What Should I Set My Smart Thermostat To in a Heat Warning?

If there is an Excessive Heat Warning on your phone, treat the thermostat as a comfort-and-safety control first and an energy gadget second. The National Weather Service uses a warning when dangerous heat is expected or occurring, which is a different situation from a normal summer afternoon where a broad efficiency tip can sit harmlessly in a blog post. [1]

For most homes, the fast answer is this: set cooling somewhere around 75–78°F if the people in the house are safe and reasonably comfortable there; use only a small setback during the worst hours; put the thermostat in Cool rather than Auto; leave the fan on Auto; and do not keep punching in 68°F expecting faster cooling. If you have a heat pump, high indoor humidity, a Nest Airwave setting, ecobee eco+ enabled, or an active utility event, those layers matter as much as the number.

Use 75–78°F as a starting range, not 2026 policy

The familiar summer thermostat advice did not come from nowhere. Trane still points homeowners to an Energy Star-style 75–78°F cooling range, with about a 4°F increase while sleeping and a 7°F increase while away, and also repeats the rough estimate that every degree above 72°F can save about 3% on cooling costs. [2] Consumer Reports similarly discusses the older Energy Star pattern of 78°F when home, 82°F when asleep, and 85°F when away, along with the Department of Energy’s broader claim that setting a thermostat back 7–10°F for 8 hours a day can save up to about 10% per year on heating and cooling. [3]

Those numbers are useful because they give the hand on the thermostat somewhere sane to begin. They are not useful when they get repeated as if every house, body, air conditioner, humidity level, and heat alert must obey the same number. Even Energy Star has described 78°F as “a starting point,” not a universal command. [4]

The policy status matters in 2026. Newsweek reported that the DOE removed a webpage in July 2026 that had advised Americans to set thermostats to 78°F in summer, and KSDK reported the DOE’s statement that Americans should set their thermostats to whatever temperature they choose. [5][6] So the honest version is not “DOE says 78.” The honest version is: inherited Energy Star-era figures still make good efficiency references, but the live decision belongs to the household, the equipment, and the conditions outside.

During a heat warning, that usually means shrinking the setback. If your normal away setting is aggressive, do not let the house climb so far that the system has to spend the evening trying to drag hot walls, hot ductwork, hot upstairs bedrooms, and high humidity back into shape. A conventional AC may tolerate a small daytime bump. A heat pump often does better with a steadier target. Either way, a heat-warning schedule should avoid turning the recovery period into a second emergency inside the house.

Heat pump or conventional AC: the setting changes

The same thermostat face can be controlling very different equipment. A heat pump and a conventional central air conditioner may both be cooling the house, but they do not reward the same behavior during extreme heat.

For heat pumps, hold the setpoint steadier than you would with a gas-furnace-and-AC mindset. Efficiency Maine tells heat pump users to pick a comfortable temperature, avoid frequent adjustments, avoid Auto mode, and use the fan’s Auto setting. [7] Elephant Energy gives similar extreme-heat advice: set the heat pump to Cool, avoid Auto mode, avoid big temperature swings, and let the system run consistently. [8]

That is not a personality quirk of heat pump owners. A deep setback can ask the system to recover exactly when outdoor conditions are worst. During an Excessive Heat Warning, the system may run for hours and still not catch up quickly, especially in a leaky house, an upstairs zone, or a room with hard afternoon sun.

For a conventional central AC, a modest setback can still make sense. “Modest” is doing the work there. If the home is occupied, raising the cooling setpoint 1–3°F during the hottest hours is a very different choice from letting the house climb into the mid-80s and expecting a fast evening recovery. If the house is empty and pets, plants, equipment, or vulnerable materials are not an issue, a larger away setback may be acceptable on a normal day; during a warning, the recovery risk rises.

Cool mode beats Auto during a heat warning

Mode is not cosmetic. Put the thermostat in Cool. Auto mode is designed to switch between heating and cooling as needed, which is not the behavior most people want during a heat emergency. If the indoor temperature dips overnight, a thermostat in Auto can make confusing decisions that fight the simple goal: keep the home livable while the outside temperature is punishing.

Leave the fan on Auto unless you have a specific reason not to

The fan setting is one of the most common quiet mistakes. Fan On feels like it should be better because air is moving. In humid weather, it can backfire. After the cooling cycle stops, a continuously running blower can re-evaporate moisture sitting on the indoor coil and push that humidity back into the house. Tom’s Heat & Air describes that fan-on behavior as a reason indoor humidity can rise, and recommends Auto for dehumidification. [9]

That matters because 78°F with reasonable humidity and 78°F with sticky indoor air are not the same room. If people are uncomfortable, sweating, or sleeping badly, chasing the thermostat number while the fan is making humidity worse is the wrong fight.

Nest settings to check before the afternoon peak

On a Nest, start with the plain controls before the clever ones: set the mode to Cool, choose the occupied cooling setpoint, and confirm the fan is not manually locked On. Then check the features that may change the cooling pattern while you are not staring at the screen.

• Airwave: leave it enabled if your home is not humid and it has been working well. Airwave can turn off the compressor while keeping the fan running to use remaining cooling in the coil, but Google says it activates only when indoor humidity is below a certain level. [10]
• Eco Temperatures: review the cooling Eco range before relying on Away behavior. Google lists Nest cooling Eco Temperature options from 76°F to 90°F. [11] During a warning, a high Eco cooling temperature may be too aggressive for pets, medical needs, upstairs bedrooms, or a system that struggles to recover.
• Safety Temperatures: use them as a backstop, not as the comfort plan. They are meant to help prevent the home from getting dangerously hot or cold, not to decide where people should sit through a heat warning.
• Schedule and Home/Away behavior: if the thermostat keeps switching into an away-saving temperature while people are home, fix that before the hottest part of the day. A smart thermostat that misunderstands occupancy is not being smart in the moment that matters.

The big Nest trap is letting an old Eco Temperature stand in for a heat-warning plan. A 76–80°F Eco cooling value may be reasonable in many homes. A value near the top of the available range can be a problem if the home is already absorbing heat faster than the system can remove it. The app will still look calm while the building is banking heat.

ecobee eco+ can help, but read what it is allowed to do

On an ecobee, the heat-warning check is less about hunting obscure threshold menus and more about understanding eco+. Do not blindly disable every smart feature; do not blindly accept every adjustment either.

Adjust for Humidity is the feature that belongs in the front of the line on a muggy day. ecobee describes it as using indoor humidity to make the home feel like the desired temperature while saving energy, rather than treating dry-bulb temperature as the only comfort signal. [12] In practical terms, that means the displayed setpoint and the felt room temperature may not line up in the simple way a traditional thermostat trained us to expect.

Time of Use is the bill-management layer. ecobee says eco+ can precool or reduce cooling when electricity is more expensive, and it cites a 2020 third-party study claiming 7–26% cooling savings for Time of Use optimization. [12] That savings figure is a vendor-presented claim, and it does not mean every home should accept an aggressive comfort tradeoff during an Excessive Heat Warning. It means the feature is powerful enough to deserve a look before the utility’s peak window starts.

Community Energy Savings is the grid-event layer. ecobee says participating customers can opt out of temperature adjustments, and its program page advertises rewards of up to $125 where available. [13] The useful part during a warning is not the reward headline; it is the control fact. If the room is getting unsafe or someone is struggling, override the event.

One caution for ecobee owners who like deeper configuration: threshold-menu behavior should be checked in the live app or current ecobee support before changing advanced settings. The everyday heat-warning decisions above — mode, fan, comfort setting, eco+ humidity, Time of Use, and Community Energy Savings — are the settings most homeowners can confirm without turning a hot afternoon into a commissioning session.

Demand-response events are allowed to surprise you only once

A smart thermostat tied to a utility program may do something different on the exact afternoon you care most. That does not make the program bad. It does mean you should know when an event is active, what the thermostat is changing, and how to take control back.

Renew Home’s Rush Hour Rewards support page says customers typically see 6–12 Rush Hour events per season, usually lasting 2–4 hours, with shorter 30–90-minute events possible during heat waves. It also says Critical Rush Hours do not precool the home, and that users can override events at any time. [14]

The override right is the one to remember. Pre-cooling can be useful when a thermostat or utility program is trying to shift load before a peak period, but it is not a universal instruction to make every house colder in the morning. Some homes shed that stored cool air quickly. Some households cannot tolerate the later drift upward. Some systems never had enough spare capacity to make the plan comfortable in the first place.

If a utility event starts and comfort is still fine, let it ride. If indoor temperature, humidity, sleep, medical comfort, pets, or vulnerable occupants are moving in the wrong direction, override. A demand-response program is supposed to reduce strain on the grid; it is not supposed to make the living room unusable while the thermostat displays a leaf.

A workable heat-warning setup

If you need a default configuration, use this as the starting pass and then adjust for the people in the home:

• Occupied daytime: try 75–78°F. If the house is humid, upstairs rooms run hot, or someone is heat-sensitive, move lower before discomfort becomes a recovery problem.
• Sleeping: do not automatically add the old 4°F sleep setback during a heat warning. Use the smallest change that still allows safe sleep.
• Away: cap the setback more tightly than you would on a normal day, especially if the system has struggled to recover before. For heat pumps, consider little or no setback.
• Mode: Cool, not Auto.
• Fan: Auto, not On, unless you have a specific ventilation or filtration reason and understand the humidity tradeoff.
• Nest: check Airwave, Eco Temperatures, Safety Temperatures, and Home/Away behavior.
• ecobee: check eco+ Adjust for Humidity, Time of Use, and Community Energy Savings before peak hours.
• Utility events: read the event notice and override if the house is no longer safe or comfortable.

If your forecast is in the truly punishing range and you want a copyable day plan instead of a settings philosophy, use our smart thermostat cooling schedule for a 115°F heat wave. It goes deeper on pre-peak cooling, occupied holds, and limiting away setbacks without pretending every home can coast through the same schedule.

When the thermostat is no longer the fix

There is a point in a heat wave where the most important thermostat move is recognizing that the system is not magic. Carl’s Quality Cooling describes the common residential design expectation as roughly a 20°F indoor–outdoor temperature difference, tied to design conditions rather than unlimited cooling capacity. [15] WRDW’s heat-wave coverage makes the same practical point from the homeowner side: lowering the thermostat setting does not make the system blow colder air. [16]

So if it is 100–105°F outside and the house is holding 78–80°F inside, that may be normal strain, not an automatic equipment failure. The system may be running continuously because it is doing all it can do. Setting 68°F will not cool the room faster; it will mostly keep the call for cooling active longer.

What should change at that point is the house strategy around the thermostat: close blinds on sun-facing glass, avoid adding indoor heat from ovens or long appliance runs, check the filter if it is safely accessible, reduce door openings, and move people to the coolest usable room. If indoor temperature keeps climbing, the system is short-cycling, ice appears, airflow is weak, breakers trip, or anyone shows signs of heat illness, thermostat settings have stopped being the right tool.

During an Excessive Heat Warning, the smart thermostat setting is a layered configuration: a reasonable starting temperature, equipment-aware setbacks, humidity-aware fan behavior, known Nest or ecobee automation, and clear override control when a utility event appears. The number matters. The layers decide whether the number actually works.

References

1. Heat Watch, Warning, and Advisory Definitions, National Weather Service
2. Should You Really Set Your Thermostat to 78 in the Summer?, Trane
3. Best Setting for Central Air Conditioning, Consumer Reports
4. What should your thermostat be set at during summer?, Fox 8
5. DOE Deletes Webpage Instructing People To Lower Thermostat to 78, Newsweek
6. DOE: Americans should set their thermostats to whatever temperature they choose, KSDK
7. Heat Pump User Tips, Efficiency Maine
8. How to Use Your Heat Pump During Extreme Heat: 6 Expert Tips for Maximum Cooling, Elephant Energy
9. How To Reduce Home Humidity, Tom’s Heat & Air
10. Learn about Airwave and how to change settings, Google Nest Help
11. Learn about Eco Temperatures and how to change settings, Google Nest Help
12. eco+, ecobee
13. Community Energy Savings, ecobee
14. Learn About Rush Hour Rewards, Renew Home
15. Heat Pump Not Cooling Enough on 100 Degree Days, Carl’s Quality Cooling
16. During heat wave, heed these tips for setting your thermostat, WRDW