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It is all about the numbers. In New Orleans if you have a 2,400 square
foot (20,000 cubic feet) house you better have at least a 4 ton air conditioner. That will
keep you cool except on the hottest days when you ought to be at the
beach or your summer house in the mountains anyway. Four tons is
48,000 BTU.
If your unit is fairly new say 5 years, the efficiency is a SEER of
12-14 and EER is about 10
which means you'll be consuming 48,000/10 watts or about 5 kilowatts
when the AC is running.
Mine runs about 70% of the time 24 hours per day during the summer
(June-October) and about 20% of the time the other seven months. Make that 18 hours per day. Entergy charges $0.10 per
kilowatt hour in the summer. Half of this is their base rate and half is fuel adjustment. So air
conditioning costs me 18*5*$0.1 or roughly $9 per day when it's hot.
During the hottest five months it costs about $250 per month to be cool.
By the way that means the newer super high efficiency AC units make
sense in New Orleans. If you spend $6,000 on a SEER 19 unit vs $2,500
for a 10, it will take you $3,500/$125 or just 28 summer months
to get your money back. With 7 summer months per year in New Orleans
you are making money after 4 years on a unit that is guaranteed for at
least ten years. After ten years you are $5,250 ahead on a $3,500
investment. That is equivalent to a 10% interest rate.
Why do you need 4 tons? One ton is equal to 12,000 BTU per hour, or 3517 watts
or 3024 Kcal per hour
The term ton goes back to the early days of refrigeration when
making ice was the goal. 12,000 BTU per hour was just enough to freeze
2,000 pounds of water in a day and make a ton of ice. (Latent heat of
fusion for water 334 kJ.kg -1 = 144 BTU per lb mass )
Four (4 ) tons is about 48,000 BTU of cooling. I could make 4 tons of
ice every day with this system. Why do I need that much power just to
keep my house cool?
Air Conditioning moves heat out of your house. So how does that heat get in?
- Leakage lets the outside in - this is the major culprit. Leaking duct work deserves its own analysis. Open
doors, uncaulked single pane windows add'em up and you get the equivalent of an 8 square
foot hole in the average house. It's like living with a window open
at all times. Watch out, if you seal your house too well you run into
other problems. Lets put leakage aside for the moment and look at the other sources of heat input.
- 10,000 BTU per hour gets through my wall and ceiling insulation - R19 or R30 means what? U the thermal conductance of
a surface is the inverse of R value. It is measured in Btu per ft2*degree F (in the nonSI world). So your R19 insulation will maintain a 19F difference for 1BTU per square foot. My 2000 ft2 attic with what was originally R25 but is now more like R10 is chewing up about 6,000 of those 48,000 BTUs I have available. My 2,000 ft2 of walls with R13 gets 4,000 more.
- 10,000 BTU per hour goes to cool unvented
appliances - light bulbs, electronics, toasters, hair dryers, irons, etc. . This might be why your
20 year old house isn't as cool as it was. You have much more stuff
plugged in. A coffee machine and a hair dryer combine to produce 3
kilowatts. My five computers are "always on" adding about 1,000 watts
to the household. We won't even talk about my TV's. Every
unvented watt you
bring into your home will require about 1 BTU of airconditioning to
move out.
- 1,000 BTU per hour on average during the daytime is needed to cool the vented cooking and cleaning spaces. The
oven and range is vented but the kitchen still heats up when in use.
Same for the
washer and dryer in the laundry and the hot water for showers and stuff
in the bathrooms. This stuff is intense but used for only a few hours
during the day and the venting helps. Lets add another 500 watts of 24
hour average cooling.
- 500 BTU per hour is consumed by people radiating body heat. An adult male in an airconditioned
room radiates / convects about 100 watts. Big guys more, kids less,
animals too.
That's only about 22,000 BTU per hour accounted for excluding
the leaks. Wow, is leakage really that big? Assuming the AC runs 70% of
the time in the summer that means we're consuming about 33,000 BTU per
hour. Even if I
missed a ton of heat sources we are spending one third of our cooling bill on
leaks. Could this possibly be right? Get out the caulk and door seals.
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