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re: water to heat/cool a house
31 jan 1997
al hartkopf  wrote:

>i think ceiling fans in every room will do the job of circulating the air
>around the room and result in an overall cooling of the room due to the
>temperature of the slab. think that will work?

conduction works well with 2" concrete walls insulated on the outside.
monolithic domes have 2" of concrete with 3" of foam on the outside, and
the temperature difference from top to bottom of a 200' diameter dome is
said to be less than 5 f. a nice solar house: the sun shines in a window
or insulated air heater and warms the room, and the warm air rises to the
ceiling and heat conducts back down the walls. perhaps these domes distribute 
solar heat too well... they might work even better as solar houses with some
way to control the heatflow, storing heat at a higher temperature in the mass
above, eg the concrete roof or a 110 f ceiling pond with some foil underneath
and a horizontal ring of form insulation replacing part of the concrete wall,
with a few vertical water pipes running through the ring instead of plain 
reinforcing rod.

where will the coolth come from, in your house? how much is needed? how
much groundwater flows under the site? how deep? perhaps you could use a
floorslab in contact with the ground, with pipes running through it, with
insulation around the perimeter and on top, and insulate the slab from the
walls of the house. foam will support 10-30 psi...

>this will be built in north carolina, west of raleigh.

looks like a great place for a solar house. january is probably the most
difficult month for solar heating in raleigh, with an average of 1130 btu/ft^2
of sun falling on a south wall and a 24 hour average air temperature of 38.9 f
and an average daily max temp of 48.9 f, according to nrel. the next step
might be to calculate the heat loss of your house using ashrae arithmetic,
by adding up each exterior surface area divided by its r-value. you may not
have much wind/air leakage with concrete walls. the average yearly air temp
in raleigh is 59.3 f, which might be close to ground and slab temps.

>the terrain is rolling hills and the altitude is a little above raleigh. we
>think that a hefty roof overhang, calculated to allow winter light in, is
>in order. still on the right track?

nrel suggests 1' of south glazing height needs 0.293' of opaque surface
above it and 0.417' of overhang above that at your 36 n latitude. 

>one issue along these lines of cool slab and cool room is condensation.
>anybody now how to predict the temperature that "dew" will begin to form
>on the cooled floor? seems like it is a temperature difference that causes
>this.

and air leakage. where else does the water vapor come from? seems like a good
idea to ventilate this house at night, and button it up during the day. july
looks like the warmest month in raleigh, with an average air temp of 78.1 f,
88 during the day and 68.1 at night. 

>...the southwest. that's the only place i ever saw those water cooled heat
>exchangers. we lived in yuma (hot!) and that thing really kept us coooolll.

sounds like you are talking about evaporative cooling. nrel says the average
humidity ratio in july is w = 0.0149 lb water vapor/lb dry air, corresponding
to a water vapor partial pressure of p/(1+0.62198/w), with p = 14.5 psia in
raleigh, and less at your higher altitude; 68 f air has w = 0.01478 at
saturation, so you might see fog at night and dew on the morning grass,
without much use for evaporative cooling (how about a roof pond?)   

>i thought about doing exactly this as it would provide a platform for
>dehumidifying the house. thats a real concern when things start to drip
>around here in july.

i don't think evaporative cooling would help this house much, but a dark
low-thermal-mass dessicant sunspace floor might absorb water vapor from house
air in the early morning, and dry and bake out in the sun with the sunspace
vented to the outdoors during the day. the sunspace floor might be lightweight
reinforced concrete insulated below, containing expanded shale which absorbs
13% of its weight in water. it might have a polyethylene glycol fountain
absorbing 11 times its weight in water... how much water vapor do you need
to absorb to keep your house at 50% rh and 75 f inside, if it leaks 1 ach
for 12 hours a day?

nick


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