Large 1986 Solar Space Heating System Marches On

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!

large 26 year old solar air heating collector

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


solar heated house
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.
solar air heating collector for house
You can see the 10 x 36 solar array  in this view of  the SW  corner.


The Collector

roof profile for solar 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.
large solar air heating collector
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.
closeup solar air heating collector
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  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  its  qualities.

After 25 years of use, the wood in the collector shows no signs of failure.


solar heating system controller
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

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  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

Cold air outlet duct from solar heater storage
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.
hot air inlet for solar heat storage
The hot air  duct in to  the top of the rock storage  bin.
Access to collector
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 access to collector and attic
Catwalk above attic insulation.
Collector blower housing under 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.


Solar Collector after snow storm
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.



Please feel free to comment or ask questions on the system...



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Gary October 14, 2011