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An oversized air conditioner blows cold air on day one. It just never dehumidifies, and it wears itself out doing it. Here is what a load calculation is, why it goes wrong most often in exactly this kind of housing, and why out here nothing checks it but you.
Sizing an air conditioner is an engineering calculation, not a lookup. The residential standard is ANSI/ACCA 2 Manual J — the eighth edition, published 2016 — and it is the only residential load-calculation method the International Residential Code and the International Energy Conservation Code reference. IRC section M1401.3 and IECC section R403.7 both require equipment to be selected (under ACCA Manual S) from loads calculated under Manual J or another approved method.
A proper Manual J takes the house as it is: orientation, glazing area and type on each elevation, insulation levels, ceiling heights, infiltration, duct location and duct leakage, the number of people who live there. It produces two numbers, not one — a sensible load, which is the heat, and a latent load, which is the moisture. In Fort Bend County the latent number is the one that decides whether the house feels right, and it is the one a rule of thumb cannot produce.
What a load calculation is not: square footage divided by 500. The tonnage on the old nameplate. What the builder put in the identical house next door. All three are in circulation and all three are how oversizing happens.
EPA’s own guidance on right-sizing is blunt about the incentive: “It is common for contractors to install oversized air conditioners because these units provide cooling more quickly, thus avoiding any chance of not meeting the cooling demand.” Bigger is the safe commercial choice. Nobody calls back in July to complain that the house got cold too fast.
The mechanism that makes it a problem is specific to humid climates. Air conditioning dehumidifies by condensing moisture on a cold evaporator coil and draining it away — and it only does that while running. An oversized system satisfies the thermostat in short bursts. EPA describes those units as short-cycling, running “for shorter periods of time than engineered for optimum operation,” and notes that “short operation times do not allow the system to effectively remove humidity” — worse, that short cycles “even allow some moisture to evaporate back into the air” from a coil that never gets a chance to drain.
Houston Hobby’s mean August dew point is 73.5°F. That is the latent load walking in through every door and every leak in the return side. In a dry climate an oversized unit is merely inefficient. Here it produces a specific, familiar complaint: the thermostat says 73 and the house feels cold and damp at the same time. People turn it down further, which makes it worse, and eventually they conclude they need a bigger system.
The system starts, runs a few minutes, stops, and starts again soon after. Long, steady run times are what a correctly sized system does in August, not a sign that it is struggling.
The classic signature. Temperature is met, moisture is not. Relative humidity indoors sitting above roughly 55% through the summer is the measurable version.
Both are humidity, not dirt. Supply registers sweating in a conditioned house means the air around them is wetter than the metal is cold.
Every start is the hardest thing a compressor does. A system that starts twice as often as it should ages roughly on that schedule, which is how a nine-year-old condenser fails.
This is the part that is specific to Fort Bend County, and it is the honest reason to ask the question yourself.
Inside a city, a mechanical permit is a backstop. Missouri City, Richmond, Needville, Meadows Place and Beasley all issue mechanical permits, and several require the contractor to be registered with the city before pulling one. That process will not audit an engineer’s spreadsheet, but it puts a second name on the work and a plan review between the quote and the equipment.
In unincorporated Fort Bend County there is no such step. The county’s own building-codes page states plainly that “Fort Bend County has not adopted zoning ordinances and does not issue Certificates Of Occupancy”, and a county verification letter puts the rest of it beyond doubt: “the County has not adopted building codes for single family residential developments.” The county’s list of what does trigger a permit in unincorporated areas — floodplain work, an enclosed structure or slab of 400 sq ft or more, land development, large fill or excavation, driveways, turn lanes, utility work in the right of way — contains no mechanical trigger at all. A full system changeout trips none of it.
The extraterritorial jurisdiction of a city does not close the gap either. The Texas Supreme Court held in Town of Lakewood Village v. Bizios (2016) that a general-law municipality cannot enforce its building codes or permit requirements in its ETJ, and the Dallas Court of Appeals extended the reasoning to home-rule cities in Collin County v. City of McKinney (2018): every municipality needs legislative authorization to enforce building codes beyond its corporate limits, and none has it. So a house in Cinco Ranch, Aliana, Harvest Green or Jordan Ranch sits under unincorporated-county rules for this purpose whatever the mailing address says.
None of that changes the license. Air conditioning and refrigeration contracting is licensed by TDLR statewide, and the requirement applies identically on both sides of a city line. What changes is enforcement: inside a city the permit is the moment somebody checks, and outside one, enforcement is effectively complaint-driven. Which means the load calculation is the only quality control on the job, and you are the only person who will ever ask for it.
Ask for the Manual J. Not “did you do a load calculation” but “can I have a copy of the load calculation with the quote.” It is an ordinary deliverable and a contractor who does them will hand one over without being asked twice.
Then look for three things on it: your address at the top, not a generic model; a separate sensible and latent figure rather than a single total; and a selected capacity close to the calculated load rather than rounded up a full ton “for the heat.” ACCA Manual S allows a margin, but a two-ton gap between the calculation and the equipment means the calculation was decoration.
If the answer is that nobody does those on residential work around here, that is useful information about the quote you are holding.
ANSI/ACCA 2 Manual J is the residential load-calculation standard, and it is the only one the International Residential Code and the International Energy Conservation Code reference. It takes the actual house — orientation, glazing, insulation, infiltration, duct location and leakage, occupancy — and produces a sensible load and a separate latent load. Equipment is then selected against it under ACCA Manual S.
Because a system removes humidity only while it is running. An oversized unit meets the thermostat setpoint in short bursts, so it never runs long enough to pull moisture out of the air, and some of what does condense evaporates back off the coil between cycles. In a climate with a mean August dew point around 73.5°F that produces a house that is cold and clammy rather than cool and dry, plus compressor wear from the extra starts.
No. The county has not adopted building codes for single-family residential development in unincorporated areas, does not issue certificates of occupancy, and has no mechanical permit trigger on its own permit list. A city cannot fill the gap in its extraterritorial jurisdiction either, following Town of Lakewood Village v. Bizios (Tex. 2016) and Collin County v. City of McKinney (Tex. App.—Dallas 2018). Inside city limits the answer is different, and several Fort Bend cities do issue mechanical permits.
Yes, everywhere in Texas without exception. Air conditioning and refrigeration contracting is licensed by the Texas Department of Licensing and Regulation, and Occupations Code §1302.251 bars performing or offering to perform it without a license regardless of whether the property is inside a city. What differs between a city and unincorporated county is enforcement, not the legal requirement — inside a city the permit is when somebody checks.
Not credibly. The inputs are physical: which way the glazing faces and how much of it there is, what is in the attic, where the ducts run and how leaky they are, how many people live there. Software makes the arithmetic fast, but the measurements have to come from the building. A load calculation produced from square footage alone is the rule of thumb wearing a spreadsheet.
It is the one document that tells you whether the equipment was chosen for your house. Get connected with an independent TDLR-licensed Fort Bend County contractor who will produce one.
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