[TriLUG] OT: thermodynamics of A/C question

[Joseph Mack NA3T]

On Mon, 25 Jun 2012, Thomas Gardner wrote:

> 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).

This worked till 2 days ago. When I opened the windows and 
turned on the fan all I did was increase the internal 
humidity from 50-75%. Since pulling water out of house air 
uses power without cooling the air, I've kept the house shut 
for the last two nights.

> Comparing electric bills makes it all worth it.
> I think I paid something like $50 last month,

but the temp for May didn't go above 80deg even once (I 
think); summer has been late this year

> 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).

it all sounds reasonable. I'd like some numbers to see if it 
saves anything.

> 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.

sounds OK. My roof is a bit high for this. If I spend 30mins 
watering the roof and it costs me $20 in time to do this, am 
I saving anything?

> 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.

maybe with your wifi programmable temp sensor.

> 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?

I can imagine if the water is a little bit acidic or basic, 
the Al coils would dissolve.

> 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....

rain water is not the same water that percolates over or 
through the ground and sits in a lake

Joe

-- 
Joseph Mack NA3T EME(B,D), FM05lw North Carolina
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