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re: looking suggestions dream house design
27 may 1996
william r stewart wrote:
>nick pine wrote:
>>...i've been trying to think of a way to make heat go down, in a house
>>with high thermal mass walls and a heat-trap type "collector," in which
>>one wall has an air heater outside the insulation, with an outlet near
>>the ceiling. does anyone have any ideas on how to do that, simply? wall
>>conduction? a ceiling fan? some sort of slow-moving water?
>
>you could use ducting and a fan, as you have mentioned before, to move the
>hot air down to your basement thermal store.
that's how norman saunders, pe, gets warm air out of his attic warmstores
and into the living space... one 1/4 hp fan to move air down from the attic
(where he collects sun thru the roof, with his "$olar $taircase"), and
another to move house air thru his basement coolstore. cliff house works
this way, with 10k pounds of water in the attic and 20k pounds of rocks in
the basement, in weston ma. he says the annual operating cost is about $100
for the fans, but i suspect that cost is a bit out of date, since a 1/4 hp
motor probably uses 250 watts full tilt, and that seems like a lot of power,
even the way norman uses them, controlling the fans to 16 different speeds
with his custom-designed controller/data logger...
norman is 80 years old now, and semi-retired, and he was a bit surprised
recently that a good rule of thumb for the cost of electricity is: 1 watt
used continuously costs about $1/year. he was also surprised that swimming
pool tube mat collectors are up to $4/ft^2. i'm sure he would be shocked
at rooftop collector prices. i doubt ever uses those. he also tends to avoid
plastic glazing, but he says he hasn't looked into that in a long time...
a 5' ceiling fan blowing air down through a hole in the ceiling might be
more efficient, to gently pressurize a central area, with some sort of large
air passages into adjacent rooms, eg a large supply grate in the floor of
the central area to allow air to flow along the basement ceiling and up
into adjacent rooms thru large floor grates, with return air grates to the
attic in the ceilings of each room... the ceiling grates might each have
a 2 watt $100 honeywell motorized damper to control the temp in each room.
but i'd like to see norman's system somehow simplified more and made more
cost-effective. it seems to me that it's better to have the water containers
on the ground instead of in the attic, so the weight and earthquakes are less
problematic, and we can use lower power fans to move air, or with the right
design, motorized dampers and natural convection, or $12 automatic dampers
with bimetallic springs, that open or close at adjustable threshold temps.
>you could then use the controls you mentioned
>earlier to open dampers to let the hot air naturally convect upwards.
seems better to just collect the heat in a walk-out basement with a glazed
south wall to begin with. but... i like one way norman collects heat in the
attic--he glazes the whole south wall of a house to make a huge low-thermal-
mass sunspace, a few inches away from the house wall itself, in some cases,
and lets the warm air rise up to the attic, where it stays trapped, like
heat in an igloo, with the entrance lower then the floor. that kind of solar
collection is completely passive, and doesn't even need dampers, but the
hot air outlet needs to be at the top of the wall, so the natural place to
store heat is in the attic, if you want to use the whole wall... hmmm.
>you could also use a low power pump for circulating water the collected heat,
>by using larger than normal pipes and have the minimum number of bends,
>elbows, and other head-adding components.
that's a good idea. you know, monolithic domes have remarkably even heat
distribution. they are made with about 2" of reinforced concrete inside
3" of foam insulation, so if the bottom heats up, say with sun through a
window, the warm air goes up to the ceiling and the concrete conducts the
heat back down to the floor level. they say a 200' hemisphere might only
have a 5 f temp difference from top to bottom... but we don't want that all
the time. it would be nice to leave the thermal mass in the dome ceiling hot,
and just bring the heat down controllably, on cloudy days. how can we do
that, a horizontal thermal break? say an insulating ring of foam, 2" thick
x 3" wide, that breaks the concrete conductivity high up near the dome
ceiling, to isolate the ceiling from the walls, with some simple low-power
way to move heat across the insulation gap?
on a cold day one might bring the heat down from the high thermal mass
ceiling into the conductive masonry walls of a dome or a room with a new
vaulted stone ceiling, or a house made with cement block walls, by moving
some water thru a few large short pipes that cross a thermal gap near the
ceiling, using a low-power, low-head pump. the pipes might be in a vertical
serpentine pattern, to inhibit natural convective waterflow.
>this is a classic problem active solar designers have been facing and
>resolving for decades.
resolving more or less well...
seems to me this "active/passive" dichotomy has limited further usefulness.
what is more important to me is cost and reliability and performance. is a
solar heating system that uses 2 watt dampers or 36 watt fans to collect and
distribute 50 kw of sun "active"? yes, if you are a purist... they used to
call these "hybrid" systems, eg john cristopher's 100% solar building in
nashua, nh, which has a cop of 50 (2% fan power, 98% solar power.) but a
hot air furnace and blower burning 1 gph of oil might use 600 watts while
running, so it might have a fossil-fuel cop of 60 or so. seems like we have
to improve heat distribution systems to raise these cops. i think we can
make a simple, reliable system with a solar cop of 10,000, with excellent
temperature control, and no backup heat.
nick
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