[TriLUG] OT: thermodynamics of A/C question

[Joseph Mack NA3T]

On Thu, 16 Aug 2012, jonc at nc.rr.com wrote:

> Follow up...
>
> Did you look at this any further?  Did you in fact do a 
> test run where you cooled the house down at night and then 
> let it heat up during the day?  I would be curious to see 
> the data.

I've been doing this all summer.

I've also abandonned the upper part of the house for the 
summer. During the day I work in the kitchen. At night I 
sleep on a mat in the lounge room. The upper floor gets to 
the 90s during the really hot days of about a month ago. In 
the morning, when I get up, the upstairs has cooled a fair 
bit (I measured it, but I forget the drop - it's still above 
the outside temp). I don't know why it drops so much - maybe 
the air vents are not totally closed off. I expect not 
cooling the upstairs will have a bigger effect on my 
electricity bill than any change in when I run the A/C.

> In the past I've turned off my AC and run hourly 
> measurements of inside/outside temp to see how rapidly it 
> heats up... I've been pleasantly surprised by how well my 
> house conserved the cooler temp.

I bring the air down to 75 when the outside air is coolest 
(in the early morning, 2-3am, when I get up to pee). and 
then reset the temp to 82 when I get up. The A/C didn't come 
on till 3-5pm even in the heat wave of a month ago. The 
house must be better insulated than I thought. The air stays 
at 82 till I reset the temp to 75 early in the morning. This 
worked well through the heatwave about a month ago. Now with 
the cooler days, the A/C doesn't come on till 8-9pm.

I have a (calibrated) thermometer stuck on the outside of 
kitchen window, where it's in the shade all day. It's 10deg 
hotter there than a thermometer in the woods about 50ft 
away. I guess the physical structure of the house does get 
hot. The kitchen window thermometer was 110deg during the 
days of the heatwave.

I do know that last year that setting the temp to 82 and 
cooling the whole house, that the A/C ran all day on the 
hottest days. I expect it only ran for 25% of the day on the 
hottest days this year. Again I expect most of that is from 
not cooling the upper floor.

I think I remember the cost for A/C for the whole summer 
last year was about $1k. The last electricity bill ,which 
included the heatwave, was $150. It's been cooler since then 
and I expect the next bill to be lower. I'll probably pay 
<$500 for electricity for the A/C for the summer.

> One other caveat to your plan - the humidity in your home.

Because of the A/C, the humidity inside is below 40% all the 
time.

Now that the worst of summer is over, the air outside is 
cool enough at night that I could open the windows, except 
that it's so humid. I'm keeping the house shut up and I'm 
running the A/C for humidity control.

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