[Greenbuilding] [BULK] Using air-conditioning periodically

Mon Jun 4 08:41:32 CDT 2007

>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.  

Lawrence Lile, P.E., LEED AP

-----Original Message-----
From: greenbuilding-bounces at listserv.repp.org
[mailto:greenbuilding-bounces at listserv.repp.org] On Behalf Of Keith
Winston
Sent: Saturday, June 02, 2007 12:49 AM
To: greenbuilding at listserv.repp.org
Subject: [BULK] [Greenbuilding] Using air-conditioning periodically
Importance: Low

The way I would think about it is, There is the cost of starting the 
compressors, in terms of  short-term high-power demand, and there's the 
wear and tear on equipment, and then there's whatever's happening with 
the temperature/comfort & cooling energy directly. My (mildly educated) 
guess is that the cost of starting the compressor is probably only 
handfuls of minutes of operation: that is, if you didn't "need" the 
system on, it would only be justified leaving it on if you were 
otherwise going to restart it within, say, 20 minutes. Otherwise, 
turning it off would be advised. That's JUST the picture of the 
compressor cost of operation, basically.

Then there's wear and tear by starting & stopping. But that's what 
central air does, so I think that would count for almost nothing: it 
cycles on and off constantly. However: newer high-efficiency equipment 
with variable speed fans and compressors (or staged compressors) will 
tend to run more constantly. Still, I would expect that actual "cost" of

additional wear and tear to be minimal. It might be most apparent on the

thermostat, since many of them are built with cheap switches since they 
AREN'T expecting them to be turned on and off.

Finally, there's the actual cooling energy. As a house heats up, it 
gains less energy from the hot outside (since delta-T is smaller). Taken

to an extreme, when the house is the same temperature as outside, it 
gains no energy. The rate at which the house gains energy is related to 
the "heat gain" calculated by Manual J techniques, at least 
theoretically, and is determined by insulation, tightness, fenestration,

etc.

So when you turn off the AC, the house heats up. As it heats up, it 
gains less heat energy with time, and so cooling it back down still 
leaves you with a small net gain. The real gain is achieved when the 
house has spent some time at thermal equilibrium, that is, the same 
temperature as the outdoors.

This same analysis applies almost precisely to the idea of turning off 
your hot water heater.

Hope that helps. And hope I didn't make a blunder as I explained it, 
since I'm dead tired! Good luck!

Warmly, Keith

> ------------------------------
>
> Message: 2
> Date: Fri, 1 Jun 2007 13:51:15 -0700 (PDT)
> From: sanjay jain <sanjayjainuk at yahoo.co.uk>
> Subject: [Greenbuilding] Using air-conditioning periodically
> To: greenbuilding at listserv.repp.org
> Message-ID: <814260.24305.qm at web27011.mail.ukl.yahoo.com>
> Content-Type: text/plain; charset=iso-8859-1
>
> Hi all,
>
> I keep hearing the argument that keeping the air-conditioner on is
better than switching it on and off.
>
> We don't use the AC - My landlord just instructed to keep it on
because he's trying to sell the place, I said I'd put it on a couple of
hours before the prospective buyers came, but his answer was it's
environmentally better to keep it running...
>
> I assume that if you keep putting it on 5 times a day that it's
probably correct, but if you only use it for an hour or 2, It makes
sense to switch it off.
>
> btw - it's central air so I'm guessing there isn't a load issue.
>
> I've seen both sides being argued,  What's the answer? 
>
> ~sanjay
>
>
>   

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