, duct location, and occupancy—not just square footage.
Sizing for extreme peak days
Problem: Sizing for 110°F days means the system is oversized 99% of the time. Better to design for 100-102°F and tolerate slightly higher temps on rare extreme days.
Solution: Use ASHRAE design temperatures for North Texas (100-102°F), not worst-case scenarios.
Ignoring attic duct losses
Problem: Attic temperatures reach 140-160°F in Texas summers. Uninsulated or poorly insulated ducts can add 20-30% to your cooling load.
Solution: Account for duct location and insulation level in load calculations. Consider encapsulated attics or moving ducts inside.
Adding "safety factor" to calculations
Problem: Some contractors add 10-20% "just to be safe." This built-in oversizing compounds other errors.
Solution: Manual J already includes appropriate safety margins. No additional factor needed when done correctly.
Not considering zoning alternatives
Problem: A 3000 sq ft home doesn't need a 5-ton system—it needs two 2.5-ton systems serving different zones.
Solution: For large or multi-story homes, consider zoned systems rather than one oversized unit.
How to Verify Correct Sizing
Did they perform a Manual J calculation?
Gave a size without measuring or asking detailed questions
Measured rooms, asked about insulation, windows, and occupancy
Why it matters: Manual J is the only accurate way to determine proper sizing
What design temperature did they use?
Doesn't know or says "worst case scenario"
Specifies 100-102°F for North Texas
Why it matters: Design temp affects sizing significantly—wrong temp means wrong size
How did they account for duct losses?
Didn't mention ducts or used a standard assumption
Noted duct location, insulation level, and estimated losses
Why it matters: Attic ducts in Texas can add 20-30% to cooling requirements
Can you see the calculation results?
Won't provide documentation or says it's proprietary
Provides printed Manual J report showing all inputs and calculations
Why it matters: Professional contractors document their work and share it with homeowners
Why is this size different from my current system?
Can't explain or says "bigger is always better"
Explains specific factors that led to the sizing recommendation
Why it matters: A knowledgeable contractor can justify their sizing recommendation
Questions to Ask Your Contractor
"Will you perform a Manual J load calculation?"
"Yes, we do a full load calculation on every install."
"We'll match what you have now" or "We size by square footage."
"What design temperature do you use for North Texas?"
"100-102°F outdoor, 75°F indoor per ASHRAE guidelines."
"We use 110°F to be safe" or doesn't know.
"How do you account for my attic duct system?"
"We factor in duct location, insulation, and estimated losses."
Doesn't mention ducts or uses a generic assumption.
"Can I see the load calculation report?"
"Absolutely, we'll include it with your proposal."
"That's proprietary" or "We don't provide that."
"What if my home doesn't cool well on extreme days?"
"A properly sized system may not maintain 72°F on 110°F days, but that's only a few days per year. The tradeoff is better humidity control and efficiency the other 360 days."
"We'll size it bigger so you never have problems" (this creates other problems).
Real Sizing Examples from North Texas
2,400 sq ft two-story in Frisco (built 2019)
Customer was quoted 5 tons by two companies. We calculated 3.5 tons. After installation: even temperatures upstairs and downstairs for the first time, 25% lower electric bills, no more "clammy" feeling.
Lesson: Modern construction needs less tonnage than older rules suggest.
1,800 sq ft single-story in Plano (built 1987)
Original system was 4 tons, always humid inside despite running constantly. Load calculation showed 3 tons was correct. New system runs longer cycles, removes humidity properly, and cut bills by 20%.
Lesson: Even older homes are often oversized. Longer cycles = better comfort.
3,200 sq ft two-story in McKinney (built 2005)
Single 6-ton unit couldn't balance upstairs/downstairs. Replaced with two 3-ton systems—one per floor. Perfect temperature control, 30% energy savings, each system lasts longer because it's not overworked.
Lesson: Large homes need zoning, not bigger equipment.
Frequently Asked Questions
What size AC do I need for a 2000 square foot house in Texas?
A 2000 sq ft house in Texas typically needs 3-3.5 tons, NOT the 5 tons that rule-of-thumb would suggest. However, actual sizing depends on insulation levels, window efficiency, ceiling height, duct location, and home age. A 2000 sq ft home built in 2020 with modern insulation might only need 2.5 tons, while a 1980s home with original insulation might need 3.5 tons. The only accurate way to determine size is a Manual J load calculation.
Why is my HVAC system oversized?
Most systems are oversized because: (1) contractors use outdated "one ton per 400-500 sq ft" rules, (2) oversizing prevents "not cooling" callbacks, (3) bigger systems cost more (higher profit), (4) some contractors skip proper load calculations to save time. Unfortunately, oversized systems short-cycle, fail to dehumidify, and wear out faster. If your home feels clammy even when cool, your system is likely oversized.
What is a Manual J load calculation?
Manual J is the industry-standard method (developed by ACCA) for calculating heating and cooling loads. It considers square footage, insulation values, window area and type, orientation, infiltration, duct losses, internal heat gains, and local climate data. The result is a BTU requirement that translates to equipment tonnage. A proper Manual J takes 1-2 hours and requires measuring your home and asking detailed questions about construction.
Can my AC be too big?
Yes, and it's extremely common in Texas. An oversized AC cools too quickly, shutting off before removing humidity. The result: your home hits temperature but feels clammy. The short on-off cycles also waste energy (compressors use more power starting than running) and wear out equipment faster. Properly sized systems run longer cycles, remove more humidity, and last longer.
How many tons of AC per square foot in Texas?
The old rule was "one ton per 400-500 sq ft" but this dramatically oversizes modern homes. A more accurate starting point for Texas is one ton per 600-800 sq ft, but this still doesn't account for insulation, windows, or duct location. Real sizing: 1,500 sq ft home = 2-2.5 tons (not 3-4), 2,000 sq ft = 2.5-3.5 tons (not 4-5), 2,500 sq ft = 3-4 tons (not 5-6). Always get a Manual J calculation.
What happens if AC is undersized?
An undersized AC runs constantly on hot days and never reaches your desired temperature. On 100°F+ days, your home might only cool to 78°F even set to 72°F. While this sounds bad, slight undersizing is actually better than oversizing—the system still dehumidifies properly and runs efficiently. The real problem is significant undersizing (more than 0.5 tons below proper size), which causes excessive energy use and equipment strain.
Should I size my AC for the hottest days?
No. HVAC systems should be sized for "design temperature" conditions (100-102°F for North Texas), not extreme peaks (110°F+). On the handful of extreme days per year, a properly sized system may only cool to 76°F instead of 72°F. That's acceptable—sizing for extremes means your system is oversized 99% of the time, causing humidity problems, wasted energy, and premature failure.
Does ceiling height affect AC sizing?
Yes. Standard Manual J calculations assume 8-foot ceilings. Higher ceilings mean more cubic feet of air to cool. Add approximately 10% capacity for 9-foot ceilings and 20% for 10-foot ceilings. However, this is already factored into proper Manual J calculations—it's one of many inputs, not a separate adjustment.
How do I know if my contractor sized correctly?
Ask to see the Manual J calculation report. A proper calculation includes: your home's measurements, insulation values, window specifications, duct system details, design temperatures used, and resulting BTU/tonnage requirement. If a contractor can't provide this documentation or sized by "matching the old system," they didn't do proper sizing. Get multiple quotes and compare sizing recommendations.
What is the design temperature for North Texas HVAC?
ASHRAE (American Society of Heating, Refrigerating, and Air-Conditioning Engineers) specifies 99-102°F outdoor design temperature for the Dallas-Fort Worth area, depending on exact location. Most calculations use 100-101°F. This means your system is designed to maintain 75°F inside when it's 100-101°F outside. On the rare days exceeding design temp, indoor temperature may rise slightly above setpoint.