Thermostat Settings Explained: What Actually Saves You Money
Most people use about three buttons on their thermostat and guess at the rest. Here's what every setting does, in plain English — and which ones actually affect your bill.
System mode: Heat / Cool / Auto
- Heat — runs the furnace or heat pump only.
- Cool — runs the AC only.
- Auto — switches between heating and cooling to hold your setpoint. Convenient in spring and fall, but in mild weather it can cause the system to bounce between modes. Many techs (me included) prefer picking Heat or Cool for the season.
Skip "Off" except when you're servicing the system — and give the system a few minutes between switching modes. Rapid heat-to-cool flipping is hard on equipment.
Fan: ON vs. AUTO
- AUTO (recommended for most people): the fan runs only when heating or cooling. This is the efficient choice.
- ON: the fan runs 24/7, circulating air constantly. It evens out hot and cold spots and keeps air moving through the filter — but it uses more electricity and can make humidity feel worse in summer, since moisture on the coil re-evaporates back into the house between cooling cycles.
If someone in the house has allergies, running the fan more can help filter the air — just know it costs extra.
Emergency / Aux heat (heat pumps only)
This is the expensive one. Emergency heat switches your heat pump off and runs on backup electric heat strips (or gas), which cost far more to run. It's there for actual emergencies — like the heat pump failing on a freezing night — not for "it's a little chilly." If your thermostat shows "AUX" running constantly in mild weather, something's wrong; that's worth a service call.
What actually saves money
- Set it and stop fiddling. Constant adjustments don't help. Pick a comfortable temperature and leave it.
- Use setbacks when you're away or asleep. Dropping the temperature while you're at work or sleeping saves real energy — a programmable or smart thermostat does this automatically so you don't have to remember.
- Don't overdo the setback with a heat pump. Huge setbacks can trigger expensive aux heat to recover. A few degrees is plenty.
- Stop setting it to 65 to "cool faster." It doesn't work faster. It just runs longer and overshoots. Set the temperature you want.
- Keep the filter clean. A clogged filter makes every setting on this list work harder.
The thermostat isn't magic — it's a switch with preferences. Reasonable setpoints, minimal fiddling, and a clean filter will beat any "trick" you've seen online. More no-nonsense homeowner guidance at HVAC DIY Help on TikTok and YouTube (@hvac_diy_help).