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Re: Stoopid question about geothermal slinkies?

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Posted by nicksanspam on September 5, 2006, 5:35 pm

Upstairs is good.

Bigger would be better. Will the greenhouse contain plants, or can it
drop below freezing at night?

You might have a passive one-way damper over an open window--lightweight
plastic film, hinged at the top. With a duct to bring down hot air from
the ceiling to enter the greenhouse at ground level.

Bad idea. Solar heat is better, with a COP of 1000 or more, vs 3 for a
heat pump. Collect 0.9x1000x12x16 = 172.8K Btu of warm air passively in
your 2-story T (F) single layer polycarbonate R1 greenhouse with 90%
solar transmission on an average 30 F January day in Phila and lose
about 6h(T-36)12'x16'/R1 Btu through the glazing.

If your airtight 704 ft^2 square space has an Rv ceiling and walls and
few windows, its thermal conductance will be 704ft^2/Rvalue Btu/h-F for
the ceiling + 849/Rvalue for the walls. At a 65 F average temperature,
it will need Q = 24h(64-30)1553/Rvalue Btu/day of heat. Rv = 30 makes
Q = 42.2K Btu. If 172.8K = 6(T-36)12x16+Q, T = 149 F, approximately.

If 75 F ceiling mass can keep the apartment 70 F with a slow ceiling fan
and a room temp thermostat and an occupancy sensor, storing heat for 5
cloudy days in a row requires that (149-75)C = 5Q, ie C = 2854, eg
2854/704 = 4 pounds of water per square foot of ceiling.

Your body heat and indoor electrical usage will help.


Posted by nicksanspam on September 8, 2006, 12:21 pm

Very nice :-)

It looks like the upper sunspace has no summer shading. You might unroll
some greenhouse shadecloth over it in summertime.

And consider making the roof flat and glazing the whole southeast side with
a single layer of R1 polycarbonate with 90% solar transmission (corrugated
Dynaglas greenhouse roofing in 4'x12' $0 sheets or 0.020" clear flat Lexan
in $00 4'x50' rolls with a 10 year guarantee) to make an 8' deep x 42' long
x 16' tall sunspace that would collect 0.9x1000x42'x16' = 604.8K Btu on
an average 30 F January day in Phila, if you trim the beloved oak a bit.

If the air near the sunspace glazing is 130 F during the day, the glazing
will lose about 6h(130-36)42'x16'/R1 = 379K Btu/day, leaving 604.8K-379K
= 225.8K Btu of useful solar heat, no?
A 42'x28' R40 130 F ceiling might lose 24h(130-30)42x28'/R40 = 70.6K Btu/day,
which leaves 225.8K-70.6K = 155.2K Btu/day for the rest of the 2nd story.
The ceiling needs a low-e surface (eg foil) to avoid overheating the room.
If it slopes up to the south, less foil is needed.

If you have 32 2'x4' windows on your 704 ft^2 space, that's 36% of the
floorspace--too many :-) You only need 2-4% of the floorspace for light.
Windows are expensive, and their framing is expensive, and they and their
framing lose more heat than walls, and they can leak air and rain and
bugs and burglars and baseballs. If you change to 64 ft^2 of R4 windows
on 42x20 = 840 ft^2 of living space, they might lose 24h(65-30)64ft^2/R4
= 13.4K Btu/day, leaving 155.2K-13.4K = 141.8K for 928 ft^2 of non-window
R20 walls losing 24h(65-30)928/20 = 39K Btu/day, leaving 141.8K-39K
= 102.8K in the solar heat budget, so you might add a FEW more windows.

If not, on a cloudy 30 F day, the 100 F apartment ceiling might lose
24h(100-30)42x28/40 = 49.4K, and the windows and walls might lose 13.4K
+ 39K = 52.4K, totaling 49.4K + 39K = 91.4K Btu/day, so the apartment
would need 5x91.4K = 457K Btu for 5 cloudy days in a row. If 75 F ceiling
mass can keep the apartment 70 F with a slow ceiling fan and a room temp
thermostat and an occupancy sensor, storing heat for an unlikely 5 cloudy
days in a row requires that (130-75)C = 457K, ie C = 8309 eg 8309/840 ft^2
= 10 pounds of water (2") per square foot of ceiling.

As an alternative to water under the ceiling, lots of fin-tube pipe under
the ceiling could heat water in a $200 1000 gallon 7' diam x 4' tall STSS
tank in the sunspace, with a copper pipe coil to heat water for showers.


Posted by tgdenning on September 8, 2006, 2:27 pm
 nicksanspam@ece.villanova.edu wrote:

This is why there aren't more solar/energy-efficient homes---thinking
too much about fancy solutions instead of the basics. :-)  (Although I
applaud you bringing up the windows.)

You mention R20 walls but the design uses uninsulated block. So all the
other stuff is irrelevant.
If this were framed with 8"nom stud walls you get R30+ all the way up.
With the insulated slab foundation as you suggested, the first floor
storage area is going to maintain >PF all on its own.

No idea what you are talking about water in the ceiling. The ceiling
can be R50 with an air space and reflective surface over the
insulation. Since there will have to be a hot water heater anyway, the
logical approach is to have  fuel-fired hot water radiant floor heat.

The solar-heated air needs to get drawn in through the already 50F
first floor not flow up under the ceiling of the living space as
shown---maybe through the first floor ceiling as plenum to avoid

Speaking of the first floor ceiling, all those steel joists as drawn
are going to be great conduits to
suck heat to the outside, which is why 14" I-joists are the proper
solution there as well as for the roof.

A very attractive design but the technical basics need to be dealt with
before the details.


Posted by nicksanspam on September 8, 2006, 6:05 pm

I noticed :-)


Posted by nicksanspam on September 8, 2006, 12:33 pm

I redid all that more simply, after looking at your nice drawings.

Heatpump bad. Solar good.

Cloudy days are like coinflips. If a house can store enough heat for
1 cloudy day, it can be 50% solar heated at best, 2 make 75% possible...
and 5 make 97% possible. At that point, the backup fuel cost is much
less important than the cost of the backup system, so you might buy
something cheap, eg a 5 kW 240 V portable space heater or some electric
baseboards or an air-source heat pump, if you want AC, which might be
rarely needed near Phila if you store coolth in ceiling mass and use
your ceiling fans--an $0 window AC would be cheaper.

I wonder which suburb you live in. I'm near Skippack.


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