[Greenbuilding] Using air-conditioning periodically

[Keith Winston]

I was just thinking about this today, and realizing that in my effort to 
be unassailable, I had probably massively overstated the time it took to 
justify turning off your AC. For exactly the reasons you describe. So 
realistically, unless you're going to turn it back on within a minute, 
probably, you should turn it off in terms of motor load.

That doesn't address the other part of the equation: if you want your 
house to be at 68F, say, and you are simply letting it swing in wider 
arcs than the thermostat would (up to 75F, say, instead of merely 70F or 
something), then the net savings of turning off is still probably 
comparatively small, since the AC now has to cool a warmer house. That 
was my other point.

Your point of doing as much cooling as possible at night is an important 
one. And your general idea, of using AC only sporadically for momentary 
comfort fixes, is of course going to use much less energy potentially 
than maintaining a consistent temp. Actually, this is similar to a 
dilemma: for many of my clients, it's relatively implausible to do away 
with central air (for resale, if nothing else...). In fact, in a tight 
well-insulated reflective-roofed ERV-equipped house, it might be 
possible to be comfortable almost all the time with a couple window 
units, which might be 1/2 as efficient but also 1/10th the cost, and 
operated only 1/4 or less as much. It depends of course if you're home 
during the day, if you can selectively cool certain areas to good 
effect, etc. It's part of why I like the mini-split systems, which in 
principle can be cheaply installed and very efficient (i.e. Fujitsu 
Halcyon 21 SEER), but they don't really fit with many people's 
expectations when you get into the more complicated cut-up house designs 
that we often have now...

Keith

Lawrence Lile wrote:
>> he way I would think about it is, There is the cost of starting the 
>>     
> compressors, in terms of  short-term high-power demand>,
>
> A compressor (or any motor) will draw 6 times normal current for about
> one tenth of a second.  If the motor has a heavy inertial load on it, it
> may take another second or two to get up to speed, which may add 10% to
> the motor amperage for two seconds.  After a minute, the average power
> is essentially the same whether the motor was started or had been
> running continuously.  This myth of the huge "surge" when you turn on
> [motor, fluorescent light, fill in the blank] is just that.  People blow
> this startup surge out of proportion, and claim that you'll save energy
> by leaving something on.  There is some effect on the life of the
> equipment, but very little on the energy use due to this "surge".  
>
> Whenever you aren't running your AC, it isn't using energy.   If you
> want to save energy on AC, turn it off.  If you want to use it for a
> short blast, say to cool things off before you go to bed, or when buyers
> are coming, go ahead and do that.  
>
> I've been using my AC for humidity control only so far this summer.
> I'll wait till evening, when it is cooler outside, and the compressor
> will work most efficiently, and give the house a blast of cold dry air
> until the humidistat gets down to 65 or 70% RH, which is about as high
> as I'd like to see it.  
>  
>