TURTLE wrote:

wrote:

In regards to the recent posting I saw about running the a/c or opening
the windows, I would like to list several statements that people have
made to me about air conditioning. The location is Texas, where the
temperature is about 75 F at night and 100 F at the hottest part of
the day.

About 88 average over 24 hours, and about 82 at night...

1. Keeping the a/c cooling the house all day uses less electricity than
turning it off and then back on in the evening...

Newton said the rate of heatflow into a building is proportional to
the indoor-outdoor temperature difference. IMO, turning the AC off
will save energy, even if only for a few minutes.

2. Running the a/c a few degrees colder at night cools the big cement
slab that the house is built on...

A 4" floorslab can store about 8 Btu/F-ft^2 with a 4-hour time constant. It
might cool from 75 to 70+(75-70)e^(-16/4) = 70.1 F after 16 hours in 70 F
air. With R20 insulation outside, RC = 20F-h/Btux8Btu/F = 160 hours, so it
might only warm from 70.1 to 94+(70.1-94)e^(-8/160) = 71.3 in 8 hours when
it's 94 F outdoors. Or less, with little air movement in the house. A slab
or a basement might be a efficient place to store coolth during a daytime
setback, since cool air falls. We might only bring coolth up into the living
space with a ceiling fan and a thermostat and an occupancy sensor as needed.

This is Turtle.

...If you turn a hvac system off less than 8 hours. It will cost you
more money to recool the house from a very high temperature to the
lower temperature than just moving up to a higher temperature on the
thermostat.

Why on earth would you say that? Do you work for Turtle Power and Light? :-)

This is basic 300 year-old physics, Turtle. Turning
an AC off for even 1 minute saves cooling energy :-)

...8 hours you may save a little but at 4 to 6 hours of down time will
cost you 4 to 6 hours of run time at 105ºF to get it back to the
regular temp. inside. Also your going to waiting about 1 to 2 hour
before you can stay in there when you come home.

But the AC setback still saves energy.

Nick