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Community and Q&A

Air conditioner settings to save energy / money

amwisner | Posted in Energy Efficiency and Durability on

I apologize in advance because I’m sure you must have an article on this, but I have searched and cannot find it.

The question is this: What is the best way to use a thermostat to reduce energy usage in the hottest days of summer (in Florida). As an example of what I’m looking for, I drafted this response to a friend who wants to know if she should “turn off” the AC during the day:

“I raise the temp at various times during the day, so it only comes on for a while and then turns off. Then at night I put it down to 78 so the house can cool down while it’s cool outside, which requires less energy than setting it down when you get home, at the hottest part of the day. To do this automatically, you can get a programmable thermostat. They usually have 4 different settings for each day. If you are home and it gets too hot during the day, you can manually set it down a little to come back on for a while. Ceiling fans can help the room feel comfortable at higher temperatures, although at some point you need the AC to run to remove heat and humidity.

“An example of settings is: 9 am – 80, 12-noon – 82, 4 pm – 84, 8 pm – 82, then when you go to bed manually set it back to 78 (if there are only 4 settings). You might think that at 84 the house will be too hot, but it will be so hot outside that the AC will be coming on pretty frequently anyway, and of course if you are uncomfortable you can always set it a little colder when you start to notice it. Or you can do smaller increments like 79, 80, 81, 80, 78.

“You’re not actually turning the AC off, so the house shouldn’t get too hot. Just don’t set the temp much colder at the hottest part of the day, because that’s when it works the hardest, and it’s going to be coming on periodically anyway as the temps get higher outside. When you get home, give your body a chance to adjust before deciding to put the temp down.”

Having the AC running all day also causes condensation/mold problems, correct?

Please point me in the right direction. (I already have the .gov reference: but it’s not detailed enough.)

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  1. GBA Editor
    Martin Holladay | | #1

    The facts are simpler than you think. Here's the story:

    1. The fewer hours you operate your air conditioner, the less energy you use.

    2. When the air conditioner is operating, the higher the thermostat setting, the less energy you use.

    Whether or not you want to suffer some discomfort in order to save energy is up to you. If you can endure high indoor temperatures, you'll save money. If you like your house to be constantly cool, that level of comfort will cost you.

    Q. "Having the AC running all day also causes condensation/mold problems, correct?"

    A. No.

  2. GBA Editor
    Martin Holladay | | #2

    The most effective ways to lower cooling costs don't necessarily have anything to do with air conditioner operation (although reducing the number of hours that the air conditioner is used, and raising the thermostat setting, certainly work).

    You might want to consider:

    1. Exterior shading devices (awnings or top-hinged shutters) for windows that get full sun.

    2. Air sealing measures to reduce your home's air leakage rate.

    3. Sealing the duct seams (if needed) of any ducts located in an unconditioned attic, and improving the thickness of the duct insulation if necessary.

    4. Improving (thickening) the ceiling insulation.

    For more information, see "My House is Too Hot."

  3. Expert Member
    Dana Dorsett | | #3

    It matters if it's a 2 stage vs single stage (and the sizing factor), since the efficiency is usually much higher when it operates at the low stage. A right-sized 2- stager will usually use less energy if you take a "set and forget" approach.

    If it's a single-stage unit with a large oversize factor (or a ridiculously oversized 2-stager) , the setback strategy works better, since the recovery ramp guarantees that it runs longer cycles. It takes at least 10-12 minutes for the system to get up to it's steady-state efficiency. If it's satisfying the thermostat in 6 minute cycles 2-4x per hour it may not hit very close to it's labeled SEER.

    Note, a right-sized air conditioner could take hours to pull the indoor temperature down from 84F to 78F on a hot day. It's worth trying to get a handle on your oversizing factor when figuring out your optimal strategy. See:

    As a general rule having the AC running all day does NOT cause condensation or mold. The exception to that would be very oversized systems running very low indoor temperatures, where it's so oversized the cycles aren't long enough to dehumidify very well, and the walls run 10-15F cooler than the outdoor dew point most of the time.

    Running the AIR HANDLER all day can contribute to mold, by driving higher rates of outdoor air infiltration. In a well designed system that's not a problem, but the "typical" system isn't very well balanced for supply/return, and the ducts & air are in the attic above the insulation with air leaks in the attic floor at every duct penetration, with duct leaks in the attic space. All of that increases the air infiltration (and cooling load) whenever the air handler is running, whether the AC is actively running or not.

    The best way to save MONEY may be quite different from the best way to save ENERGY, depending on how your electric rates are structured. Utilites that have time of use rates or offer "rush hour rewards" for allowing them to back off on your AC settings when the grid is being maxed out (this usually requires a WiFi thermostat, and subscribing to the program, if offered) can save a lot of money by pre-cooling the house when energy is cheap, letting the AC idle through the expensive-electricity hours.

  4. amwisner | | #4

    Thanks. I figured I would get "it depends" type of answers. I am aware of all of the above, but I am specifically asking how to be energy efficient while keeping the house reasonably comfortable for people to be in all day at the same time. Maybe it would help to assume average home and AC system energy efficiency. I guess to clarify, this is in opposition to keeping the house at 78 degrees all day and night, while temperatures outside reach the high 90s. Assume that the system is approximately right-sized.

    There are many internet pages that say to keep your thermostat setting at the same temperature all day long because it's too stressful to the system and burns too much energy to "turn off" the AC and then turn it back on later (which for most people is 5 - 6 pm, at peak heat), but there's nothing on a detailed strategy to keep the house comfortable while having the AC run minimally.

    I am aware that using ceiling fans helps comfort, to a point.

    I am aware that the system has to run continuously for a certain amount of time in order to effectively dehumidify. With my proposed schedule, assuming very high outside temperatures which increase during the day, and average (or better) home efficiency, the system would turn off for a while and then come back on and run for a while because outside temperatures would be continuing to increase during that time.

    I am aware that a 2-stage system is more efficient than single stage. Is there a way to re-set a single stage to run all low speed all the time?

    I assume that I am correct that it's easier, and thus more efficient, to cool the house back down after it's cool outside, say after 10 pm, than to maintain ideal temp (say, 78) through at peak heat.

    I apologize if I'm just not getting it, but I've been studying this for years, including reading your site, and what I am proposing makes sense to me.

    Thanks for the clarification regarding condensation/mold.

  5. Jon_R | | #5

    If your AC system causes high humidity somewhere, then periodically turning the AC off long enough to let that area dry can prevent mold growth.

    The best answer to your question will come from measurements. Try various things and measure kWh/degree day. Repeat enough to reduce errors (eg sun and wind).

    If you can tolerate just a few hours of higher temp, then yes, the best time for those are when it's hottest outside.

    A thermostat with more hysteresis will probably improve efficiency (the tradeoff being greater temperature fluctuation). But it could cause a system to switch to higher output (reducing efficiency).

  6. GBA Editor
    Martin Holladay | | #6

    Q. "I am specifically asking how to be energy efficient while keeping the house reasonably comfortable for people to be in all day at the same time."

    A. The suggestions I made in Comment #2 are in response to that question.

    Q. "Is there a way to re-set a single stage to run all low speed all the time?"

    A. By definition, a single-stage appliance has just one speed. There is no such thing as "low speed" with a single-stage appliance.

  7. Expert Member
    Dana Dorsett | | #7

    Answering the probably-intended question...

    "Is there a way to re-set a TWO stage to run all low speed all the time?"...

    ... the answer is "usually". You'd have to consult the manuals for both the AC and the thermostat. (Sometimes the thermostat controls when the second stage is engaged, sometimes not.)

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