What is a Passive Annual Heat Storage (PAHS) System?

John Hait's book Passive Annual Heat Storage, Improving the Design of Earth Shelters provides a detailed description of a PAHS system,  illustrates how to design and build one, and includes numerous warnings about how to avoid mistakes. For a summary overview of PAHS, see Umbrella Homes.

To better understand how a PAHS system works and what it does, it is helpful to elaborate on each of the four words, Passive Annual Heat Storage. For those without a scientific background, some of the theory behind PAHS may seem somewhat complex, so I've tried to simplify the explanations. John Hait devoted several chapters to the subject, and I will use only a few paragraphs, so please bear with me.

Passive: A properly functioning PAHS system should require a minimal amount of fossil fuels for heating and cooling, such as gas, oil, or coal. Warmth can enter the living area when the sun radiates energy in through the windows, when the sun radiates energy onto the doors, windows, and brick siding installed over the exterior walls, when heat energy is conducted in through the walls, doors, and windows that are in contact with warmer outside air, when warmer air leaks in through cracks or flows in through ventilation systems, and when heat energy is conducted in from warmer thermal mass surrounding the living area. Thermal mass is the large volume of concrete, steel, water, and soil surrounding the living area, which, in turn, is surrounded by an insulated umbrella. Warmth can leave the living area when interior objects radiate energy out through the windows, when interior objects radiate energy onto the exterior walls, doors, and windows, when heat energy is conducted out through the walls, doors, and windows by contact with colder outside air, when warmer air leaks out through cracks or flows out through ventilation systems, and when heat energy is conducted into the surrounding cooler thermal mass. The large volume of thermal mass enclosed within the insulated umbrella can store huge amounts of heat energy from the living space (warm up) and can release similar amounts of heat energy to the living space (cool down) as needed to moderate the inside temperatures.

Annual: A PAHS heating and cooling system is influenced by the annual climatic conditions surrounding the house and thus never quite reaches a steady year-round operating state. However, if an adequate amount of insulation is used in the umbrella and in the exterior walls, if a sufficient number of windows are properly placed to admit sunlight when needed, and if adjustable awnings and insulated drapes or curtains are used to block sunlight and heat transfer when not needed, a comfortable mean annual temperature within the living space can be obtained, which will vary by only a few degrees Fahrenheit (°F) over the year—a little cooler in the winter months and a little warmer in the summer months. On a daily basis, insulated drapes or curtains can make a big difference. Windows have lower resistance to heat energy flow or heat conduction (R-value) than do walls and doors, and they allow various types and amounts of radiant heat energy to more easily pass through in both directions. Cold windows absorb more radiant heat energy from people than they give back, so a person will feel colder when near cold windows. Insulated drapes will substantially reduce heat transfer through the windows and increase their effective R-values, thereby making the living space feel more comfortable. Drapes can also be used to temporarily block unwanted sunlight from occupied areas for increased comfort.

Heat: Heat energy is a mysterious entity. Each atom of everything around us—solid, liquid, or gas—has a specific amount of vibrational energy. Temperature is a measure of atomic vibrational energy level; the higher the vibrational energy in a solid, liquid, or gas, the higher its temperature and the warmer it will usually feel. Heat energy always wants to flow from warmer to cooler atoms or from warmer to cooler materials in general. Heat transfer processes are very complex and consist mainly of conduction, convection, and radiation. They are illustrated in Hait's book. Conducted heat flows between adjacent atoms and materials in contact, such as the warming or cooling sensations one feels when touching warmer or cooler objects. Convected heat energy moves as heat energy is carried in moving gasses or liquids passing over surfaces or intermixing to transfer by conduction, such as when a fan blows warm or cool air past your body and you warm up or cool down. Radiated heat energy is emitted from all atoms on the surfaces of all objects and is absorbed by all atoms on the surfaces of all objects, similar to the way antennas emit and absorb electromagnetic energy. You feel the warmth of the sun, because it radiates more energy to you than you radiate back to it. And you feel cold standing next to a cold wall or window, because you radiate more energy to them than they radiate back to you. Hait also describes a fourth method of heat transfer, important to the PAHS systems, which he calls "heat transport." In heat transport, heat energy may be carried into or out of a porous solid, liquid, or gas by the flow of another liquid or gas through it. It is similar to convective heat transfer. The biggest danger to a PAHS system is uncontrolled groundwater flowing through the soil under the insulated umbrella and adding or removing heat energy when it is not wanted.

Storage: As noted above, everything around us—solids, liquids, and gasses—is composed of atoms, which constitute mass. Atoms come in different sizes and glob together to form millions of different molecule types. Atoms and molecules may be packed together very loosely to very densely,  where mass or weight per unit volume describes a material's degree of density. Generally the denser a solid, liquid, or gas, the more atoms it will have per unit volume and the more heat energy it can store and release per degree of temperature change. The amount of heat that can be stored or released per unit volume and per degree of temperature change is called specific heat. Water, concrete, iron, and soil are high specific heat materials and can store and release large amounts of heat energy per degree of temperature change. These materials are said to have high thermal mass. Air and insulation are low specific heat materials and can store and release only small amounts of energy per degree of temperature change. These materials have low thermal mass. Each solid, liquid, or gas also has a thermal conductivity rate, which is a measure of how fast heat energy is transferred from warmer atoms or molecules to adjacent cooler ones. It takes a finite amount of time for a warmer atom or molecule to transfer some of its vibrational heat energy to an adjacent cooler atom or molecule. Scientists have determined that it takes about six months for heat to travel about twenty feet through soil, and this delay is the primary mechanism behind the PAHS system, which will be described later. A PAHS-based earth sheltered house contains thousands of tons of high thermal mass material in and under the floor, in and behind the walls, and in and over the roof. This material can store huge amounts of thermal energy from the living area and give it back when needed. The thermal mass of the surrounding water, concrete, iron, and soil acts like a giant flywheel, slowly absorbing and releasing vibrational heat energy from and into the living area to keep it comfortable.

Every place on Earth has an average annual temperature, which may range from more than 80 °F near the equator to well below zero °F near the poles. The average annual temperature could also be taken as the average of the average daily, weekly, or monthly temperatures at a location over an entire year. In the Peoria, Illinois area the average annual temperature is near 51 °F, which means that the soil temperature about 20 feet below the surface will remain close to 51 °F year-round. Temperatures in the soil surrounding a typical earth-sheltered house in the Peoria area would fluctuate up and down around 51 °F, falling in the colder months and rising in the warmer months. If the temperature in the living area were allowed to follow the fluctuating soil temperature, it wouldn't be very comfortable for the occupants. That is why insulation is usually placed on the inside or outside walls, roof, and floor to isolate the interior from the soil and why auxiliary heating must be provided to moderate the inside temperatures.

Covered Insulated Umbrella Along the South Side of the Earth Sheltered House

Insulated Umbrella Along East Side of Earth Sheltered House Ready for Dirt

Part of Insulated Umbrella Covering the Earth Sheltered House

More of the Insulated Umbrella Covering the Earth Sheltered House

In an earth sheltered PAHS house the insulation is moved out from the walls, roof, and floor into the surrounding soil to trap a large volume of thermal mass between these surfaces and the insulation. Two layers of insulation are typically sandwiched between three layers of 6 mil or 0.006 inch thick plastic vinyl. The above photos show part of the insulated umbrella placed around and over the earth sheltered house. Insulation is placed above the roof about two feet underground and extends about 20 feet out beyond the walls of the structure. It is also spread out from the exposed walls, two feet or so below ground level, to about 20 feet. The insulation layer is thickest over and near the house, and gets thinner farther out. In this application the insulation thickness varies from 5 inches down to 1.5 inches. The thicker insulation over and around the structure prevents most of the heat from passing through it, where the temperature differential is the largest, and the thinner insulation farther out also prevents most of the heat from passing through it, where the temperature differential is much smaller. Soil temperatures near the outside edges of the insulation will be about the same as temperatures in the ground farther out, but the temperatures under the insulated umbrella progressively closer to the walls, roof, and floor will approach a nearly constant and more comfortable value, which is controlled by the occupants. Temperatures remain stable near and inside the structure because it takes about six months for heat to travel 20 feet through soil, so the effects of cyclic annual temperature changes in the soil at the outer edges of the insulation never appear at the shell of the house, which is 20 feet away from the uninsulated soil at the edges of the insulated umbrella.

Brick Walls Going Up Around Windows and Patio Door on South Side of House

Windows Surrounding the Front Door on East Side of House

Our House In The Hill uses several methods to adjust the average inside temperature. The primary heating method allows the sun's radiant energy to pass in through the south and east facing windows shown above and uses insulated drapes to keep the accumulated energy from escaping at night and on cold, cloudy days. Adjustable insulated drapes help to control the flow of radiated, convected, and conducted energy through the windows in either direction. In the warmer months awnings are swung out over the south-facing windows to block direct sunlight, and they are retracted in the colder months to allow the sunlight in.

The sun's radiant energy entering through the windows is the primary source of heat, but its availability is controlled by several factors. Thus the sun's energy may not always be sufficient to maintain a comfortable living space. Cloudy days may hinder heating when it is most needed. Our house has about 200 square feet of effective glazing oriented 15 degrees east of south and about 86 square feet of effective glazing oriented to the east. Effective glazing is taken as 80% of gross window glass area to account for light blockage around the edges. The house sits on a southeast-facing hillside, so it receives full sun in the morning. Hills and trees block the sun after 3:00 p.m. or so in the colder months. Orienting the south-facing wall 15 degrees east allows more sunlight in through the windows in the earlier hours before it gets blocked by trees in the afternoon.

Masonry Stove in Entryway With Plenum Above Left and Lots of Thermal Mass

Part of the Air Distribution System Above the Kitchen and Away Room

Sunroom Between House and Garage Awaiting Polycarbonate Roof

A masonry stove, shown above, is located in the entryway between the house and garage. Its several tons of thermal mass and the many more tons of thermal mass in the walls, floor, and ceiling around it, will store heat from the stove and from the sun for later use. A plenum is located in the brick wall adjacent to and above the stove, and ductwork from the plenum (see the second photo above) runs throughout the house to distribute heated air to all the rooms as needed. Also shown above is the 16 foot wide by 12 foot deep sunroom that will have a 16 foot wide by 12 foot deep sloping clear polycarbonate roof, oriented 20 degrees south of east. The sunroom and the entryway directly behind it sit between the house and garage.

View From Sunroom Through Windows Into House

View From Sunroom Into Entryway Through 12 Foot Wide Double Patio Door Opening

During cooler months, the rising sun shines directly into the sunroom and reaches back into the entryway through ample doors and windows. See the above photos. In the last photo above, plywood scaffolding rests on the horizontal beams and obscures an additional 12 foot wide by 2 foot high transom window opening above the doors. Opening the two windows between the house and sunroom and the two sliding doors between the sunroom and entryway (not yet installed in the photos above) will allow air to circulate from the house through the sunroom and on into the entryway and plenum, so that warm air can be distributed throughout the house. The only cost for this auxiliary heat will be the electricity required to run the air distribution fan.

Top View of House Showing Air Tube Layout, Except the Tubs Entering the House

The exterior walls of the house are insulated to R40 and covered with bricks on the outside. The walls are airtight, so uncontrolled heat loss or gain by air leakage is minimized. Fresh air and additional heating and cooling within the living area is controlled by a set of eight 6-inch diameter air-tubes buried in the ground. The above image shows the approximate tube layout except that the tubes running into the house are not shown. This air-circulation system will be described in more detail in another post. The R40 insulation determines the steady resistance to heat transfer through the walls when a constant temperature difference exists between the outside and inside. But the bricks' ability to absorb, store, transfer, and release heat energy in response to the sun's radiant energy, air movement, and changing outside temperatures, gives the walls an apparent higher dynamic resistance to heat flow. Thus the dynamic R value may be somewhat higher than the steady R40 value, and this can save additional heating and cooling energy throughout the year and make the house feel more comfortable. Increasing R values keeps the inside wall surface temperatures closer to room temperature, so we feel more comfortable near them.

Reducing humidity to comfortable levels in the warmer months without the aid of auxiliary energy is a challenge for a PAHS system. Primary dehumidification is accomplished using the eight air tubes configured in heat exchanger fashion. To form a heat exchanger, the tubes are divided into two groups of four each, with the upper four placed directly above and in contact with the lower four over much of their average 200 foot lengths. Air always flows in opposite directions through the two sets of tubes, and there is usually a temperature difference between them along much of their length. So if one set of tubes is bringing in warm, moist air and the other set is taking out cool, dry air, the warm air will give up some of its heat to the cool air along the way by conduction. If the temperature drop in the warm air is large enough, some water might condense out in the process, but the humidity level will be near 100% if it wasn't there already. This air entering the house would feel cold and clammy, not a good prospect at all.

As suggested in John Hait's book, we used a large cold storage area to further cool the incoming air so more moisture could be condensed out and the humidity reduced to comfortable levels. In the image above, the north arrow points up and to the right at about a 45° angle. On the north side of the attached garage, buried deep in the hillside, is a large root cellar with a separate 40 foot by 40 foot insulated umbrella embedded in the ground above it. The root cellar can be seen to the right of the garage. The various tubes terminate in two 4' by 4' pits, one located in the garage and the other is in the root cellar. Two sets of four tubes each come in from the south (the curved set of tubes outside the house) to the pit in the garage. From there, two sets of four tubes each go (north) to the other pit in the root cellar. Some 4-inch diameter tubes are buried deep around the root cellar (not shown in the image) to circulate cold winter air through the large thermal mass surrounding it. This cooling system will remove large amounts of heat energy from the thermal mass surrounding the root cellar, and the insulated umbrella will prevent heat from flowing back into the mass from above when the outside temperatures moderate.

When additional moisture is to be removed from the incoming air, it is first diverted through one set of four 35 foot long tubes that form one half of the heat exchanger connecting the two pits. From the pit located in the root cellar, the incoming moist air then circulates through four 6-inch diameter tubes (the rectangular loop from and back into the pit in the image) passing through the cold thermal mass and is further cooled, losing even more moisture. These four tubes reenter the pit and deliver the cooled air, now containing much less moisture, back to the four returning heat exchanger tubes to be reheated by the incoming warm air and delivered into the living space. The condensed moisture flows from the tubes into the pit and exits through a drain to the outside.