Peak Summer Is When These Myths Matter Most

Mid-July is when air conditioning transitions from a comfort amenity to a life-safety system for many Americans. The heat waves that peak in the weeks around the summer solstice — when the thermal lag described in the 5/26 piece places temperatures at their seasonal maximum — make the difference between an air-conditioned and non-air-conditioned environment the difference between safe and dangerous for vulnerable populations.

It is also when the myths about air conditioning — how it works, how to use it efficiently, whether it’s harmful — are most consequential. Believing that turning the thermostat down further cools the house faster, or that air conditioning causes illness, or that fans are equivalent to AC in extreme heat, leads to decisions that cost money, reduce comfort, and in some cases compromise health. Mid-July is exactly the right moment to address them.

Myth: Setting the Thermostat Lower Cools the House Faster

This is the most universally held air conditioning misconception, and it is wrong in a specific and instructive way. A central air conditioning system has one cooling speed — it runs at full capacity or it doesn’t run. Setting the thermostat to 65°F rather than 72°F does not make the system work harder or faster; it simply means the system will run longer before shutting off, ultimately reaching a lower temperature than the 72°F setting would have produced.

The analogy is a car accelerator: pressing it to the floor doesn’t make the engine larger, it just keeps it running longer. An air conditioner set to 65°F will reach 72°F at the same time as one set to 72°F, then continue running to reach 65°F — consuming more energy in the process without cooling the house any faster during the period you actually care about.

The practical implication is that setting the thermostat to the temperature you want, rather than lower than you want, produces equivalent comfort with lower energy consumption. Arriving home to a hot house and setting the thermostat to 60°F will not cool the house faster than setting it to 74°F.

Myth: Leaving AC Running All Day Is More Efficient Than Turning It Off

This myth is the opposite error from the thermostat myth, and it is also wrong — though the degree of wrongness depends on climate conditions and home characteristics.

The argument for leaving AC running all day is that the system has to work harder to cool a very hot house than to maintain a moderately cool one. This is partially true but misses the larger picture: a house that has been allowed to warm to 85°F during an eight-hour absence requires less total energy to cool back to 72°F than running the AC continuously for eight hours at 72°F. The energy saved during the absence is greater than the energy required to re-cool the house in most conditions.

The exception involves homes with specific characteristics: very poor insulation that allows the house to heat extremely rapidly when AC is off, or very high humidity conditions where the AC’s dehumidification role matters — a house allowed to reach high humidity levels can take longer to dehumidify than to cool, and the muggy feeling that results can persist even after the temperature returns to the setpoint.

Programmable and smart thermostats exploit this asymmetry effectively: setting the temperature to rise during the work day and drop 30 minutes before typical arrival produces comfort without the energy cost of continuous cooling. Most Department of Energy guidance suggests that raising the thermostat 7 to 10 degrees while away for eight hours produces meaningful energy savings in most climates.

Myth: Air Conditioning Makes You Sick

The belief that air conditioning causes colds, sore throats, or respiratory illness is extremely widespread and almost entirely wrong — with one important nuance.

The common cold and other respiratory viruses are caused by viruses, not by cold air exposure. Simply being in a cool, air-conditioned environment does not cause viral illness any more than being outside in winter causes viral illness. The association people observe between AC use and feeling unwell has a different explanation.

What air conditioning does cause — and this is the legitimate core of the myth — is dry air, mucous membrane drying, and in poorly maintained systems, circulation of dust, mold spores, and other particulates. A dry, air-conditioned environment can irritate nasal and throat mucous membranes, producing dryness and mild irritation that feels like the beginning of a cold without being one. Poor air filter maintenance in AC systems can circulate dust and allergens that trigger allergic reactions that also resemble a cold. These effects are real but are not viral illness and are addressed through filter maintenance and, if dryness is severe, a humidifier.

The one genuine respiratory risk from air conditioning involves Legionella bacteria — the cause of Legionnaires’ disease — which can grow in poorly maintained cooling tower systems in large commercial buildings. This is a real hazard that building operators are responsible for managing through water system maintenance protocols, and it is categorically different from the benign household AC that most people are using when they worry about getting sick from their air conditioner.

Myth: A Fan Is Just as Good as AC in Extreme Heat

The 4/29 heat myths piece covered this specifically — fans work by enhancing evaporative cooling from the skin surface, which requires that the air moving across the skin be below body temperature. When air temperature exceeds body temperature (approximately 95 to 98°F), a fan moves hot air across the skin and can increase heat gain rather than cooling. This myth is dangerous enough in the context of extreme heat to repeat.

The nuance that the 4/29 piece established is worth restating here in the AC context: fans are effective and valuable supplements to air conditioning at temperatures below body temperature. Running a ceiling fan with the AC allows the thermostat to be set several degrees higher while maintaining the same comfort level — the wind chill effect of the fan compensates for the slightly higher air temperature. Fans used in this way meaningfully reduce AC energy consumption without compromising comfort.

What fans cannot do is substitute for AC when temperatures exceed body temperature. On days when the heat index exceeds 95°F or 100°F, air-conditioned environments provide protection that fans cannot replicate.

Myth: Closing Vents in Unused Rooms Saves Energy

Closing supply vents in unused rooms seems intuitively logical — if you’re not using the room, why cool it? This logic is incorrect for how forced-air cooling systems actually work, and closing vents can actually increase energy consumption and damage equipment.

Central air conditioning systems are designed and balanced to distribute cooled air across a specific number of vents and return air pathways. When vents are closed, the air pressure in the duct system increases — the same volume of air is being pushed through a smaller number of openings. This increased pressure causes several problems: it can force air through duct leaks and gaps that would otherwise remain sealed, wasting cooled air into unconditioned spaces like attics and walls. It increases the load on the air handler fan motor, which was designed to move a specific volume of air and operates inefficiently when that flow is restricted. And it can cause the evaporator coil to freeze — a malfunction that reduces system efficiency and can damage compressor components.

The efficiency strategy that actually works is to keep interior doors open to allow air circulation throughout the conditioned space, and to use programmable setbacks that raise the temperature during unoccupied hours rather than closing vents in specific rooms.

Myth: AC Should Be Set to the Lowest Comfortable Temperature

Energy consumption by AC systems scales with the difference between outdoor temperature and the indoor setpoint — a larger temperature differential requires more energy to maintain. Setting the thermostat to the lowest temperature you find comfortable maximizes energy consumption; setting it to the highest temperature you find comfortable minimizes it.

Most people have more thermal tolerance than they realize — the specific temperature they consider comfortable is partly habitual and partly influenced by the temperature they encounter when they first enter the house. People who maintain 68°F setpoints often find that 72°F is equally comfortable after a week at the higher setting, as their expectation adjusts. The 7°F to 10°F setback during absence that saves meaningful energy applies at any starting setpoint.

The Energy Star recommended summer thermostat setpoint — 78°F when home, higher when away — is warmer than most American households maintain but is consistent with what most healthy adults find tolerable. The gap between the Energy Star recommendation and actual practice represents both genuine comfort preference and habitual overcooling that efficiency advocates argue can be reduced through gradual adjustment.

What Actually Maximizes AC Effectiveness

The interventions that most effectively improve AC performance and comfort while minimizing energy consumption are the same building envelope improvements described in the summer heat weatherproofing piece: reducing solar heat gain through windows with exterior shading or window film, improving attic insulation to reduce the heat flowing downward from a superheated attic, sealing air infiltration pathways that bring hot outdoor air into the conditioned space, and using ceiling fans to distribute cool air and allow slightly higher thermostat setpoints.

The air conditioner handles the heat that gets inside. The building envelope determines how much heat it has to handle. Making the building envelope more effective — rather than simply running the air conditioner harder — is the highest-return strategy for both comfort and cost.