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Re: Not home(turn off A/C), home( then turn on A/C) question? - Page 2

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Posted by nicksanspam on July 24, 2004, 11:22 am

This seems unlikely to me.

I'm afraid this makes little sense to me. Would you like to explain further?
We could calculate the bed temp after 16 hours at 70 F and 8 at 90 F, which
would be closer to 70. But most beds are far less massy than the one you seem
to suggest. They are mostly air (the box springs), a little steel, and fabric
(the mattress), so they would tend to cool quickly to 70 F in early evening.  

To a first approximation, a room with 80 F walls and 70 F air should feel
the same as a room with 75 F air and walls, according to the ASHRAE 55-2004
thermal comfort spec. Initially overcooling the air in a house (or turning
on a ceiling fan or a sprinkler) to make up for initially higher mean radiant
temps from walls and furniture may use less energy than cooling the house to
a constant temp for an hour or two before the setback ends.

We discussed drywall. Nobody mentioned fiberglass. Wood doesn't have much
thermal capacity by volume, as you can see from the bed example. Neither
do most insulations.

Ceilings are better for heat storage because they can be a lot warmer than
the room, especially with a low-e surface. Floors can't, to the same degree.

Well? IIRC, you claimed to have an EE degree...

Can you be more quantitative?

Historically-speaking, most people quickly tire of doing that. Automatic
systems seem preferable.

Concrete blocks with about 5 Btu/F and lots of surface. What's underneath?

A warm air path... How could this possibly work passively? You might make
the slab thicker to store more heat, altho you can't make it much warmer
than the room temp without cooking the occupants. Massy ceilings are better.

"Direct loss," with miserable performance, compared to indirect gain.

You may be the rare kind of guy who will actually operate shutters
twice a day for years and years...

Is this the passive part? :-)

Sounds like they need to be 16-20" thick, on average, vs 12" R48 SIPs.

Kachadorian's. I wonder where this house will be.

"The center of Hell." :-)

Was this an adobe house? Was "the better part" 3 days?

Maybe the warm walls made the beds feel uncomfortably warm to you.

How did the house store so much heat in a couple of days without AC?
Why did it take so long to become comfortable again? With insulation
outside thermal mass, it seems to me the inside of the house should
warm slowly without AC and cool quickly with AC.


Posted by Bob Ward on July 24, 2004, 11:42 pm
On 24 Jul 2004 07:22:48 -0400, nicksanspam@ece.villanova.edu wrote:

How do you get the bed to cool more quickly than the air in the room?

Posted by Rod Speed on July 24, 2004, 11:46 pm


He never said anything like that.

Posted by Timm Simpkins on July 25, 2004, 3:37 am

= -168/ln((75-70)/(90-70))



I wasn't using my figures.  It was stated that a bed that has risen to a
temperature of 90 degrees will reach 71 degrees after 8 hours in 70 degree
air.  Since thermal mass works both ways, the speed is the same for it to
reach 90 degrees again.  So, if his figures are correct, and a bed takes 8
hours to cool 20 degrees, it also takes 8 hours to heat.

As far as them being only air steel and fabric, yes, the box springs are
like that, but most matresses are filled with fairly tightly packed padding,
usually cotton I believe.  That cotton has a much higher thermal mass than
wood, steel, and fabric of the box springs.


The air in the room would feel that way, but if you have furniture to sit on
or lay on, that 80 degree heat directly on your skin will not feel like 75


Actually wood is a fairly good thermal mass.  I don't have the figures right
here, but I believe it was rated at 2.78.  Whatever it was, it was close to



I didn't ask that question, and that has nothing to do with electrical

Where the house I'm building will be off the grid, energy efficiency is
something that is quite the necessity.  If I get tired of it, it wouldn't
take anything to make light actuated motors to open and shut the doors.

Compacted sand or gravel with a 6 mil poly vapor barrier.


May I suggest a book?  The Passive Solar House by James Kachadorian.  If you
have a massy ceiling, the heat stored in the ceiling is exposed to the cold
outside air when the sun goes down.  While it may work to a degree, it isn't
very efficient.  The exposure to the outside cold draws any heat transferred
by the sun quite fast, especially if the ambient temperatures are rather
low.  That's not to mention what would happen if the roof were covered in
snow.  With a solar slab design, you expose the slab to the solar energy
during the day and you insulate it from the cold during the night.  This
way, the majority of the heat dissapation will happen inside the house, and
not to the outside.



I don't understand where you get the direct loss from.

See above.  Light actuated timed relays connected to a motor can be easily
and cheaply made.

Well, I don't care how good you build a house, there is no way you can have
everything perfectly the temperature you want it.  I guess I should have
explained that the air circulation would be a method for the air to
efficiently circulate through the house.  The only time it would be active
would be those times the furnace needs to be turned on.  Otherwise, the
circulation happens due to heat difference.

That depends on the wood you use.  You can get anything from .8 R per inch
for oak to 1.48 R per inch for cedars.  I'm building mine out of pine which
has a 1.35 R per inch rating.  That doesn't even include the thermal mass
gains in an area with mild climate.

Actually, that is the one I found to be most helpful.  Didn't think you had
read it.

That house was not very well insulated.  It was built in the '50s.

Posted by Nick Pine on July 25, 2004, 12:32 pm

I'm afraid this still makes little sense to me.

Keep up :-) That was 20-minute drywall, vs your mythical 121-hour bed.

My 60"x80"x8" queen size mattress weighs about 60 pounds. Fully-packed with
cotton fiber at 95 lb/ft^3, it would weigh 95x60x80x8/12^3 = 1728 pounds.
I've never ripped it open to see what's inside, but I feel steel springs with
about 1/2" of padding on top and bottom. The ASHRAE HOF says cotton fiber has
a specific heat of 0.319 Btu/lb. The mattress might have 60x0.319 = 19 Btu/F
of heat capacitance. If the box springs add another 19, we have C = 38, with
G = 1.5x2x60x80/12^2 = 100 Btu/h-F (losing heat from 2 sides), RC = 0.38 h,
so this bed would heat or cool to the room air temp in about 2 hours (5 time
constants), vs a week. You seem to have trouble admitting when you are wrong,
esp to Mr. Speed.

The ASHRAE HOF says Hem Fir with a 12% moisture content weighs 24.5-31.4
lb/ft^3, with 0.39 Btu/lb-F, so it stores about 11 Btu/F-ft^3. You might
say that's "close to 2.78" :-)

You made a less-precise claim: "A passive solar house can loose as little
as 10 degrees overnight with an outside temperature in the teens."

Of course it does :-) Remember all those Rs and Cs? Hollow concrete blocks
(weighing 32 pounds at 0.16 Btu/F-lb, with 128 in^2 of face area) store
about 6 Btu/F-ft^2 (think 6 farads), so a 32'x32'x8' tall block house with
1024 ft^2 of walls has C = 6K Btu/F... 82 ft^2 of windows have 82/4 = 20.5
Btu/h-F of thermal conductance, plus 1024-82 /R20 = 47 Btu/h-F of wall
conductance, making RC = C/G = 6K/67.5 = 89 hours, like this, in fixed font:

11 ---www------- T   If T(0) = 75, what's T(16)?
           --- 6K

Possibly not :-)

People have been trying to do that for 30 years. Oddly enough, they
haven't succeeded. The doors fail to seal well, the system is way too
expensive, and so on. Good luck.

That's a start.

We need insulation over the mass.

It can be a lot more efficient than what you propose.

How? Where's the insulation? Carpeting over foamboard might help...
But then you could only heat the slab indirectly, with warm air.

Big holes in the bucket.

It's a more correct way to say "direct gain" :-)

Can you explain exactly how warm air naturally circulates under the floor
due to heat difference? :-)

Ah yes. Dynamic R-values :-)

Still wondering.


Article 18035 of alt.energy.renewable:
From: nick@ece.vill.edu
Subject: Re: Passive Solar Home/Solar slab
Date: 14 Apr 1998 03:29:32 -0400
Organization: Villanova University

Kachadorian's book missed the point here and there...

I've seen this done with walls as well: stack up hollow blocks aligned
so air can flow vertically through the hollows, and make holes in the
inside face of the wall at the top and bottom (or use perpendicular
U-shaped blocks under a thicker wall, with insulation on both sides)
to let room air flow in and out of the wall. This increases the amount
of wall surface exposed to room air, which makes the wall a better
room-temperature thermal store, by decreasing the internal series
resistance of "the heat battery." Also, a wall allows natural airflow,
vs a floor, which needs a fan...

But this can be further improved: if the masonry can somehow be kept
at a higher temperature, it can store more useful space heat, and
provide better room temperature control. A 30 pound concrete block that
cools from 75 F to 70 F releases about 25 Btu of heat as room air cools
from 75 to 70. The same block cooling from 120 to 80 F might release
about 200 Btu (8X more) of heat while keeping a room at exactly 70 F.

"Living inside the heat battery" limits the upper storage temp, which
limits how much heat can be stored, and it means the living space air
has to follow heat battery temperature swings.

I've been in Richard Komp's house in Maine, which has a hollow block
floor warmed by solar room air (he calls it a "hypocaust," like an
ancient heated Roman bath floor) with less insulation underneath than
Kachadorian's later floors. His J.C. Whitney 12 V fan and PV panel
force air under the floor, which might be a nice home for mice, were
it not for his pet, Ernie the ermine. Dr. Komp (president of the Maine
Solar Energy Association, and author of "Practical Photovoltaics")
doesn't seem to worry much about mold or dust. I guess there's a vapor
barrier under the floor, and his house air tends to be dry in the winter.

How about making the thermal mass water in sealed containers, with about
twice the specific heat by volume of solid masonry, and putting the
containers inside a compact easily-insulated closet/sauna/warm room
that sits on the ground? It could be more easily cleaned, that way.

Windows between the outdoors and 24-hour heated living space lose heat
at night and on cloudy days, so there's a dilemma: the more windows,
the more solar gain on a sunny day, but the more loss on a cloudy day.

Putting most of the windows on a low-thermal-mass sunspace avoids this
dilemma, since we can capture the heat of solar-warmed air in some
thermal mass inside the house on a sunny day, and stop air circulation
and let the sunspace get cooler at night.


Article 14649 of alt.architecture.alternative:
From: nick@ufo.ee.vill.edu (Nick Pine)
Subject: Re: What would be the most energy efficient, safe, Low maintenance
house available today?
Date: 21 Dec 1999 04:13:05 -0500
Organization: Villanova University


I reviewed his book in manuscript form.

I doubt it, but even so, one might do a lot better.

Warm air rises. Why would it want to flow under the floor? Lots of warm
air needs to touch Lots of thermal mass surface to raise the slab temp
on a sunny day without overheating the house and ensure a low day/night
temperature swing. During a cloudy week, such a house gets exponentially
colder and colder, without woodstoves and so on.

Removing most of the solar glazing to a low-thermal-mass sunspace (one
that gets cool overnight and stays cool during a cloudy week) with an
insulated wall between the living space and the sunspace allows the same
or more solar gain when warm air flows between the sunspace and the house
during the day, but reduces the nighttime and cloudy-day heat loss from
the living space...

Dr. Rich Komp (author of Practical Photovoltaics and president of the
Maine Solar Energy Association) says warm hollow floors like his
(which predates Kachadorian's) aren't new. Romans built hypocausts,
hollow floors heated with warm air from hot water or fires. So did
Chinese peasants. Warm hollow floors make good homes for dust and
varmints. Rich's friend Ernie the Ermine takes care of that problem.

Living inside the heat battery, we are subject to its temperature swings,
and if there are no temperature swings, there is no solar heat storage.

We can't charge the slab up to a high temperature because we have to
live with it in the room. The same amount of thermal mass at a higher
temperature stores more useful heat than lower temp mass, and it allows
keeping a constant room temp until the mass cools to something close to
that room temperature.

Floor slabs don't usually have much insulation between themselves and
the room air, and they are difficult to insulate because of their shape.
The same amount of insulation applied to a cube with equivalent mass
lowers the rate of heatflow a lot more.

And water stores about 3X more heat than masonry by volume. It can
also be cheaper and more useful, even in sealed containers.

So, we can do a lot better.

Nick (now plumbing a $20 1500 gallon 275 pound rainwater tank)

Article 11207 of alt.solar.thermal:
From: nick@acadia.ee.vill.edu (Nick Pine)
Newsgroups: alt.solar.thermal
Subject: Re: Thermal Mass/Passive Solar Information
Date: 16 Mar 2002 09:50:13 -0500
Organization: Villanova University

I seem to recall that this slab uses a fan. It might make more than a
40% heating fraction, with lots of airflow and slab channels (ie heat
transfer surface) and a carpet on top of some foamboard over the slab and
a low thermal mass sunspace with separate 100 F air ducts between the
sunspace and the slab and lots of house insulation, eg 12" R48 SIPs.

Then again, we might make the house walls hollow block with the holes
lined up so air can naturally flow vertically through the walls, with
lots of insulation (eg Dri-Vit) outside the block.

But either way, we seem to have lots of little inaccessible nooks and
crannies to attract dust and spiders and varmints.

Storing heat for 5 cloudy 30 F days in a house that cools from 75 to 65 F
means 650+(75-30)exp(-120h/RC), so RC = -120/ln((65-30)/(75-30)) = 477 h.
A 48'x48'x8' house with an 8" 25 Btu/F-ft^3 slab with C = 48x48x8/12x25
= 38.4K Btu/F needs a max thermal conductance G = 38.4K/477 = 80.5 Btu/h-F,
or 57.5 for 3748 ft^2 of R65 walls and ceiling, after subtracting 4% of
the floorspace as R4 windows, with no air leaks or internal heat gain.
Sounds Herculean...

Making the house 32x32x16' tall with a 17K Btu/F slab and 2048 ft^2 of
5 Btu/F-ft^2 block walls makes C = 27.2K Btu/h-F, so G = 27.2K/477 = 57,
or 36.5 for 2990 ft^2 of R82 walls, with no air leaks. More Herculean.

Sounds good to me, with a hydronic slab.

Harry Thomason trickled water over a roof under a layer of plastic and
a layer of glass, with a 2/3 solar heating fraction for the US Customs
House in Richford, VT. Trickling a 50% 2:1 CaCl2+LiCl solution over a
dark asphalt shingle roof under a single layer of corrugated polycarbonate
plastic might be a painless retrofit, with 4x4 sleepers. The desiccant
solution could store more cloudy-day heat than the same volume of water,
with less thermal loss due to condensation under the glazing.

Sounds good. Big Fins in a sunspace, with "thermal cogeneration," as the
sun heats the panels and their "waste heat" (warm air) heats the house.

Duane Johnson mentioned a recent patent for a "CLFR" (Concentrating Linear
Fresnel Reflector?) IMO, this kind of thing requires more tinkering than
engineering. Cutting kerfs in 2x4s, trying to avoid meltdown, with some
PVs under 3 suns in the shallow water pool...

Somebody should build one and write a book. How about you?
You might do a LiCl house and a bubblewall house as well.

Solar house heating is pretty ho-hum for most people these days.
Letting oil prices rise or making house heating a competitive sport
like America's Cup racing would help (no outboard motors allowed :-)


Article 14451 of alt.solar.thermal:
From: nick@acadia.ee.vill.edu (Nick Pine)
Subject: Re: Hair brained solar cistern???
Date: 8 Jan 2003 14:43:44 -0500
Organization: Villanova University

Not if you used any numbers or knew shit about physics or cared about cost
and performance and simplicity :-) I just reread K's book. Lots of whopping
mistakes. For instance, he thinks a house needs 2/3 ACH for health, 27X
more than the 0.025 ACH Swedish standard.

Page 17 says "As you can see, the reduction in solar benefit increases
exponentially as you rotate the home's orientation away from true south."

Page 30 says:

  If this combination of poured concrete slab over horizontally laid blocks
  is ventilated by air holes along the north and south walls, air will
  naturally circulate through this concrete radiator when the sun is out...
  the south wall will be warmer than the north wall... air that is next to
  or alongside the south wall will rise. Warmed air will then be pulled out
  of the ventilated slab, and the cooler air along the north wall will drop
  into the holes along the north wall. This thermosiphoning effect will
  naturally continue to pull air through the Solar Slab.

Page 49 says "Incorporate an air lock entrance" with miniscule energy savings
  except for a department store, or a house with a huge active family.

Page 53 describes "reflective" foil smack up against plywood:

  The interior foil face will reflect heat back into the room, even though
  it is sealed inside the thermo-shutter... The outside foil face of the
  insulation contained within the wood veneers will reflect the sun's summer
  heat back out the window.
Page 94 belies the natural air circulation described on page 30:

  The duct shown running down the middle of the bgase under the poured slab
  is included in all cases. It should always be used as the return-air duct:
  do not reverse the air flow pattrern shown on the control diagrams. By
  using the Solar Slab as part of the return-air duct system, the Solar Slab
  will constantly assist the furnace by preheating the return air. Even if
  the home will be heated with a woodstove and emergency electric furnace,
  the return duct should be included and the air mover hooked up per the
  appropriate control diagram...  

Page 101 says:

  2. Size of electric heating system = 9.25 kilowatt=hours,
     with an annual consumption of 7.616 kilowatts,

Page 102 says:

  The calculation for the electric backup option determined that
  we would need 9.25 kilowatts per hour for the Saltbox 38...

Page 106 says "The theoretical minimum temperature to which a home with
  a Solar Slab will drop is the ground temperature under the solar slab..."
  (Yes, that will keep the pipes from freezing in most parts of the US,
  if a perfectly airtight house with infinite insulation :-)

Page 107 says "it also costs more to cool air than to heat air," as if
  K. is unaware of evaporation, night sky radiation, or the phrase
  "coefficient of performance."

Page 137 ignores one-way passive backdraft dampers:

  It may seem that a sunspace that is gathering enough heat to become
  90 degrees Fahrenheit on a cold, 15-degree but sunny winter day would
  be beneficial to the home. And yes, it can be beneficial. However,
  the same overglazed sunspace that accumulated all that heat during
  the cold but sunny day will need lots of added heat when the sun goes
  down to prevent it from freezing, which means that the sunspace or
  greenhouse will tend to draw heat from the rest of the house as its
  flow of solar heat reverses course, back out through the glazing.

but his solar slab is one good way to store overnight heat from inexpensive
passive air heaters or a low-thermal mass sunspace that can add valuable
floorspace to a house. One drawback is dust--it's hard to clean the rough
passages in the hollow concrete blocks. Another is fan power. A vertical
thermal mass (eg a chimney with extra flues open at top and bottom) might
store and release heat to a house with no fan power at all...


Article 14457 of alt.solar.thermal:
Subject: Re: Hair brained solar cistern???
Date: Thu, 09 Jan 2003 06:12:20 GMT
Organization: Villanova University

Well, page 46 says:

   Let there be no misunderstanding about where the fresh air makeup
   is coming from. The walls and roof of your home should be very
   tightly constructed... Fresh air will enter your home through
   controlled or deliberate openings... not through gaps in the insulation
   or poorly sealed windows and doors.

A 3,000 ft^2 house with 2/3 ACH has 267 cfm, enough for 18 full-time
occupants, using the 15 cfm/occupant ASHRAE standard.

K. doesn't mention heat recovery, although he talks about an
"air exchange or ventilator system." HRVs seem useless for most
US houses, since the natural air leaks can easily supply most of
the ventilation air. A 3,000 ft^2 house only needs 30x60/(3000x8)
= 0.075 ACH for 30 cfm. We might turn on a ventilation fan if
the house feels stuffy or the RH exceeds 60% in wintertime...

We might store more heat with better room temp control by
circulating 100 F air from a sunspace under the floor...

That might help, but the concrete block holes are rough. Some people
suggest casting 4" PVC pipes into a concrete floor...

If a solar house needs, say, $00 per year of electrical energy
to operate, we might heat a superinsulated house at the same cost,
and forget about fans and mass and glass...


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