By mid-June, most San Antonio homes run cooling equipment more hours than not. Our cooling season stretches close to six months, August highs sit near 96 degrees, and the humidity holds through nearly all of it. Thermostat advice written for Phoenix or Denver falls apart the second you try it on a century-old bungalow here.
The short answer
The best thermostat setting in San Antonio comes down to two numbers and one rule.
Hold the house near 78 degrees while you’re home. Let it climb to 82 or 85 when the place sits empty eight hours or longer, and start pulling it back down before anyone walks in. Older homes with leaky envelopes take a shallower setback, closer to 3 or 4 degrees, for reasons the neighborhood section gets into.
Leave the system running. Do not shut it off.
That last part is where people lose the money they thought they were saving. Raising the setting while you’re gone captures nearly all of the savings on the table. Killing the system captures a bit more on paper, then charges you back in humidity and worn parts.
What the research actually found
The U.S. Department of Energy puts a number on the setback. Adjusting your thermostat by 7 to 10 degrees for 8 hours a day cuts heating and cooling costs by up to 10 percent a year.
Patrick Phelan, a mechanical engineering professor at Arizona State University, breaks it down smaller. Every degree you go up saves roughly 3 percent on cooling. So 78 at home and 85 at work is a 7-degree swing, which lands right inside the band the DOE measured.
Short absences are wasted effort. Elizabeth Hewitt, who studies energy use at Stony Brook University, said it about as plainly as it can be said: if you’re gone fifteen minutes to run to the grocery store, you don’t get any gain. The house has to sit empty long enough to actually drift.
Why shutting it off backfires in a humid city
Your air conditioner has two jobs. It drops the temperature, and it pulls water out of the air. Switch it off for nine hours in July and the second job stops.
The EPA recommends holding indoor relative humidity below 60 percent, ideally between 30 and 50, because mold gets comfortable above that line. Walk into an 84-degree house with soaked air and your system has to strip out the heat and the water both, which takes far longer than most people expect.
Your equipment pays for it too. Phelan notes that a cooling system needs 15 to 30 minutes of run time before it reaches peak efficiency, and that hard cycling leads to more frequent repairs. We replace capacitors and contactors all summer on units that get slammed from a dead stop to full load every afternoon.
A setback saves you money. A shutoff just moves the bill from the electric company to your repair guy.
Eight moves that make the setback work harder
The thermostat is one lever. These are the others, roughly in order of what they give back:
- Close blinds and curtains on west and south glass before noon, not after the room already gets hot.
- Skip the open-the-windows-at-night trick. It works in dry climates. Here it loads your house with moisture your system has to pull back out tomorrow.
- Run ceiling fans only in rooms someone is sitting in, since a fan moves heat off skin without lowering room temperature.
- Change the filter monthly during cooling season. A loaded filter stretches every recovery cycle.
- Put reflective film on your worst west-facing windows.
- Seal the attic hatch and any recessed can lights. Both leak conditioned air straight up.
- Keep furniture and rugs off supply and return registers.
- Check your attic insulation depth. Older homes here are routinely running well short of what current code calls for.
The sweet spot shifts by neighborhood
Gregor Henze, an architectural engineering professor at the University of Colorado Boulder, points out that heavy homes hold their cool while drafty homes lose it fast. San Antonio has both, sometimes on the same block.
Woodlawn Lake and the 1920s bungalows
Abel lives out here, so we work these streets constantly. The stock around the lake and over toward San Pedro Springs Park runs to 1920s Craftsman bungalows: wood frame, no thermal mass, and ductwork retrofitted decades after the houses went up. These houses shed cool air fast, so a 3-to-4-degree setback is the honest ceiling until the envelope gets addressed. Push one to 85 and the system runs from four o’clock to bedtime chasing it.
Castle Hills and the 1950s ranches
Long, low houses on quarter-acre and half-acre lots, usually on a single-stage system that was sized before somebody added a room. The spread-out footprint takes a deeper setback fine. The far bedrooms come back last, though, and that’s a zoning conversation rather than a thermostat one.
Stone Oak and the 1990s two-stories
Stucco and vaulted ceilings, with a second floor that collects everything the first floor gives up. These take the deepest setback of the three and need the earliest recovery. Wait until you’re pulling into the driveway to start cooling and the upstairs won’t be right until after dinner.
Time the recovery around the grid
CPS Energy runs conservation events on summer weekdays, generally between 1 p.m. and 10 p.m., briefly shifting enrolled thermostats a few degrees. That window is worth knowing even if you never sign up. It marks the hours when power costs the most to produce and when your system fights the hottest, brightest part of the day.
Start the recovery before that window closes, not after. Pre-cooling a little earlier costs less than clawing back ten degrees at 6 p.m. with a superheated attic radiating down through the ceiling.
CPS Energy also runs a WiFi Thermostat Rewards program that pays bill credits to customers who enroll a qualifying smart thermostat and allow those brief adjustments. You keep override control at the thermostat and in the app.
What has to be right for the strategy to work
A schedule can’t rescue a system that’s fighting the house it sits in. Before you blame the setback, check these:
- Equipment sizing. Oversized units cool fast, shut off before they dehumidify, and leave you clammy at 74 degrees.
- Duct integrity. Leaky attic runs dump cooled air where nobody lives. Sealing and repairing them is often the single biggest change we make on an older home.
- Refrigerant charge. A low charge stretches recovery time until the setback stops paying for itself.
- Coil condition. A dirty evaporator or condenser coil does the same thing, quietly.
- Blower and airflow. A weak blower or a crushed flex duct turns a 30-minute recovery into a two-hour one.
- Thermostat placement. A stat on a sunlit wall or above a lamp reads high and runs the system past the point it needed to.
- Whether anyone programmed it. Plenty of houses have a programmable thermostat that nobody ever set up.
Want the schedule handled properly? A thermostat that can actually run a setback needs the right wiring behind it, plus staging matched to your equipment and a schedule built around when your family is really home. That work rides along with any air conditioning installation in San Antonio, and it’s worth sorting out before you spend another summer fighting a stat nobody set up.
When the thermostat isn’t the problem
If your house can’t hold 78 on a normal afternoon, no setting will save it. That’s a capacity or airflow or charge problem, and a new thermostat won’t touch any of them.
We’re family-owned, licensed in Texas as a Class A air conditioning and refrigeration contractor, EPA 608 certified, and BBB A+ rated. We’d rather tell you the truth than sell you a box. Bring us the symptom. We’ll test the full cooling cycle and tell you what failed in plain English, and the repair carries our 90-day warranty.
Call or text 210-763-4698, Monday through Saturday, 7 a.m. to 7 p.m.