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CHANGING THE CURRENT

CHANGING THE CURRENT

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Craig Muccio

About "Ask the Energy Expert"

Craig Muccio runs FPL's Conservation Research & Development Program and crunches the numbers to figure out how you can save by managing your energy use.

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Turning the A/C up or leaving it at the same temperature. Which is better?

By Craig Muccio

Asked on: July 5, 2013 by Question via Facebook

Is it more economical to turn the A/C up during the day and down at night, or leave it at the same temperature all day? 

The urban myth that you should leave your thermostat at the same temperature all the time has been around for decades. The truth is, it takes a lot of energy for your air conditioner to run all day, maintaining an inside temperature that’s significantly lower than the outside temperature. On the other hand, if you set your thermostat higher in the morning, the unit will run less during the day, saving you money on your FPL bill. Even if the air conditioner has to catch up when you bring the temperature back down later, it is still more economical to keep your thermostat set higher during the day.

By the same logic, if you are in the habit of lowering the thermostat before bedtime because you prefer a cooler room while you sleep, your cooling costs increase by up to 5% for every degree you lower the temperature! To use less air conditioning at night, turn on ceiling fans – they make you feel cooler even though they aren’t technically cooling the house. Of course, when heating in winter, lowering the thermostat setting saves you money because your heater runs less. For more energy efficiency tips, visit www.FPL.com/tips.

 

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Comments [45]

  1. Mike r.

    I think this article misses the point entirely. The point is that it's hotter during the day than at night and therefore you must keep th ac running durin the day to keep cool, but at night you don't always need to because its already cooler. The idea is that if you keep the air on the same temp all day and all night the air will keep coming on and tuning all day and night- thereby makin you pay more than if you turn the air off at night but keep it on all day. There's always someone home at my house and no way am I going to leave the air at 80 degrees during the day even if nobody is home. I have music equipment and the worst thing for music equipment of ANY kind is to keep the house warm and humid. Ill just keep my house comfortable and pay the extra 10 dollars a month thank you.

  2. John Ford

    Great story

  3. John Ford

    Great article. Thanks

  4. Liz

    What's the deal?! My bill in the past four months has been a hundred dollars more. Last year the same thing happened on the same month then around October it goes back to a hundred dollars Less. I know something is not right because I have been pregnant through the fall and winter and I have been putting the air really cold even during the winter then when I had the baby in the spring I started feeling cold so I put the air like eight degrees higher and my bill started coming a hundred dollars higher!!! Nothing else in my home has changed!! I think I'm going to have to keep a daily log of my meter!

  5. Erik

    With all due respect to the author, that's an overly simplistic answer ... First things first, no mater how old your HVAC system is spend the $20 for a 7-day (5+2 preferably) programable thermostat. This isn't a simple black and white problem because Florida is in both a sub-tropical and tropical zone meaning the average relative humidity is over 70%. So, simply turning off your HVAC in any typical home built since the 1940's will cause more harm than good. I'll save you all the diatribe on modern architectural characteristics of homes. The best advice for maintaining the health of your home while saving energy (and hopefully money) is to gently raise and lower the inside temperature as needed. When you aren't home for long periods have the thermostat set between 78-81. About an hour before your due home, set it for what you find comfortable. The same is true just before bedtime. Never set cooling lower than 68 because it's virtually unattainable and will over work your system. Of course, there's nothing wrong with occasionally turning the system off and opening some windows, but be mindful of the humidity. Also, as noted, ceiling fans can be great energy savers by dropping the heat index, however leaving them on in unoccupied rooms does nothing except waste energy.

  6. HVAC Dave

    I do HVAC for a living- system designs, repairs and fixing those "$29.95" service calls. Allow me to reply to several posts on here: Marlene Burns - No, there is no FPL rebate for a 13 SEER system. Rebates start at 14 SEER and above. The article had sound advice with regard to temp set back when the living area will be vacant for more than a couple of hours. Less then that the savings if any are minimal. There are several manufactures of thermostats that have an "Intelligent Recovery" feature. The t-stat will figure out over the course of a few days how long it takes for it to reach the particular set point at a specific time. No more guessing if you have to set your t-stat to come on at 4pm to reach 76 degrees at 6pm when you get home. This ONLY good for programmable t-stats. Now a word on programmable t-stats...... Do you leave and come home at the same time during your work week? If not, are you at home most of the day and have the t-stat set for one temp and another at night? If you answered "Yes" to both of these, a programmable t-stat will benefit you. If you answered "No" to both, don't spend the extra money on a programmable t-stat because you will constantly be battling to override the program settings. A programmable t-stat is not what you need for the one week a year you are on vacation. Jim Carson - Have a GOOD HVAC company evaluate your system, if you have a regular home/vacant schedule get a programmable t-stat. Also, a t-stat is simply an on-off switch for your system based upon temp. It DOES NOT get any cooler any quicker by lowing the set point. Instead of setting it at 71 before you go to bed, set it to something more reasonable like 75 or 76 and sleep through the night. C. Mullenax - Yes, this method works for multistage HVAC systems. I have a 2 stage 18 SEER heat pump and with my intelligent recovery t-stat, it works GREAT!

  7. Greg C

    I know that it is more efficient to set back heating at night when burning fossils fuels that allow the heating system to catch up when higher temp kicks in. Does the same hold true for heat pumps systems where the heat output in cold weather just isn't that hot and it seems the temp just can't catch up? Thanks

  8. Scott Smith

    This question and answer brings up an issue I have wondered about for years. Why don't they make a thermostat that would adjust itself based on the outside temperature rather than the time ??? Or it could adjust to a temperature difference. Example: Set at 76 degrees, if the outside temp went up to say 86, would adjust inside to 77. If outside temp went to 90, inside would go to 78. Outside temp 94, inside to 79. etc. It would still feel cool, and be dehumidified. Why all the thermostats based on time ???

  9. Laura

    My electric bill is higher every month. Is it possible fot someone from FPL go to my home and do some type of survey to tell me whats to lower the bill?

  10. sue

    Monika, You sound like you are doing lots right. I think the thing that is different is the outside temperature. Your A/C needs to work against a higher outside temperature in August than in June, and nights are nolonger below 80 in August, so the A/C needs to work all 24 hours, not just in the day. In a couple of months your bill should be dropping by $20/m just by doing what you are doing. (My old utility would include average monthly temperatures on the bill, just for this reason.)

  11. andy

    This post is an attempt to debunk a myth, however the post consists of little more than opinion. A few facts or case-studies may help. I believe a company such as FPL may have a piece of equipment capable of measuring power usage. I'm not questioning the expert who wrote this, however an expert would understand the need for fact as opposed to waffle.

  12. Kersi F. Munshi

    It's also helpful to turn the thermostat up when there is no one in the house. And, if you can wait to take your shower after returning, also turn off the water heater when there's no one in the house.

  13. Arpad Szasz

    The answer is correct, but it does not explain the reason, and amazingly I have never seen it explained anywhere else. Let's combine everyday science with pure logic: The AC pumps out heat, and the more it has to pump out as a total amount, the more your electric bill will be. Therefore we must look at total amount of HEAT that is pumped out. This amount equals to the total amount of heat that leaks into the home. The total amount of heat leaking in depends on the average temperature difference between inside and outside. Keeping the home cool all day increases the average temperature difference, therefore it increases the total amount of heat that leaks in, so the amount of work the AC does is also increased.

  14. Gary

    Absolutely correct. This sounds like an A/C operational issue. There is not reason it should take this long to cool a house unless 1. The system is malfunctioning (any number of reasons to include low on refrigerant, clogged coil etc.), not maintained correctly, or NOT EVEN COMING ON. 2. A unit that is working properly but drastically undersized for the house. (unlikely) 3 Or....Like my ol' man use to say, close the front door cuz you're air conditioning the whole neighborhood!! 4. Refer to #1 (most likely)

  15. John Parsels

    Since I'm home all day, why would I raise the temp? It is already set at 80 degrees.

  16. JoAnn Mock

    We have 3 zones. We leave the guest wing at 80 at all times and close the door to the rest of the house. The other two zones are set at 77 during the day but at night we turn up the main house to 80 and close our bedroom door leaving our bedroom at 77. Are we saving any money by doing this?

  17. Marsha Sherman

    I would like to know what it costs to keep my desk-top computer on by the hour. Is it better to allow it to go into hibernate when it's not in use, or shut it off?

  18. Chris W

    For years my wife and I have gone around and around on this. Everyone has "heard something" from someone. Would the talented mechanical and thermal engineers at FPL simply write a "white paper" with a decent technical analysis and model including heat inputs, thermal masses, thermal resistances, efficiencies at different temperature differentials, etc.. for a typical SFl home? Maybe a model on an Excel spreadsheet or simulator? Put it out there so we can use good old mathematics to lay this argument to rest once and for all. Thank You!

  19. Alan

    I think Craig's answer is correct for families who work during the day and are not at home. So, it would be logical for these people to turn up the temp while they are at work so they do not waste energy cooling the house when it is empty. But this strategy is not appropriate for retired people (like myself) who are at home a good part of the daytime hours. We need to have the house cool all of the time, and so we need to keep the AC at a lower temperature. Is there any strategy that you can recommend for us?

  20. kent

    Is it better to wrap (insulate) a water heater that is an uncooled garage?

  21. David

    I'm no expert but I adjust my thermostat to 79 or 80 during the day. The unit cycles all day but not excessive. At 4 or 5pm I turn it down to 76. At 9pm it goes to 73. It always cycles on and off like it is supposed to. At 6am it's back to 79 or 80. My bill has that 20.00 diff. I'm guessing higher temps, oven usage, higher humidity, guest staying over, extra showers/lights and TV. My home is 1800 sqft. I can't complain. Are your appliances new? How high is your water heater? Hope this was useful and remember, this works for me. I'm no expert. lol. Just trying to stay cool cause I have MS and SAVE A BUCK.

  22. Nita

    Is a digital thermostat more accurate than an old fashioned regular thermostat? Is it more economical to have a digital thermostat? Are there more problems with a digital thermostat?

  23. tOM

    It isn't mentioned anywhere in this advice but I assume it is for people who work and are out of the house during the day. I'm retired and I am in and out of the house several times during the day, so it doesn't make any sense to change the thermostat each time I go out. I keep it at a constant 76 all the time.

  24. gooden

    What temperature does FPL recommend for A/C use?

  25. Todd King

    How much should one drop the temp during the day over the night time temp. We had a new heat pump installed and they stated no more than 5 degrees due to the stress on the unit. Previously I believe I saw on your website to have a change of 10 degrees primarily for vacation houses. BTW ours is a vacation house.

  26. Donna Dupuy

    What about turning it completely OFF when you leave and then back on when you get home? Doesn't it make the compressor work harder and use more energy than setting the thermostat higher/lower?

  27. Jill

    In the winter is it better to turn the a/c off or just raise the temp to keep it from running?

  28. Krys

    I also just bought a new system and your a/c guy should have told you about any rebates but I was informed they only apply to high efficiency units which are 16 seer or higher.

  29. Mike Kelly

    Well now this is interesting because FPL's policy is/was, "Set it and forget it" at 78.

  30. Betty Kish

    I leave my thermostat at 79 all summer with the ceiling fans going. Last month my bill was 76.00. Can I save money by changing the thermostat as you advised?

  31. Jim

    It's been getting warmer outside. Maintenance of a constant indoor temp (81, in your case) requires more energy with higher outdoor temps.

  32. Caroline

    I assume this is assuming a working day, where higher temperatures during the day is ok. But what about stay at home folks? What sequence would you recommend? tkxs

  33. kenneth nassor

    This answer is not clear, I do not know if I should change the thermostat!

  34. Jessie Odell

    Turning it up during the day does in fact save energy, however it also makes your home or office more uncomfortable.. How about the power companies all lower energy prices to make it more affordable for consumers and take smaller bonuses, rather than multi million dollar ones so we can afford our power bill with out suffering at higher temperatures? I know can't help but think if there was a car used by fpl rather than the chopper seen coming and going daily you all could save a lot of energy too. I understand though a car isn't as comfortable and luxurious as the chopper.....

  35. Monika

    I am at work all day and I have my A/C set on 81/82. I have to say I am not a big A/C fan and can't stand very cold temps. I also don't like the A/C blowing on me at night time or any other time. Same with the fan. So when I get home I turn it to 80 or 79 or even 78 - depends on how I feel. I only turn it up, because I like to be a bit cool when I get out of the shower. I like to take hot showers all the time. I have been doing this for the last 3 month, all the time the same temps and my bill goes up about $ 20.00 per month....WHY? And at night time I keep the A/C on 81 again. What am I doing wrong to raise the $$$$ on the electric bill?

  36. Scott I.

    I have a 3 year old RUUD dual compressor AC unit 3 ton 19.5 seer. I adjust mine from 75 to 81 degrees, using a programmable thermostat, while I am away at work. I have it set to cool back down to 75 at 6:30. Since my thermostat accounts for 1 degree per 30 minutes, it starts cooling down around 4. This seemed to be the best for overall energy costs. But my AC guy who came out said my unit was not designed to do that and I should leave it on all day at 75, which I disagree with. He said I should try it and see if my bill comes down. I guess he is part of the so called myth. Also, FPL recommends to not let the temp get any higher than 81 due to humidity and possible mold problems, so I use that. But maybe that is for seasonal residents while they are away?

  37. Scott

    Jim, Very good points- from what you just have just described it sounds like you have a Delta T issue. This will need to be checked by a licensed HVAC contractor. (16-20 degree temperature split), if your temperature split is below 16 degrees it will take longer to cool your home and will add to higher energy usage, poor performance, and high humidity in your home. YES, cooling takes time--- but within reason--- we call this time to temp. However in your case it’s taking too long and most likely the relative humidity could be high or above RH 55% in your home making you feel even more uncomfortable. Please have this checked just because you here and feel the A/C running doesn’t mean it’s working at 100% efficiency. The FLP recommendation is sound and considered best advice because I would recommend the same thing. However it does not fit everyone’s case or if the home foot-print has cooling issues to start with could make things more uncomfortable. Please check the following: 1. Condition of HVAC system, age, size, and maintenance. 2. Duct-work in the un-conditioned space is it sealed, designed correctly, and well insulated? 3. Attic insulation should be around 12" to 15" thick or R-38 equivalent for Florida climate. 4. Thermostat calibration. 5. Is the attic properly vented to control conduction and convection? Regards, Scott

  38. C. Mullenax

    Is this assessment true even for the new two-stage compressors where presumably the recovery period would kick in the high-stage, versus maintaining temperature using primarily the low-stage during the day if set at a constant temperature (or perhaps a smaller increase)?

  39. Ed Hall

    Best process is to raise it up by 15 degrees to keep it from coming on often. Then when you get home lower it by 10 degrees, and about 2 hours before bed lower it by 5 degrees so it is at the temp you like. By the time you go to bed you should feel comfortable. Even if you take a hot shower. One thing to keep in mind is this only works if you have a automated system and you keep your doors and windows weatherproofed. We are all human and do forget to adjust manually.

  40. Marlene Burns

    I just bought a new air conditioner, Fridgidaire brand, 13 seer, 2 1/2 ton. Is there a rebate for the newer energy efficient ? If so, how do we apply for it? It was $3,500.00, thank you for any information you have.

  41. Jim Carson

    You'll need to do more to persuade me. Here's a typical scenario of my following your advice: 1) Set thermostat to 80. Leave for work. 2) Come home at 6pm. Turn air conditioner back down to 74. 3) Three hours later, inside temp is still 76. 4) At 11pm, turn thermostat down to 71. Go to bed with only a sheet for cover. Toss and turn. 5) 3am: Air conditioner finally catches up to 71. Wake up to cold air. Pull on comforter. Try to get back to sleep.

  42. TOM

    The best corollary I ever heard was, by the same logic that you shouldn't turn off the AC, you should also never turn off your oven if you intend to bake something later.

  43. James

    I have ignored this "Urban myth" for decades. Simple common sense and logic should be all that is needed to bring more sense into the matter, but it does not. There are many occurrences where colleagues and friends have brought up this myth, though they can counter with no argument or logic that overrules mine, I don't make a difference and they simply go on believing what they've "heard"...

  44. Bohumil Mokovsky

    Duh....

  45. robert reichart

    This is absurd advice,,,, the contents of the home retain heat and cool nesss,,,,, leave it alone but not at 70 d F

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