Why most quotes still use the rule of thumb
The 'one ton per 400 square feet' rule is fast, free, and usually wrong. It was rough back in 1985 when most homes had similar windows, single-pane glass, and 4-inch wall insulation. Today's Houston homes vary wildly - same square footage can have R-13 or R-21 walls, single-pane or low-E double-pane glass, west-facing or shaded exposure, 8-foot or 14-foot ceilings.
ACCA (Air Conditioning Contractors of America) publishes Manual J - the technical standard for residential cooling and heating load calculations - and has since the early 1980s. It is the right way to do this. The Texas Department of Licensing and Regulation requires licensed HVAC contractors to follow accepted engineering practice; Manual J is that practice. Companies that skip it because they are 'too busy' are skipping the part that determines whether your home will be comfortable for the next 15 years.
What Manual J actually evaluates
A proper Manual J load calculation, done either with software (Wrightsoft, Cool Calc, Elite RHVAC) or a careful spreadsheet, accounts for:
- Wall, ceiling, and floor R-values (insulation actually installed, not just code minimum).
- Window area, orientation (N/S/E/W), glass type (single, double, low-E, tinted), and exterior shading.
- Air leakage (ACH50 from a blower door test, or estimated from age and construction).
- Internal gains: occupants, lights, appliances, electronics.
- Indoor design conditions (typically 75 deg F at 50% RH for Houston).
- Outdoor design conditions for your specific zip code (Houston ASHRAE 1% design dry bulb is approximately 96 deg F).
- Latent load (humidity removal) separate from sensible load (temperature drop).
The output is two numbers: total cooling load in BTU/hr (which divides by 12,000 to give tons), and the sensible-to-total ratio, which tells you how much of that capacity is doing dehumidification. In Houston, that ratio matters - a lot.
Why oversizing is worse than undersizing
Most contractors would rather 'play it safe' and put in a bigger system. That logic was a hangover from when systems were undersized in the 1970s. In a humid climate, oversizing is a disaster:
- Short cycles. An oversized AC hits the thermostat setpoint in 5-7 minutes and shuts off. The evaporator coil never gets cold enough for long enough to wring water out of the air.
- Clammy comfort. The air is cold but the relative humidity stays at 60-65 percent. You crank the thermostat lower; bills go up; the room still feels wrong.
- Mold-friendly conditions. Sustained indoor RH above 60 percent feeds mold growth in cabinetry, behind drywall, and inside ductwork.
- Component wear. Compressors hate frequent starts. An oversized system can see 8-12 starts per hour vs 2-3 for a properly sized unit. That is 4x the start-cycle wear over equipment lifetime.
What a real quote looks like
When Avatex quotes a replacement system, you receive in writing:
- A printed Manual J load calculation (cover sheet with cooling and heating loads in BTU/hr).
- A Manual S equipment selection (how the chosen system matches the load).
- A Manual D duct verification (existing duct sizes vs the airflow the new system needs).
- The AHRI certificate confirming the indoor and outdoor units are a matched pair.
- Model numbers, SEER2 ratings, and any matching tax-credit or rebate documentation.
If a contractor cannot or will not provide these, that is a tell. We are happy to walk you through any other quote you have received side-by-side - it is the fastest way to see whether someone has actually done the engineering, or just guessed.