Closed vents, 65-degree settings and routine duct cleaning: AC myths that cost you money

No, and no: the two myths I hear most

Closing vents in rooms you do not use does not save money, and setting the thermostat far below the temperature you want does not cool the house any faster. Closing vents raises pressure in the ducts and reduces airflow through the whole system, and an air conditioner cools at the same rate whether the thermostat says 74 or 65; the lower setting only makes it run longer and overshoot. What does save money is moderate thermostat setbacks when you are away, a clean filter, sealed ducts and regular maintenance.

I am Brandon Quigley of Quigley Heating & Air in Farmers Branch, where my family has worked on heating and cooling systems since 1945. Myths in this trade are stubborn because each one sounds like common sense. Here are the ones that cost homeowners the most, and what I tell people instead.

Myth: closing vents in empty rooms saves energy

It feels logical. Close the vents in the guest room and the extra cold air goes where you need it. In practice, your system was designed to move a certain amount of air through all of its ducts. Close several vents and that air has nowhere to go. Pressure builds in the ducts, more air leaks out of seams and joints into the attic, and less air crosses the indoor coil. On the cooling side, low airflow across the coil is one of the classic causes of a frozen coil.

On our zoning page we put it plainly: opening and closing registers by hand is not zoning, and it can cause leaky ductwork and low airflow. Real zoning uses motorized dampers in the ducts, a control panel and a thermostat in each zone, so the system adjusts how much air it moves. If you genuinely have rooms you rarely use, or floors that fight each other, zoning is the tool designed for the job.

Myth: a lower setting cools the house faster

You come home to a warm house, so you set the thermostat to 65 to speed things up. An ordinary air conditioner has one speed of cooling. Setting it lower does not make it work harder; it just tells it to keep going longer. The house reaches 74 at the same moment either way, and then keeps cooling past it, wasting energy and leaving you cold.

Variable-speed systems do adjust their output, but they are designed to reach and hold the temperature you set, not to sprint toward a lower one. The better approach is to set the temperature you actually want, or to let a programmable or smart thermostat start cooling the house before you get home.

Ceiling fans have their own myth attached. A fan does not lower the temperature of a room; it moves air across your skin so you feel cooler. That means a fan can let you set the thermostat a little higher and stay comfortable, but a fan spinning in an empty room is only using electricity. Turn fans off when you leave the room.

Myth: shutting the system off all day is always cheapest

Raising the temperature while the house is empty does save money, and ENERGY STAR estimates that homes with high heating and cooling bills, or that are unoccupied much of the day, can save about 100 dollars a year with a certified smart thermostat. The key word is raising. Turning the system completely off on a 100-degree afternoon lets the house, the furniture and the walls soak up heat for hours, and the system then has to run hard into the evening to catch up, often while the house stays uncomfortable.

A moderate setback, a few degrees warmer while you are out, with the thermostat bringing it back down before you return, gets you most of the savings without the struggle. One caution for heat pump owners in winter: deep setbacks can make the system call on its expensive backup heat strips to recover, so use a thermostat set up for a heat pump.

Myth: low refrigerant just needs a top-off

An air conditioner does not use up refrigerant the way a car uses gas. It circulates in a sealed loop. If the level is low, there is a leak. Adding refrigerant every summer without finding the leak buys a few weeks or months of cooling and guarantees the same call next year. The EPA notes that system leaks harm the environment and result in increased operation and maintenance costs.

On older systems the math gets worse. The EPA states that after 2020, only recycled, reclaimed or previously produced R-22 can be used to service existing equipment, which has made that refrigerant scarce. The right answer is to find and repair the leak, or, if the leak is in a failing coil on an old system, to talk honestly about replacement. A company that tops off your system year after year without mentioning a leak is not doing you a favor.

Myth: a bigger air conditioner cools better

People sometimes ask us to go up a size when they replace a system, especially after a hard summer. Bigger is not better. An oversized air conditioner cools the air quickly and shuts off before it has removed enough moisture, so the house feels clammy at the right temperature. It also starts and stops more often, which wears the parts that start the system. The right size comes from the house, its insulation, its windows and its ducts, not from the hottest week you remember.

Moisture matters a great deal here. A Dallas summer is long, and on humid days a system that runs in short bursts leaves the air damp and sticky even when the thermometer looks right. A correctly sized system, and especially a variable-speed system that runs longer at lower speed, spends more time pulling water out of the air. That is why many people find they are comfortable at a higher setting after a properly sized replacement than they were with the oversized system it replaced.

Myth: ducts need cleaning on a schedule

We clean ducts, so you might expect me to recommend it every few years. The honest answer is that it depends on the house. The EPA states that duct cleaning has never been shown to actually prevent health problems, and it does not recommend routine cleaning. It suggests considering cleaning when there is substantial visible mold inside the ducts or on other system components, when rodents or insects have infested them, or when ducts are clogged with excessive dust and debris or particles are blowing out of the supply registers.

Remodeling dust, pets and allergies in the household can all be good reasons to have the ducts inspected. A sales call that promises better health from cleaning alone is not. If you notice a musty smell or water stains near ducts or ceilings, the bigger question is where the moisture is coming from, and a mold inspection may come before any cleaning; this guide on musty smells and ceiling stains explains that process. Cleaning moldy ducts without fixing the water source just schedules the next cleaning.

Myth: if it is running, it does not need maintenance

Systems rarely fail all at once. Capacitors weaken, coils collect dirt, drains slowly clog and refrigerant leaks a little at a time, and the system keeps running, less efficiently, until the hottest week of the year finishes the job. A clean filter is the first defense, and my guide on HVAC maintenance you can safely do yourself covers the homeowner's side.

The technician's side is the tune-up: cleaning the condenser coil, checking refrigerant levels and electrical components, measuring air temperatures and clearing the drain. That is where the weak part gets replaced in April instead of failing in August. It is also why the service our clients value most is not a repair at all; it is maintenance.

A related myth is that the most expensive filter is always the best. The EPA advises using the highest-efficiency filter your system's manufacturer recommends, and that qualifier matters. A filter that is too restrictive for your equipment can starve the system of air just like a dirty one. Check the manual, or ask your technician at the next visit.

Myth: a new system will fix the hot rooms

A new outdoor unit connected to the same leaking, crushed or undersized ducts will deliver the same uneven air, just more efficiently. ENERGY STAR notes that ducts are often major energy wasters and that sealing and insulating them can improve heating and cooling efficiency by as much as 20 percent. In many homes the problem room is fed by a long duct run through a hot attic, or the upstairs simply has more heat to fight than the system was ever set up for.

If you have a two-story home where the second floor never catches up in summer, my guide on fixing a hot upstairs walks through ducts, returns, insulation and zoning. Any of those may matter more than the equipment.

Getting a straight answer for your house

Myths spread because they are simple. Your house is not. If your system runs constantly, your bills keep climbing or one room never feels right, a technician who measures before recommending will tell you more than any rule of thumb. If the system is running but not cooling at all, start with my guide on an air conditioner that runs without cooling.

We serve Dallas, Farmers Branch, Carrollton, Coppell, Frisco, Garland, Highland Park, Plano, Richardson and University Park, and we service all brands. Tell us what the house is doing, and we will tell you what we find, including when the answer is something simple you can fix yourself.

Questions people ask me