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
>> > it doesn't save money.
>> corse it does.
perhaps you don't disagree with that.
>> > 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
>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
where i live, it's nice for a few weeks a year, with daytime setbacks
and night ventilation with cool dry air, when possible.