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re: more fun with air conditioning
9 aug 2004
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.

would you say this about a heating setback, with an ordinary furnace?

>> >> 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.

time or energy?

>> >> 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.

you said acs are rated at 95 f outdoors, and capacity might decrease from
36k btu/h to 33 or 31 at 105 f. what about cop? and... if an ac must work 
at 105 f in late afternoon to cool a house back to 70 f after a setback,
it might have to work at _115 f_ to keep the house 70 f in the warmer part
of the day, with no setback.

>> >> 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

this ac setback seems to save lots of energy.

>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.

...1,200 or 1,500?

>> 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.

this ac setback seems to save lots of energy.

>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. 

why 2,880 btu/h, vs 2,400 or 5,327?

i forgot the house was 99% rh inside? :-) that seems like a different case.

how did the house air get that way? perhaps it's built in a florida swamp?
key west is very damp in august, with w = 0.0185, but the average temp is
only 84, with an 89 f daily max and a 95 record high... 84 f at w = 0.0185
is only 73% rh, but the dew point is 74, so you might make the house air
99% rh for a while if you ac'd it to 70 f, then opened all the windows.
but why would you want to do that?

i believe the setback would still save energy, with the windows closed.
houses leak air, and that can require dehumidification, but they leak
less air (via stack effect) if the indoor-outdoor temp diff is smaller,
eg during an ac setback. we could quantify this in excruciating detail.

>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.

why 15,120 btu/h, vs 14,926 or 17,261?

>***now your forgot about the derating of the hvac system when you operate
>it in ambiants of over 95f outdoors.

we might agree that _helps_ setbacks save energy. (and say "you forgot.")

>the hvac system is rated at the btu rating at 95f 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 110f 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.

but it still needs fighting, even without a setback. imo, latent loads are
just ongoing heat loads which don't affect setback savings unless we open
the windows during the setback and get condensation (vs humid air) inside
the house.

>***now you had said to turn the system back on before you got home to get
>the temperature down to 70f inside the house. now as you have a derated
>system for the laten heat and derating of the 110f 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.

why 4 hours, vs 3 or 7?

>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.

i think you will find it still saves money.

>*** 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 80f
>inside the house and when your super heat is very high the compressor will
>use more electricity or watts during this time.

so... if it's 105 f outdoors, a house that's 80 f indoors will use more ac
electrical power than a house that's 70 f indoors? :-) and even more, if
the house is 105 f indoors?

>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.

newton might be surprised :-)

>*** 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.

perhaps we can help each other. you don't seem to understand newton's law
of cooling: the amount of heat that flows through a wall is proportional
to the size of the wall and the indoor-outdoor temp diff. if the indoor-
outdoor temp diff is 0, no heat flows. 

>> >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 72f inside but
>> >i would have to deal with high humitity in the house

you seem to have trouble spelling the word "humidity" :-) koko the gorilla
has a 1000 word vocabulary. wwii "basic english" for islanders had 800 words,
eg "howitzer." "humidity" seems like a basic hvac word. how many words does
an hvac criminal need to be successful at his or her trade? must they be
spelled correctly? you also seem to have trouble with "laten," "anbiant,"
"rize," "removial," and "tom."

>> >and will have to run my system at lower temperature than 72f or
>> >maybe down to 65f to get the water vapor out of the house. with
>> >a 90%rh a house at 60f will feel very warm inside. with a 10%rh
>> >and 95f 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.

when did you stop beating your wife? :-)

>***now either ashrae 55-2004 is full of shit or your full of shit.

you might more properly say "you're full of shit." but there's at least
one other possible explanation. without meaning to seem disrespectful,
i submit that you may be wrong (gasp.)

here's a quote:

  standard 55-2004, "thermal environmental conditions for human occupancy,"
  is a revision of standard 55-1992. the standard specifies conditions in
  which a specified fraction of the occupants will find the environment
  thermally aceptable. the revision is a consensus standard that has
  undergone public and ashrae review; it incorporates the relevant research
  and experience gained since the 1992 revision... intended for use in design,
  commmissioning, and testing of buildings and other occupied spaces and
  their hvac systems and for the evaluation of thermal environments... in
  close agreement with iso standards 7726 and 7730... based on an adaptive
  model of thermal comfort that is derived from a global database of 21,000
  measurements taken primarily in office buildings. 

>as i see what you said of 60f 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 60f
>in any home. it would be muggey as hell in there. your off the wall here son.

dad!!! i never knew. you must be very old. please remember me in your will.

the ashrae 55-2004 spec has a minimum comfy temp of 67.3 f. below that, most
people are "cold." you said 60 f is "very warm." maybe turtle is right, and
thousands of people mistakenly said they were too warm vs too cold at 60 f, 
and the 31 comfort experts on the 55 standards committee were also wrong.

>***now you say 10%rh at 95f that you or maybe ashrae 55-2004 says this
>is very warm to the average person.

the ashrae 55-2004 spec has a maximum comfy temp of 82.2 f. above that, most
people are "hot." you initially said 10% rh at 95 f is "very cold," and now
you say 10%rh and 95f is "a very good condition and would feel pretty good
to anybody." maybe turtle is right, and thousands of people mistakenly said
they were too cold vs too warm at 95 f, and the 31 experts on the ashrae 55
standards committee were also wrong. you might buy a copy of ashrae 55-2004 
(a 28 page standard), and mail in the attached enclosed "form for submittal
of proposed change to this standard under continuous maintenance." for sure,
your opinions will be duly noted. 

>***now you expressed a need to control the %rh in a home.

i think that was you.

>we have controls now days that you can set the %rh and the temperature
>all with a thermostat hanging on the wall.

i think i said that.

>***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.

it seems to me that a well-designed hvac system would operate at a 100%
duty cycle at the local coincident summer dry-bulb/wet-bulb design temp,
which would only occur for 1% of the year, eg for 88 non-consecutive hours. 
i mentioned this before, but you seem to have ignored it.

>***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

ah yes. the value of same-sex blowjobs. i'll leave that to you and paul :-)

>[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.

it seems very useful to be able to walk up to a system, smell the charred
paint, look at the blown fuses and thermostat settings, and so on. 

>***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.

thanks. would that you had a clue as well! (in the hortatory subjunctive.) 

>the books are only half the way and the other half comes with ojt. get out
>in the field and see for yourself.

no thanks. my heart is too pure.

>p.s. nick -- does lost in the 60's mean anything to you?

i'm a frayed knot.

nick




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