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TURTLE wrote:

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.

Newton died in 1727. These days, few people argue with his Law of Cooling.

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 reasonable 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.

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.

OK. Let's try 1.5... An 18K Btu/h AC might cool the house from 105 to 70 F
in (105-70)1200/18K = 2.4 hours. Keeping the house 70 F for 8 hours would
require 8(110-70)300/18K = 5.4 hours of AC operation... 2.4/5.4 is a 55%
energy savings, not unlike the previous 55% savings.


***Now all your calculation look good to cool it back off as to saving half the
energy to do so. There is a few things you forgot. Laten air or heavy moisture
content air is hell to cool off. On a 1.5 ton hvac system at 99%RH inside the
house. The btu rating of the hvac system will be reduced by 2,880 btu's. just
for moisture removial during the turn back on of the system when the moisture
content had been let to rize. So your 18,000 btu unit is now 15,120 btu system
till it gets the moisture out of the house.
***Now your forgot about the derating of the hvac system when you operate it in
ambiants of over 95ºF outdoors. The HVAC system is rated at the BTU rating at
95ºF ambiant of the condenser. In the High % RH inside the house and you have
only the rating 15,120 btu system. Then you put the 110ºF Ambiant temperature on
the condenser which will derate the btu rating by 12% of the 15,120 btu system
you have now. It comes to 13,305 btu ability you have now and have to cool the
house with. Now as I can see your hvac system will take longer to cool the house
off than the 2.4 hours as you had said. If the system staied on and keep the
moisture down it would not have to fight the laten heat of the moisture all at
one time.
***Now you had said to turn the system back on before you got home to get the
temperature down to 70ºF inside the house. Now as you have a derated system for
the laten heat and derating of the 110ºF outdoor temperature. You would need tom
turn it back on about 4 hour before coming home and this would shorten your down
time down to about 4 hours. Now you will be running the system 4 hours so that
you can turn it off for 4 hours. So if you turn it off for 4 hour to save some
money. It will still be running at a 50% run time running all the time or
turning it off to feel like you are saving something.
*** Now When hvac system that are being run at high anbiants of the indoor
temperature you super heat will be off the chart till it gets below 80ºF inside
the house and when your super heat is very high the compressor will use more
electricity or watts during this time. By letting this condition happen you will
be burning more power than running at normal indoor temperature. The higher the
temperature is indoors and outdoor will definately cost more to operate.
*** Now with all of this left out. You seem to not have a clue to what is going
on in a hvac system operation in a home.














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...

The ASHRAE 55-2004 comfort standard says most people would find 90% RH
at 60 F "very cold" (Y = -3.2, with 99.6% of people dissatisfied) and
10% RH at 95 F "very warm" (Y = +2.8, with a 97.4% PPD.) Maybe you need
a comfortstat vs thermostat. It might automatically lower the air temp
to compensate for a higher RH or Mean Radiant (wall) Temperature.


***Now first your going to have to leave them drug alone here for they are
getting to you and your concept of what is going on here.
***Now either ASHRAE 55-2004 is full of **** or your full of ****. As I see what
you said of 60ºF at a 90%RH is very cold to the average person. Nobody in their
right mine is going to put up with 90%RH at 60ºF in any home. It would be muggey
as hell in there. Your off the wall here son.
***Now You say 10%RH at 95ºF that you or maybe ASHRAE 55-2004 says this is very
warm to the average person. At 10%RH and 95ºF is a very good condition and would
feel pretty good to anybody. The only time this condition would feel warm to a
person would be when it got over 98.6ºF the body temp..
***Now you expressed a need to control the %RH in a home. We have controls now
days that you can set the %RH and the temperature all with a thermostat hanging
on the wall.
***Now Nick if the hvac system was designed correctly and not over sized as you
can only deal with. The %RH and the temperature will go hand in hand as being
correctly set if the system is left on 24 hours a day. All hvac systems are
designed to be left on 24 hour a day and not off and on like you think they
should be run.







Nick


***Now Nick when you start discussing the design of a hvac system with people
who install, service, and work on them. You need more info on the business
[really see what your talking about ] than what comes out of a book. The Book
can only take you so far and then you have to go see what the thing is really
doing when it is running.
***Now Nick try to get a clue when discussing hvac system and how they really
work, but I will say you are good at the book part of the business. The Books
are only half the way and the other half comes with OJT. Get out in the field
and see for yourself.

TURTLE

P.S. Nick -- Does Lost in the 60's mean anything to you?


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