Annualized geo solar (AGS) for passive heating in cloudy, cold regions

Hello! I've caught the live-off-the-land bug like you all and I just love learning how easy all the solutions are, and yet they're not mainstream at all.
Anyways, I did a search on here and didn't find anything about annualized geo solar. Does anyone have any experience with, or opinion of this design method.
You can read about it here: http://www.greenershelter.org/index.php?pg=3
It sounds very similar to "Passive Annual Heat Storage" (PAHS) but a simpler and cheaper way of implementing it.
I'll be "getting to work" in northern Michigan where there isn't much sun during the winter months so a standard passive solar design wont cut it. My plan is to
build a fully passive, sustainable, greenhouse with a 1000gal aquaponics system inside. Since I'll be raising tilapia fish, they don't like it when it gets too chilly
as it does most of the year in Mi. Ill also have a rocket mass heater as backup.
So this AGS system sounds like a dream come true but it's hard to find much info on it besides that one site. Or if there's any other building methods you're
aware of for my particular climate I'd love to hear about them.
Thanks! Keep up the great work.

Thanks for the interesting link Levi. Check out the insulating Thermal Mass post as it covers some of this stuff. I agree that the methods in the links seem too good to be true because I feel they are. There is no science or research to back anything up just some interesting theories. Think about it this way; if one could store water that was brought to the boiling point(water has more more storage capacity than dirt) below a home or greenhouse to the depth of 20', how long would you expect it to heat the home or greenhouse without any additional heating in a cold MI winter. The answer would depend on the Airtightness and Insulation levels of the structure above it (and the insulation surrounding heated water). Even built to Passiv haus standards, roughly R50 walls R70 roof for your area, I would expect the heat to be gone in about one to four weeks depending on the Delta T. There's a much more clever way of using the ground for heating and cooling; ground source heat pumps.

Levi,
It is hard to convey all which one thinks he knows in a few short sentences but I’ll do my best to state what I believe is correct. I’ll start by saying that in an endeavor such as is being considered the devil is in the details. And there are a lot of details which will affect the implementation and success of any solar harvesting project. Fossil fuels are like beating someone up to take what you want whereas solar harvesting is like talking a farmer into selling a piece of his land.

I think the concepts behind PAHS and AGS have merit but they are not fully fleshed out with details. It’s still a work in progress. Heck, we can hardly get conventional housing made which performs as it is supposed to let alone implement all the concepts of PAHS or AGS in a workable manner when needs can vary so widely. Having said that, we all need to be working toward proving or disproving the concepts.

The AGS design calls for a metal roof as a solar collector. Over the years I have built about 8,000 ft2 of greenhouse but I’ve never had a metal roof on any of them. lol. What will be used as a collector, the roof of an adjacent building? The Mica Peak AGS does not have a solar collector for its roof so I’m assuming the solar collector is the low building in front of the home. Also, is the greenhouse glazing going to have an insulation system of some type? Besides fish, what will be growing in the greenhouse in the winter, in other words, will all of the glazing need to be open to the sunshine during the winter days? Will the greenhouse be sunken into the earth like a Walipini greenhouse?

I am also having trouble finding information on this concept. I would like to build a house with the AGS design to supplement the heating needs. I can find plenty of complicated math on thermal conductivity of different types of soil and the effects of moisture but I am having trouble figuring out how to size the thermal mass and at what depth the thermal mass should be to heated by the heat tubes. Designing for the time it takes for the heat to move through the system for an accurate six month delay is puzzeling me. I feel like the solar collector can be adjusted in size after building the house but the thermal storage under the house will be very difficult to change. Any ideas.

It seems the don stephens web site is no more. But I did find this document of his.

Just recently I was scolded for speaking for other people than just myself so - this is my interpretation of what Rob Roy says on the subject, and I will leave the

''proven science'' found in his books for others to find and interpret .

That part of the below ground structure of a Wafati within its Umbrella- is exposed to a weather temperature of a climate much further south ! Because the ground
temperature less than 6' down is 55º year round That part of my house is wintering over in Lower Virginia or North Carolina where the Averaged Winter Air Temps
are 55º !

Now, part of the structure Is Not wintering over down South! We must super insulate that section and religiously watch construction and use techniques to guard
against bridging, poor installation jobs, and carelessness in use allowing cold air entry !

Air exchanges are important to protect the insulation from performance loss due to water vapor entrapment ! Bridging again.

But, and it is a Big But, with careful structure design we can gain and store both Solar Heat Gain (trombe walls) and add much of the heat energy produced by our
personal energy uses to the structure -not forgetting to add the 100 watts of energy we add just simply by being alive !

Again these are my interpretations of other peoples words, and I am not speaking for them, merely mentioning the source, and my interpretations !

For the Good of The Craft ! big AL

Permie guru Geoff Lawton has a new video out on this very topic. Geoff Lawton - Heating Your Glasshouse This is not a complex system, but just the basic 2nd law of thermodynamics and can be done on what ever scale your situation calls for. If you are going to dig a 3 foot footing for an attached lean-to type of greenhouse (which is what I am planning) then this could be a perfect system for here in Northwestern Montana. The concept is simple. And I don't think we get very far when we over think to the point of becoming paralized. If you have doubt, create a "heat cofin" or similar small structure. It simply becomes an economy of scale as in any good design. For example, Swales work. They work if they are small, or huge. They work. The system is simple, elegant and effective. So is AGS. Don't get wigged out on the physics (as I kept telling myself all through my undergrad!) and give it a shot. I found that working on the calculus and mechanics is sometimes best when I have a shovel in my hands, and not a calculator and pencil.

Great info. everyone. Danette, any progress on your project? I'm designing an attached AGS type greenhouse for a potential client right now and may be getting too caught up on the physics myself! but any further info. would be appreciated.

We have a financial challenge and have been pretty much stopped cold on a lot of the work here. Now that winter is about to set in, I am just wanting to get my sheep and livestock guardian dogs through in good shape and go at it all again in the spring. sigh* Money always seems to be a challenge.

Hi Folks,

I'm new to this forum. Have checked it out from time to time, but mostly hang out over and Donkey32.proboards.com. Only so much time in most of my evenings with two kids wanting to wrassle…

Anyway, i am glad to have finally tracked down some discussion about AGS. Thought i'd share a bit…

I have built one large house using Don Steven's approach, and consulted on another. I then designed and helped build another using a closed bottom battery (insulated bottom and sides and partially on top to ensure the 10 ft lag effect), and more recently i have installed a smaller similarly closed battery in a 600sq.ft addition. In this latter case i have set a 12" diameter duct along one end of a 3' deep battery, and heavily insulated around it to encourage the heat to migrate across the 10 ft or top-insulated mass to reach 6ft of uninsulated mass-to-floor-slab interface.

I switched to closed (bottom and sides) battery because here in the pacific north wet sub-terrainian moving water tends to draw heat away. The first place i built (Don's style) was on a mild slope with very well-draining sandy-rocky soil. The battery has 5 sensors showing that it harvests and transfers the heat as we hoped it would… but as soon as a major deluge of rain hits (as in 3 straight weeks of heavy rain) the temps through the whole battery drop sharply. We had originally planned to install a 4' deep curtain drain distal to the waterproof insulated cape, but did not for a few silly reasons. Might still add that feature. But we are not sure if it will do the trick.

The second house build with a Don style AGS was in a similarly constructed house (light clay walls, well insulated… etc) but on high ground with mild slope dropping away all sides. This battery offers up a steady 15c to 18c throughout the long cloud-covered months of our temperate rainforest winter. Topped up with a little masonry heater/cook stove this system is working very well.

The first build with the enclosed battery (a 1600sq.ft light clay 'big' cabin) we didn't put temperature sensors in the battery, but with about 700watts of surplus microhydro adding heat to the sub floor slab, and a 6" CSA RMH burning less than 1/2 cord a year the house is toasty warm.

Lots more to share about all of these projects… but just thought i'd get a start by saying the above and: AGS, PAHS, and Bob Ramlow's Solar Sand Bed approach all take a page from the ancient indigenous knowhow. People stayed warm enough in Tipi's and Pit houses because the central hearth fires sent roughly half of their heat into the surrounding earth. Nothing like soaking up conductive heat…
Masonry heaters and RMH's demonstrate the same principles. Decoupling heat generation from delivery via predictably (enough) slow conductivity through earthen mass…

I look forward to more discussion on all this.
but for now sleep time

If anyone is interested in reading the original Greenershelter site it can be found on the way back machine https://web.archive.org/web/20110716102644/http://www.greenershelter.org/index.php?pg=1


Hey Patrick I'm glad to hear people are having success with the AGS idea, I'm in the preliminary stages of thinking about designing a house and have always wanted to integrate AGS into the design, would love to get more details on what you have learned from involvment in these builds.

Some of the images from the wayback machine:

more from the wayback machine

last bits from the wayback machine

Found a very well documented project utilizing AGS, will be interesting to see how it develops.... http://diygreenbuildingwithjerry.blogspot.ca/2014/07/timeline-annualized-geosolar.html

Thank you all for keeping this forum live! Valuable!
I've come to conclusion I will build my insulated garden shed utilising Passive Annual Heating System (PAHS).
Why insulated shed? I am in Latvia, and in the winter time we may get down to -30C (-22F). Because the water oxidiser (there's a lot of iron in the water), water filter and softener will be located in the shed (and not in the house) - I cannot allow subzero temperatures in there. Also, various things to be stored have plastics that are better stored in ambient temperature, also - things like pressure washer and freezer (to store frozen berries, mushrooms etc) deem for ambient temperatures. Due to limited space and aesthetic desires, we cannot afford earth-ship type of building. And, to preserve stored things from radiation, no windows. So, a heat source is needed. Green one, of course. Also, as this is a weekend house, I want the heating as care-free as possible. In theory, PAHS suit perfectly. Confidence grows watching Geoff Lawton visiting Community Greenhouse in Invermere, British Columbia, Canada: https://permaculturenews.org/2015/01/09/how-to-build-a-geo-solar-greenhouse/ (or, the same in Vimeo https://vimeo.com/168761278  ) .

So I am kindly looking for some ADVICE - is there anyone who would suggest on how to:
1. calculate needed soil amount to have enough for thermal mass "battery"
2. speed of heat travelling through the soil (so that heat release timing is adjusted to seasonal needs*)?
3. What type of soil shall I use**?

* - Nov-Mar need for heat source, Mar-Oct would be good for collecting the heat, I guess. Our location is 56°57′N, next to the sea; for comparison Invermere is 50°30′N, continental climate. The shed will be square shaped well insulated airtight building H3,8xW6,4xD4,8m (in feet: H12,5xW21xD15.7).
** - gravel would be the easiest for me to get, then sieved sand and then the rest.

Many thanks in advance!

Karlis

And, yes, I will share the result (pictures, how is it performing etc.) with you all.

Any more info/updates from Karlis or Pat Amos or others who have built/are building would be so great!!!

Sorry I´m completely new to your forum - but I have the original text from Don Stephens re AGS. Can I just insert this into your forum here if anyone interested?

Cheers
Marie

As long as there are no copywrite issues that would be great

David

Well, not that brilliant in english, but cannot see any restrictions. Can however see that it is not available on internet anymore for free......until I find out how to copy the paper (Look for "Tokyopaper" with the titel: ANNUALIZED GEO-SOLAR as compared to PASSIVE ANNUAL HEAT STORAGE
by Don Stephens) here is the short version of his on PAHS. And that one is still to be found on the internet and in a more readable version:

Traditional "Passive Annual Heat Storage" (PAHS), as originally promoted by John Hait, in his 1983 book of that name, is a very simple-sounding, totally passive (meaning without fans, pumps or other mechanically-aided heat movement) DIRECT GAIN solar approach, designed to provide up to 100% of heating needs.
To achieve it, one builds a high-mass, UN-INSULATED (except where exposed to above-ground air-temperature fluctuations), largely earth- covered, usually poured-concrete house shell, with SEVERAL FEET of earth over it and earth against much of the walls. This is then covered with a moisture-barriered insulation "umbrella" over that roof earth and berms and extending out 20 feet (sub-grade) all around. Finally it requires a cap of sod and/or planted earth, or other durable material, protecting the umbrella from ultra-violet degradation and other trauma. With his designs, air exchange and supplemental soil heating are often achieved with thermo-driven passive air-flow through earth tubes.)
Solar heat comes in the windows, SUMMER and winter and is (hopefully) absorbed quickly enough, by the mass of the structure and surrounding earth, to prevent "excessive" overheating of living-space air. In cold "poleward" climates, they tend to work best if there's enough summer solar penetration to raise daytime indoor temps to 75° to 80°F. (24° to 27°C.), which is, unfortunately, warmer than many people enjoy for summer.)
When that solar-heated indoor air is warmer than the surrounding earth (summer and sunny winter days), heat moves through the earth-contact portions of the building shell (by conduction) out into the surrounding soil, to be stored there. In winter and cool spells, as heat is being lost through windows and exposed walls, causing the indoor air temperature to drop below that of the surrounding previously-heated earth, that stored warmth naturally moves (slowly) back through the shell and radiates into the living spaces, maintaining winter temperatures (optimally) up to 65° to 68°F. (18° to 20°C.), again, cooler than many people prefer in winter.
Because of its density, each cubic foot of dry earth holds over 1,000 times as many BTUs of heat, per degree, as a cubic foot of air, so the heated soil provides great reserves of warmth to resist the indoor air's slow cooling.
However, since "Classic PAHS" depends on a major overhead earth mass and rapid indoor-air-to-under-umbrella-earth conduction to prevent living spaces from overheating during solar gain, this approach doesn't really lend itself to building with wood or other more-insulating materials, to foam-sandwiched concrete thin-shells, or to above-grade designs. So, for those wanting to pursue these approaches, the potential for satisfactory performance from Hait's type of PAHS is seriously compromised, at best.
________________________________________
Another kind of passive annual heating technique has been advocated, for homes in milder climates, by members of the Baggs family of architects (and described in their their book, AUSTRALIAN EARTH-COVERED BUILDINGS, copyrighted 1991.) They use a "time-lag, surface-charged" approach derived, in part, from the experience of opal miners living in their excavated tunnels beneath the south-central Australian desert. To accomplish this, they advocate placing at least six feet of dry earth on top of the building. That way, summer sun warming the roof-top earth surface has to migrate downward for six months before passing through the uninsulated concrete ceiling, to warm the living spaces below in winter. (Likewise, winter cools the earth surface and that cooling draws warmth from the house below six months late for summer comfort.)
An advantage of this approach for hot-summer regions is that it allows designs to have overhangs sufficient to completely preclude warm-season direct sun penetration. On the other hand, major drawbacks include the massive structural demands of accommodating that much earth load and the costs of addressing them. And from an environmental perspective, there are major consequences due to the amount of concrete use this implies.
________________________________________
Still a different kind of annualized solar has been explored in England and elsewhere, involving ACTIVE summer heat collection by water-filled solar panels, and the transfer of that heated water into huge, insulated, usually- underground, storage tanks, from which that warmth is recovered six months later by various passive or mechanical means.
The Earth Centre by Zedworks is a fine example of this technique. This kind of "contained" and precisely-measurable system appeals to mechanical engineers, but the initial expense of constructing the huge vault and the often-complex collection and distribution systems can be major barriers.
________________________________________
Annualized Geo-Solar (AGS ), on the other hand, is both simpler and far less expensive. It takes advantage of the extended, predicable time-lag that naturally occurs when deposited heat disperses through defined distances in dry SUB-STRUCTURE earth and the ease with which it then radiates up from a conductive floor.
Since this warmth is typically acquired from ISOLATED GAIN solar- capture sources such as sunspaces, greenhouses, thermosyphon collectors or even plenum space beneath a metal roof surface, problems of unwanted indoor overheating during summer collection periods are avoided. And because the earth-charging transfer medium is usually air, it is more easily controlled and contained. As a result, far greater choices in design and materials are possible without compromising basic system performance.
(I say isolated solar is "typically" the heat source, but one can also use a range of others, such as an outdoor, summer-fired wood-stove or pottery kiln, extraction-tubes in a "hot" compost pile or what-have-you, for input to the system. One could also divert any unwanted attic or near-ceiling build-ups of summer heat into the under-slab dispersal tubes, thus storing this excess warmth for seasons when it will be more appreciated, while reducing or avoiding the need for costly air conditioning.)
The basic elements of an AGS system consist of:
1. Any WELL-INSULATED (in the above-grade or shallow-earthed, planted portions) STRUCTURE, designed to minimize heat losses and gains, with a conductive floor that facilitates heat transfer, at least in heat-return zones .
2. Some ISOLATED HEAT SOURCE (typically air-based summer solar, although one could use another energy supply and/or transfer media.)
3. INSULATED TRANSFER DUCT/PIPE segments to carry the heated medium (air or whatever) from heat source to the dispersal zone earth beneath the house with minimum loss.
4. UN-INSULATED DEPOSIT DUCT/TUBE SEGMENTS imbedded in the dispersal zone, where heat is transferred to...
5. ...An adequate mass of DRY EARTH for storage and for time-lagged transmission, before moving up through....
6. ...the CONDUCTIVE FLOOR MATERIAL and radiating into the living spaces.
7. A CONTROL ON unwanted premature HEAT RETURN, either by the time required for it to travel through the VERTICAL DISTANCE between deposit site and the slab above, or by the HORIZONTAL DISTANCE between a deposit site directly beneath insulated areas of floor slab and the nearest un-insulated floor areas where one wants heat to conduct up through the floor. This latter approach is usually the easiest answer (no deep ditches/less diggable soil depth required); typically this means running the dispersal ducts/tubes under the insulated central portion of the floor, so the heat must travel out horizontally for 6 months (about 9'-10') before reaching perimeter uninsulated areas of the slab (the areas above which most of the heat loss through windows, doors and exposed walls also usually occurs.)
8. An AIR OUTLET OPTION - either a solar chimney (for a totally passive flow, where other factors make that feasible), an extraction fan (sometimes PV-powered), and dampered exhaust outlet, or return of the medium to the isolated heat source, for rewarming.
9. PERIMETER SUBGRADE MOISTURE-DIVERSION/INSULATION CAPE, extending from the structure's outside walls out to about 20'-24' from the deposit tubes/ducts, to prevent heat from short-cutting back outside, instead of coming up through the floor. (This often actually means just a 6' to 8' band of perimeter insulation, since most of that 20'- 24' distance is actually under the house - a major cost savings and landscape benefit not enjoyed with PAHS 20' edge extensions.)
10. SIMPLE CONTROL SYSTEMS that regulate when the flow is activated and when all exhaust convection is blocked (to prevent the unwanted venting of precious earth-stored heat.)
11. A few SENSOR POINTS to monitor performance and, eventually, determine whether it's necessary to restrict the amount of summer charging, to prevent possible winter over-heating.
All this may sound more complex than PAHS, but it's actually less expensive, more controllable and allows far more design and construction flexibility. And with its potential to meet 100% of your winter heating needs, while keeping you toasty warm in winter and cool in summer, it offers tremendous future freedom and long-term savings on energy bills!

Ohh brilliant - its here: https://web.archive.org/web/20110726131759/http://greenershelter.org/TokyoPaper.pdf

Cheers

I've done a walk through of various greenhouse/solar combinations in a few other places here.

TL;DR: If you want to keep the fish happy, don't do it in a green house.

Net annual heat requirement for a greenhouse

= 24 * Footprint * DD /2R  with the answer in BTU.

So your typical 2 ply plastic hoop house 30 x67 feet  So Footprint = 2000 square feet.


DD = degree days of heating.  This is normally calculated against a 70 F reference.  So if the temp is 69 for 1 day, that's  a degree heating day.  If it's -30 for 2 days that's 200 degree heating days.

R is the average R value of your greenhouse.  For plastic, it runs about R1 per layer of plastic.

The 2 comes from an approximation that for every square foot of floor there is 2 square feet of roof or wall.  Pit houses, and chinese style ones get away from that.

The 24 comes from the conversion of days from the degree days to BTU/hour that R values are measured in.

A square foot of double plastic green house in my 10,000 degree day heating climate takes 240,000 BTU/year

1 BTU changes 1 pound of water 1 degree F.

So if we limit our temperature chnge to 20 F it takes 12,000 pounds (240,000/20) of water to store a year's worth of heat.

At 60 lbs/cubic foot (yeah, it's bigger than that, 62.5.  Cut me some slack) that's 200 cubic feet of water.


You aren't going to store the whole year's heat.

Let's figure that even on a cloudy day we're getting close to enough sunshine, so we're really only heating it at night.

That cuts the requirement in half.  100 cubic feet.

Let's do a higher efficiency roof -- R 4, isntead of R2.  50 cubic feet.

Let's seperate the greenhouse from the water storage, and try to store water at a differential of 100 degrees instead of 20 degrees.  10 cubic feet.

Let's use this hotwter tank building as the north wall of our greenhouse, and change that 2 factor in a 1.4 factor.  6 cubic feet.

So now for each square foot of green house we want a squarefoot of hot room filled with barrels of water.

That is do-able.  Google "Nick Pine" solar heat storage.

Is it the best answer?

Meh.

Here's a better one:

Make a high effiency rocket stove type heater and use it to heat a surplus service station gasoline tank full of water.  They are typically 20,000 gallons = 160,000 pounds of water.  Raise the water temp 100 degrees you have 16,000,000 btu.

On a -30 day, you need 2400 BTU/square foot so your 2000 square foot green house needs 4,800,000  So you have to fire up the stove once every 3 days.  If the temp is only down to 20F you fire it up once a week.

How much wood?  Various charts put wood at 20 million to 30 million per cord of wood.  So you'd go through a fair bit of firewood.


A million BTU is about a gigajoule or 10 therms.  Right now I'm paying 3 bucks a gigajoule for natural gas.  So those bitter cold nights would cost me about 15 bucks a night.  Or if I heated strictly with natural gas about 1500 bucks a year.

Sure off grid is worth it?

Maybe a trout pond would be easier.

Yes, Marie, yes! I already though nobody will reply to my post. Did wait for some months, and then lost the interest in checking it out. And today, 2 months after Maria did post her comment, I pop in and - yes, yes, yes! Thank you! At least, any indication of the speed of heat traveling through sand (and that is horizontally 9'-10' in 6 months) !!!
So, you are an angel, Maria (I hope I am right about gender here)! Thank you so much! I need to build it this summer, no chances. Will get it working only for next winter, though, as the construction will be finished in, say 2-3 months - and that is when the summer will be gone already.
But - I will definitely post the result here! Mainly, of course, because of Marie!

Marie Hjort wrote:
Annualized Geo-Solar (AGS ), on the other hand, is both simpler and far less expensive. It takes advantage of the extended, predicable time-lag that naturally occurs when deposited heat disperses through defined distances in dry SUB-STRUCTURE earth and the ease with which it then radiates up from a conductive floor.
Since this warmth is typically acquired from ISOLATED GAIN solar- capture sources such as sunspaces, greenhouses, thermosyphon collectors or even plenum space beneath a metal roof surface, problems of unwanted indoor overheating during summer collection periods are avoided. And because the earth-charging transfer medium is usually air, it is more easily controlled and contained. As a result, far greater choices in design and materials are possible without compromising basic system performance.
(I say isolated solar is "typically" the heat source, but one can also use a range of others, such as an outdoor, summer-fired wood-stove or pottery kiln, extraction-tubes in a "hot" compost pile or what-have-you, for input to the system. One could also divert any unwanted attic or near-ceiling build-ups of summer heat into the under-slab dispersal tubes, thus storing this excess warmth for seasons when it will be more appreciated, while reducing or avoiding the need for costly air conditioning.)
The basic elements of an AGS system consist of:
1. Any WELL-INSULATED (in the above-grade or shallow-earthed, planted portions) STRUCTURE, designed to minimize heat losses and gains, with a conductive floor that facilitates heat transfer, at least in heat-return zones .
2. Some ISOLATED HEAT SOURCE (typically air-based summer solar, although one could use another energy supply and/or transfer media.)
3. INSULATED TRANSFER DUCT/PIPE segments to carry the heated medium (air or whatever) from heat source to the dispersal zone earth beneath the house with minimum loss.
4. UN-INSULATED DEPOSIT DUCT/TUBE SEGMENTS imbedded in the dispersal zone, where heat is transferred to...
5. ...An adequate mass of DRY EARTH for storage and for time-lagged transmission, before moving up through....
6. ...the CONDUCTIVE FLOOR MATERIAL and radiating into the living spaces.
7. A CONTROL ON unwanted premature HEAT RETURN, either by the time required for it to travel through the VERTICAL DISTANCE between deposit site and the slab above, or by the HORIZONTAL DISTANCE between a deposit site directly beneath insulated areas of floor slab and the nearest un-insulated floor areas where one wants heat to conduct up through the floor. This latter approach is usually the easiest answer (no deep ditches/less diggable soil depth required); typically this means running the dispersal ducts/tubes under the insulated central portion of the floor, so the heat must travel out horizontally for 6 months (about 9'-10') before reaching perimeter uninsulated areas of the slab (the areas above which most of the heat loss through windows, doors and exposed walls also usually occurs.)
8. An AIR OUTLET OPTION - either a solar chimney (for a totally passive flow, where other factors make that feasible), an extraction fan (sometimes PV-powered), and dampered exhaust outlet, or return of the medium to the isolated heat source, for rewarming.
9. PERIMETER SUBGRADE MOISTURE-DIVERSION/INSULATION CAPE, extending from the structure's outside walls out to about 20'-24' from the deposit tubes/ducts, to prevent heat from short-cutting back outside, instead of coming up through the floor. (This often actually means just a 6' to 8' band of perimeter insulation, since most of that 20'- 24' distance is actually under the house - a major cost savings and landscape benefit not enjoyed with PAHS 20' edge extensions.)
10. SIMPLE CONTROL SYSTEMS that regulate when the flow is activated and when all exhaust convection is blocked (to prevent the unwanted venting of precious earth-stored heat.)
11. A few SENSOR POINTS to monitor performance and, eventually, determine whether it's necessary to restrict the amount of summer charging, to prevent possible winter over-heating.
All this may sound more complex than PAHS, but it's actually less expensive, more controllable and allows far more design and construction flexibility. And with its potential to meet 100% of your winter heating needs, while keeping you toasty warm in winter and cool in summer, it offers tremendous future freedom and long-term savings on energy bills!

very interesting