A/C usage strategy

sub3marathonmansub3marathonman Solar Expert Posts: 300 ✭✭✭
I have been wondering about how to run the A/C for the least amount of electricity used. I have always heard that it is better to let the house heat up in the day, then cool it off at night.

BUT, if your SEER is overall 18, your A/C may be 12 when working hard and 24 when in the easy mode. So you would be loosing that efficiency by cooling all at once. But if you've saved enough by not running the A/C at all, then it would still be worth it. What I've tried to do occasionally is to just step it down manually, but that is difficult to catch at just the right times. I wish it was possible to just set the A/C unit so it couldn't go into the inefficient hard-working mode, but I haven't been able to find that out.

Comments

  • System2System2 Posts: 6,290 admin
    Re: A/C usage strategy

    There is a problem with the equation to solve this. The variable "human" is not a single variable but variable variable and of course there may be more than one which can result in several setpoint changes within short periods of time.

    If we remove this variable for now we can see that there are still other factors to take into account.

    The first thing is to not run the A/C at all. Maximum efficiency, lowest power draw.

    To keep in the no A/C mode it is best to cool during the night, then seal it off when the temps outside being to increase above those inside. The better insulated the area is the better this will work. So add insulation.

    So lets say all the insulation that can be done has been done.

    And the area is being cooled at night with lots of fans to cool off everything so the thermal masses inside get to the lowest temperature seen outside.

    Now you are starting the day off at the best possible position.

    If the objective is to keep the lowest temp possible, then start the A/C right away.

    If some temp increase is acceptable then delay turning the A/C. If there is a maximum acceptable temp then the A/C will likely have to be turned on before that point, particularly if the A/C is too small to handle the max heating load.

    The greater acceptable temp increase the less power being used.

    As for the idea that the A/C is less efficient when moving more heat (drawing more power) that has not been my experience with large units, some very large.

    Most A/C units I know of are much more efficient when fully loaded. Part of the energy of starting the unit goes to cooling down and warming up the various parts. This is why units are often undersized it keeps that to a minimum and makes the most of the heat exchangers. Generally the greater the temp differential on them the more efficient (there is graph that will show how each unit operates).

    Undersizing also allows the unit to run at rated speed. This means less loss in the system as the percentage of energy (heat/cool) lost through all the various means is smaller than that being moved. Move less heat and a higher percentage is lost (even though it may be the same amount).

    From the A/C point of view it likely prefers to run wide open and not throttled back.


    During the heating of the day the area may increase to the point that A/C is needed.

    Depending on the size of the A/C unit you may want to start it and let it run at 100% for a while an then shut off.

    If you can lower the temps such that the continued heating from outside does not result in temps above the acceptable maximum then that is likely going to be best.

    If the unit has to start again, then again you will want it to run at maximum for a set period and shut down.

    When the outside temp starts to drop heat will still be coming into the area but at some point you want that heat to be removed by fans and ventilation and not by the A/C. The sooner the A/C is shut off and ventilation started the better.

    Now we have to put humans back in.

    Humans will not like the cold blast of air from the A/C unit only to have it shut off and the room heat up again.

    If the cooling is needed for other people, and not equipment, then that, IMO, should decide how the unit is operated.

    So I suggest setting it up so it is comfortable, stable temps and then work around that. Start ventilation later in the day so temps are kept comfortable, fans not too loud and so on.

    Then work with that. Add insulation (to area and equipment), shade, different paint color and so on.

    Bottom line, all the extra heat that gets in has to be moved out, if the A/C is used to do that it likely is going to be most efficient doing that at full load.
  • Solar GuppySolar Guppy Solar Expert Posts: 1,980 ✭✭✭
    Re: A/C usage strategy

    Its negligible in the Seer rating or efficiency, your talking maybe a 15 degree difference for the condenser to operate in day vs night in Florida, maybe a few hundred watts difference, you can just look at the Xantrex SCP midday, vs night and see the actual power difference ( assuming your AC is on the load side of the XW-6048 ) , mine is and its not that big of a difference

    This is one reason GeoThermal is nice, fixed temperature for the exchanger. For you in Bartow, all efforts should be spent on the structure not the Heatpump. R40 Minimum insulation for the attic, radiant barrier, proper roof venting, double pane windows, insulated doors ( including to the garage ) and when you insulate, make sure its the whole house, around here the builder skip over the garage.

    I'm keeping a 2600sf home cooled with a 3 Ton Seer 15 unit now without problems just by having the R40 in the entire attic
  • RCinFLARCinFLA Solar Expert Posts: 1,416 ✭✭✭✭
    Re: A/C usage strategy

    Just to add something not mentioned, condensing water vapor saps more energy then just cooling the air. When air is humid there is less air cooling. If house is sealed up it should not rise much in relative humidity from having A/C shut off during day.

    If you are not at home during the day it is best to shut off A/C or at least run thermostat temp up to 78-82 degs F.

    The day setting may be your preferrence on how long it takes to cool back down when you get home.

    First thing is get a programmable thermostat that you can set up to automatically change the temp based on time of day. One that reads out hours of active operation is also nice.
  • Dave AngeliniDave Angelini Solar Expert Posts: 6,072 ✭✭✭✭✭
    Re: A/C usage strategy
    I have been wondering about how to run the A/C for the least amount of electricity used. I have always heard that it is better to let the house heat up in the day, then cool it off at night.

    BUT, if your SEER is overall 18, your A/C may be 12 when working hard and 24 when in the easy mode. So you would be loosing that efficiency by cooling all at once. But if you've saved enough by not running the A/C at all, then it would still be worth it. What I've tried to do occasionally is to just step it down manually, but that is difficult to catch at just the right times. I wish it was possible to just set the A/C unit so it couldn't go into the inefficient hard-working mode, but I haven't been able to find that out.

    You might want to read the thread in this group on the Sanyo Mini Split. The concept being that you run the unit in a low power mode only, all day long, powered by the sun. Probably would not work well in climates that do not cool down at night. Another strategy...
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
     http://members.sti.net/offgridsolar/
    E-mail [email protected]

  • BB.BB. Super Moderators, Administrators Posts: 32,204 admin
    Re: A/C usage strategy

    The thread is here:

    Sanyo mini split AC (inverter/variable speed)

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Dave AngeliniDave Angelini Solar Expert Posts: 6,072 ✭✭✭✭✭
    Re: A/C usage strategy

    Thanks Bill ! Will summer ever come to Northern california? In this neck of the woods we are now usually watching the big waterfalls from the snowmelt above 9,000 feet. Heck, we are adding serious snow still!
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
     http://members.sti.net/offgridsolar/
    E-mail [email protected]

  • BB.BB. Super Moderators, Administrators Posts: 32,204 admin
    Re: A/C usage strategy

    I don't know--it has been cool down near the coastline too... My wife's friends/family believe that we are going to be in for one killer summer heat wave if this follows earlier weather patterns. :confused:

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: A/C usage strategy

    it's quite hot right now over my way as temps are in the 80s. have no fear as it will come sooner or later to you. i think they are calling for 87 or 88 tomorrow and my a/c is on.
  • PhotowhitPhotowhit Solar Expert Posts: 5,697 ✭✭✭✭✭
    Re: A/C usage strategy

    I want to add a thought for off grid systems, I like running the A/C during the day particularly once the battery's have received 95% or so of their charge, even if I'm not at home. It cools down the thermal mass and makes cooling in the evening much easier.

    Hot early here in Misery (Missouri) we've reached 90 a couple days, 88 today. overnights have still typically cooled down, one evening/moring low around 74.

    My money would be onb a scorcher, only time will tell.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
  • sub3marathonmansub3marathonman Solar Expert Posts: 300 ✭✭✭
    Re: A/C usage strategy

    The Sanyo Mini-Split got me thinking about the efficiency factor of the A/C unit. I hadn't even thought of the difference in efficiency between day and night, which unfortunately there isn't much as Solar Guppy pointed out.

    But if I run my A/C many times but only for a short period, I stay in its 24 SEER zone, but if I run the A/C one time at night to cool off the entire day's heat, it will run and run at 12 SEER high cooling mode, thus negating the efficiency of the dual-speed setup.

    So I was just wondering, in terms of total electricity used over a day, which method was better. From reading things here though, it looks like it might be a "six of one, half dozen of the other" situation.
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