Design - Update On the Thermal Performance of the GeoSolar System

The fenced area in front of the house is the solar collector,
 the downstream air intake for the GeoSolar system

The analysis of the of the house temperatures during 2022, the first partial-year of occupancy, was incomplete and inconsistent as we positioned and repositioned thermometers in an attempt to capture the best data.  Consequently, this report uses data only from the two full years of occupancy --  2023 and 2024 -- that were tracked with identical protocols.   2025 will not be included until the end of the year.

The chart below is a distillation of a more detailed comparison of 2023 and 2024 but it sufficiently summarizes the data in a user-friendly way.  The "feel good" temperatures were calculated as follows:

               Room temperature - Ground temperature x  0.40  =  X

                Ground temperature  +  X   =  Feel Good Temperature

As described in a previous post on the thermal performance of the house, the room temperatures were taken from a thermometer on the wall in the middle of the house.  Ground temperatures were registered by lowering a thermometer to a depth of 5 ft through a pipe protruding from the living room floor.  0.40 is a multiplier that reflects the fact that the temperature of the thermal mass (the soil surrounding the pipe under the house) has a greater effect on the comfort level in the house than does the air temperature -- actually slightly more than twice the effect of air temperature.  As an example, the hottest recording for 2024 was taken on August 1 when the air temperature was 86 and the mass temperature was 74, rendering a tolerable feel good temperature of 76.

The striking feature of the year-to-year comparison is the similarity between them.  In the table, deviations of more than three degrees are highlighted with colored font -- blue when the temperature for 2024 is lower than in 2023 and red when the opposite occurred.  Over half of the year-to-year values deviated less than one degree; five less than two degrees.  If the figures in future years are similarly consistent, it would signal that the GeoSolar system has reached equilibrium.  

The conduits for the system are widely separated under the house and therefore widely separated when they emerged to daylight behind the house.  My thinking was that, by leaving their terminals individually accessible, some could be deactivated if the thermal mass began to overheat.  But, since the passive functioning of the conduits didn't work as intended, they had to be connected above ground and run to a temporary solar chimney so that a fan could be used to pull air through the pipes from the solar collector end.  The temporary system worked so well that it was recently replaced by a submerged system that allowed successive "Y-ing" of the pipes so that only one large pipe entered the solar chimney from each side.  And the ugliness of the exposed pipes was eliminated.  But burying the pipes precludes future changes which, based on bimonthly thermometer readings for two and a half years, seems now to be a non-issue.

The summer feel good temperatures in the upper 70s might be too extreme for some folks who are  accustomed to air conditioning.  As the chart shows, the hottest temperatures occur in the late summer and early fall and last only four to six weeks.  The high temperatures are manageable like they were in the days before air conditioning -- by having high ceilings, keeping the air moving with fans and opening the windows at night.  And it occurs at that time of year when nighttime temperatures are dropping enough to make window opening as effective as it is earlier in the summer.  

Winter feel good temperatures dipping below 70 degrees last longer, actually from the first of the year until late spring.  But hovering in the upper 60s, as they do, is a healthy range and would probably be comfortable for most folks accustomed to conventional HVAC systems.  In our view, any temporary inconvenience, in either winter or summer, is a welcomed trade-off for the savings -- thousands of dollars per year -- in electricity costs. (The details regarding the cold spring temperatures and the warm late summer temperatures, are thoroughly parsed in a previous post.)

 

The photo at right shows the temporary solar chimney and the conduits that originally came to daylight individually.

The photo below shows the permanent solar chimney with the conduits running to it underground.  Like the temporary chimney, it contains a small furnace blower that pulls air from the solar collector in front of the house through the nine conduits under the house.  Despite the PV array in the background being the smallest the vendor would sell, it provides all of the power needed by the house, including for the 240v tools in the garage-workshop.  The electrical requirement for the new chimney blower therefore becomes an inconsequential add-on.

Electric bills divide costs between power consumption and various administrative costs.  Our power consumption for the entire year of 2024 amounted to $160 -- not absolutely passive solar but close.  "At 19 cents per kilowatt-hour, the current price of electricity is the highest on record going back more than forty-five years" according to a July 31, 2025 article in the St Louis Post Dispatch, further validating the wisdom of our project.  What is especially satisfying is that, with reverse metering, the array has already paid for itself.  But the diminutive size of the array is possible only because of the passive solar design of the house.  If its nearly 3,000 sq ft were conventionally-built, it would require a solar array that was significantly larger, more expensive, require a longer pay-off period and, even then, probably never supply the amount of electricity the house would need. 

Construction - Passive Solar Heating System Completed

The solar heating system, after ten years of trial and error, has finally been completed.  It was originally designed to be entirely passive but it did not work as planned and a fan had to be added.  (Click on any photo for better viewing.)

The original design comprised an elaborate solar collector in a pit in front of the house that would harvest sun energy during the warm months and send it as heated air through nine conduits that are slanted slightly upwards and spread out under the house.  Heated air passing through the conduits would warm the thermal mass under and behind the house so that it would in turn warm the house during the cold months.  However, we soon learned that the legacy cold in the soil in which the conduits were buried pushed cold air downward through the conduits.  And the air in the collector was not hot enough or voluminous enough to reverse the flow.  The warm air would have to be pulled through the conduits from above.

Standing on end at the right, visually aligned with the bottom of the
PV panels in the background, is the short section of culvert before
 being partially buried in about the same location.

If air had to be pulled through the conduits, they could no longer emerge behind the house individually; they needed to be extended and converged at a "solar chimney" containing a fan.  Accordingly, a temporary chimney was cobbled together and pipes were run above ground to it.  A small furnace blower on a timer was installed in the chimney.  By the time the ugly temporary arrangement was replaced with the final iteration, it had worked well for two summers.

Since the 4" PVC conduits were to be entirely buried, they were shorten back to where they were horizontal and below grade then, with couplings, transitioned to 6" PVC pipes that ran underground to the new solar chimney.  On the way to the chimney, the 6" pipes were consecutively wyed together so that only one pipe entered the chimney from each side. The vertical pipe inside the chimney that received them was a piece of 12" corrugated culvert buried deep enough to receive the pipes through holes in its sides then protrude about 2 ft above grade.

The stick-built housing enclosing the culvert and holding the blower rested on top of the culvert as well as on legs supported by

concrete pads.  The opening through which the fan blew the air, sucked from the conduits, faced north and was fitted with a door that could installed during cool months to keep cold air from falling into the conduits and diluting the hard-won heat stored in the thermal mass.  Another opening, that was large enough to allow replacement of the fan was caulked shut but could be accessed in the future. 

Our PV array is the smallest the vendor would sell us but, as we estimated years ago, turns out to be about right for our needs.  So the demand placed on it by the "non-passive" solar chimney fan has not been consequential, considering that our gride sourced electricity during the entire year of 2024 amounted to only $105.

As long as the conduits were individual and accessible above grade, there remained the possibility of capping some of them during the warm months which might be advantageous if the thermal mass gets warmer year-over-year as seems tentatively to be the case.  If so, perhaps the simplest answer will be to reduce the inflow of air by capping some of the conduits leading out of the collector.

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OVERDUE SHOUTOUT

It doesn't take much scrolling through our blog to appreciate the number of men and women who have contributed to our house building project.  Shown at the right is the plaque hanging in our entry foyer that acknowledges the many volunteers and contractors who have been involved in this ten-year project.  (It can be enlarged for better viewing by clicking on it.)

And our plumber friend Bob Morgan, who came into our lives too late to be included on the plaque, took the lead for the project described here.