You set the thermostat to 74 and come back to find it at 78, or 85, or a schedule you never made. It feels like a malfunction, and occasionally it is. More often, something is changing the setting on purpose. Here is how to find out what.
Start with the obvious: a schedule
Most programmable and smart thermostats run a schedule, and a schedule overrides a manual change at the next time period. If your change “reverts” at a particular time of day, that is almost certainly the cause. Look for a hold option. Temporary hold keeps your setting until the next period. Permanent hold keeps it until you cancel it.
Smart thermostat features
Smart thermostats can change temperatures on their own by design:
- Learning features build a schedule from your adjustments and may apply it later.
- Away or eco modes triggered by geofencing on a phone, or by motion sensors, raise or lower the setpoint when the house looks empty. A phone with location turned off, or someone who left their phone at work, can trigger them at the wrong time.
- Other household members with the app on their phones.
Utility demand response programs
Many utilities, including in the Sacramento area, run programs that adjust enrolled smart thermostats during peak demand on hot afternoons in exchange for a bill credit or rebate. If your thermostat moves up a few degrees on a hot weekday afternoon and returns later, check whether it was enrolled, perhaps by a previous owner or during a rebate signup. The app or the utility account usually shows these events, and most programs let you opt out of individual events.
Low batteries
Weak batteries cause strange behavior in many thermostats: resetting to defaults, losing the schedule, or jumping to a preset temperature. If your thermostat uses batteries, replace them first, even if the low battery warning has not appeared.
Power interruptions
After an outage, a tripped breaker, or a blown fuse on the furnace control board, some thermostats restart with a default setpoint and schedule. Check whether the change lines up with a recent outage. If it keeps happening with no outage, the thermostat may be losing power intermittently, which points to wiring or a board problem. See a furnace blowing its fuse.
Where the thermostat sits
If the setpoint is not changing but the system behaves as if it were, the thermostat may be reading the wrong temperature. Direct afternoon sun, a lamp, a TV, a kitchen, or a supply vent nearby will all do it. Compare its reading with a separate thermometer placed beside it.
When it really is the thermostat
- The display flickers, freezes or goes blank
- It will not hold a setting even on permanent hold with fresh batteries
- It reads several degrees off from a thermometer beside it
- Buttons stop responding
At that point replacement is usually cheaper than repair. A standard thermostat replacement is a short visit, and wiring questions are covered in our thermostat not working guide.
A note for heat pumps
If you have a heat pump and the thermostat shows auxiliary or emergency heat turning on by itself, that is usually the thermostat bringing in backup heat because the heat pump is not keeping up, not a random setting change. See heat pump running constantly.
What we do
If the thermostat itself or its power supply is the problem, we check voltage at the thermostat, the wiring and the control board, and replace the thermostat if needed. Our diagnostic is a flat $69 and it is credited toward the repair. See also thermostat blinking Cool On.
Frequently asked questions
Why does my thermostat keep changing temperature on its own?
The most common causes are a programmed schedule overriding your change, smart thermostat learning or away modes, a utility demand response program, low batteries, or a power interruption resetting it.
Why does my thermostat set itself to 85?
Often an away, eco or vacation mode, a schedule period, or a utility program adjusting the setpoint during peak demand. Check the thermostat app or settings for recent events.
Can the power company change my thermostat?
If the thermostat is enrolled in a utility demand response program, yes, within the program’s terms during peak events. The utility account or thermostat app usually shows enrollment.
Can low batteries cause a thermostat to change settings?
Yes. Weak batteries can cause resets, lost schedules or jumps to default settings. Replace them first.
How do I stop my thermostat from changing temperature?
Use permanent hold, turn off learning or away features if you do not want them, check for utility program enrollment, and replace the batteries.
When should I replace my thermostat?
When it will not hold settings with fresh batteries, reads several degrees off, or has a flickering or unresponsive display.