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TURTLE wrote:
The location is Texas, where the temperature is about 75 F at night
a nd 100 F at the hottest part of the day.
About 88 average over 24 hours, and about 82 at night...
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.
Well first I see you don't work on hvac system and know what the run times
are
for a properly sized hvac system verses a cool down time for a indoor temp.
of
about 105ºf down to 70ºF to 75ºF.
How would the house get from 70 F to 105 F in 8 hours on a 100 F day?
Assuming it could (which would save lots of AC energy), and assuming
it had resonable insulation, it would have very little thermal mass,
so the AC could cool it back to 70 F very quickly.
Now if you have oversized hvac system like 5 tons on 1,500 sq. ft. house.
Your answer would be ok, but a properly sized system would cost you big
time on a 4 hour down time.
The setback would still save energy, unless the AC becomes a lot less
efficient (has a lower COP) with a higher indoor-outdoor temp diff.
A 1500 ft^2 house with 300 Btu/h-F of thermal conductance could warm from
70 F to 105 F in 8 hours on a 110 F day if RC = -8/(ln((105-110)/(70-105)
= 4.1 hours, which makes C = 4.1x300 = 1200 Btu/F, not much. A 36K Btu/h
AC might cool the house from 105 to 70 F in (105-70)1200/36K = 1.2 hours.
Keeping the house 70 F for 8 hours would require 8(110-70)300/36K = 2.7
hours of AC operation... 1.2/2.7 is a 55% energy savings.
Nick
This is Turtle.
This all looks good on paper but in the real world with a over sized hvac system
as you say a 3 ton on 1,200 sq. ft. house. A properly sized hvac system will
have a 1.5 or maybe 2 ton at most on the 1,200 sq, ft. house. 3 ton 36K btu
rating system when properly sized will never be on a 1,200 sq. ft. house.
Now you say the 3 ton 36K btu rating is used in your calculation here. All hvac
system are rated at 95ºOutdoor temperature and when the ambiant goes above the
95ºF level your BTU rating falls a good bit to maybe 31K or 33K btu's at 105ºF
outdoor temperature.
Now with all your calculations that the hvac system is perfectly tuned and all
coil are clean and serviced regularly. There was some research done in
California on systems being properly charged with freon, Clean coils, and
running at what they should be putting out. More than half was not operating at
what they should have been and was not putting out the BTU rating stated by the
manufactor. A lot of the system checked was running at about 70% of what they
should be putting out.
Now to you have a big ass 3 ton hvac system on 1,200 sq. ft. house. I personally
have a 2,250 sq. ft. home and cool it with a 3.5 ton 14 seer system. I have let
my cooling system be off for being gone for about 8 to 10 hours and on a 105ºF
day. My house inside went up to about 98ºf inside and when i turn on my
perfectly tuned system on. It took 3.5 hours to pull the house back to 72 ºF.
The first 2 hours you can not stay in there for it being too hot. Now if i would
install a over sized hvac system and have a 5 ton or so. It would not be but a
hour or so to get back down to 72ºF inside but I would have to deal with high
humitity in the house and will have to run my system at lower temperature than
72ºF or maybe down to 65ºF to get the water vapor out of the house. With a 90%RH
a house at 60ºF will feel very warm inside. With a 10%RH and 95ºF inside the
house will feel very cold. If you don't properly size the system you will have a
nitemare with %RH to deal with.
Now here is one that will never fit with your calculation on the cooling of the
house. All properly designed and sized hvac systems are designed and installed
to have a 90% run time which are designed to be run all the time and not turn
them off. On a properly sized / designed hvac system if you started at 100ºF
inside temperature could easily have a 8 hour run time to get it cool enough to
be called cool enough to live in it. So if it is designed correctly your theory
is out the window. If you have a oversized system you could maybe somewhere near
the recovery time needed to get the house back down in temperature, but with no
humitity removial like it should be.
Now you had said a house inside with no air on will not go up to 100ºF+ with in
8 hours -- out in the direct sun light, 105ºF outdoors, Roof area temperatures
running about 190ºF with the direct sun light on it, house with the average R-19
rating on it, and most all houses are not air tight. In 8 hour, I would be
surprized to not see it 100ºF.
Now one last point here. Your calculation will be for the air inside the house
and not for the metal , cloth couches , Rugs / carpet, and furniture which will
hold and release heat over the next 4 to 8 hours and you will have to remove
this extra heat held by these items as the next 8 hours of operation time goes.
Nick i live in the real world and you must live in the Paper world.
TURTLE
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