What Your Thermostat Settings Actually Do
Why this matters
A thermostat is a switch with opinions, and most of its buttons do something a little different from what their labels suggest. A fan left on "On" can make a house more humid in summer. Emergency heat on a heat pump can make the next bill look like a mistake. Setting the temperature to 85 to warm up faster does not warm the house any faster. None of these are failures; they are settings doing exactly what they are designed to do. Knowing what each one does keeps you comfortable, keeps the bill predictable, and keeps you from calling for a repair the system does not need.
The screen, labeled
Every brand lays it out differently, but nearly every thermostat has these same controls. Here is a generic one, set the way a heat pump home might be on a winter morning.
+------------------------------------------+
| INSIDE 66 F SET TO 68 F | <- 1
| MODE: HEAT | <- 2
| FAN: AUTO | <- 3
| [ AUX HEAT ] | <- 4
| SCHEDULE: WAKE 6:00 AM 68 F | <- 5
| HOLD: OFF | <- 6
+------------------------------------------+
1 inside reading vs the target you chose
2 HEAT / COOL / AUTO / OFF / EM HEAT
3 AUTO runs the fan only with heat or cool
4 lit: backup heat is helping right now
5 the next scheduled change
6 HOLD freezes the setting, ignoring 5
Mode: heat, cool, auto, off, and emergency heat
- Heat and Cool do what they say: the system only heats, or only cools.
- Auto switches between heating and cooling on its own to keep the house inside a range you set. It is handy in spring and fall. Most thermostats keep a gap of a few degrees between the heat and cool targets so the two do not fight.
- Off turns heating and cooling off. On many thermostats the fan can still run on its own setting.
- Emergency heat (EM HEAT), on a heat pump only, is covered in its own section below, because it is the setting that most surprises people.
Fan: Auto versus On
Auto runs the indoor blower only while the system is heating or cooling. This is the right default for most homes.
On runs the blower all the time, whether or not the system is heating or cooling. It mixes air between rooms and runs more air through the filter, which some people want. The costs:
- Higher humidity in summer. During cooling, the indoor coil is wet with water it has pulled out of the air. When the compressor stops and the fan keeps blowing, much of that water evaporates right back into the house before it can drain away. A house that feels cool but clammy with the fan on "On" often improves just by switching to Auto.
- More electricity. The blower motor runs every hour of the day instead of only during heating and cooling cycles. How much that adds depends on the motor; newer variable-speed blowers use far less than older ones.
- Drafts in winter. Air moving through ducts in a cold attic or crawlspace can come out of the registers feeling cool between heating cycles.
Some thermostats have a Circulate setting that runs the fan for part of each hour, which is a middle ground.
Hold versus schedule
A schedule changes the target temperature at set times, such as cooler at night, warmer before you wake.
A temporary hold (often what happens when you press up or down) keeps your new temperature only until the next scheduled change, then the schedule takes over again.
A permanent hold keeps your temperature until you cancel it, ignoring the schedule completely. If the house is suddenly too warm or too cold at the same time every day, look for a schedule you forgot about. If the schedule seems to have stopped working, look for a permanent hold someone set.
What a setback is worth. The U.S. Department of Energy estimates you can save as much as 10 percent a year on heating and cooling by turning the thermostat back 7 to 10 F from its normal setting for 8 hours a day. "As much as" is the important part: the savings are larger in a leaky house in a harsh climate, smaller in a tight, mild one, and they depend on the kind of system, which leads to the one big exception.
Why cranking the setpoint does not heat or cool faster
Most furnaces and air conditioners have one output: they are either on or off, and while they are on they deliver heat or cooling at a fixed rate. Setting the thermostat to 85 instead of 70 does not make that rate higher. It only tells the system to keep running longer, so the house overshoots and you walk back and turn it down. The same is true in reverse: setting it to 60 on a hot day cools no faster than setting it to 72.
There are two partial exceptions. A two-stage or variable-speed system may run at a higher stage when the gap between the room and the target is large. And a heat pump may bring on its backup heat when the gap is large, which is faster but much more expensive to run, as the next section shows.
Emergency heat and auxiliary heat on a heat pump
A heat pump moves heat from outdoor air into the house rather than making it. Because it moves heat instead of making it, it typically delivers two to three or more units of heat for each unit of electricity it uses, depending on the outdoor temperature and the equipment. Most heat pumps also have a backup, often electric resistance heating strips in the indoor unit (or a gas furnace in a dual-fuel system).
Auxiliary heat (AUX) comes on automatically when the heat pump needs help: during a defrost cycle, in very cold weather, or when the house is far below the target. The AUX light on the screen means it is running right now. That is normal in its place.
Emergency heat (EM HEAT) is a manual setting that shuts the heat pump off completely and heats the house with backup heat only. It is for when the outdoor unit has failed or is damaged, typically until a tech can come. Electric resistance strips deliver one unit of heat for each unit of electricity, so running on emergency heat for days instead of the heat pump can make the bill jump sharply. Use it when the tech tells you to, or when the outdoor unit is clearly not working (iced into a solid block, damaged, or not running at all), and tell the tech you switched it.
A setback that cost more than it saved, worked through
The setup. A heat pump home in a cold winter. The owner set the schedule to 68 F during the day and 62 F from 10 p.m. to 6 a.m., following the setback advice. In the morning they noticed the AUX light was on from 6:00 until about 6:50 every day.
What was happening. At 6:00 the target jumped from 62 to 68, a 6 degree gap. That large a gap is exactly what tells many heat pump thermostats to call in backup heat, so the electric strips ran for about 50 minutes alongside the heat pump each morning.
What that 50 minutes costs, in energy. The figures below are illustrative, to show the size of the effect; your strips and your heat pump's efficiency come from your installer or the equipment labels.
strip heat (illustrative) 10 kW
run time 50 min = 0.83 hr
strip energy 10 kW x 0.83 hr = 8.3 kWh
same heat from heat pump 8.3 kWh / 2.5 = 3.3 kWh
(assuming 2.5 units of heat per unit of electricity)
extra energy each morning 8.3 - 3.3 = 5.0 kWh
over 90 cold mornings 5.0 x 90 = 450 kWh
To see it in money, multiply the 450 kWh by the rate printed on your electric bill (R cents per kWh). The point is the comparison: the strips used about two and a half times the electricity the heat pump would have used for the same heat (8.3 against 3.3 kWh).
What they changed. They cut the overnight setback to 65 F and moved the wake-up time to start at 5:00 so the heat pump could recover slowly before anyone was up. Checking the screen the following week, AUX came on only briefly, on the coldest morning. Many newer thermostats have an "adaptive recovery" or "smart recovery" feature that does this for you, starting early and avoiding the strips; if yours has it, turn it on.
What would change the answer. With a gas furnace, or a heat pump with gas backup (dual-fuel), the original 6 degree setback would usually save energy, because there is no expensive strip heat to trigger. With a newer cold-climate heat pump that handles low temperatures well, the installer may recommend little or no setback at all. Ask whoever installed your system which you have.
Small things that cause big confusion
- Dead batteries. Many thermostats run on batteries even when wired. Weak batteries cause blank screens, missed schedules and short cycling. Change them once a year.
- Placement. A thermostat in sunlight, above a lamp, or next to a drafty door reads the wrong temperature and runs the house to match. Moving it is a job for a tech.
- A "wait" or snowflake symbol. Many thermostats delay restarting the compressor for a few minutes after it stops, to protect it. Waiting is normal.
- Changing the system type in the settings menu. Installer settings tell the thermostat what equipment it controls. Leave those to the tech; a wrong setting can run the wrong equipment or none.
Where your part ends
Your part: the mode, fan, schedule and hold settings, the batteries, watching which lights come on and when, and writing down what the screen said when something went wrong (a photo of the screen is ideal). Call a tech when the screen is blank with fresh batteries, the system runs nonstop without reaching the target, AUX or EM HEAT stays on for days, or the outdoor unit is not running in heat mode. Turn the system off at the thermostat and call if a heating or cooling cycle comes with a burning smell or a bang. If a CO alarm sounds, get everyone outside first and follow How to Respond When Your Carbon Monoxide Alarm Goes Off. Wiring, installer settings and replacing a thermostat on a heat pump or high-voltage system are a licensed tech's work.
References
- U.S. Department of Energy, Energy Saver guidance on thermostats and setback savings
- See related: Your First Winter with a New Heat Pump
- See related: Normal and Not-Normal Sounds from Your Heating and Cooling
- See related: When to Stop and Call a Licensed Pro Around the House
- Your thermostat's user manual for its schedule, hold and recovery features