12 Air Conditioning Mistakes You Didn’t Know You Were Making

Just about everyone in San Diego has an air conditioner, even if it’s of the window variety. In order to keep your energy bills low in the heat of the summer, you’ll want to avoid these air conditioning mistakes you didn’t know you were making.

Air Conditioning Mistakes

1. Closing vents in unused rooms

Two weeks ago, a man walked into our offices in Lakeside and asked the following question:

“My wife says closing the air vents to unused rooms in our house is bad idea, but I think it’s okay and good way to save money. Can you tell me which one of us is correct?”

To which I proceeded to tell him that his wife was right. Many people, including myself in years gone by, have closed their air vents in unused rooms to attempt to lower their air conditioner bill.

Instead, what you’re doing is putting a strain on your system.

Your system was carefully chosen to manage your whole house. By closing off an air vent, you’re destabilizing your air conditioner.

I once heard it put this way:

Imagine you’re walking at a brisk pace, breathing through your mouth and out your nose. If you close one nostril, you either move less air (depriving yourself of needed oxygen) or you work harder to do it.

The same is true of your air conditioner. Air was meant to go out that duct. Closing it off doesn’t mean air no longer travels through the ducting, it just comes back on itself, forcing the air to flow at an imbalance and creating undue pressure on the system.

You’re not actually saving any money, and in fact you are ruining your system in the progress.

When I explained this to the gentleman, he left with his head down.

I’m sorry, sir, but your wife is right, and you’re not winning that argument. Closing air vents is one the air conditioning mistakes that nearly everyone makes.

2. Running the AC when no one is home

This one seems to obvious, but yet so many people neglect to get it right.

Either they simply forget to turn off their air conditioner, fail to program the thermostat, or just don’t want to come home to a hot house.

The first reason is actually a no-no, the second is plain lazy, and the last one is wasteful.

Let me explain.

With today’s technology, there’s no reason why you shouldn’t have a smart thermostat that allows you to—at a minimum—set an hourly schedule.

If you know no one will be in the house from 7 AM to 4 PM every single day, then reprogram your thermostat accordingly.

Don’t turn your system completely off, that will waste a lot of electricity. It’s less expensive to keep a house cool than it is to cool a hot house.

But what you want to do is turn the temperature up about 5-8 degrees when you’re gone.

Normally keep the AC at 75? Turn it up to 82 while you’re gone. Then have it set to re-engage about 30 minutes before you get home. If all goes well, it’ll be ready for you without all that unnecessary energy loss.

AC Mistakes leaving windows open

3. Opening your blinds and windows

This is one the air conditioning mistakes I especially struggle with.

You see, I love sunshine. It’s the reason I moved to San Diego in the first place.

But opening all the curtains and blinds in the home let’s in a crazy amount of heat from the summer sun, thereby raising the temperature of your home.

Increased temps means your air conditioner has to work harder to keep it cool.

As much as it pains me to say, it’s best if you keep your blinds closed, at least during the hottest parts of the day.

Are you home in the morning, but not in the afternoon? Considering opening those blinds and enjoying the sun while you’re home, but then closing them every time you leave. No sense in leaving them open if you aren’t there to enjoy them.

Or perhaps you can open them, but only slightly. Just enough to let some light in, but not enough to allow the sun’s rays direct entry.

Do as much as you can to eliminate sources of unwanted heat in the home and your air conditioner will thank you.

4. Not scheduling your thermostat

We talked about this one already, but it bears repeating.

Schedule that thermostat!

If you still have one of those old fossils that has the slider stick or the dial.

Yeah, you know what I mean.

Those imprecise “set it and hope you got it right” ones. Then when you realize it’s too hot or cold, you have to make little tiny adjustments over a couple of hours.

And once you find that ideal temp, when anyone goes near the thermostat you’re like:

But you know what? You don’t have to be that person anymore.

You don’t have to have an ancient, archaic thermostat from the 1950s.


Get yourself a nice Ecobee 4 or Nest Learning Thermostat.

Both of these give you precision temperature (har har, that’s the name of our company). Plus, they’re completely programmable to do whatever you want them to. Want to turn your AC on from your phone right before you leave work? You can do that.

Better yet, the learning thermostat will use GPS to learn your work habits and automatically make the switch for you.

Creepy right?

5. Setting the temperature too low

This is another major blunder that unfortunately just about everyone falls into.

My stepfather routinely set our air conditioner to 60 degrees.

You read that right.



Yeah, it was like an icebox in our house.

The worst part was that he’d leave the heater on 60 as well during the winter, so it was basically an icebox all year around.

The problem is that your air conditioner wasn’t designed to run constantly, 100% of the time.

It was made to cool your house to a certain temperature and then turn off, allowing the house to warm a little before re-engaging.

This reduces strain on the system and saves you money.

When you set the temperature so low, the air conditioner will never be able to achieve that number.

Imagine being on a hamster wheel with a nice bowl of ice cream just outside, all you need to do is keep running and you’ll get there! But you never do.

You run and you run and you run, and you never make it, and eventually you get tired and worn out, but you never reach the prize.

It’s tiring and disheartening.

That’s what you’re doing to your AC. Not only are you spending more on your energy bill, but you’re wearing your system out faster.

Just don’t do it!

Set that temperature to a reasonable number, like 78.


But that’s hot!

It’s not when it’s 110 outside.

Can you tell I live in El Cajon?

air conditioner too cold

6. Using bathroom/kitchen vent fans

This is one the air conditioning mistakes you’re probably not thinking about. To be fair, it’s not a grievous mistake that will cost you thousands of dollars, but when you’re scrimping, every penny counts.

There’s nothing wrong with running your vent fans when they’re necessary. However, if you’re like me, you tend to leave them on for an hour or more after you’ve left the room.

The whole time vent fans are running, you’re sucking up your cold air and spitting it outside, forcing your AC to work more.

7. Failing to clean your air filter

Most people are aware that air conditioners have air filters, but those same people don’t really think much about cleaning them.

Sure, if you’ve ever accidentally pulled it out and seen how dirty it is, you might stop what you’re doing and give it a rinse.

But if you’re pulling your AC out year to year and just sticking it in the window and running it, the continued dirt build up will make your unit inefficient and less effective.

For mini-splits or central air, regular cleanings are crucial. Mini-splits are notorious for being finicky. Treat them like you would a new car or a really old car you’re trying to squeeze a few more years out of.

Central air systems already demand costly energy bills. Failing to clean the air filter will only drive that price higher for the same result.

8. Expecting too much from your AC

This isn’t so much a mistake as it is an informational bit. When it’s 80 degrees outside, you could reasonably run your AC and expect it to cool your house to 72.

Personally, I don’t run my AC unless it’s at least 90 degrees outside. In which case 72 degrees is at the very bottom rung of possibility with the air conditioner.

On those crazy hot days of 100-115 degrees, your air conditioner simply will not be able to achieve 72 degrees no matter how hard it runs.

Air conditioners, at best, can lower the temperature around 20-25 degrees. If you’re annoyed it’s 110 and your house is only 85 degrees despite the AC being set to 72, I have news for you. You might think your system needs replaced, but truthfully it’s operating at peak performance, and in fact it might be working overtime to get you that cool 85.

The mistake here is not in dollars, but in your expectations.

Understand the capabilities of your system and and plan accordingly.

9. Choosing the wrong size system

Many people mistakenly believe that getting a larger unit than is necessary for your space means it will be cooler/faster/more efficient etc

The truth is, improperly sizing your air conditioner can result in shorter lifespan and more costly repairs to the unit.

If your house is 1,000 square feet and you purchase a system that treats 2,000 sqft, yes, it will be more powerful. And, yes, it will cool faster.

The problem is that your system will be starting and stopping so often it will wear itself out and grow unreliable over time. It may not be an immediate problem, but it is one that will come to bite you in the end.

Conversely, trying to save a buck by choosing a weaker system will do nothing but ensure your unit has to work harder resulting in the same effect.

Air conditioning systems need to be properly sized to run efficiently. It’s for this reason that AC installation should never be a DIY project. Call your local HVAC installer to help you make the right choice.

10. Installing your thermostat in the wrong spot

setting thermostat too low air conditioning mistakeUnless the man in the photo is 10 feet tall, one should never have to stoop to set a thermostat. Because thermostat installation is often becomes a DIY project, this is one of those air conditioning mistakes that can be a real problem.

Thermostats aren’t only for setting the temperature, they’re also the thermometer/sensor.

Because hot air rises and cold air sinks, placing a thermostat too low may give an inaccurate reading of the air’s temperature. This results in you being either too hot or too cold, depending on the season.

Additionally, placing a thermostat near a door or in direct sunlight will also affect the reading and thus the functionality of your unit.

Ideal thermostat locations are in a central area of the house, away from external doors and sunlight, and placed at about 5 feet from the floor.

11. Ignoring regular maintenance

Your air conditioner may be your savior on those hot days, but it’s no Superman. It’s not indestructible.

It is just a piece of mechanical equipment after all. It’s a combination of thousands of tiny parts. It only takes one tiny broken part to throw off the whole system.

Sure, it may continue working, but not at peak performance, resulting in a less than consistent temperature for you and shorter lifespan of your system.

Regular, yearly preventative maintenance is key to keeping your air conditioner in top shape.

Ideally, you’re calling your service technician about a month or two before your regular hot season. They’ll come inspect your unit and make recommendations if repairs are needed.

If you ignore this advice, you may find yourself without air when you need it most.

12. Avoiding replacing your old system

In tandem with the last item, sometimes your system just needs to be replaced.

No amount of servicing will bring it back from the dead. Sure, it might keep it running just one more year, but there is a point where enough is enough.

You might be trying to scrimp and save every last dollar and don’t want to drop several thousand to replace your system.

But, instead, you’re spending hundreds of dollars on a dying system and losing hundreds more in energy bills.

Sometimes it’s just time to say out with the old and in with the new. Not doing so might make this one of the most costly air conditioning mistakes you can make.

Need Help?

Precision Temperature is San Diego’s trusted heating and air conditioning specialists.

If you need any help with maintenance, installation, or just have questions, we’re here for you. Contact us today and one of our knowledgeable staff members will do their best to assist you.

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