[TriLUG] OT: thermodynamics of A/C question

[Thomas Gardner]

Getting back to the original thought (cheap and easy ways to reduce
your A/C bill), here are some random thoughts:

I've been doing the same thing (two box fans blowing out from two
fairly centrally located windows at night, and opening selected
windows to get the house cooled down as much as possible at night,
then closing everything up at sun-up).  I've got the thermostat set so
that it comes on around 84 degrees.  Yeah, we sweat a little sometimes,
but you'd be surprised how quickly one acclimates.  Fans help a LOT.
The way I see it, I grew up without central A/C, and it didn't
kill me.  Now that I think of it, I didn't have central A/C until
we moved into this house in '96.  Yet, somehow I managed to survive
even when the outside temps went up into the 105 range....  So far,
the A/C has only started coming on late in the afternoon/eve for the
last couple days.  Comparing electric bills makes it all worth it.
I think I paid something like $50 last month, whereas I think it was up
over $200 for the same period last year.  I forget the exact numbers,
but it was definitely cause for celebration.

I've also been doing the other thing Joe initially suggested for
a couple years in my new shop (programmable thermo, set very cool
for a half hour before sunrise, and hot the rest of the day).
Although in really hot weather, it never reaches its cold temp (70)
in that half hour, the A/C almost never comes on during the day
(set to 85).  Since I don't spend huge amounts of time in the shop
yet, this seems to work fine.  I override it sometimes, but by and
large, its usually pretty good.

The point is, in my experience, both of the things you mentioned
seem to work very well.

As for other things I've thought of for A/C bill reduction, the first
one is super-easy:  I'm thinking I need to go out in the afternoon and
just hose down the roof a little.  I gotta figger the roof is what,
like a billion degrees, and if the water temp is something like 60
degrees or so, that's got to help.  I'm thinking it would probably cost
less than a quarter or so in water to wet down the roof (and I have a
LOT of roof).  I don't have data for it, but I find it hard to believe
that won't either save me more than a quarter on my electric bill,
or make the house a little more comfortable.  Either way, it's worth a
quarter and the little time it takes to go out there and hose it down.
I need to start doing that....

Of course, building some automatic system to cycle itself on and off
whenever the attic temperature is above XYZ would be nifty and all,
but I'm not sure it would be worth either the initial expense or the
risk of messing it up and having to pay for fixing the house when
you end up with a leak.

Another thing I've wondered about is:  Besides building something
around my A/C unit (with plenty of airflow, of course) to give it
some shade, what about using the cold water from the city to cool
those coils outside?  Run a little line out there, rig it up with
misting nozzles like I remember from my days when I worked in the
greenhouse, arranged in a ring around the unit so that the mist
sprays up all around it.  Then use an electric valve to turn it on
whenever the A/C comes on, and the fan on top of the unit will suck
all that cool mist over the coils to cool them off that much faster.
Again, the water costs next to nothing, but I'd bet it would cool the
house down faster, thus the system would have to run less for the same
thermostat setting.  Then again, if you ruin the unit (corrosion in
general and calcification on the coils being two concerns that come
immediately to mind), where are all the savings?  Seems like the
thing should be designed to allow water to get sucked in, though.
After all, they put those things out in the whether....

$0.02,
tg.