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re: ac short cycling - ask nick pine
6 jun 1996
marc l o'brien wrote:
ask nick pine? **i'm** not a freon pumper! :-) but if you like, i'll try
to help. first a word about greenhouse shadecloth. i've almost convinced
the local newspaper, the collegeville independent, "neutral on nothing--
accept and defend the truth, whatever the source!" which is 122 years old
this week, to hang a 24' x 32' piece of 80% sun-absorbing red greenhouse
shadecloth over the barn red south wall of their converted barn with r11
insulation and 84 ft^2 of double pane windows. the windows alone will pass
600 btu/ft^2/day of sun into the building in august, ie 50k btu/day, or 10
hours of 5,000 btu/hr ac, when the average daily max outdoor temp is 85 f,
with a daily night min of 66 and a 24 hr average of 76. this stuff costs
about 20 cents/ft^2 from stuppy at (800) 877-5025, sewed to order with
hems and grommets in about 2 weeks, and it comes in various colors (black
being my favorite :-) so this should save them about a dollar a day in
ac cost and make the place more comfy inside...
>paul milligan (pjm@nando.net) writes:
>> there is no quantifiable time period that defines 'short cycling'
>>in this context ( as opposed to certain other failure mode contexts we use
>>the same term to describe ).
nonsense. less than 10% on-time is a short cycle. next question? :-)
> changing the subject slightly, cold room design normally
>works on 16 hr out of 24hr. lots of spare capacity here and time for
>off cycle passive air circ defrost. off for considerably more time
>than the 8 hrs above, then open products like beef become too moist
>and soft to work on.
i suppose that's not much of a problem in the uk these days.
>back to the home system, if the home contains a large thermal mass
>then during initial temperature pull down the unit will run
>continuously as normal.
sounds reasonable to me...
>then when room temperature first reaches the
>set point the ac cycles but the room mass would not yet have
>dropped to set point and so will quickly warm the room air again
>bringing the ac back on. the ac will appear to short cycle until
>the rooms thermal mass more closely reaches set point.
so the duty cycle will gradually decrease over time...
>then once the room mass is within the thermostats differential temperature
>range it affects cycling time depending on the ratio of that mass
>to rate of room heat ingress.
i think i see what you are getting at here. what we really want is a house
with a large rc time constant, no? perhaps one with a huge amount of thermal
mass, and a reasonable amount of insulation. suppose those two brothers who
built their stone house with 13 million pounds of rocks, including rocks for
the roof (written up in mother earth news recently) had foamed the outside
with 4" of urethane? hmmm. rc = r30/3000ft^2x13x10^6x0.16 = 20,800 hours or
28 months or 2.4 years. open it up for a few days once a year in the winter,
if it begins to get too warm, and open it up for a few days every summer if
it gets too cold. no need for heat or ac. the average yearly temp where i
live is about 55 f, so it would be open more often in summer. if we start
the house out at 70 f in july, by the end of our 5500 dd heating season, it
will have lost 13.2 million btu, enough to cool the rock walls 6 degrees f.
> so, if the room mass versus heat ingress is low then the
>unit will cycle frequently.
> if the room mass versus heat ingress is high then the unit
>will stretch its off and on cycles.
that makes sense to me.
>nick pine would explain i'm sure.
sure. repent now! give up your freon habits!
hope this helps.
nick
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