Question about air conditioning and power usage
July 16, 2021 12:25 PM   Subscribe

Baby it's hot outside. So, I run my central air conditioning. It is so hot outside that whether I set the temp to 70 or 75, it will run all the time. So, if it is running all the time, does it matter what temperature it is set to in terms of power usage or cost? Is there any incentive to set the temperature higher if it is going to run regardless?
posted by AugustWest to Home & Garden (11 answers total) 5 users marked this as a favorite
No. An A/C is either on or it isn't. If it's on all the time it's on all the time.

Relevant SLYT.
posted by caek at 12:27 PM on July 16 [2 favorites]

By the way, I learned that AC units are sized to reduce the indoor temperature a certain number of degrees down from the outdoor temperature. So, it is sized for 20° reduction, then you get it to comfortable temperature when it is in the 90's but you will never get the house below 80° if it is over 100° outside.
posted by metahawk at 12:36 PM on July 16

It would be better to turn the temperature higher, especially if you are going to be out of the house for a while. Better to let the unit rest and then cool the house back down than to run all the time, which isn't great for it. I recently also learned that air conditioners are designed to cool the house by 15-20 degrees. Lots of a/c companies have this kind of information on their websites. If it's not true, I haven't found a good source disagreeing with it.

If it's 98 degrees, set your thermostat to 78 or 80 and use a box or ceiling fan or other ways to stay cool. Don't just run your a/c constantly.

Consumer Reports says you'll save money if you move up the temperature a bit, too.
posted by bluedaisy at 12:57 PM on July 16 [1 favorite]

It's true that AC is either on or not, so once the setting is within the range that causes it to never reach its setpoint, it will run forever and it doesn't matter how much lower you turn the setting. It is also true that this really isn't a great idea. You should either turn the temperature high enough that the AC can reach the setpoint and have some time to cycle off, or repair/upgrade the AC so that it can reach the desired temperature rather than running constantly. It's not just a lot of energy usage, but it's hard on the equipment because it is not designed to run that way.
posted by primethyme at 1:13 PM on July 16 [1 favorite]

So, it is sized for 20° reduction, then you get it to comfortable temperature when it is in the 90's but you will never get the house below 80° if it is over 100° outside.

No, they are sized by square footage, and how cold your room gets and remains has more to do with insulation than with how long your air conditioner runs. An oversized central AC will cool a home down 20F+ in 20-30 minutes which is a bad thing, as it is hard on the equipment and doesn't remove enough humidity.

If you AC cannot get your house down to a reasonable temperature (and 75F is reasonable) without running all the time, you need an energy audit and an equipment service. Running your central A/C that much will also run your electric bill up something crazy, so get the stuff done ASAP.

Also, if you have a relatively new A/C they have variable speed compressors and fans, and then minor differences in temperature can result in different amounts of electricity usage. Older units don't have variable fans so they either running or not.
posted by The_Vegetables at 1:30 PM on July 16 [3 favorites]

BTW, you can check how well your A/C is running by putting your hand up to a vent or with a $20 laser temperature measuring gun if you want to be accurate. The temperature through the AC coming out of the vents should be 15-20F degrees colder than the intake temperature (perhaps what metahawk was referring to, but the intake temperature is inside the house, so it is variable and will decrease as the AC works). If it's not, you probably have a refrigerant leak or your equipment is not working properly.
posted by The_Vegetables at 1:37 PM on July 16 [2 favorites]

Our solution to all of these problems is to turn the A/C on when the thermostat registers just a few degrees above our ideal temperature. That way, it runs constantly only for a short time until the house is cooled to the set temperature, then cycles on and off to keep it there.
posted by DrGail at 2:06 PM on July 16

When an AC isn't cooling well, the first thing to do is check the outside unit and make sure it is clean and has good air flow. It is made up of very fine fins that need to pass air to disperse the heat, and all sorts of fluff outdoors plugs it all up. If it's just dirt and dust plugging it, you can clean it quite well by just spraying through the fins with a garden hose. If it's a little worse you may have to make sure it's powered off so the fan doesn't start, and then take the covers off to clean it manually and then spray through the fins with the hose. Don't bend or mess up the fins, but do spray them clean.

If it still doesn't cool your house adequately when you're having typical weather for your area, call in an expert.
posted by fritley at 4:11 PM on July 16

It's not just a lot of energy usage, but it's hard on the equipment because it is not designed to run that way.

This is bad advice.

Turning on is very stressful for the AC motors used in most air conditioning systems as the starting current draw is very high (practically a dead short) and the lubricant is not properly distributed in the system until the pump gets going. Running continuously is not a problem and frequent short cycles are. (Short cycles are also bad for comfort and prevent proper dehumidification.) So a properly sized air conditioning system will run continually at the design temperature, intentionally. If you’re out west, the recent heat wave was design temperature and then some, so running continuously is absolutely appropriate for a well designed system and will not damage it.
posted by doomsey at 4:13 PM on July 16 [6 favorites]

Maybe but your electric bill will suffer if your ac runs continuously. As far as I can tell short-cycling is running for 10 minutes to 30 minutes, and off for the same amount of time. If your ac runs for longer than 2-3 hours when you have been home and the indoor temperature before it starts is reasonable, you need to get it checked.
posted by The_Vegetables at 9:05 AM on July 17

I've lived in Texas my whole life and we use ac from like April to October at least. It is rare that it runs more than 4 times an hour, and usually it's on for 10 minutes or less at a time.
posted by RustyBrooks at 10:24 AM on July 17

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