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re: am i saving money on ac?
25 jun 2002
james  wrote:

>i often cool the house at night with fans drawing cool damp outside
>air.  i then use the ac to dehumidify the air.  i try not to use the
>ac during the day.  sometimes i run the ac all night down to 68 and
>leave it off during the day until it's 80 inside.

an ac might be more efficient when the outdoor air is cooler at night. 
 
>am i better off just to leave the ac on at 75 without using cool damp
>night air?

i'd say "probably not," unless the ac has to run a long time for
dehumidification, which might indicate that the outdoor damp air
is condensing into a puddle or being absorbed as liquid into some
surface (eg bare concrete) somewhere inside the house. 

say the night air is 60 degrees at 80% rh for 12 hours, and it's 90 f
for the rest of each day, and your house has no internal heat gain,
with a thermal conductance of 500 btu/h-f and 10k btu/f of thermal
capacitance, so the time constant in outdoor air is 10k/500 = 20 hours
with the windows closed. say you have enough airflow and thermal mass
surface to make the house very close to 60 f every morning.

in case 1, you ventilate the house every night with grainger's $115 4c853
86 w 21k cfm 48" ceiling fan under a 2 watt motorized damper in the ceiling.
this uses 12hx88w/1000 = 1.056 kwh per day. in the morning, you fire up
the ac as needed (and modified for efficient dehumidification) to dry
the house air so it will have at most, say, 60% rh when it warms to 80 f...
60 f air with 80% rh has humidity ratio wm = 0.0089 pounds of water per
pound of dry air, and 80 f air at 60% has wa = 0.01334, so warming the air
to 80 f reduces its rh to wm/wax80 = 53%. comfy. no dehum needed. and
after 12 hours, the house air only warms to 90-(90-60)(1-exp(-12/20))
= 76 f, so you don't need ac for cooling either. 

in case 2, you don't need ac at night, but running it 12 hours per day
removes 12(90-75)500 = 90k btu from the house. with a typical cop of 3,
this requires 90k/3/3412 = 8.8 kwh, ie 88 cents at 10 cents/kwh.

nick




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