Dear PoPville – Leave the A/C on or off During the Day?

Photo by PoPville flickr user Christopher Michael Poole

“Dear PoPville,

Today, I returned home from work to find my apartment 92 degrees! It’s awful, I think it’s hotter inside than out on the street right now (at 7pm). At this rate, it probably won’t be a comfortable temperature (i.e.upper 70s) in my apartment until I go to bed.

My issue here is a disagreement with my roommate. He leaves for work later than I do and turns off our A/C system for the entire day. I’ve asked him to keep it on and adjust the temperature to something reasonable (like 83 degrees) so that it’s not sweltering hot when I return home from the office (I’m usually home first). We have central A/C, so it’s easy for us to set the temperature and let the system regulate itself.

He thinks that leaving the A/C system on during the day is wasteful. I think it doesn’t really matter, as the A/C system will need to blow at full blast for multiple hours in the evening instead of gradually regulating itself throughout the day.

Who’s right in this situation?”

122 Comment

  • Don’t go straight home today, let him be the first one home and see how he likes it.

  • I think you are probably right, although it probably will take some tinkering to figure out the most efficient temp is to leave the thermostat at all day.

    Not sure how it works in apartments, but for homes (if you have one of the new electric meters) you can monitor your daily usage on Pepco’s website.

    • Wow, people here do not know their physics. No offense intended, but barring second order effects (like the conditioner running less efficiently over time – or startup costs each times it turn on – which could go either way, but probably mostly cancel out) it’s clearly more efficient to leave the ac off during the day and run it like crazy while you’re home. Plus, the electricity you use is more efficiently generated at night (thanks to lower overall loads on the grid).

      Which is not to say who is right or wrong on the comfort vs. cost to your pocketbook and the environment. That’s a decision you have to make for yourself.

      But, on the technical question, it is very clear your roommate is correct.

      • Well, Mr. Science, the utilities seem to disagree with you. See below.

        • This wouldn’t be the first time that whatever the utilities said disagreed with science.

        • Yes, as the original anon (in this particular chain), I’m not sure that I’d view the people who sell you electricity as the most credible source of information about how to use it most efficiently. Utilities do sometimes have an incentive to encourage people to reduce their use of electricity for load balancing reasons – but they make money by selling electricity – so their incentives are complex. More importantly, much of their information is often wrong, and they are often managed by morons (listen to Bloomingdale folks complain about WASA some day…).

          In this case, people keep talking about how it takes more electricity to cool a house from 92 to 70 than from 80 to 70. Well duh. But that’s not the real comparison. Because it takes electricity all day to maintain the house at 80. Your house gains more heat per minute the greater the degree of difference to the outside. So if your house is at 80, and it’s 96 outside, your house is trying to gain quite a bit of heat every minute – heat that your AC has to battle to expel. On the otherhand, if you let your house heat up to 96 where it sits for a few hours at the high, in balance with the outdoors, your house doesn’t gain any heat during the hours the outdoors spends at the day’s high. Over all, over a 24 period, your AC has to expel much less heat if you leave it off during the day – because your house gets to spend more time at or near equilibrium with the outside.

          And that works even though if your house is well insulated it still won’t get up to 96 during a day of no AC – the more time it spends nearer to equilibrium, the less energy you’ll lose overall.

          • You also have to factor in the issue of peak demand, which has less to do with your house and more to do with how power stations are sized. Utilities like electricity are sized to meet society’s demand which will only occur about 2% of the entire year during the very hottest and very coldest days. So if you run your AC during a summer day (while you’re gone), you’re contributing directly to the increased size of the power grid and using electriity when its at a premium. It’s better for the environment to keep it off during the day and use it more in the evening. Even if you use 10 times as much electricity you won’t be contributing to the peak demand.

      • Turning on/off in hot weather puts a lot of stress on the unit, when a house/apt has heated up all day. and then has to cool down. Not good for the long-term of the appliance and it doesn’t save money. I’ve tried different regimes and the best thing is to turn it up so that it isn’t constantly running (I do 78 degrees) and then go to a cooler temp when home, although most of the time, I only turn it down to 76. I should note that I draw the blinds/drapes during the day, too.

        • It saves electricity to leave the AC off during the day – whatever you do with it at night. Your house gains less therms per hour the closer it is to being in equilibrium to the outside. The further out of equilibrium it is (i.e. if you are keeping your house cooler), the more therms per hour your house gains – and thus the more therms per hour your AC unit has to pump out in order to keep your house cool. You have to pump way less therms per day if you leave your AC off during the day.

          That being said, the effects on lifespan of the compressor vary depending on the compressor. Leaving it running for a long time (i.e. if you left it off all day and are now cooling your house back down at night) is hard on many compressors. On the other hand, startup is the moment of greatest stress on your compressor, and your compressor has to start and stop quite a bit all day to keep the temperature constant if you leave it on. Plus, to keep the temperature constant, your compressor has to run more total during the 24 hour period.

          On balance, it always saves electricity to leave your AC off during the day, and it often, but not always, does a better job preserving the life of your AC unit. On the other hand, you have to come to a warm house.

    • ah

      The most efficient is not to run A/C at all.

      So the question is how much do you want to pay for your comfort. If your A/C won’t get the apartment to a comfortable temperature quickly when you return, then you need to set it so it will be comfortable when you use it. You are balancing costs with comfort, and your roommate seems not to care much about comfort.

      Why not get a programmable thermostat. Set it to go to 82 when you leave in the morning and return at night. Your roommate can be “happy” in the warm air between teh time you leave and he leaves.

  • You are correct, your roommate is wrong. Leave it on but turn it up; otherwise it’ll just have to run forever when you get home. Besides, that kind of heat can’t be very good for your furniture, perishable food, etc.

  • Emmaleigh504

    According to some PSA I heard when I lived in Florida, leaving the central ac set to 82 when you are at work or on vacation is the most energy efficient. I don’t remember who put out the PSA, but that’s what I do.

    • Emmaleigh504

      Another temp fact: 68 is supposed to be the best temp for your body to fall asleep.

    • Trust me, I lived in Florida for a while. After you adjust to the tropical heat, 82 feels like 72 with AC.

      That setting is specific to Florida and hot areas. I think someone said it better, above… find your right spot and tinker with it.

      • I lived in Bangkok for several yeasr it only marginally increased my tolerance for heat and humidity. I also lived in Atlanta even longer–the heat and esp. humidity just kept me indoors.

  • He is right as a technical matter about which is more efficient, though if you can’t sleep because it is so hot then spending a little extra money and carbon to keep it cool is probably worth it (but that’s your call). The larger the difference between inside air temperature and outside air temperature (with outside T higher), the less efficient an air conditioner is. That means that letting your house get warmer makes the AC more efficient at cooling it, as does waiting until evening to run it when temperatures outside are a little lower. Most air conditioners are also more efficient running flat out than cycling on and off.

  • Get a programmable thermostat. Cool less when you’re away, anticipate your return home with a cool-down an hour before, and save some juice while you sleep. Ultimately, the system is on all the time, so maybe roomie won’t like it, but if your comings and goings are pretty regular, you can each get your concerns partly addressed for the sake of a $35 purchase from Home Depot.

    • saf

      We do this with both heat and ac. It keeps the place comfortable and the bills reasonable.

    • Pepco was also recently giving them away (with installation services), if you enroll in their energy cycling program, which might appeal to your roommate as it has a potentially even greater environmental impact. I’d contact them to see if they’re still running that program.

      • DON’T sign up for PEPCO’s program. This allows them to turn off your AC unit without your permission during peak usage. So if you were home sick or working from home, you could find yourself without AC.

        • gotryit

          If you really want to save the environment, then you would sign up for the program. That’s what will make a serious difference. And save you a few dollars.
          You can only save a little without impacting your comfort.

        • I have the PEPCO thermostat and it won’t make a noticeable difference for a reasonably sized apartment. They will cycle it off during peak times, but they don’t just shut it off all day. And they give you some rebates. I’ve never noticed when they cycle my AC off, even when I’m home all day.

        • The Pepco program can only turn your a/c off for minutes at a time, which reduces the temp by a few degrees. You can choose how much to cycle your a/c. For instance, if you elect 50% cycling, your compressor will operate half the time it did during the hour prior to the conservation period. The fan will continue to circulate air even though the compressor has reduced operating time. (you can do 50%, 75% or 100%). Total conservation periods over the season usually amount to less than 1% of the year and typically only lasts 3-6 hours on the days it is in place.

          Depending on the level of participation you choose, you may or may not notice that an Energy Wise Rewards conservation period is even occurring. During the course of a conservation period, the temperature in a typical home may rise 1-3 degrees (50% level), 2-4 degrees (75% level), or 4-7 degrees (100% level). Variables such as home insulation, shaded windows, the addition of ceiling fans, and the amount of home foot traffic are some of the things that can also affect the temperature.

          Please learn about the program before advocating that people not sign up. Sure, maybe people don’t want to do 100% if they don’t want their a/c turned off, but they have other choices.

          • Disagree. Don’t do this. The rebates from Pepco are tiny, and you can achieve the same result for yourself in terms of energy savings by programming your own thermostat. Do you really believe that the company is looking out for your best interests? Would you allow your bank to regulate how much of your money you could withdraw? Allow your supermarket to decide what you buy? This program, along with smart meters, are being pushed by Pepco in order to increase their long term profits under the guise of energy savings.

          • ah

            +1. I looked closely at this. The savings are miniscule on overall rates. And then they shut you off on the day you most want A/C.

            These types of programs are much more sensible for large electricity consuming businesses that can simply shut down on really hot days.

        • We did sign up and haven’t had any issues at all, plus we got a free thermostat and the ability to monitor our usage online. It was a good deal for us, but everyone should check it out for themselves. They’ve got plenty of info out there, including some of the info shared by other commentators.

          • The Pepco program and thermostat are good. The manual, both printed and online versions, are terrible. Be sure to get good instruction from the installer on how to program and especially, how to override the default settings.

    • We usually just have it set to “auto” a certain temp at night. He turns it to the “off” position when he leaves for work in the AM.

      We have a programmable thermostat with four different time/temp combination settings. I need to learn how to set this thing! Unfortunately, our apartment did not include the directions. Will google the model tonight.

      • Get thee to the Google and figure out that programmable thermostat!

      • My programmable thermostat has the instructions “imbedded” into the side of it – it took me a while to notice them, but check to see if yours has a little tab or something you can pull on around the edges – the instructions are on a little laminated card that slides into the unit itself. Mine is a Honeywell that is 5 or 6 years old.

    • I’ve heard GREAT things about the Nest programmable thermostat. A little pricier than other models, but this is something you can control via your smartphone/internet if you are away, or if you plan on coming home earlier/later than expected. I plan to get one once I have some extra $$$.

      • A Nest is over $200. Pretty sure if you’re just renting it’s not worth the investment. If you own the place, then go for it!

        • You can alway take it with you and put the old one back.

        • It took me 5 minutes to take off the old thermostat and put on my Nest. I just kept the old one to put back once I move. The thing saved me over $200 last summer, so it already paid for itself.

      • It is definitely better to leave it on but at a higher temp while you are away. The Nest thermostat is based on that principle – turning the air/heat on and off manually is less efficient than having a standard range of temperatures. We have a Nest in our 4 floor, 2000 sq ft townhouse (two of them actually – we have two zone hvac). It’s pretty awesome – you can create really custom schedules, it has an auto away function, and after a few months it learns your system’s efficiency so it will begin to heat or cool in advance to reach the target temp on schedule. You can also adjust the schedule and current temp from your smartphone (maybe a website too?). There are other fancy things if your hvac unit supports it – like ours will keep the blower on for a bit after the a/c or heating unit shuts off, to push all the cold/warm air out of the ducts and into the house.

        Like the person above mentions, it can move with you to the next place. It was really easy to install, just pry the old one off and attach the wires per the diagram.

  • the REAL answer is that there is no clear answer unless you do a bit of experimentation. I assume you have Pepco… so all you need to do is log on to your account and you can pull out the hourly energy use data. Do a couple of days of experiments (run one day the way you would like it, one day the way your roommate would like it, and then some sort of in between setting). then just add up how much energy you used. NOTE that you will need to have three days in a row of relatively similar weather (cloud cover, temperature).


    • I tracked my useage 2 summers ago with my central AC (relatively new heat pump in petworth row house). I tracked by reading my meter each morning for 2 weeks, varying the temp, run time, etc. The least energy used by far was turning off during the day, and running when I got home (I wound up programming to come on 2 hrs before I got home for my comfort).

  • I set my central AC for 80 when I leave the house and I leave the window unit in the bedroom at 70 all day. Seems to work well and my bills are much lower than when I only had the central ac.

  • My home inspector told me to just leave the A/C at one temperature, and it would be the most (cost) efficient. It takes a lot of energy to rev up your A/C after it’s been off, or set very high. For your unit to cool itself off from 92 degree to whatever you change it to at night takes a lot of energy.

  • My thermostat is programmed to let the house warm up during the day, and then cool it down before I get home from work. It does the same thing when I sleep, and then the opposite in winter. I paid about $40 for the thermostat. It pas for itself in a single month in the summer.

  • Buy an $80 programmable thermostat, and have it kick on the AC while you’re still at work. Ask the landlord to pitch in and he can keep it, or take it with you when you leave.

    • This. Your roommate is wrong, but this is a good compromise.

      Otherwise, move. Your roommate is a passive aggressive snarklepuss.

  • The fact remains that it gd hot as hell outside this week. I get turning it off nearly all other weeks during the year, but seriously, its just a few weeks a year we have to really deal with the 95+ heat. Seems that there should be consideration to the fact that this request is not the norm 12 months of the year.

  • Compromise: Get a heavy duty timer from home depot for $11 and have it come on an hour before you get home. Leave it off most of the day, but also close the blinds for all the windows to block direct sunlight and turn off computers and other heat-generating appliances that you don’t need.

    The air conditioner is least efficient when the air outside the coils is also hot, like at 3pm, when you are not there anyway. So it is still better to blast it in the evening than to leave it on during the peak heat of the day when no one is even there.

  • Does your roommate turn the ignition off at ever red light when driving?

  • Generally, I’ve read that just setting it higher say to 85 is better. Trying to cool a 90+ degree room to 72 degrees puts a greater burden on the system. Thank god I live in the basement. I find it goes up maybe 5 degrees during all day while im gone with no AC even when its like yesterday.

    • This was my understanding.
      Having your air conditioning completely off when you’re not at home is also not a good idea because you’re likely to get mildew, at least in your bathroom.

  • laurelo

    I interned for a utility company (in their community relations department in the summer–talking to consumers about how to keep their bills low and manage their energy efficiency) and what you’re recommending is exactly what you should do. The amount of energy the A/C has to use to compensate for the drastic change in temperature is much more than maintaining your house at a lower temperature all day and then modifying once you’re home to an even lower temperature. There was some math equation we would share about for every five degrees change on your thermostat, you’re adding X percent to your bill, but that escapes me at the moment.

    • I work with building technologies and you’re right on. Leaving the air conditioner running during the day, albeit at a higher temperature setting, is more efficient than turning the air off. It requires less energy to cool a room from 82 degrees to 70 degrees than it does cooling the room from 90 degrees to 70 degrees. This is a similar concept to using a heat pump that takes in ambient air to heat up water in a water heater: it’s way easier to regulate temperature fluctuations within a smaller range of temperatures than it is to regulate temperature over a larger range.

      • This is the wrong example. The question isn’t amount of energy required to cool from 82 to 70 degrees vs 90 to 70. The question is amount of energy to cool from 82 to 70 PLUS the amount of energy to keep the temperature at 82 all day vs 90 to 70. My studies monitoring my meter show that the latter uses less energy.

  • I turn mine up from 72 at night to 78 during the day while at work. It takes about 5 hours to bring it back down to 72 in time to go to sleep.

    During days like I think you are correct. You should leave it on but at a higher setting.

    • 5 hours seems like a long time to cool down 6 degrees! Perhaps you have a very large home or old HVAC, but it only takes about 1/2 hour for my HVAC to cool my place about 5 degrees (albeit, my place is only 600 sq ft…)

  • gotryit

    Enroll him in thermodynamics 101 and home ec 101. This guy isn’t making much of an impact. These “rules” about which is more efficient are small potatoes. Now if he forgoes AC and just uses a house fan, now we’re talking.

  • You need to get a new roommate!

  • Neither plan would really save more energy than the other. But leaving it at 85 during the day will save you money in other ways.

    1. Your furniture, floors, and wall paint would fall apart because of the huge daily temperature swings
    2. Your A/C’s compressor will doubtless explode one night because you’re making it work for like six hours straight instead of a few minutes ever half hour or so
    3. Your refrigerator will break, or at least ruin your food. The fridge compressor can only cool the inside as long as the outside temp (i.e., the inside of your apartment) can absorb the heat it’s pumping out. If your apartment is 95 degrees, your fridge can’t do shit.

    Your roommate sounds dense. Try telling him that you’re both right, but that leaving it on all day will ruin his schlitz and hot pockets something.

    Or, better yet, move.

    • *leaving it off all day will ruin his stuff, not the other way around. I think you get it.

    • So basically pennywise, pound foolish.

    • I call baloney on the damaged property, etc. I don’t have AC at all and find my fridge works perfectly fine. The only damage I’ve noticed is that command sticky hooks don’t hold as well with the humidity.

      • Yeah. I spent 2.5 summers here in DC without AC. No problems with the fridge.

        • saf

          We got ac this year. We have had the house since 1990.

          So, 22 years, no ac, fridge worked.

          Also, all those years before ac existed and before it became common in houses – people did not drop dead all over the place. Neither did fridges.

    • I agree that it’s the VARIATIONS that will damage things, not necessarily not having a/c. But the best part of the comment above is that it will burn out your unit to make it run for hours at a time – not a huge deal for a renter since it’s not YOUR unit, but if it breaks you’re gonna be sad. In heat waves you can wait weeks for an a/c repair appointment!

  • andy

    I think you should consider how long it takes to cool the apartment when you get home. If it is only a few minutes to get to a temperature at which you won’t sweat, I would fully turn it off during the day. I would actually turn it off a little bit before leaving, just leave before it gets warm inside the apartment.

    If it’s actually true that your AC won’t cool your apartment until 2-4 hours after you turn it on (i.e., you return at 7:00 p.m. and it is cool, finally, 9-11:00 p.m.), I might actually consider a better AC unit or consider leaks or something, because that’s a really long time.

    All that said, I’m willing to go along with factual arguments regarding energy efficiency, if it takes into account turning off the AC entirely for part of the day (and isn’t using keeping the house at 72 degrees every minute of every day as a counterfactual). I would also like a programmable system that basically turns off the system and turns it on just in time for my return home, but in the absence of that, I’m a fan of turning everything off for part of the day.

  • I know that guy!

  • If you have the money and can afford it, get a Nest thermostat. Set it up and you can control the unit using a smart phone. If you don’t grant him access he will be powerless to do anything about it.

    If he insists on turning off the AC after you leave the house, never fear, using your phone you can control it from where ever you are. He turns it off? Presto Chango – you turn it back on. The thing is amazing. Just bought a house with one that was already installed. So amazing when I am in bed at night and it gets to hot. No need to get up and walk your lazy ass to the thermostat, control is right there on your phone. First world living rules!!!

  • Any advice if you have window units with no temperature control? I’ve always left mine off, but the apartment is always roasting when I come home at the end of the day.

    • gotryit

      You can get timers and other controls like that that go between the plug for the window unit and the wall. Check home depot in the electrical section. There may even be some with built in thermostats.

  • There is no way of knowing until you experiment. Btw the Pepco website has a report where you can view your daily usage by kWh. Try it for a few days and see which one is better.

    OR Just tell him to stop being a cheap bastard and leave the thermostat on 80 during the day.

  • Programmable thermostat is great for this. And this just reminded me that I have an extra brand-new one (I accidentally double-ordered from Amazon) if anyone wants to buy it and save me the trouble of mailing it back to Amazon. Lux brand – model TX1500E – works great, easy to install. I’ll sell it for whatever Amazon is charging – think it is around $34.00.

  • +1 on programmable thermostats. Set to a higher temperature, but don’t shut off entirely.
    Another big factor ppl don’t often think about is window treatments. Block out as much light as you can.

  • pablo .raw

    There are many factors to consider in this situation like how tight is the house? If you live in an air leaky house, then you’ll be losing a lot of conditioned air to outside and therefore paying for it. Also not sure if someone has mentioned it but there’s also the fact that air conditioner gets rid of air moisture, without it, there’s the possibility of mold growing during humid summer days.

    • +1. Mildew can quickly grow on the clothes in your closet. Wood expands and contracts – your floor and furniture will be damaged over time. I would be most concerned about the health aspect of this situation.

      • My mom turned off the A/C in her house because she was staying mostly at her boyfriend’s pl,ace… and the (vinyl?) flooring in her bathroom contracted and no longer fit properly.

  • I used to shut my window units off each day when I went to work and it was a long time before the house was comfortable when I got back. Someone recommended keeping them on, but at a higher temp, during the day, so I tried it as an experiment. Not only was my house more comfortable, my electric bill dropped from $200/month to $100 – $150/month, so I presume I am using less energy as well.

    I had to replace the window units this year and got new models with a “sleep” mode. I use that at night and during the day and it works great. The unit automatically raises the temperature 4 degrees over theh hour after you hit the “sleep” button, then 8 hours later it goes back to the original temp. Works like a charm and my electric bill was only $56 last month. Waiting to see how big it is following this heat wave.

    • Thanks all! Such helpful suggestions. I’m going to try a timer or control that can go between the windown unit and the wall.

  • I think you should capitalize on the heat and this hip hot yoga bullshit but turning your apartment in to a hot yoga den. when your annoying roommate gets home and asks “why are all these hot sweaty hipsters in my living room stretching their collective crotch all over my swank west elm couch?!” your signature blank stare will have all the answers he seeks.

  • I got a Nest before last summer. $200 up front, but my power bills were about $200-300 less over the summer, so it paid for itself. Now this summer it’s just saving me money and I just turn the AC on before I leave the office so it’s cooling down when I get home.

  • holy fuck the amount of misinformation in this posting is astounding.

    • andy

      we could always use your help

    • Please enlighten us? I’m serious, I’d like to know what’s wrong in here.

      • A lot of the discussions about efficiency factors on air conditioners – as in, people claiming that the energy required to re-cool an area from 90s down to 70s outweighs the energy saved by raising the temperature during the day, and… you see, it just get’s complicated to run through without a short lecture on basic thermodynamics, efficiency factors, leakage, basic econ, etc. I tried to start a couple of times, but I think that anonymous 4:46 sums it up.

        • Here’s a quick test. Can you explain the difference between BTU and BTU/hr? How does a BTU/hr (cooling) relate to a Watt?

          If you understand this, then I’ll listen. Otherwise, you’re regurgitating old plumbers tales that are just as likely to be BS as true.

        • so how do you deal with your thermostat?

          • it’s programmable, but someone is always home.

            If they weren’t, then I would raise the temperature a bit during the day.

      • see Anon 2:41 for the best answer

    • thanks for clearing it up.

  • When figuring out the ‘costs’ of leaving the A/C on or off, many forget to consider the potential damage to items that they own. In weather like this, depending on how hot your particular apartment gets, you could come home to melted cosmetics, damaged foodstuffs, melted candles, etc. If pets are involved, it’s a no-brainer.

    I have two A/C window units in an apartment that gets a lot of sun. One is left on ‘energy saver’ during the day with a fan to circulate throughout the apartment. I close all the blinds and place a cardboard panel in the two windows that get most of the sun. It stays comfortable enough for the furry friends and to not feel overly hot upon my return.

  • Get a NEST thermostat that you can contro via wifi with an app on your phone. That way you can turn the thermostat on before you get home so it has a chance to cool down. Both you and your roomate win.

  • Any suggestions for how to best handle a narrow 2-story rowhouse with central air, that never comes close to regulating the upper level? There’s consistently a 10 degree difference or more – often freezing downstairs and sweltering upstairs. We’ve bought a window unit for the master bedroom and just leave the AC running at what we consider a comfortable temp for downstairs, but it feels incredibly wasteful (not to mention, still insanely hot in the hallways and other bedrooms upstairs). Clearly dual zone would be great and is part of the long term plan, but in the meantime looking for a solution with less up-front cost.

    • Our upstairs is always warmer as well. We keep the vents closed in the basement (where it’s always the coldest) and close a couple on the first floor – this is usually enough to keep the upstairs cooler.

    • If you haven’t done so, take a look at your attic insulation and see if it needs replacing.

    • you might also have a damper closed… somewhere in the system. lots of variables here as all central air systems are installed a bit differently. Remember when cooling, it’s best to force the cool air to the second floor and let it settle down to the first floor. When heating it’s better to force the air to the first floor and then let it rise to the second floor naturally.

  • I turn the AC off when I leave but have fans around that I turn on, along with the AC, when I get home. The fans cool you as the AC gets rolling. I feel leaving the AC on all day is very very wasteful. (Caveat: If you “do” leave the AC on all day, make sure the windows are closed. It’s easy to do, but can lead to major condensation buildup [he says from experience]). Also, the fastest way to get cool is to take a cold shower. Works like a charm, even in a super hot place.

    • P.S. I cool my place only to 80 degrees, and during the day it heats up to 85 or 86 — not such a big difference. I agree with earlier posts that if the spread is very great, probably a good idea to keep the AC on an adjusted temp all day. But I recommend gradually raising the temp and getting comfortable with the heat. That works to eliminate the (I think) dreadful contrast between your inside temp and the outside temp in summer. if you can live in a mildly warm place, you feel much more comfy outdoors in the heat.

  • Solution: get a new roommate. This one is broken.

  • Does it really matter who’s right or wrong? Obviously, you need to compromise to continue happily living together.

    Thankfully, my husband and I are pretty consistent as far as air conditioning regulation. We like it cold – probably too cold (and a lot colder than it sounds like most on here would like it). We normally sleep with it around 70/71 and then I move it up to 74 and 75 during the day when we’re gone – both during the summer and winter. We have two big, furry, older dogs, so we can’t completely turn it off or put it much higher. I grew up in Central Florida – we lived in our house for about 5 years before we had air conditioning installed (not green, just poor). I think that’s part of the reason I like it so cold. Also, I can attest the heat – but mainly the humidity – can do a number on the natural materials in your house (wood, clothes, etc.).

    • What type of damage do clothes sustain? I assume it varies by material. I’m really curious about what you’ve experienced.

      • Mold and mildew. If it sits for awhile (like clothes I would have at the back of my closet), it’s impossible to get the stench out unless you bleach the hell out of it. Towels too. Actually, towels were the worst because they would never get dry.

        • Offtopic but The mold/mildew comes out easily in the sun (light fabrics that won’t fade) or white vinegar FYI

  • We need real world data! Please run an experiment. Do it your roommate’s way for one pepco billing cycle, then do it your way for one pepco billing cycle, try to keep the rest of your usage constant, and compare BTUs (or whatever) on your bills. (Not a perfect experimental design if outdoor temperatures vary from one cycle to the next, but you get the idea). No one can answer this question in the abstract. It depends on your particular system, how much insulation you have, etc.

  • Is the guy pictured the guy who needs help? If so, I can replace your current roommate, good lookin’!

  • Heat flow rate into your home is a function of the temperature difference. The colder it is in your house, the greater the heat flow rate into your house. So keeping your AC on all day versus turning it off will clearly cause the heat flow into your home to be greater. So if you keep your AC off, the amount of heat that has entered your home is actually much much less than if you keep your AC on. Keep in mind, the AC doesn’t prevent heat from entering your home, all it does is absorb the heat that has already entered your home and then rejects that heat back outside. So you are right, your roommate is wrong.

    And when it comes to moisture, hot air does have the potential to carry more moisture than cold air, so having it really hot in your place will allow for a condition of very high moisture…If you take a can of pop outside in this high heat and humidity, the reason it gets moisture on it is because the air around that can has cooled off and has reached the dewpoint where the air is completely saturated and can no longer carry the amount of moisture that is in it, and so the moisture condenses onto the colder surface. That same thing happens on your AC unit’s evaporator coil.

    • My apologies for the incorrect who is right statement. From an energy perspective, the roommate is right, the OP is wrong. My above explanation is why.

      • sorry dude… your answer isn’t complete either. you are making too many implicit assumptions about the system (i.e. house).

        • I didn’t assume anything about the house. It doesn’t really matter.

          We could get much more detailed about this, but just discussing the basics is sufficient to conclude which method is more energy efficient given the information the OP has provided.

          If you don’t think so, I’d like to hear why.

  • You are right leave it on. The energy ( that your roommate claims he is saving) will be lost when you return home and turn the a/c on full blast. Now the a/c unit must overcome the heat and tends to stay on longer. The compressor in an a/c unit is a large motor and it draws a lot of current (the stuff you really pay Pepco for) the longer it runs the more Pepco is happy

  • brookland_rez

    I leave mine set to 72 when I’m gone and I turn it down to 68 when I’m there, but I like it cool in my house.

  • From an environmental standpoint, it’s definitely better to turn it off. The annual peak of electricity usage in the US is on hot summer weekday afternoons., due to a/c demand. Most of our electricity here is coal-generated, and coal plants are extremely inflexible in their output – they take days to ramp up or slow down. That means you have to constantly be generating enough electricity to satisfy peak, 24/7, even though peak is only a relatively small portion of the day. Otherwise there will be brownouts. So, much of the electricity they generate is wasted.

    So, anything you can do to cut electricity demand in those few peak hours on weekday afternoons actually goes a long way to reducing coal burning. That is why many utilities, including Pepco, are starting programs to temporarily shut off people’s home a/c units during the day when they are not home. It’s a way of evening out demand, and it’s much cheaper for the utility (and much better for the planet) than building a new coal power plant.

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