Changing Thermostat Temperature

The thermostat is the only part of the HVAC system most people touch, and how it gets used has more effect on the electric bill than the efficiency rating of the equipment. In San Antonio, where the air conditioner runs seven months a year, a few habits around temperature changes can save real money and keep the system from wearing out early.

Small changes, big runtime

Every degree you lower the thermostat below the outside temperature in summer adds roughly 3 to 5 percent to cooling cost. Going from 78 to 72 on a 98 degree day is not a small change; it is 20 to 30 percent more runtime. The system does not cool faster when you set it lower, it only runs longer, so setting it to 65 to "cool the house down quick" does nothing except overshoot and waste money.

The setback strategy that works here

The Department of Energy's general advice is to set back the thermostat when you are away. In San Antonio's humidity that has a limit. Letting the house climb to 85 while you are at work means the system has to remove eight hours of accumulated heat and moisture from the drywall, furniture, and slab when you get home, and the house feels sticky for an hour even after the air reaches 76. A setback of four degrees, from 76 to 80, captures most of the savings without the recovery penalty. Program the system to start cooling 45 to 60 minutes before you return.

Leave it alone once it is set

Constantly nudging the thermostat up and down is the habit that costs the most. Each adjustment triggers a new cycle, and startup is the hardest moment in a compressor's life. Pick a temperature, program it, and let the system hold it. If a room is uncomfortable at the set point, the fix is airflow or duct balancing, not a lower thermostat.

Fan on auto, not on

Running the fan continuously keeps air moving but it also re-evaporates the water sitting on the coil between cycles and pushes that humidity back into the house. In this climate, auto is the better setting for most homes. The exception is a house with a serious hot-room problem, where continuous fan can even things out at some cost in humidity.

Winter settings

For heating, 68 when home and 62 to 64 overnight or away is comfortable for most families and easy on a gas furnace. Heat pump owners should avoid large setbacks; a heat pump recovering from a big drop will kick on the electric auxiliary strips, which cost three times as much to run as the heat pump itself. Keep winter setbacks to two or three degrees on a heat pump.

Smart thermostats

A smart thermostat learns the schedule, pre-cools before you arrive, adjusts for humidity, and sends an alert if the house temperature climbs unexpectedly, which is the earliest warning of a failed system while you are away. Most models also report runtime, which is a useful way to notice a system that is starting to struggle. Installation takes under an hour and we can set it up during any service visit.

If the thermostat is the problem

A thermostat that reads several degrees off, loses its schedule, or triggers short cycling can make a healthy system act broken. Placement matters too; a thermostat on a wall that gets afternoon sun or sits near a supply vent will never control the house well. If yours seems off, our diagnostics visit includes thermostat calibration and placement review. Call 210.549.4839.