Subject: Re: AC Short Cycling - Ask Nick Pine

From: [email-masked] (Nick Pine)
Date: 6 Jun 1996 12:15:17 -0400

Marc L O'Brien <[email-masked]> 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 ([email-masked]) 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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From: [email-masked] (paul milligan)
Date: 7 Jun 1996 21:47:23 GMT

In article <[email-masked]>,
   [email-masked] (Nick Pine) wrote:
~~>Marc L O'Brien <[email-masked]> wrote:
~~>
~~>Ask Nick Pine? **I'm** not a Freon pumper! :-) 

~~>>paul milligan ([email-masked]) 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? :-)

	yeh, here 'tis - back that one up, buddy !  :-0

	How can you make such a statement without working within parameters
of load and delta ?  ( hint - you can't )

	Short cycle ( again, in the context of system usage, not failure
modes )is not a question of percent of running time vs total period, it
is rather the pattern of on/off cycles, and their relative time structures.

	Example #1- hot, hot day, leaky house, poor insulation:

		Unit runs 5 minutes, satisfies, calls again in 5 minutes,
runs 5 again, etc.  That _is_ short cycling, and yet run time is 50 % !

	Example # 2 - 80 degrees outside, dryish, evening

	Unit runs 5 minutes to acheive 77 inside, satisfies, shuts off. 
Does not call again for an hour.  That is _not_ short cycling, but run 
time is <10 % !

	Example 2B - same as # 2 above, but set point is 70 - unit runs for
20 minutes, satisfies, and does not call again for until the next afternoon
( seeing as evening has continued to fall, and the temperatures to drop ).
Run time for that period is %trivial, but it is _not_ short cycling !  :-)

	I repeat : 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 ).

	Your statement regarding 'nonsense' will taste better with a little
salt, my friend !  :-)

Paul

_________________________________________________________
No Disclaimer needed - This is my own PPP account, and my
employer doesn't care what I think anyway !  :-)
_________________________________________________________

The SEHVAC FAQ is at: http://www.elitesoft.com/sci.hvac/
My humble personal pages are at: http://www.webbuild.com/~pjm/index.html

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From: [email-masked] (Nick Pine)
Date: 8 Jun 1996 07:16:22 -0400

My short cycling guru friend paul milligan <[email-masked]> wrote:
>~>>paul milligan ([email-masked]) 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 ).

>of load and delta ?  ( hint - you can't )

Oops :-)

>is rather the pattern of on/off cycles, and their relative time structures.

Hmmm.

 
 
>Does not call again for an hour.  That is _not_ short cycling, but run 

Good examples. Perhaps we should say "short cycling is a condition in which
an AC cycles more than 6 times per hour." Or "short cycling is a bicycle trip
of less than 3 miles."

Here are some more examples:

        Example # 3A - 80 degrees outside, 5K Btu AC in an 8' cubical room
at 80 F inside, with 6" concrete walls with a heat capacity of 22 Btu/F-ft^3,
surrounded by 2" of polystyrene foam with an R-value of 10. The walls have
a thermal mass of 8x8x6x6/12x22 = 4224 Btu/F, so the unit runs for 3x4224/5K
= 2.5 hours to achieve 77 inside, satisfies, shuts off. The room with a time
constant of RC = R10/(64x6)x4224 = 110 hours takes t hours to warm to 79 F
where 79 = 80 + (77-80)exp(-t/110) ==> t = -110 ln((80-79)/(80-77)) = 154 hr, 
ie 6 days, then the AC comes on again and cools it to 77 F in 2x4224/5K =
1.7 hours, and so on. Is that short cycling?

        Example # 3B - 80 degrees outside, 5K Btu AC in an 8' cubical room 
at 80 F inside, with 2" polystyrene R10 foam walls with a heat capacity of
0.5 Btu/F-ft^3, surrounded by 6" low-thermal-resistance concrete walls. The
inside walls have a thermal mass of 8x8x6x2/12x0.5 = 32 Btu/F, so the unit
runs for 3x32/5K = 0.0192 hours or 1.15 minutes to achieve 77 inside, shuts
off, the room with a time constant of RC = R10/(64x6)x32 = 0.83 hours takes
-0.83 ln(-1/3) = 1.16 hours to warm to 79 F, the AC comes on again and cools
it to 77 F in 2x32x60/5K = 0.77 minutes, and so on. Is that short cycling? 
 
        Example # 4A - 86 F outside 8 hours a day, 67 for 8 hours a day, and
77 for 8 hours a day, 8' cubical room with 6" concrete walls on the inside,
initially 77 F, surrounded by R10 foam. Phila owner takes AC to junkyard, gets
1 cent/pound, and buys a $5 100 Watt window fan at a garage sale. During the
night with the fan on, the concrete walls have a surface air film thermal
conductance of about 2 Btu/hr-F, so the room has an RC time constant of about
1/2/(64*6)x4224 = 5.5 hours. During the day, buttoned up, the room temp rises
to T(8) = 86 + (77-86)exp(-8/110) = 77.6 F, and during every night the owner
turns on the fan for t hours, where 77 = 67 + (77.6-67)exp(-t/5.5), so
t = -5.5 ln((77-67)/(77.6-67)) = 2.14 hr, and so on. Is that short cycling? 

        Example # 4B - 86 F outside 8 hours a day, 67 for 8 hours a day, and
77 for 8 hours a day, 8' cubical room with 6" concrete walls on the inside,
initially 77 F, surrounded by R10 foam. Wise Phila owner removes three piece
suit, dons shorts and T-shirt, sells fan and lives in 77.6 F room during the
day, 76.4 F room at night. 

        Example # 5A - 86 F outside 8 hours a day, 67 for 8 hours a day, and
77 for 8 hours a day, Phila owner and loony brother build stone house with
13 million pounds of rocks, including rocks for the roof (see recent Mother
Earth News) and foam the outside with 4" of urethane. RC = 20,800 hours, so
if the house starts out one summer morning at 65 F, by the end of an 86 F
day, the indoor temperature rises to T(8) = 86 + (65-86)exp(-8/20,800)
= 65.00692 F. 

        Example # 5B - From June through August in Philadelphia, when the
average outdoor temp is 74.7 F, the indoor temperature in this house with
a 29 month time constant rises to about 74.7 + (65-74.7)exp(-3/29) = 66 F,
while the brothers open their house sometimes at night to their low-thermal-
mass sunspace with its solar-dried dessicant floor to remove some humidity,
while venting the hot sunspace to the outside on sunny days. A layer of
wood chips or vermiculite or crushed stone or a thin layer of concrete over
foam might work as a dessicant floor...

        Example # 5C - September arrives, with an average daytime high
temperature of 77.6 F, and the owners get ready for winter in this house
with 3,000 ft^2 of walls with 2 million Btu/F of thermal mass with an indoor
surface thermal conductance of about 2 Btu/F, ie an indoor time constant of
1/2/3,000x2 million = 333 hours or 2 weeks, by opening the windows for t
hours during the later part of the month, to raise the house temperature
to 73 F, where 73 =  77.6 - (77.6-66)exp(-t/333), so t = -333 ln(0.39)
= 308 hours or 12.8 days.

        Example # 5D - Winter arrives, with about 6,000 heating degree days,
at a 70 F base temperature, and the house loses 24x6,000x3,000/R30 = 14
million Btu, and cools from 73 to 67 F. The average daily min temps in May
and June are 53 and 62. The brothers open the house up on 10 cool nights in
early spring to allow it to cool to 65 F, and the summer begins again.

        Example # 5E - Loony brothers wear sweaters in winter, T-shirts
in summer, and leave the south-facing windows closed all year :-) 

Nick

PS: Lent 700 ft^2 of 13.9 cent/ft^2 black 80%-solar-absorbing shadecloth to
the local newspaper, The Collegeville Independent, "Neutral on Nothing--
Accept and defend the truth, whatever the source!" so they could see how their
comfort and AC bills change when they hang it over their barn red south wall
with R11 insulation and 84 ft^2 of double pane windows. They are worried about
the building inspector, so they are thinking of stenciling it with big red 
letters saying "Vietnam--1964-1973," to exempt it from the building code as
self-expression (their publisher's a lawyer :-) They might add "Desert Storm--
1990-1991," in red poster paint, so it will bleed in the rain.

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From: [email-masked] (Nick Pine)
Date: 6 Jun 1996 12:15:17 -0400

That makes sense to me.

Hope this helps.

Nick

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From: [email-masked] (paul milligan)
Date: 7 Jun 1996 21:47:23 GMT

Paul

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From: [email-masked] (Nick Pine)
Date: 8 Jun 1996 07:16:22 -0400

Oops :-)

Hmmm.

 

 

Nick