[Greenbuilding] How to not short-circuit passive solar/in-floor hydronics

[Laren Corie]

> From: Shawna Henderson <shawna at abridesign.com>

> Here we have a disastrous conflict: passive solar maxed out and in-
> floor heating throughout slab-on-grade house (or any other type of
> house where direct gain is in direct conflict w/in-floor).
> Overheating galore, solar gets discredited, as does low-temp
> hydronics. Not twit who didn't think it through. Or maybe twit too.
> Who knows?
>
> I put forward two options for avoiding disaster with controls:
>
> 1. Place thermostats for direct gain zone close to windows so they
> respond to change in air temp more quickly, ie, turning off when air
> close to direct gain windows starts to warm up

Hi Shawna;

This situation is why a direct gain house should never have a fancy
sensor system for a hydronic slab, that reads outdoor temperatures.
I have always used simple room thermostats on my hydronic slabs,
with a 1F hysteresis, instead of the standard 2F.  That will solve the
problem of the backup system going on, when the Solar is already
heating the room.    Just turning the thermostat down will work too,
or programming in a lag, but clients usually need more complicated
solutions, to make them happy again, after a problem arises..

   It is not nearly as simple the other way around, to stop the direct
gain from overheating the space, that already has its floor charged
by the backup system.   The problem with this house, is that in the
design process, when the hydronic backup was chosen, it was not
realized that it would negate the floor as direct gain thermal mass.
There was no direct gain thermal mass added, to replace the slab
area that was taken away to be used as the distribution system
for the hydronic backup.  The Solar design was blown, by not
having adequate dedicated direct gain thermal mass. Creating
it, is how to correct the deficit.

  I am not sure from your post if this is a real house, or just a
group design challenge.  However, there are many solutions,
that together can fix the situation, even in an existing house.

1) If the hydronic system is zoned, turn off (or at least down)
the zone for the direct gain slab area.    Instant.....no conflict.
But, I suspect that if this obvious situation was missed that
several others were, as well.  So, further corrections may
be needed to reduce temperature swings, thereby more
effectively storing heat.

2) Spread/difuse the overheating to the houses remaining
integral thermal mass, via convection, either forced or passive.

3) Intercept the sunlight just inside the windows, so that it will not
reach the floor. This can be done either with dark surfaces, to ab-
-sorb light, turning it into heat that will be carried up to the ceiling,
where it will distribute itself around the house, without overheating
the living space level (according the American Bureau of Standards,
to over-heat a room the floor need only reach 78F, or the walls 84F,
but the ceiling (over 6'-8") can reach 115F). This greater temperature
rise also allows the relatively small thermal mass of the ceiling to store
a great deal of heat.

    The other way is to use light colored reflectors, that will shine the sun-
-light up to the ceiling, onto the walls, and/or back deeper into the house.
Either one of these can be accomplished with horizontal blinds.  For even
more flexibility you can get mini-blinds that are flat black on one side, and
white (or polished metal) on the other. Take your pick, convection or light. ;O)

4) If the design is botched even worse than that. you can add thermal
mass.  The old-school fix-up is water tubes in front of the direct gain
windows. You could also build a series of masonry walls, perhaps on
45° angles to the glass, to let morning light penetrate the space, but to
block/store afternoon gain. The window side can even be glazed to force
the heat to travel through the masonry, creating a time lag.   Or, just a
short wall, behind the glass, about hip high.  Another old-school solution
would be to enclose water drums, inside this wall, and make it into a seat.
Furniture is probably going to keep most of the sunlight off the floor,
anyway.  That is one of the many big problems with direct gain floors..

Personally, I like mass up in the attic, in small water containers,
but that requires easy to work with space, up there.  A thermal
storage of two-liter PET plastic water bottles could be built into
the interior walls, betwen studs, to passively convect.  Too bad
paraffin drywall isn't available, so a couple of layers of it could
be put on the ceiling.  You might be able to put two-liter PET
plastic soft drink bottles, filled with water, between the ceiling
joists, held up by a netting or hardware cloth, and even blow
air through the joist spaces (inspector would have fits, thinking
it was a fire hazard, even though PET plastic water bottles
in the ceiling are actually a passive fire suppression system),
controlled by a  standard programmable wall thermostat,
run in series with a cooling mode thermostat, in with the
bottles, set at about 82F (so it will only operate when
there is enough heat).  A Stratojet ($20 from Solar
Components) could blow the warmed air down to
the floor level, in a nice narrow stream, by the wall.

Direct gain houses need to be tuned, like the rigging of a sail
boat.  Flexibility and several solutions, should be part of the
original design.

> 2. Zones and controls to 'strip' heat from direct gain areas
> and send it to cooler areas of house.

I would forget about using the hydronic system to redistribute..
It's already too late, once the air is overheated.  The problem
is no coupled mass for the Solar gain, not the heat in the slab.

> This is assuming that you need a heating system at all...;-)

  Especially a hydronic slab, that charges mass.  Why store
heat from fossil fuels?  However, direct gain systems in cold
cloudy climates need a backup.    The south facing windows
barely (if) break even in the coldest months, and the nighttime
loses are really impressive.   If they collected enough heat for
most of the 24/7 demand, the space would be miserably hot
during the gain times. Using the living space as the collector
box, is just a bad idea. I hope we have grown wiser, since
the early 1980s. See-through collectors are nice, but not
living in a hot/cold box. Trying to live between the glazing
and the absorber is asking for discomfort.

> Anyone else anticipate or see this predicament coming up?
> I would like to have some examples of best practice (or any
> practice) on  this. Any links to research/monitoring docs
> would be greatly appreciated.

I have done dozen of hydronic slabs, in direct gain rooms,
However, I never relied on the slab to absorb the sunlight.
I both reflected it to higher mass, and intercepted it at the
windows. Then I convectively moved it to a storage, UNDER
the radiant slab. That way the stored solar heat, was "behind"
the radiant slab.  And, the pancake heat sink was always
returning cool air into the overheated direct gain space.
However, I quit doing that about twenty five years ago,
because I hated the aesthetics of the fan noise, while
the house was supposedly heating itself  "naturally"

The problem presented here, is simple. The design lacks
an understanding/implimentation of one of the most basic
principles of Solar thermal design. That is: Thermal mass
is a cooling system, for the collection area, and needs to
be designed as such.  Storage is just a nice side effect.

 My advice is to optimize your windows for daylighting,
rather than maximizing them, for Solar overheating. Then,
use a low mass sunspace/porch as your Solar collection
area, isolated from the living space.      Also, isolate your
thermal storage from the living space.   My preference for
twenty five years has been up in the attic, so there is no fan
or controls needed, to charge it, and you can quietly bring
down a little heat to the living space, as it is desired, using
small fans and/or pumps, and programmable thermostats.
At the same time, you get DHW heating, and stack effect
summer ventilation, as well as wind sheilding and shading
of the south facing living space windows.   Plus, a really
great place to bask in the sun, in the middle of winter.

-Laren Corie-
Climate Responsive Home Designs Since 1975