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re: not home(turn off a/c), home( then turn on a/c) question?
23 jul 2004
timm simpkins  wrote:

>what rod is failing to recognize is the thermal mass of everything in the
>house.

rod wrote:
 
>> > it doesn't save money.
>>
>> corse it does.

perhaps you don't disagree with that.

and...

>> > the amount of stored heat in a house is a lot to overcome.
>>
>> pity the amount stored doesnt change much

maybe this is where you disagree. you seem to be saying that
the contents of a house remain warm for a very long time after
an ac setback, making the house so uncomfortable to be in that
the setback isn't worth the money savings. 

>while this might not be much of an issue if the entire contents of the house
>were made of plastic...

that sounds nice, for effective setbacks.

>...it does make a difference when things are made of every day materials
>such as wood, cotton, metal etc.  depending on the thermal mass of the
>materials in your home, and even your walls, the heat can be stored for
>lengthy periods of time, and released slowly back into your home.

... 1/2" drywall stores 1/2 btu/f per square foot. a slow-moving airfilm
has a thermal resistance of about 1.5 btu/h-f-ft^2, which makes rc = 0.33
hours, ie 20 minutes. if you expose 90 f drywall to 70 f air, after 1 hour,
it should be 70+(90-70)e^(-1/0.33) = 71 f. 

>one obvious place this heat is stored is in a bed.  a bed that gets to
>90 degrees throughout will take sometimes a week of a desired temperature
>to loose enough of its heat to be comfortable to sleep on again.

if your bed temp falls from 90 to 75 f ("comfortable") over a week (168 h),
in 70 f air, 75 = 70+(90-70)e^-(168/rc), so rc = -168/ln((75-70)/(90-70))
= 121 hours. an upper airfilm conductance of 1.5 btu/h-f would make c =
rc/g = 121/1.5 = 80.6 btu/h-f, eg a waterbed that's 12x80.6/64 = 15" deep, or
a 39" deep concrete bed, or a solid wood (hem-fir) bed 95" deep (which would
require a serious diet or a high ceiling.) starting from 70 f, after 8 hours
in 90 f air, the bed would reach 90-(70-90)e^(-8/121) = 71 f. 

>walls of a home that has been kept at higher temperatures will feel warm for
>a couple of days depending on the materials it is constructed of.

walls that is constructed of water might do that. a clever thermostat might
precompensate, keeping the room comfy with cooler air during that time.

>concrete floors are also excellent heat storage devices.

warm air rises. concrete ceilings can work, but water is better...

>all of this heat is attracted to the cold when the air conditioning is
>turned back on and makes its way back into the room.

maybe we need heat scarecrows.

>these are the same principles that make passive solar heating so effective.

"direct loss houses" have mass inside outsulation. houses with low-mass
isolated sunspaces and hot massy low-e ceilings can perform a lot better. 

>thermal mass is the great storage battery for the heat that enters in
>through the windows.

and windows are huge holes in the heat storage bucket.

>...a passive solar house can loose as little as 10 degrees overnight with
>an outside temperature in the teens.

how much would it loose by dawn if the concrete block walls were 75 f at dusk,
with r20 insulation and 8% of the floorspace as r4 windows, on an 11 f night?

>it would take 5-6 days to release all that energy at that rate if all the
>windows were blocked off.

windows are bad news, thermally-speaking. low r-values compared to walls.
lots of labor and thermal bridging. they can admit sun, bugs, rain, and
burglars, vs outdoor cameras and flat screen tvs or computer projectors. 

>while a cool temperature on the surface of a particular solar mass will
>make it release that heat more readily, it will still do it slowly, and
>it can take days.

it should take about 5 cloudy days, for a high solar heating fraction.
direct gain floors need insulation above, but how would we heat them?
and how would we avoid losing heat through the windows at night and on
cloudy days?

>sitting on couches would be uncomfortable, laying in your bed would be
>uncomfortable, standing too close to a wall or laying on the floor all
>would be similarly uncomfortable.

the numbers don't seem to support that. what's your experience?  

>all in all, the truth of the matter is that every environment is different,
>and depending on the outside climate, the thermal masses inside, the length
>of time the ac is off, and many other factors, it may save more money, but
>it won't necessarily be comfortable for several days.

maybe yes, maybe no...

>if an air conditioned house isn't comfortable, what's the point in having
>air conditioning?

where i live, it's nice for a few weeks a year, with daytime setbacks
and night ventilation with cool dry air, when possible.

nick




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