[TriLUG] OT: thermodynamics of A/C question

[Glenn Hennessee]

I'm doing pretty much the same thing. Put a fan blowing out of the
house at night and open a window so cooler outside air is sucked in.
With my fan I can get the temp at night down to about 5 degrees above
outside temp. A larger fan/more fans would probably lower that
difference. [I lived in a house for a year with a whole house fan and
it was a marvelous thing. Open a couple of windows, turn on the fan
and in 15 minutes you were down to almost outside temp.] Close the
windows in the morning and keep as much light(=heat) out as possible
too. I haven't turned my AC on yet this year and the inside temp tops
out at ~82. A couple of years ago I added a lot more insulation in the
attic and put in soffits. That helped a great deal with both heating
and cooling. I doubt that spraying the roof with water will help much
if the attic is well insulated.
glenn

On Mon, Jun 25, 2012 at 11:34 AM, Thomas Gardner <tmg at pobox.com> wrote:
> 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.
> --
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-- 
Glenn