This large solar space heating system was built in 1986 as a part of
the original home construction, and has been in continuous use ever
since -- 25 years, and is still going strong.
The system is a solar air heating system with a 10 ft high by 36 ft
wide solar air heating collector. A rock bin in the basement
provides thermal storage for the system.
The system has required zero maintenance over its 25 year life!
The house and solar system were built by Mike Smith. This is
his own house, and he has
lived in the home since it was built. The house is located in
Rhode Island. Mike is also one of the
originators of the
Mooney Wall
-- a low cost, high R value wall that is good for both new construction
and retrofit.
Thanks very much to Mike for taking the time to put together these
pictures and description of the system!
Pictures and Description from Mike
The house was built in 1985. We picked a lot on an East -West
street
with the
lot on the South side so we could orient the public ( Hollywood side )
to
the North and the Solar side to the rear.
The 8' x 8' cupola is a viewing feature. We had water views to
the West & North.
You can see the 10 x 36 solar array in this view of the SW corner.
The Collector
This is the roof profile. The collector is on the 60 deg
slope on the lower gambrel roof.
The upper portion of the roof is a truss,
creating a low attic.
The collector array is a 10x36 Hot air collector
developed by Solar Homes , Inc in the late '70's.
After many changes, this is the system we settled on.
Close up of collector showing cosmetic
paint peeling.
The Absorber plate is galvanized 26 ga. steel, the flow system is a
double 2x3 maze with a low center 12" dia. inlet and a high center 12"
dia. outlet.
The inlet divides the flow into 4 parallel streams, flowing in the maize
formed by the double 2x3's.
From the roof out , the construction is as follows:
- Truss/ roof frame.....
- Roof sheathing ( plywood ).
-
ThermoPly 4x10 sheathing ( aluminum covered cardboard about 3/16" thick ).
- The flow chamber 3 inches high built up by the 2x3 maize.
- The galv. absorber plate painted with flat black Rustoleum High temp.
- A 3/4" furring to support the glazing.
- Solar Components aluminum channels to support and trim the glazing.
The right picture above is a closer view on the collector. It shows
evidence of peeling paint. The paint peeled after the first year, and
has not gotten any worse since. The paint failure was probably due
to insufficient preparation. We degreased the galvanized plate with vinegar
but it should have been done more thoroughly.
The glazing is low -iron 46x120 single glass at 9/64 inch thick. This
was the biggest change from our earlier systems. The earlier systems used
Kalwall double glazed 1
1/2" thick panels. We gave up the extra insulation for better
solar transmission and longer life of the glazing.
The collector design was from Solar Home, Inc. They were a solar
products manufacturer/distributor from about 1976 to 1990 or so. selling to homeowners
and contractors...Solar
Solar Homes probably sold a
hundred systems....my company installed maybe
15-20. My partner, Spenser Dickinson and his company, Midbay Co, probably installed the same
number. We had 3-4
architects using our systems, and maybe 5 –6 other
builders using our systems. The Federal government
withdrawal of support pretty much put an end to Solar
Homes, Inc.
The collector is entirely built on site.
The collector is a back pass collector with all airflow behind the absorber.
The aluminum channels that hold the glazing are fastened to 3/4 x 2 1/2
furring nailed thru the absorber into the double 2x3 absorber maize.
The Thermoply sheathing material kinda grew on us. We started using
it as a backer on top of the roof sheathing. The more we used it,
the more impressed I was with it’s qualities.
After 25 years of use, the wood in the collector shows no signs of failure.
Controls
The controls are a Goldline DLX 30 (the original controller)
with a digital display.
Close up of the controller.
The controls are a Goldline DLX 30 (the original controller)
with a digital display.
The digital display provides temperature readings for thermistors located in
the collector, and the top, middle, and bottom of the rock storage bin.
The 124F showing on the display is the the collector temperature for 8am this
morning.
The differential temperature at which the blower comes on is adjustable.
We use 20F degrees between the bottom of the rock storage and the collector
temp.
Rock Bin Heat Storage
Here is the 8x10 rock storage bin in the basement. There is a chamber in the
bottom raised by 8" block with a steel screen supporting the rock. The bin
is filled with 4" tailings from a sand & gravel supplier.
The hot air from the collector is blown into the top of the rock bin, and
then sucked thru the rock, heating the bin from the top down. The air
exits the bottom of the rock bin and then goes back out to the bottom of the
collector array.
mold in th rock bin was never an issue...there was never moist air
introduced....it was always solar hot air....extremely dry...too dry.. we
have to humidify the house during the heating season. we use an Emerson
Moist-air 4 gallon in the front hall
Ducting and Attic
Here's the outlet at the bottom of the rock bin
leading to the 12" dia. flex duct going to the attic
and the bottom of the array.
The hot air duct in to the top of the rock storage
bin.
Access to the small attic in the top part
of the Gambrel roof.
The hot air duct leaves the rock bin as shown in the center picture
above. Originally there was also a DBH ( Distribution blower ), controlled by a
thermostat. with a go/no go setting ( if the storage wasn't hot enough ).
If the thermostat called, it would turn on the Distribution blower and blow
thru a flex duct system to the rooms in the house.
This blower failed about 10 years ago.
We found that the heat rising thru the duct and radiating out of the rock
storage bin was pretty much just as effective as the
blower system, so we never repaired it.
The back up heat in the house is electric radiant cove heat. Each room has
it's own thermostat and cove heat, so, we can turn off / turn down unused rooms.
The System is called a "System 60" because we intended to provide about 60
% of the heating needs of the home.
If you go up the staircase into the cupola, you will see an attic
access door into each side (right picture above). This leads to a catwalk raised above
the 24" of cellulose ( it was 12" originally... but we
upgraded it about 4 years ago )
Catwalk above attic insulation.
The collector blower and enclosure are
under the celluose.
Here is the catwalk above the 24 inches of attic insulation (left picture
above). Looking down to the gable end you can see
the back of the ThermoPly sheathing .
All the walls were sheathed with ThermoPly... which I understand was ( is ?
) used extensively in housing in the mid-west.
We also used ThermoPly as the base for all of our collector systems.
In the right picture, buried under that cellulose is the CBH ( Collector Blower housing )
which contains the Original Grainger 1/2 hp
blower that we installed in 1985. It has never been serviced. It has never
failed. It has never been so much as oiled.
When we turn the system on in the fall, the blower turns on as soon as the
sun strikes the collectors and the differential ( delta-T ) gets to 20
degrees.
The distribution blower system failed, but we found we didn't need it or even
want it.
If I were doing it again, I would probably leave the rock storage bin out and
blow the hot air directly into the house, using the house mass
for storage.
Final Thoughts
All -in all , we are very happy with the system, we had some 40 degree nights
the past week, so the system was turned on about 2 weeks ago
(about October 1).
From 1986, our first winter in the house to this winter is 25 years...
still plugging away. I really like hot air systems for their longevity and
low maintenance.
Since 1985 we have done zero maintenance to the system, we have not replaced
so much as a screw, nor any of the control system, nor any of the blower system.
If we were building today, I'm not sure we'd have a solar heating system. With our super insulation (in 1986 we did double wall with FG batts &
cellulose )
and a Mooney wall, we could get by with a very small heating system and
combine it with central air. So these collectors might very well be a PV array
.
This is an interesting question that Mike
raises: On a super insulated home, is a PV array plus a mini-split
heat pump (or equivalent) competitive with a solar thermal heating
system?
I'm working on a comparison, and should have something in a week or two.
Gary
Here's a typical winter scene after a snowstorm. When the sun comes out ,
the collectors will clear themselves by about 10 am.
Mike Smith
October 10, 2011
Mike runs M. F. Smith Associates General Contracting, and can be foundhed at
M. F. Smith
Associates...
Thoughts on Mike's System
First, it says something about quality of the design and the care with which
it was built that the system has been performing for 25 years with no
maintenance. Amazing!
If you were to do a large air heating collector like this today, you would
want to consider one of the flow through absorber designs. The flow
through absorbers are likely to to be somewhat more efficient. A lot of
thought would have to be given to getting an even airflow distribution.
The use of rock bins for storage is less common today, in part because of the
fears of mold growing in the bin. On the other hand, I've not heard from
anyone who has actually had a problem, and its a nice simple form of storage
that will never flood your basement.
From Mike: Mold in the rock bin was never an issue.
There was never moist air introduced. The air flow through the rock
bin is always solar hot air, which is extremely dry. Too dry -- we have
to humidify the house during the heating season. We use an Emerson
Moist-air 4 gallon in the front hall.
Mike's final comment on the tradeoff between a solar thermal system and a PV
powered heat pump system for new construction is an interesting tradeoff to
explore. I'm attempting an evaluation of how a solar thermal system plus a
PV for house electrical loads compares to using an all PV approach with
something like a mini-split for HVAC -- I'll put a link to it here when its
done.
Once again, thanks very much to Mike for providing this rundown on the
system.
Gary
Comments
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