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Hollow concrete and radiant heat with passive solar collector

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Posted by Dennis on November 19, 2005, 11:40 pm
I have been considering different building methods for a small cottage,
(~20' x 26' interior).  I have already dismissed ICFs and am thinking
of using hollow core concrete blocks with solar heated air running
through the cavities.  This air would be separate from the regular room
air.  The floors would also use concrete blocks which I think would
offer a very comfortable radiant envelope with built-in thermal mass.

I was originally thinking of pex tube hydronic in-floor heat until I
saw this link http://www.blockjoist.com/  that uses a special truss
system with concrete blocks and top coat of regular concrete.  If the
hollow cores in the floor were in line with the cores in the walls they
might make for a good thermo-syphon conduit.

I could cover the exterior with foam board and siding.  Although I
would prefer field stone for the exterior, (lots of it laying around my
place), it would require another wythe of concrete on the foundation
wall down to the footing, which may be too costly.  This would make for
a fairly solid sandwich wall structure that would be nearly impervious
to the elements and vapor movement.

One of my other concerns is freezing during extended power outages,
(this is in northern Wisconsin).  If I avoid using hydronic heating I
won't have to worry about that system freezing and can plan my plumbing
to allow for fast drain down for the time I am not there.  However, it
would still be nice to have it maintain an above freezing temperature
to avoid having to bring home all the things that could be damaged,
(canned goods comes to mind).  If I could maintain about 40F with a
passive system it would be easier to get comfortable after arriving
late on a Friday evening when the outside temp is -20F.

A friend of mine heats his place with wood, (24' x 32' with loft) and
it takes half the weekend to get comfortable with several stokings at
night.  Regardless of what type of other heat source I use I want
things to be comfortable in a few hours and I thought this whole hollow
core thermal mass passive thermal idea might work.

I would probably use the entire gable end south wall as a collector
with few or no windows.  The air would syphon through the attic above
the second story loft, down the walls, through the floors and back
through some sort of plenum in the basement floor to the bottom of the
collector.  I'd also like to work it out so the air flow is in opposite
directions in every other core of the floor to minimize any hot/cold

If the south wall has 9 vertical feet of collector space and a 22 x 11
foot triangular area above that on the 12 pitch gable I should be able
to build a (9' x 22') + (11' x 11') = 319 sqft collector.  I don't know
enough yet to calculate any more results from this.

If anyone has anything to add I would appreciate it.  I also have not
calculated the cost difference of building with block as opposed to
conventional materials.  If anyone has used the block truss floor
system I would also appreciate any feedback on costs/performance of
that as well.


Posted by Solar Flare on November 20, 2005, 1:00 am

How do you know these construction block holes will not
be full of mortar, as they should?

How will you connect one tube with the next and still
support the roof or are they all in parallel?

for a small cottage,

and am thinking

heated air running

from the regular room

I think would

built-in thermal mass.

in-floor heat until I

special truss

concrete.  If the

in the walls they

siding.  Although I

it laying around my

the foundation

This would make for

nearly impervious

power outages,

hydronic heating I

can plan my plumbing

there.  However, it

freezing temperature

could be damaged,

about 40F with a

after arriving

is -20F.

32' with loft) and

several stokings at

I use I want

this whole hollow

as a collector

the attic above

floors and back

the bottom of the

flow is in opposite

minimize any hot/cold

space and a 22 x 11

I should be able

collector.  I don't know

I also have not

as opposed to

truss floor

costs/performance of

Posted by Dennis on November 20, 2005, 1:59 am

First, I will know if they are full of mortar if I am the one laying
them.  Second, nothing says that they must be filled.  I could fill
every 3rd core, this is common practice.  In fact, the empty cores are
often filled with styrofoam beads.  In addition to that, if I use the
double wythe foam sandwich wall with field stone above grade, that I
mentioned, I wouldn't have to fill any of the cores if the exterior
wythe was the structural member.  If I understand everything correctly
at this web sight http://www.blockjoist.com/  these blocks also are not
filled with mortar.

I guess I'm not quite sure what you are asking.  I haven't worked out
every tiny detail yet.  However, the roof will be supported by some
type of rafter system to allow for usable living space, ie. two
bedrooms.  It might even be possible to use the block joist system for
this.  A little cutting of block and/or strategic placement will allow
the horizontal block cores and the vertical cores to line up.  There
are also three sided blocks.  Maybe I could use three sided blocks at
the corse that intersects with the floors.

I'm really interested to know if anyone has tried the block joist
system and if anyone who knows a lot more about solar collectors and
thermal mass than I do thinks this may work or not and why.


Posted by nicksanspam on November 20, 2005, 7:22 am

In northern Wisconsin, eg Eau Claire, where NREL says 430 Btu/ft^2 falls
on the ground and 790 falls on a south wall on an average 16.8 F December
day with a 25.3 F max. Not an easy solar house heating climate.

I've seen that, with room air entering and leaving block walls through holes
at the top and bottom. Mice and dust come to mind, and a low temp swing.
Mass with a higher temp swing can store more heat. Water can be cheaper
than concrete and it stores about 3X more heat by volume.

Warm air rises, so getting solar heat into a floor is difficult.

How much?

You might just drain the pipes.

You might maintain 40 F with electric heat, or the loss from a large heat
storage tank on the ground. You can heat SIPs a lot faster than concrete.
Or 9" TGI walls with poured cellulose insulation.

I like the idea of massy ceilings for heat storage. Or a low-e ceiling
surface above fin tube pipe, with a big heat storage tank on the ground.
With a slow ceiling fan and a room temp thermostat and an occupancy
sensor to warm a room as needed.

The air is unlikely to flow naturally below the base of the collector.

A square foot of R2 sunspace glazing with 80% solar transmission might
collect 0.8x790 = 632 Btu and lose 6h(70-20)1ft^2/R2 = 150 on an average
December day, ie 482 Btu net, so a 9'x22' sunspace might collect 95.4K,
or more, with an enclosed solar staircase roof.

With 48 ft^2 of R4 windows and 694 ft^2 of Rw walls and 520ft^2/Rc of
ceiling conductance and 12+694/Rw for windows and walls and 94.5K
= 24h((65-16.8)(12+694/Rw+(Tc-16.8)520/Rc), 64.3/Rw+(Tc-16.8)/Rc = 6.45.
With R32 walls, Tc = 16.8+4.44Rc. Rc = 40 makes Tc = 194 F, theoretically.

If Tc = 120 and the ceiling can still warm the cottage at 80 and it loses
5x24h((100-16.8)520ft^2/R40+(65-16.8)(12+694ft^2/R32)) = 325K Btu over 5
cloudy days and (120-80)520P = 325K, P = 15.6 psf of ceiling water, ie
a 3" depth. Or put fin-tube pipe under a low-e ceiling with 15.6x520 = 8112
pounds of water in a 130 ft^3 tank on the ground, eg a 4'x8'x4' deep tank.


Posted by Dennis on November 20, 2005, 7:01 pm
 nicksans...@ece.villanova.edu wrote

Yes, I've read a book about this concept.  However, I am considering a
closed loop system in which the room air does not mix with the air in
the cores.  If the system was sealed mice would not be able to get in
and any concrete dust would stay in the cavities.  I understand that
water is cheaper and a better thermal mass storage system.  The biggest
issue I have with it, in this remote location, is that I don't want any
water that could freeze due to a system failure.  I'm also curious if
HDPE tubes burried several feet under the basement floor might work for
thermal heat storage.  I gotta dig that hole anyway, I may as well have
it dug deeper while the guy is there with his back hoe.  This is on a
hill top 20 feet above the water table.

If the system is a closed loop I'm not sure this will be a problem.
I'm only just learning about this stuff, so help me to understand.
Also, I was not very clear in my first description of what I envision.
I'll do my best to describe it without a drawing.

This cottage would have a basement floor, main floor/basement ceiling
and second floor/main floor ceiling with concrete blocks lying on their
sides, connected to each level via the cores in concrete block walls.
In the 45 degree angle slanting wall area of the loft/second floor
there would be a ceiling about 7 feet high.  The area above this second
floor ceiling is where the heated solar collector air would first go
after leaving the south wall.  If you were standing outside looking
north at the solar collector and could see through the walls and could
see the air moving it might look like this.

As the air is heated in the collector it rises into the attic area,
which is sealed and ducted through the rafters to the walls.  This
displaces relatively cooler air through the ducts to the walls.  By
blocking or partially blocking some of the cores I believe it could be
directed at each intersection.  The path of air flow for one core might
go down an east duct to knee wall to floor.  At the floor/main floor
ceiling it moves west to the opposite wall, down the west wall to the
main floor/basement ceiling, back through the floor to the east wall,
down the east wall to the basement floor, through the basement floor
into a large duct area running north to south under the center of the
basement floor to the south wall, up the south wall cavities back to
the lower entrance of the solar collector.  Does this make sense?

I guess I understood a thermo-syphon with a solar collector to function
similar to a DC electrical circuit.  If the air is run through ducts
aren't they like electrical conductors?  As long as they return to the
source to make a complete circuit does it matter if some of them are
physically lower than the collector?

I'm not sure yet.  Perhaps 4", maybe more.

Yes, that would be easy enough to do, especially If I don't have to get
rid of 40 gallons of hot water.  Maybe I should install point of use
water heaters.

I agree and have considered SIPs.  My concerns about SIPs, (which I
think are way cool), is the lack of availability in the area, the need
for contractor installation with an expensive crane and crew make it a
non DIY/local help project.  What are TGI walls?

This sounds like something I might like to try, maybe in my garage, or
a house that I occupy regularly.

See my thought on this above.

I appreciate your input.  You seem to be the most knowledgeable poster
in this group on how to cost effectively incorporate solar heating.
Like I said before, I am just learning.  One of my biggest
misunderstandings is in some of the calculations.  I can grasp the
concepts when explained, but many of the variables are foreign to me.
Is there a file I can get that has this information?

There are some other reasons why I am currently stuck on masonry
construction, (yes, I could change my mind).  I like the strength it
offers, it is resistant to things like storms and fire and is low
maintenance.  The most likely causes of damage that I have seen are
freezing pipes and burglary/vandalism.  This is a place I go to relax.
I don't want to worry about it or be saddled with a lot of maintenance
like with the mobile home that is there now.

I would also like to make steel shutters to cover the outside of the
windows that would have sliding bolts on the inside that would slide
into the wall for security when I am not there.  I could also make
steel entrance door covers that have locking similar to some of the
doors on Navy ships, which would also only be used when not there.  By
making it extremely difficult to break in the thiefs will most likely
move on to easier targets.  The weakest link then would be an angry
vandal, ("how dare he try to keep me out"), who might take his
frustration out on the solar collector, causing the building to freeze.

So anyway, that is what I would like to accomplish a few years from
now.  A small, cozy, aesthetically pleasing, story book cottage.  One
that will last for generations.


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