San Antonio homes run cooling equipment close to six months a year, so most owners here buy a diagnostic before they buy anything else. Trouble is, the word means two things. At one shop it means measured readings written down next to what they should have been. At another it means twenty minutes with a flashlight and a tablet full of replacement options.
The short answer
A real diagnostic produces numbers.
A tech opens the equipment, tests the electrical side, measures the refrigerant circuit, checks airflow, and records each reading next to what it should have been. Then he says which one is out of range. A verdict with no number attached to it is an opinion, however confidently it gets delivered.
So ask him what he measured.
The trade already wrote down what gets measured
None of this is a matter of taste. The Air Conditioning Contractors of America publishes ANSI/ACCA 4 QM 2019, Quality Maintenance of Residential HVAC Systems, and ANSI/ACCA 9 QIvp 2016, HVAC Quality Installation Verification Protocols. Both put tolerances on the job.
QIvp calls for measured superheat within 5 degrees of the manufacturer’s value for that model, subcooling within 3 degrees, airflow within 15 percent of design, and amperage and voltage inside the equipment maker’s specs. Standard 4 is the maintenance-task standard, and its instruction on capacitors is to replace any that are bulged, split, incorrectly sized, or off spec. That last one means metering the part against the rating printed on the can.
Every one of those checks produces a number a homeowner can be shown. None can be done by looking at the unit.
Why charging by feel gets the wrong answer
Refrigerant is where guessing costs the most. Piotr Domanski and Vance Payne of the National Institute of Standards and Technology, with Hugh Henderson of CDH Energy Corp, measured what common faults do to a system. In NIST Technical Note 1848, a 30 percent undercharge cut cooling capacity by roughly 15 percent and efficiency by about 12 percent.
Overcharging is sneakier. The same study found a 30 percent overcharge barely moved capacity while pushing total power draw up around 6 percent. Top off a system that didn’t need it and you hand the owner a higher bill and a house that feels the same.
The air temperature at the vent won’t settle it either. California’s Title 24 energy code verifies charge through superheat or subcooling rather than a temperature split, and for good reason: split moves with humidity and airflow. It’s a screening number that tells a tech whether to keep looking, not a charge test.
Refrigerant doesn’t get used up. If a system is low, something is letting it out, and finding that is the job you paid for.
The EPA tells homeowners to locate and repair leaks instead of topping off leaking systems.
The readings to ask for before anybody quotes a repair
Ask for these by name. A tech who ran a full diagnostic already has them:
- Line voltage at the disconnect and under load, against the data plate.
- Amp draw on the compressor and both motors, against nameplate rated load.
- Run capacitor microfarads, measured against the rating printed on the can.
- Superheat and subcooling, with your model’s target beside each.
- Return and supply air temperatures at the coil, plus the wet bulb.
- Total external static pressure both sides of the blower, against its limit.
- Condenser and evaporator coil condition, with a photo if the coil is somewhere you can’t see.
- Drain line, float switch, and safeties tested under power, not glanced at.
Why “your compressor is shot” is the call to question
The compressor is the most expensive part in the box, and it gets blamed for failures upstream of it.
There’s a real difference between a compressor that won’t start and one that can’t. Before anyone condemns it, a tech should have checked line voltage under load, the contactor for pitting, the run capacitor, and whether the internal overload opened because the motor is hot. An overheated compressor in 100-degree air will refuse to run, cool down over an hour, then start like nothing happened.
Locked rotor amps and winding resistance settle it. A few minutes separates a part that failed from one that got framed.
How this plays out in two San Antonio pockets
Same procedure, different questions, depending on what’s in the yard and what’s overhead.
Stone Oak and the 1990s and 2000s two-stories
Master-planned since the mid 1980s: stucco two-stories with vaulted ceilings. Plenty of that equipment is at or past the end of its service life, and that’s the house where “it’s shot, you need a whole system” shows up before anything is measured. A share of that older equipment still runs R-22, which the EPA stopped allowing to be produced or imported on January 1, 2020. They run on recovered and reclaimed supply now.
So the diagnostic here answers two questions: what failed, and what shape everything else is in. Ask for the refrigerant type off the data plate and readings on the healthy components too, because that’s what tells you whether the repair is worth making. Static pressure and a tested drain safety belong in the same visit.
Fair Oaks Ranch and the newer master-planned homes
Newer builds out here run large: tall ceilings and big glass, and either a zoned system or two systems splitting the house. The complaint is rarely a dead unit. It’s one room or one floor that won’t hold.
That’s an airflow and zoning diagnosis, so the readings shift: static pressure, room-by-room supply temperatures, damper travel, zone board operation, and which system serves that room. Adding refrigerant to a correctly charged system to chase a hot bedroom is how a comfort problem becomes a compressor problem. Newer equipment often still carries manufacturer parts coverage, so model and serial get pulled before anyone sells you a part.
How the diagnostic fee should be structured
Understanding what an AC diagnostic includes is half of it. The other half is how you’re billed.
One flat diagnostic fee. No separate trip charge on top of it. No “inspection package” that costs more than the fault-finding it replaces. And it applies toward the repair when you go ahead, so testing stops being a profit center and becomes the front end of the job.
Holding a scary bid from a chain shop? Bring it. We’ll test the system and tell you whether that bid matches what the equipment is doing. Most of our AC repair work in San Antonio starts with that visit. A system on a regular schedule gives cleaner numbers, which is part of why how often to service your AC is worth settling.
Signs you’re on a sales call instead of a service call
Any one of these can have an innocent explanation. Two or three together, ask questions:
- A replacement quote arrives before a measurement does. Nobody knows what a system needs in the first ten minutes.
- Refrigerant goes in with no leak search. That’s a repeat visit sold as a repair.
- The compressor gets condemned with no electrical test. Capacitor and contactor and line voltage all come first.
- A trip charge, a diagnostic fee, and an inspection package. One visit shouldn’t generate three line items before any work happens.
- Jargon that never resolves into a number. Ask what “low on freon” measured out to, and you should hear a superheat or subcooling figure.
- No license number on the paperwork. Texas rule 16 TAC 75.71 requires it on every proposal and invoice, with the TDLR regulated-by statement.
- He won’t write down what he found. Anything measured can be repeated by the next guy.
When the honest answer really is replacement
Sometimes it is. A dead compressor on a 20-year-old unit, or a coil leaking in three places on refrigerant that’s no longer produced. We’ll say so.
The difference is you’ll see how we got there. We’re family-owned, licensed in Texas as a Class A air conditioning and refrigeration contractor with a Combined endorsement, license TACLA00102222C, EPA 608 certified, and BBB A+ rated. Abel Ybarra has been in the trade since 2004. We’d rather lose a system sale than sell you one you didn’t need. Repairs carry our 90-day warranty, and financing is available when the honest answer is the costly one.
Call or text 210-763-4698, Monday through Saturday, 7 a.m. to 7 p.m.