Relative AC Efficiency

[Alphonse Q Muthafuyer]

Brothers Abe and Erb live next-door to each other in smallish single-level houses that are identical in all regards. Both have central AC.

In the hot, HOT summer months, both set their thermostats to 82 F.

Around 2 pm, Abe sez "The Hell With This!", turns the AC off, goes down the nice, cool basement and reads a book until 6 pm, at which time he turns the AC back on (82 F) for the remainder of the evening.

Erb fiddles not with the AC: leaves it set at 82 all day/nite.

All other things being equal, which bro. (if either) makes more efficient use of AC energy (i.e. uses less elec. for any given day)? And, of course, "Why?".

Thx, AQ

"The monkey and the baboon was playing 7-up. The monkey won the money but he scared to pick it up. The monkey stumbled, mama. The baboon fell. The monkey grab the money and he run like hell!" - from "Dirty Motherfuyer", Roosevelt Sykes, around 1935

[Mark Lloyd]

[snip sig with missing sig separator of "-- "]

I'm currently about as close as possible to the center of the big south central hot spot (it got to 109 degrees in east Texas). My AC has been running constantly all day, and the temperature is still 5 degrees above the thermostat setting.

Considering the above, If I turned it off for 4 hours, it couldn't run any more than constantly afterward.

[Red Green]

Mark Lloyd wrote in news: [e-mail]:

Have to agree with ya when it's at that point Mark.

Apparently the hot spot is headed east for Tuesday. Back up to 102/104 in eastern NC.

[Rick-Meister]

If only it were that simple.

Efficient cooling is dependent on two variables: Btu's of cooling and humidity. It take MUCH more energy to cool humid air. So, if it's hot AND humid outside and you turn off the A/C, only to turn it back on later, you'll be using much more energy to get rid of that extra humidity. Of course, if your A/C is working non-stop in either case, then it's a non-issue.

[trader4]

I think you're assuming here that when he turns off the AC, he's also opening the windows to allow more humid air inside. That wasn't stated in the post, only that he turns off the AC for 4 hours and retreats to the basement. If he doesn't open windows, then the amount of humid air that enters the house with or without the AC on is probably similar.

With the windows closed, I'd say there is no question leaving the AC off for 4 hours and then turning it back on results in less energy usage, just as setting back a heating system for any amount of time results in energy savings. That holds true unless there is some other unique factor at play. An example of that would be a heat pump system, where if you turn it back in heat mode too much, backup electric resistance heat could be used during recovery.

[Mark Lloyd]

and it is humid here. I'm not in Arizona where the temperature is higher but its dry.

[Zyp]

The difference is comfort. When the air conditioning is turned off, the money meter stops spinning and you're uncomfortable. When you turn it back on, your comfort level goes up. Clearly if you set the thermostat to some temperature that's unreasonable, then the system will run continuously. Say

74 degrees when its 110 outside. [Unless it's designed for that perimeter.] System design has it's limitations, just like anything else.

Efficiency is something most folks have a hard time understanding. They tend to get efficiency confused with performance. If you relate miles per gallon with efficiency and speed and time with performance as in a car, then you might have a better understanding of the term.

[Zyp]

Another misunderstanding. Humidity and cooling effect. Humidity is controlled by LATENT HEAT removal. Latent heat removal can not be measured with a thermometer. SENSIBLE HEAT removal is that heat which can be measured with a thermometer. Most air conditioning systems designed for comfort have *about* 75% of their cooling capacity in sensible heat removal and 25% in latent heat removal. Although systems run for a long time and may not be moving the thermometer very far, they are continuously removing moisture from the conditioned air making it more comfortable for us. [Humidity is that component that makes everyone feel sticky and wet.]

[Stormin Mormon]

Shut system off saves money.

When the AC is running, the indoors is more cool. More cool means that more heat pushes in from the outside (higher delta t). Also more humidity pushes in, higher delta h, if there is such a measurement.

[hallerb]

[e-mail]...

abe saves a little money since his is off for hours

[Alphonse Q Muthafuyer]

Heat buildup occurs faster in a cooler house.

Also might want to consider AC cycling. It takes an amperage spike to start a compressor ...

Say, with AC on and t-stat at 82, AC runs 5 min. then shuts down for 10. That's 4 starts and 20 min. of operation per hour. Abe's AC runs for maybe 40 min. when he turns it back on at 6 pm, but the system starts/cycles about 18 less times and total run-time is less.

Cheers, AQ

"The monkey and the baboon was playing 7-up. The monkey won the money but he scared to pick it up. The monkey stumbled, mama. The baboon fell. The monkey grab the money and he run like hell!" - from "Dirty Motherfuyer", Roosevelt Sykes, around 1935

[jmeehan]

Not nearly enough information to even give a good guess. Things like the insulation size and efficiency of the A/C units at various temperatures (inside and outside temps) number of door openings, internal heat sources (like cooking, number of people TV sets etc, Ground temperatures in the basement .... It just goes and goes .