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A 100% passive solar heated bungalow

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Posted by David Delaney on February 19, 2004, 6:41 pm
I have radically rewritten the page of my web site
that sets out the goals and rational of the solar
house I am designing.  (Actually, this is the
first time that it has been reasonably complete
and comprehensible.) See the text below.

The links to the detail pages have been suppressed
by the process of copying the text here, but this
page, with its links, may be seen at

David Delaney, Ottawa, Ontario


A 100% passive solar heated bungalow

The notes and documents referenced below record my
thoughts about the design of a solar house. I live
in Ottawa, Ontario, Canada, 45.3N, 75.6W. I want a
house that gets all its space heat and most of its
hot water heat from the sun.  For various reasons,
I want a house that will be comfortable during a
prolonged power outage in the winter.

The house will be necessarily unusual.  Since I
want it to be maintainable without specialized
knowledge, its systems must be very simple. It
must require no unusual mechanical or electrical
components. The fewer mechanical and electrical
components, the better.

Ottawa  has a cold cloudy winter.  December is the
worst case month for solar design. On average
there are 824 heating degree-days in December,
giving an average ambient temperature of -10C
(14F).  I will use a design average temperature of
-18C (0F).  The December solar radiation on a
vertical south facing wall is 2.15 kWh/m2/day (682
Btu/ft2/day) (RETScreen method. Data from NASA
satellite observations calculated and presented at
http://eosweb.larc.nasa.gov/sse/ .) These data
imply that the house will have to be very well
insulated. It will also have to have a large heat
collector (large air heater) and store heat
obtained during a sequence of good sun days and
deliver it to heat the house for fairly long
periods of no sun, say 10 days, in a specialized
heat store separate from the living space.

My work is based primarily on the work of Norman
Saunders and William Shurcliff. William Shurcliff
was an innovative solar thermal thinker and a
great documenter of solar thermal ideas. Norman
Saunders was one of the most creative of the solar
heating pioneers. His goal was to provide all
space heat and much domestic water heat from solar
thermal energy--no backup heat, even in the cold
and cloudy winter of Massachussets.  This goal
requires storing heat for later use. Shurcliff
describes three of Saunders's houses in "Super
solar houses--Saunders's  100% solar, low-cost
designs",  Brick House Publishing, 1983. These are
active, not passive, solar houses--they require
fans. Here is an excerpt describing one of these
houses, the Cliff House.

Norman Saunders's Cliff House as described by
William A. Shurcliff, (pdf) (Reproduced by
permission of William A. Shurcliff.)

Although admirable, the Cliff House is far from
meeting my goals. It needs fans and complicated
electronic controls,  which I might be willing to
use for convenience, but which would have to be
unnecessary for basic comfort when the power
fails. I want a house that can be completely
passive and comfortable during extended power
failures. I include local solar electricity in the
category of power that might fail, primarily
because I want the heating system to be
conceptually shallow. No one needing to restore
heat should have to understand solar electricity. 
Although the Cliff House will not work in a power
failure, some of its principles seem likely to be
useful in achieving my goals. The most promising
features of the Cliff House are its use of a
sunspace on the south wall as a solar air heater,
its use of air as a heat transport medium, and its
use of natural convection to move hot air from the
air heater to an overhead heat store and to return
cool air from the heat store to the air heater.

Air as a heat transport medium offers the
possibility of a simple solar heat collection
system that cannot freeze or leak, and requires no
plumbing.  A thermosyphon (natural convection) air
heater with an overhead heat store eliminates the
need for dampers and control systems to keep cold
night air out of the heat store, while still
operating without human attention.

One of the drawbacks of the Cliff House (from the
perspective of my requirements) is that only part
of its heat store is located overhead.  Its main
heat store is located under the first floor slab.
The basement heat store is composed of
approximately 50 tonnes of small stones. It
provides both a very large thermal mass and a very
efficient transfer of heat from hot air to its
thermal mass. Although the attic thermal mass of
the Cliff House, which is composed of 11 tonnes of
water in 50 drums of water (55 US gallons each), 
has approximately the same thermal mass as the 50
tonnes of stones in the basement,  it aborbs heat
from the air only slowly, because of the
relatively small surface area of the drums of
water. The 50 tonnes of small stones in the
basement store, on the other hand, have a very
large surface area. The Cliff House has a powerful
blower to drive a large volume of hot air through
a duct from the attic water store to the basement
stone store.  Since blower runs at full power only
when the air heater is very hot, and runs at much
lower power most of the time (it runs 24 hours a
day), the Cliff House needs a complicated
electronic control system.  (There are other uses
for the control system that I will not go into

I intend to dispense with the basement heat store
and the control system of the Cliff House. I will
make up for their loss by increasing both the
total thermal mass of the attic heat store, and
its heat-transfer surface area.  The attic thermal
mass will consist of two components, 11 tonnes of
water in 50 drums, and 30 tonnes of concrete
blocks stacked to provide a large heat-transfer
surface area. The reduced mass of the concrete
blocks (30 tonnes, as compared to the 50 or more
tonnes of stones in the basement of the Cliff
House) will be made up by larger temperature
excursions in the concrete block stack. The
decision to place such a large thermal mass
overhead poses some architectural difficulties.

The thermal mass of the heat store will probably
be a lot bigger than necessary.  The reasoning 
and data required to decide how big a thermal mass
should be in order to be just slightly bigger than
necessary are beyond me.  The heat store design
must ensure that the only drawbacks of its thermal
mass being  too big are in the cost and the space
it consumes.  Being too big must not degrade its
performance. The keys to good performance of a
heat store with an oversized thermal mass are

thermal stratification of the thermal mass, so
that heat may be stored and retrieved over a wide
range of states of the thermal mass at a
temperature not far below that at which the heat
was received from the solar air heater, and
priority to the house and the domestic hot water
for claims on high temperature heat from the solar
air heater.

When these conditions are met, the occupants of
the house will 1) not have to wait for warmth on a
sunny day while a cold thermal mass heats up,  and
2) will not have a cold night and morning after a
moderately sunny day that started with a cold
thermal mass.  Thermal stratification also
maximizes the usability of the heat in the store.

The ceiling and upper walls of the house (a high
ceiling bungalow) will be heated by natural
convection from the heat store controlled by
manual dampers. Distribution of heat downward from
the ceiling area will be by thermal radiation and
a low power ceiling fan.  Thermal radiation alone
should be sufficient to keep the house liveable
when the ceiling fan is not working.

The pages at the links below give more detail to
the thermal principles I plan to incorporate in my

1) General thermal scheme: Organizing the air flow
between a thermosyphon solar air heater and a
thermal mass located above it.  Norman Saunders
reduced the energy loss from the solar air heater
of his Cliff House by having the cool air enter
the air heater from the top and descend next to
the glazing. The pattern of the air flow in the
Cliff House requires the north-south thickness of
the air heater to be greater than the north-south
extent of the elevated thermal mass. The linked
page describes a way to remove this constraint
while still having the cool air descend next to
the glazing. The air heater can be much thinner,
and/or the north-south extent of the elevated
thermal mass can be much larger, or the thermal
mass can be positioned more centrally in the
building.  I believe this idea is new.

2) Thermal mass concepts:

A two-part heat store with large thermal mass, low
resistance to air flow, and large heat-transfer
surface area. Thermal mass of drums of water on
top of a stack of concrete blocks. Describes an
improved physical arrangement of the two
components of the two part heat store. (I intend
to use this improved arrangement. A concrete-block
thermal mass with large air-concrete surface area.
Describes the concrete block component of the two
part heat store in detail.

4) Ventilation scheme: Ventilation for a solar
heated building with a large thermosyphon air
heater. No fans during  power failures means no
heat recovery ventilator (HRV) during power
failures. Here's a novel way to get a lot of fresh
air into an entirely passive solar house: I
believe I am the first to suggest it.


Nick Pine is a source of inspiration. Although he
does not necessarily agree with all of my
decisions, he has given extremely valuable
feedback at crucial points in the process I am
following. Find him in his lair at


Posted by Steve Spence on February 21, 2004, 1:59 pm
Are you going for the full Arts & Crafts treatment? We just spent a weekend
in a A&C bungalow, which was heavily influenced by Stickley. Extreme cold
(-30F) and wind (45 mph gusts) outdoors, but the place was sealed up tighter
than a drum and there wasn't a draft to be found.

Steve Spence
Renewable energy and sustainable living
Donate $0 or more to Green Trust, and receive
a copy of Joshua Tickell's "From the Fryer to
the Fuel Tank", the premier documentary of
biodiesel and vegetable oil powered diesels.

Posted by David Delaney on February 21, 2004, 5:04 pm
 On Sat, 21 Feb 2004 08:59:33 -0500, "Steve Spence"

I'm not familiar with the Arts & Craft treatment,
or with Stickley.  I take it I should be.

The restriction to bungalows derives from 1) the
elevated heat store whose lowest part must be
above the top of the air heater, 2) the reliance
on natural convection from the heat store to
discharge it for space heat 3) the consequent
inability to heat anything below the bottom of the
heat store by air movement 3) the consequent
limitation to heating only the upper part of the
house by air movement, 4) the need to rely on
radiation from a hot ceiling to heat the lower
part of the lowest, and therefore only, storey.

The restriction to bungalows could be eliminated
by controlling the air flow so that hot air has to
flow under most of the ceiling of the first storey
before rising, cooled somewhat, to a second
storey. Conduction through the first storey
ceiling to the second storey floor would also have
to be controlled. Too much precision required for
a first project with these ideas. Hence, bungalow.

I am designing a box-like house to keep thermal
design simple, envelope integrity and reliability
simple, and costs low.

David Delaney, Ottawa

Posted by Steve Spence on February 21, 2004, 5:59 pm



for some ideas.

Steve Spence
Renewable energy and sustainable living
Donate $0 or more to Green Trust, and receive
a copy of Joshua Tickell's "From the Fryer to
the Fuel Tank", the premier documentary of
biodiesel and vegetable oil powered diesels.



Posted by nicksanspam on February 23, 2004, 1:00 pm

Warm air rises, no? I still don't understand how this works...

What makes the ceiling hot? How does heat move from the heat store
above the ceiling down to the ceiling level? Conduction?


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