Some of the most repeated heating and cooling advice raises your bills instead of lowering them. Here is what the sources actually say about the myths that cost homeowners the most.
Licensing, permit, lien and tax rules vary by state and city and change over time. This guide is general information, not legal or tax advice; check with your local building department, state licensing board or a professional for your situation.
Checklist
- Closing vents in unused rooms does not save energy; it raises duct pressure, pushes air through leaks, and can damage the heat exchanger or coils.
- Setting the thermostat back 7 to 10 degrees for 8 hours a day can cut heating and cooling costs by up to 10 percent a year.
- Cranking the thermostat does not heat or cool faster; it only makes the system run longer and use more energy.
- Oversized furnaces and air conditioners often come from rule-of-thumb sizing instead of a Manual J load calculation, so ask for the numbers.
- General efficiency guidance points to an annual professional service visit, and vents should stay open and unobstructed.
Heating and cooling eats up nearly half of the average American household’s energy bill, more than $900 a year. That makes HVAC advice tempting, especially the free kind offered by neighbors, coworkers, and internet commenters. Some of it is harmless. Some of it raises your bills or shortens the life of your equipment. Here is what the sources actually show about the beliefs that cost homeowners the most.
Does Closing Vents in Unused Rooms Really Save Energy?
This is probably the most repeated HVAC tip in existence, and it fails on every count. Your heating or cooling system produces the same amount of air no matter how many registers you close. The equipment does not know that you shut a vent in the guest room, so it keeps pushing the same volume of air into the same ductwork.
What actually happens is a pressure problem. Closed registers cause air to back up into the ducts, and the extra pressure forces conditioned air out through duct leaks into attics, crawlspaces, and wall cavities you were never trying to condition. Rooms with closed vents also leak more air through openings in walls and windows. To make up for the heat that escaped, the furnace runs longer cycles, which raises your costs.
The wear gets worse. Furnace components need a minimum volume of return airflow to stay at a safe internal temperature. Choking off supply vents reduces that airflow, which can overheat the heat exchanger, the most expensive component in the system, and cause cracks. A cracked heat exchanger can also allow carbon monoxide into the home. In cooling systems, closed vents can freeze the coils and damage the compressor. Fixed-speed blowers move less air under the added pressure, and variable-speed blowers use more energy compensating.
The better move is the opposite of the myth: keep all supply and return vents open and clear of rugs and furniture.
Myth vs. Fact: Cranking the Thermostat Heats or Cools Faster
Myth: Setting the thermostat to 90 in January warms the house faster.
Fact: A furnace or heat pump runs at the same capacity regardless of where you set the dial. A higher setting does not speed anything up. It just keeps the system running longer, past the point where the house is comfortable, and higher settings in winter and lower settings in summer increase energy demands. You pay for overshoot you never wanted.
If you want real savings, the Department of Energy points the other direction: set the thermostat back. Lowering it 7 to 10 degrees for 8 hours a day, typically while you are at work or asleep, can save up to 10 percent a year on heating and cooling. Recovery does take longer, since the system has to close a bigger temperature gap when you return, but that still uses less energy than holding the house at your comfort setting all day while nobody is home.
Tip
The longer and deeper the setback, the bigger the savings. In one commercial example, setting thermostats 10 degrees lower during unoccupied nights and weekends produced 20 to 40 percent energy savings. Setbacks of a few hours or less may not produce meaningful savings, so aim for long unoccupied stretches.
Is Bigger Always Better When Replacing a Furnace or AC?
Myth: Upsizing the new unit guarantees comfort.
Fact: Right-sizing beats big every time, and the numbers matter. Proper sizing comes from a Manual J load calculation, which measures how much heating and cooling your specific house needs. Manual J calculates the sensible load and the latent (humidity) load separately, in Btu per hour, and adds them together.
Plenty of contractors skip the calculation and use a rule of thumb of 500 to 600 square feet per ton. High-performance homes often come in around 1,000 square feet per ton or more. In one documented case, a Manual J claimed a home needed 368 square feet per ton, far beyond either figure, and got there by inflating the window area and plugging in a much worse window rating than the home actually had. The calculation said the upstairs zone needed 1.5 tons; the contractor wanted to sell 2.5.
Before you sign, ask whether the estimate includes a Manual J calculation and what numbers it used. Put the sizing and scope in writing; the items in what a home improvement contract should say apply directly to an HVAC replacement. And since HVAC is a licensed trade in many states, verify credentials first using the steps in how to check a contractor’s license.
Myth vs. Fact: Annual Tune-Ups Are a Waste of Money
Myth: Yearly service visits are just a way for companies to bill you.
Fact: General energy-efficiency guidance points to a yearly service visit: a trained technician should examine and clean your heating and cooling equipment at least once a year. A visit is also a good time to confirm that supply and return vents are unobstructed, check for gaps around exterior doors and windows that need weather-stripping or caulk, and find out whether your walls and attic have enough insulation for your climate zone.
Where skepticism belongs is with strangers who show up uninvited. Be wary of door-to-door crews pushing an immediate full replacement after a storm; the patterns described in door-to-door repair scams after storms show up across the trades, HVAC included.
Myth vs. Fact: A Tight, Efficient House Needs No Fresh Air
Myth: Sealing the home airtight solves everything.
Fact: Airtight homes save a lot of energy, but pollutants can build up without adequate exchange with outdoor air. Building science guidance from the Department of Energy says every energy-efficient home, whether brand new or older, needs whole-house mechanical ventilation to keep indoor air quality where it should be. Kitchen and bathroom exhaust fans help, but they run in short bursts and only clear contaminants close to where they start. Whole-house systems come in four types: exhaust, supply, balanced, and energy recovery, which limits energy loss by transferring heat between the outgoing and incoming air.
Myth vs. Fact: A Thermostat Is Just an On-Off Switch
Myth: A fancy thermostat cannot save much money.
Fact: Thermostat settings drive how often your system runs, and smart thermostats automate the savings. Many learn your preferences and build a schedule that shifts to energy-saving temperatures while you sleep or are away. Geofencing lets the thermostat detect when you have left for the day and adjust accordingly. Models carrying the ENERGY STAR label have been independently certified to save energy, with certification grounded in real-world field data, and they drop into a low-power standby mode between uses. Before buying, confirm the thermostat is compatible with your heating and cooling equipment.
Quick Facts Worth Remembering Before Your Next Service Call
- Keep every supply and return vent open and free of rugs, curtains, and furniture.
- Seal gaps around exterior doors and windows with weather-stripping or caulk.
- Have a qualified professional inspect and clean the system at least once a year.
- Ask for the Manual J load calculation behind any replacement estimate, and get it in writing.
- Insulation and air sealing improvements that meet the 2021 International Energy Conservation Code may qualify for a federal tax credit.
- If you are replacing equipment or upgrading insulation, ask your utility about available rebate programs.
If a new installation does go wrong, the documentation and escalation steps in when a contractor job goes wrong apply to HVAC contractors the same as anyone else.
Keep reading
- What a Home Improvement Contract Should Say: the written terms that protect you on an HVAC replacement, from scope to payment schedule.
- How to Check a Contractor’s License: verify that the person sizing and installing your system is legitimately licensed in your state.
- When a Contractor Job Goes Wrong: what to do when an installation is faulty, delayed, or abandoned.
Homeowners ask
Does closing air vents in unused rooms save energy?
No. Your system produces the same amount of air no matter how many vents are closed. The trapped pressure forces conditioned air out through duct leaks, makes the system run longer, and can overheat the heat exchanger or freeze cooling coils.
How much can a thermostat setback really save?
The Department of Energy says setting the thermostat back 7 to 10 degrees for 8 hours a day can save up to 10 percent a year on heating and cooling. Savings grow the longer and deeper the setback, but setbacks of only a few hours may not produce meaningful savings.
Is a bigger furnace or air conditioner better?
No. Equipment should be sized with a Manual J load calculation based on your home, not a rule of thumb. Oversized systems cost more up front and short-cycle, and some contractors inflate the calculation to justify larger units.
Are annual HVAC tune-ups worth the money?
General efficiency guidance points to a yearly service visit, with a trained technician examining and cleaning the heating and cooling equipment. Be more skeptical of door-to-door crews who push a full replacement on the spot after a storm.