Passive Annual Heat Storage
with Slow Water Table Movement
Water has always been an issue with underground houses or even the portion of aboveground homes down at their foundation and at basement levels.
The PAHS concept came out of the Rocky Mountain Research Center in Missoula MO in the mid 80s. By that point in time PAHS was too late to benefit from the energy crisis momentum of the 70s and emerged, unfortunately, after underground houses had acquired the reputation for leaking and a lot of related expensive solutions.
PAHS creator, John Hait is a researcher with a diverse range of interests. Though his PAHS design avoided the water drainage problems other underground designs had struggled with, there was still the potential for moving groundwater to carry away the heat his design was storing. His clients during the days when he was personally promoting PAHS were primarily survivalists, one of the few groups to sustain an interest in underground houses after the energy crisis ´curiously´ faded.
Via the internet I got a chance to discuss some questions I had with John and he gave me the address of the only one of his clients willing to share their experience. Survivalists tend not to be chatty but this client was a retired water works employee and very public spirited. Speaking to Lin Bleeker of South Dakota was very helpful, particularly the fact that there had been no water problems, no plant or animal complications with the umbrella and his heating expenses for the last eleven years were under $100 total... an average of less than $9/year over 11 years of Dakota winters. Since his home is one of Pearcy's underground concrete-over-metal-web designs of fairly substantial size, his satisfaction was particularly reassuring. Except for the errant groundwater problem. In the Dakotas the problem there is the catastrophic loss of groundwater to support unsustainable choices in agricultural use as well as the usual poor water usage practices in standard plumbing design. They were near running on empty, at which point life there becomes untenable though, of course, the "leadership" needed to deal honestly with the nightmare are nowhere to be found so though the heat storage problem was amply demonstrated, the watertable movement issues were still open.
The PAHS water handling strategy is an insulation umbrella. Hait reasoned from some of the thermal performance data drawn from research and experience in the 70s that he could (and did) create a dry, thermal storage area around an underground house by layering plastic sheeting and styrofoam insulation board in a sandwich about 4-6 inches thick, at a depth of about 2 feet beneath the ground surface and extending about 20 feet beyond the perimeter of the house. To our advantage, this system is better installed by the owner for quality and savings. The materials are inexpensive but they require attention to properly layering them like a system of shingles and taking some care while earthcovering them. The key to the water control is being able to drain that water table to daylight somewhere away, and below, the heat storage area. But what if you didn´t have a building site on a rise or hill. The land we were considering is very flat.
At the depth of the insulation umbrella the plastic/foam is clear of roots, most frost, light and is non-biodegradable. The earth beneath the umbrella stabilizes at a new temperature creating a sort of heat-sink in which the house is embedded. In the summertime the heat entering the house dissipates into the sink, reaching the 20 foot perimeter by the time the season changes to cold weather and then providing warmth for the house to draw on as the winter above draws heat from the house and secondarily from the storage... creating a year-long flywheel effect. The layers of plastic, the appropriate slopes and the escapes at the edges, all divert surface water from entering the heat storage area, as well as sheltering the house from everything but errant watertables. So we've been consulting the catalogs of soil types from the soil conservation folks for the nature of the soil and its watertable at our site.
Consulting the Soil and Water Conservation Maps for the county shows that our lot is an Avonburg soil, except for the back of the lot where the map shows Rossmoyne soil. The water table rises to within 1.5 to 2 feet of the surface in the winter months so the question that needed resolution was whether the groundwater was also moving laterally since a up-and-down movement would deliver the lower heat up to the house in the winter.
The water table doesn´t appear to be a threat to the heat because Avonburg is "a very dense sponge" according to the local soil scientist, Dan LeMaster (740-772-1711 ext 110). This is glacial till and supersaturated because it was under 100´ of ice 150,000 years ago, making it very dense and, what little space between particles there is, is mostly filled with water except near the surface where evaporation over geologic time has dried it out.
The groundwater moves very slowly laterally because the sponge is so dense that the heat will pretty much stay under the home... Research has shown about 1.4" per day about 83"-90" down from the surface. That would move mid-level heat laterally 20´ between the time retrieval starts til it finishes, under a house designed with a 65´ by 80´ footprint and an umbrella with over a 100´ span. There is naturally no insulation below the floor (only a vapor barrier and gravel), but we will be making the drainage lines under the footers 2´ deep and putting a gravel-filled irrigation ´moat´ around the umbrella with a 5´ deep center, which should draw down the groundwater and slow its rise. The illustration shows that, even on the side of the umbrella where the cooler ground water is approaching the heat sink area, the ground water has the potential to work well with PAHS. The water table moves at about the same rate as the heat is drawn out of storage by the house during the retrieval phase; the water rises toward the slab carrying its heat from the edge of the now shrinking storage area while cooler water rises into edges of the receding heat storage area. As water table begins to drop, the heat-input season begins. (See illustration)
In fact, with its higher specific heat (1.0 cal/g/degC) compared to sand and clay (0.2 cal/g/degC), the presence of water in the soil should increase the heat storage capacity of the PAHS heat sink and enable the heat acquisition phase to be particularly effective.
In regard to drain placement, Chris, of the county´s conservation team, reported that a french drain in Avonburg soil draws the water down from a distance of about 15´ around the drain, beginning slowly as the water table rises above drain level and drawing more strongly as the table rises higher. Our footer drains are 25´ apart, drawing 12.5´ on each side, and the drains in the moat are only 20´ from the footer drains, so all "draws" easily overlap.
Because the direction of flow is based on the character of the bedrock, which is likely in the tens of feet below the surface, the exact the direction for this location is unknown though it is likely eastward since the "cincinnati arch" that generally shapes the local bedrock slopes downward as you move away from Cincinnati. In addition, both direction and flow rate can be affected if there were a "sand lense" beneath the location, which would eliminate the problem as the water table plummets into the lense. Our solution is to design additional solar heating capacity into the house. Another possible solution was to use the subslab gravel as a rock path that can be heated daily by an active solar convection collector with its thermostat regulated fan for flexibility of control.
Dan LeMaster, the Ohio soil scientist who was referred to me by our county soil and water conservation staff, was very helpful and had a whole raft of local anecdotal observations that would indicate that the 20´ umbrella apron is likely our minimum, the most indicative was the fact that the county´s deepest water pipes are not affected by a cold temperature front moving down through the soil during an extended cold weather period for about 2 weeks so, driven by a mega temperature difference, the temperature wave travels the distance from the surface to the 6´ deep pipes at a rate of 4 inches a day. Driven by only a few degree temperature difference instead of a mega difference, the PAHS heat storage and retrieval is anticipated to move its "front" 20 ft in 6 months which is 1.3 inches/day, indicating that a supersaturated soil is likely to have a rate of thermal change consistent with the behavior of PAHS systems where the design has been successfully implemented as reported out west.
Analyzing the colder edge approaching the heat storage
The represented water table level moves horizontally until December, at which point it rises as well, continuing horizontally until three months later.
The iso-temperature regions are represented with the house-temperature area receding at the rate premised by the PAHS theory.