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For environmental and cost reasons, I'd like to use my AC unit less. In situations where passive cooling is not practical, I am trying to determine the most efficient way to do this. (I believe the unit is a Trane XR14 heat pump, installed 2009.)

My fiancee and I live in a brick ranch house with a brown roof, about 1,200 sq feet. We live outside Philadelphia, so during the summer it can consistently get above 90. During the day, when we are at work, I close the windows and shades and set the AC high (80 F). I keep it on when we're gone because we have two cats and a dog. I get home around 5:20 PM. At night, it will eventually get below 80 outside. At this point, I am trying to determine which is more efficient: to set the house to the expected night temperature (expecting that the cooler air will be trapped in the house and keep the AC from triggering more frequently during the night after the initial run down to, for example, 77, and taking longer to turn on the next day) or keep it at the current interior temperature (to reduce the time the AC runs at night).

A concrete example: let's say it gets to 77 outside and the house is 80. Would it use less total energy to set the AC to 77 or 80? My goal is to use the least amount of energy possible in order to lower the monthly electric bill.

Lots of people have mentioned comfort vis a vis humidity, but for the purposes of the question, I'm more concerned about cost as I'm pretty heat acclimated and I can just run a small bedside fan if it gets too gross at night.

  • Are you asking about absolute energy consumption? If so the answer seems clear: whichever is higher (and will result in less runtime). You're not really comparing apples to apples then, though. – isherwood Aug 8 '18 at 15:32
  • I suppose. I'm not sure of the exact meaning of the term, but if it's exactly what it sounds like, then yes. Whatever will use the very least amount of electricity total. I will edit my question accordingly. – Robert Miller Aug 8 '18 at 15:41
  • What color is your roof? – Harper Aug 8 '18 at 17:27
  • Brown. I'll update the question. – Robert Miller Aug 8 '18 at 17:34
  • Is this Trane XR14 heat pump sufficient to heat the house in the winter? Presumably it has backup electric resistance heating. Does the unit give an indication when the resistance heat kicks in? – Jim Stewart Aug 8 '18 at 21:16
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Setting your AC to a higher temperature will at the very least use no more electricity than setting it to a lower temperature. In most cases, it will use less electricity, and how much less depends on how much higher you set it.

In your situation with the inside temp at 80, if the outside temp is 77 and you set the AC to 77, it will run to cool the house from 80 to 77. At that point, it may or may not run, depending on how fast the inside of your house warms up. Temperature, insulation, wind, and other factors determine how quickly your house loses its conditioned air, and therefore how often the AC needs to run to maintain the set point.

If you instead set the AC to 80, it will only run if the inside temp gets above 80. With an outside temperature of 77, that really can't happen (yes, there can be some heat released from the roof and other structure, but that heat will be there regardless of what you set the AC to).

Note that none of this takes into consideration your comfort. 77 and humid may be much less comfortable than 80 and dry.

  • To clarify: in my scenario, the outside temperature is cooler than the inside temperature. – Robert Miller Aug 8 '18 at 15:59
  • @RobertMiller whoops, read that backwards, let me edit – mmathis Aug 8 '18 at 16:00
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    Depending on the thermostat type a 3 degree delta may not make much difference as that is close to the dead band for a mechanical stat, but higher will save $ in the long run in the summer similar to turning down the heat in the winter. – Ed Beal Aug 8 '18 at 16:16
  • House is 80 F and outside is 77 F. Run the a/c rather than try to get outside air in to cool the house. The a/c condensing unit would use very little power because 77 F air flowing over the condenser coils will lead to very low pressures on the high side of the compressor. Low pressure on the high side means low power consumption. – Jim Stewart Aug 8 '18 at 16:46
  • What is the design of the condensing unit? SEER? Does it have a 2-speed condenser fan or even modular (dual) compressors? When operating at partial load units with these features are more efficient than a standard system. But even a basic single compressor with one-speed fan uses a lot less power when the outside temp is lower. – Jim Stewart Aug 8 '18 at 16:58
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In answer to your concrete example, the answer would be you would use less energy if you kept your t'stat at 80.

Unfortunately that's not the whole answer. I spent about 30 minutes trying to figure out how to answer this question with out publishing a technical 3000 word article on energy management. You need to take several different things into consideration. That would include:

  1. The insulation value of your dwelling both the wall R value and the ceiling. Also the type of foundation.
  2. The general orientation of your house. North, south, east or west.
  3. Glass exposure, how much and its orientation, and R value.
  4. The EER value of your duct work as well as the EER of your AC unit and the general air circulation character of your dwelling.
  5. The style of your house, ranch, zero lot line, two story, etc.
  6. How your attic is ventilated.

And on and on and on.

Getting more specific to your question, one of the things you need to take into consideration is the relative humidity of your house and how it affects your comfort level. From a mechanical engineering aspect, the term HVACR is heating, ventilation, air conditioning and now refrigeration, it's the conditioned air I am trying to address. Meaning because of the heat index (air temperature and humidity combined), it may be more advantages to run the AC at 77 just to remove the moisture from the air which will make you more comfortable due to the lack of humidity (lowering your heat index). Or get a dehumidifier.

Hope this helps to point you in the right direction.

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I (b. 1940s) have always lived in hot climates (Texas, Louisiana, Florida) and I well remember 1950s Dallas TX with no a/c. Modern a/c units are amazingly more efficient than earlier ones.

I am nursing our 27-year old Carrier 12 SEER R-22 unit 42k btu/h or 3.5 ton w/ single-speed condensing fan. When the condensing unit cycles on it draws 107 A for a fraction of a second the lights dim very briefly in the kitchen, then the current drops down to 20 A (less for lower ambient temp). The 40 A circuit breaker for the condensing unit accepts the brief excursion to 107 A (LRA 107 A) and doesn't trip. Our unit is a little undersized for our 2000 sq ft house, and this makes it more efficient because it doesn't cycle off much in the middle of very hot days.

By contrast there are new units that have a special "soft start" and these don't have a high current spike when they cycle on--these ramp up from a few amps to the steady state current. Not only that but they can reduce the flow of refrigerant and will get into a continuous low power state in which they use less power when full cooling is not needed.

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It may help to find out the natural air flow direction. 30cm wool strings temporarily fixed in door frames observed for some days before sun rise disclose the flow directions. If known, a fan at an incoming and outgoing window at different rooms will help to exchange the air (in accordance with the measured statistically normal direction ), which should be done via timer for ca. 1 or 2 hours before sun rise, for example from 4 am to 5 am. At that time, air will be cool and will be having the lowest quantity of humidity - statistically in most locations of this planet.There are also mechanical kits to open/close windows via timers. Dry air needs less energy to get cooled down compared to humid air. Outside window shutters at the south side avoid heating up the air inside during day time. At windy days, a candle or cigarette helps to find gaps in the house hull, if put near doors, windows, electric outlets etc. To run 2 (or more) fans will only need a tiny amount of energy compared to the A/C. It also helps to have a light air movement above the bed area during sleep. F.e. a fan taken from an old PC run at reduced voltage (=silent) with horizontal air flow ca. 70cm above the bed improves the air condition and helps to prevent mosquito bites.

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When looking at total power consumption, don't fret about the brief start up currents. The big picture is to reduce the amount of time that the big loads are actually running. Hense, the best options are setting the temperature higher or just using the system fan to circulate the air in the house. Even better then those would be opening windows (screen closed) during the night and closing them in the morning. Heavy blinds also to block the sun from heating interior surfaces. Keep in mind a slightly oversized air condioner uses less total energy because it isn't running constantly. Another option is invest in a window air conditioner for localized cooling. That way the big system just isn't used.

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Assuming that you had a the most efficient thermodynamic cooling system available, the next frontier to exploit is a network connected thermostat: it can provide you with efficiency (and comfort) gains by strategically and selectively engaging the cooling according to a strategy (temperature schedule). Additional non-time (schedule) strategies include automagically turning the device on when the smartphone enters a geographic 'fence'.

An ceiling fan over the bed allows you to stay cooler at a given temperature: you may find that bringing the humidity down to an acceptable level coupled with the fan delivers significant reduction in energy consumption.

Advice: seek a thermostat that can provide you with usage data: this provides the necessary feedback for making better decisions and is handy for posting questions.

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