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Community and Q&A

Higher solar heating fractions???

user-716970 | Posted in Mechanicals on

I find it hard to believe that with today’s building and insulating technologies, that most or even all of a home’s space and hot water heating can not be provided by solar… I realize that the issues of heat storage and controlling the release of this heat need to be dealt with. I am intrigued by Robert Starr’s (www.radiantsolar.com) ideas for an insulated earth bed storage system. Read the DOE’s report on this system…they seem to be rather enthusiastic. (note that it was done in the 80’s). They really seem impressed with the systems ability to harvest and store heat at much lower temps, thereby significantly improving the efficiency. In his book “Solar Water Heating”, author Bob Ramlow briefly discusses the sand bed (high mass) storage system and he seems to think that it works well, although it does require some user input to control the system well. Any thoughts ???

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Garth,
    People have experimented for years storing solar heat in sand beds or large (2,000 - 5,000 gallon) tanks. Here are the problems:

    1. Sand beds don't work -- they generally don't ever get warm enough to be useful. Nevertheless people who build them usually swear they work.

    2. Very large water tanks aren't cost effective.

    3. In most colder areas of the U.S. and Canada, there isn't much useful sun when you need it most (in November, December, and January).

    4. It's hard to cost-effectively store thermal heat for more than three or four days, although many people have spent thousands of dollars trying.

  2. user-716970 | | #2

    Martin
    1. The DOE report that was done for the radiantec people seems to be unbiased and encouraging. They did not report any problems getting the sand bed hot enough, even with Vermont's weather. In fact the problem seemed to be how to regulate the release of the heat on warmer than normal days.
    2. I agree, although a smaller tank would be required for domestic hot water...
    3. There should still be enough to heat a super insulated home. Getting the collectors to any temperature above room temperature, should allow some heat to be harvested and stored.
    4. I am not convinced as of yet...

  3. GBA Editor
    Martin Holladay | | #3

    Garth,
    The report you're referring to dates back to the early 1980s. The report was written by Robert Starr, who happens to be a neighbor of mine. I remember stopping by the house used for the study when Bob was building it; I still drive by it all the time. (By the way, the solar collectors have been removed from the home's roof by its current owners.)

    The study was conducted by Robert Starr, who also owns Radiantec and sells solar thermal equipment. It's a little bit different from a disinterested research study conducted at a university.

    Suffice it to say that the study is decades old, and it raises as many questions as it answers.

  4. Robert Riversong | | #4

    With a superinsulated house and modest passive solar design, 50% of heating load can be aquired for free from the sun without extra expense, without diurnal temperature swings, without difficulty in controlling indoor temperature, without heat stratification, without differential heating of multiple storeys, and without any active energy inputs or controls or maintenance issues.

  5. user-716970 | | #5

    Robert,
    Thanks for weighing in. I agree that superinsulated and modest passive solar should be done first. Now, what is stopping one from using solar hot water for the remaining 50%? I still am not ready to write off the high mass storage systems as unworkable....Bob Ramlow says he can make them work in Wisconsin...but I suppose this could be marketing as well...

  6. Rick | | #6

    Garth, you're on the right track. The sun is the source and storing it with mass is still the best way. Don't write anything off based on anything that has been written here. Keep investigating and learning. But location is (amost) everything. Where are you? You mentioned Bob R. and Wisconsin and how he can get solar thermal to "work" there--but it "works" better in some other places, and even worse in others. There's no wonder Martin sounds almost bitter, and why solar collectors may have disappeared from Vermont roofs. Look at this link
    http://www.wunderground.com/history/airport/KMPV/2008/10/1/MonthlyHistory.html#calendar

    and then browse through the winter months for 2008. Browse more locations in addition to archived, regional data. You can't have any solar with the sun, and Vermont looks, and has historically been, pretty bad for sun in the winter. I am designing a system for Maryland, and it doesn't look a whole lot better here. It also appears that historical insolation data might be optimistic. In recent years, winters are not as sunny as historical averages (global warming, clouding up?). But then, check out Eric Doub's place in sunny Colorado (better winter sun than Florida-not even to mention some snow boost). He has backed away some from solar storage, and jumped on the use-the-electric-grid-as-a battery bandwagon, but I doubt if he would say that the solar thermal mass storage that he has used and is still using is not worth it. Besides, although the U.S. is severely spoiled and so ready to take the currently easy, convenient approach of using the grid as a battery, it remains to be seen if that remains an attractive approach as penetration increases and utilities still need to satisfy greedy stockholders. I now believe that there was/is much to be learned from those in the 1970's (and now) who would be self-sufficient to a large degree.

    Solar hot water space heat is not cheap and easy, particularly for retrofit. As you know, you still need to reduce load, first and foremost. But most people talk about economic payback, and is it worth it, is it cost effective, but all bets are off when it comes to that, and, make no mistake, that payback analysis is nothing BUT a bet. I watched DOE websites change fairly rapidly in recent years as to what represents "cost effective" payback as related to insulation levels. Do what YOU think you can afford and is best for your future, and consider the myriad, ever changing, payback analyses as secondary. As they say with investing in stocks, you may find you can do better than the experts.

    Oh, one other thing, you should probably not expect that other 50% space heat from solar hot water that you talk about. The sun may be cooperative, but there are losses. An analysis I did for my place with Energy Plus--super insulated, tight (1 ACH for retrofit) envelope, plenty of collectors and superinsulated water storage for cloudy days--gave me 85-90% of space heat and dhw. I generally believe in that analysis but, again, it is based on historical sun data, and also I'm sure it is not perfect in other ways (although E+ is rather thorough if you are thoroughly commited to suffering its clumsiness). Whether my E+ analysis is good or not, that 100% goal is elusive. There's a lot to consider, but I say don't stop considering it.

  7. user-716970 | | #7

    Hi Rick
    Thanks for your post. I am located in southern Saskatchewan...one of the best places in Canada for winter or year round insolation. And we often get the snow benefit as well...I am planning to build from scratch so adding an insulated "sand box" below my slab would be easy to do and would not add much, or any incremental cost. I would think that in order to be able to start storing heat in the late summer, that the sand box would have to be very well insulated. Not sure the R levels required but am still looking into this...

  8. Rick | | #8

    Garth,

    Just in case you have not seen this. http://www.daycreek.com/
    There's a sand box there and some good detail, although you have to search around a little for it.
    I've been keeping an eye on that site for a few years now, and the sandbox is still running.
    I haven't searched every corner of the website, but I don't think there's a whole lot of specific performance information there.

  9. Robert Riversong | | #9

    Another, lower-tech approach to sand-bed storage is to incorporate a passive solar thermal mass and mechanically-heated radiant slab (whether solar or otherwise heated) with a shallow, frost-protected (SFP) foundation.

    If the SFP foundation is formed with a perimeter grade beam (rather than monolithic), insulated vertically and horizontally outward, and a floating slab over rigid insulation (and slab-edge insulation), vapor barrier and 14" of compacted sand (50 tons/1000sf), then the heated slab will not only radiate upwards with a reasonable response time, but also charge the subslab sand and concrete grade beam (and the subsoil beneath and around that). That is how a SFP foundation functions: as a subsoil thermal storage system.

    While the sublab thermal storage media will always remain less than indoor set-point temperature and hence not be a heat source, it will instead by a heat sink which will minimize the downward delta-T and reduce the heat load of the building.

    This is a simple, completely passive, and cost-effective way to reduce heat load to the point where a higher percentage of heat (> 50%) can be passively received from the sun. If the radiant floor is heated partly from solar thermal collectors, then an even higher solar contribution can be attained.

    A SFP foundation detail for my modified Larsen Truss wall system can be seen at:
    http://www.builditsolar.com/Projects/SolarHomes/LarsenTruss/SFP%20House%20Detail.jpg

  10. GBA Editor
    Martin Holladay | | #10

    Robert,
    You wrote, "While the subslab thermal storage media will always remain less than indoor set-point temperature and hence not be a heat source, it will instead by a heat sink which will minimize the downward delta-T and reduce the heat load of the building."

    I'm confused. Evidently the heating system (which may or may not include solar collectors) is putting heat into the soil under the slab. If "the subslab thermal storage media will always remain less than indoor set-point temperature," when will the homeowner get some of the heat back? Evidently, the homeowner won't. It's a one-way trip for the heat from the slab to the soil.

    What if, instead, you installed thick sub-slab foam, reducing the heat loss? It would require less heat to heat your house.

  11. Robert Riversong | | #11

    The advantages of a SFP foundation include: minimal site disruption and excavation, minimal use of high-embodied energy concrete (and elimination of the need for a concrete contractor), and reduction of the use of high-embodied energy foam insulation compared to a full foundation with slab. But it requires a designed steady-state downward heat loss to maintain above-freezing ground temperatures. It is obviously unsuitable for wet or highly-conductive soils, but appropriate in sites with well-drained aggregate (relatively non-conductive) soils.

    To install thick sub-slab foam would undermine the SFP foundation. That would necessitate either a full frost wall, which would not require exterior insulation but then would dramatically increase slab-edge heat loss (unless you could manage to use about double the slab-edge insulation as you put under the slab), or a full basement foundation which would dramatically increase the heated volume of the house and require additional heat inputs.

    In VT, the deep ground temperature is 42°-44°, which is significantly higher than the average winter air temperature. Once the sub-soil under a SFP foundation is warmed (say to the average of soil temperature and indoor temperature, or 55°, then the downward delta-T is reduced to 10° and subsequent heat loss downward is minimized.

    The combination of relatively non-conductive soil and good winter snow cover, along with the vertical and wing insulation required for the SFP system, creates a relatively insulated sub-soil mass which maintains a constant temperature with little heat input.

    Increasing the amount of subslab insulation in a SFP foundation would require increasing the R-value of the vertical insulation and the width of wing insulation (and I've seen no data on how to determine that). I would love to see a ground-temperature monitoring study of various combinations of insulation levels correlated with overall envelope heat loss with a SFP foundation, but I know of none.

    While my building system doesn't reach the extremes of PH standards, it creates a house which ranks in the top 5% of all VT Energy Star homes, with a design heat load of less than 1.5 Btu/DD·SF.

  12. user-716970 | | #12

    Robert
    Thanks for the post. The problem that I envision for your approach of using a mechanically or solar heated radiant slab, along with a large amount of passive solar, would be that the slab would often be fully charged at the time that the passive solar needs that mass to regulate itself, resulting in overheating... I have read many sources that say that passive solar and radiant floors don't play well together, especially in very good envelopes.

    The foundation that I envision, would be much like the one shown in your link, except that the sub slab insulation would be moved down below the sand. Any excess solar heat from the DHW system could be dumped into the sand bed probably starting in late summer. This could be done using flat panel collectors and a PV powered circulating pump and some simple controls. By harvesting energy at such low temps, the system becomes very efficient. My hope would be that if the sand bed could be raised to the indoor setpoint early enough in the fall, that it could be maintained without adding much in the way of any extra solar input. I am thinking that for this to work, the sand bed itself would have to be very well insulated. Maybe someone has some modeling software that could shed some light here??

  13. Robert Riversong | | #13

    Martin,

    Another way to answer your question (which seemed to ignore the fact that I stated explicitly that the subsoil thermal storage was NOT a heat supply), is that by reducing the envelope heat loss, the available insolation becomes a higher solar heating fraction (which was the topic of this thread).

    And any soil-based heat storage system, however well insulated, is also losing heat to the surrounding soil. One way to decrease the rate of heat loss is to add petrochemical foam insulation with its high embodied energy and high global warming contribution. Another way is to decrease the delta-T. The somewhat discredited Envelope House did this through increased cost and complexity, and so does a SFP foundation but at decreased cost and complexity.

  14. GBA Editor
    Martin Holladay | | #14

    Robert,
    You claim that you make better use of the solar thermal system's heat "by reducing the envelope heat loss." However, I contend that you have expanded the thermal envelope to include a lot of soil, thereby increasing the thermal envelope's heat loss. If instead you established the thermal envelope directly under the slab, you will actually do more to "reduce the envelope heat loss." You're right, however, that such insulation must be correctly detailed by including adequate perimeter insulation, which I would always advocate.

  15. Robert Riversong | | #15

    Garth,

    While it's true that a fully-charged radiant slab would be less available for passive solar thermal storage, it would be somewhat self-regulating since the warmer the floor the less solar absorption. If the floor has some reflectivity, then insolation can be absorbed by the cooler walls and ceilings and furniture.

    But I agree that combining a radiant slab with a passive solar mass strategy somewhat reduces its effectiveness. However, in a low energy-demand house, the lower required slab surface temperature makes the floor available to solar absorption without overheating.

    I think a sub-slab thermal storage system has some promise, but requires careful engineering, a more active heating system than I like to use, could be far more difficult to regulate than a 4" mass floor, could result in a cold floor at various times in the heating season, and has the potential to be a water reservoir of not perfectly sealed.

  16. Robert Riversong | | #16

    Martin,

    To the contrary. A sub-slab massive heat storage reservoir would be a signficant expansion of the thermal envelope, not so much in volume as in heat capacity. Since I place R-10 foam under my slab, in addition to the R-10 slab-edge and foundation perimeter insulation, my thermal enclosure ends at the slab base. You might consider the subslab soil to be "semi-conditioned" space.

    But the steady-state heat loss to the soil, unlike to the air around the house, diminishes as the heating season progresses, the ground temperature rises, and the snow cover increases.

  17. user-716970 | | #17

    Robert
    I have a hard time imagining why such a large mass would be more difficult to regulate than a slab. Once it gets up to room temperature, ( if that is even possible) it would be very hard to move it from there, and it should be fairly easy to maintain it's temp with minimal input (assuming very good insulation). If passive solar input is too large, this could result in some control problems.

    Would rigid fiberglass insulation (Roxul Drainboard ?) be a possible candidate for sub sand box use?

  18. user-716970 | | #18

    Here is a link to an interesting report done by John Straube and Chris Shumacher http://www.civil.uwaterloo.ca/beg/Downloads/Insulation_Study.pdf

  19. Robert Riversong | | #19

    Garth,

    A large themal mass is always more difficult to regulate (maintain thermal equilibrium) than a smaller mass. In large commercial masonry buildings this is known as thermal inertia. Any deviation from thermal homeostatis requries a much longer time to correct and the inertial (or lag) effect requires carefully-callibrated differential controls.

    Additionally, as I've noted, if there is not enough heat available to bring a high-mass floor to greater than room temperature, then it will have the opposite comfort effect of a radiant floor. Wintertime human comfort requires warm feet and cool heads, with the mean radiant temperature having at least as much impact on comfort as air temperature.

    And what's your point in linking to the Straube/Shumacher study? It supports the advantage of thermal insulation immediately under a radiant slab.

  20. user-716970 | | #20

    Robert
    I appreciate your patience in answering all my questions and off the cuff speculations. I am definitely out of my league here, (I had to look up homeostatis :)) The reason I added the link to the Straube report was because I thought that it graphically showed how heat moves under a basement or slab....

    I think that I finally get that too much mass can be a rather large problem to deal with. I do think that it has been an informative discussion (at least from my viewpoint). Thanks again.

  21. Robert Riversong | | #21

    I should add, however, that much of what is perceived as a problem is based on the modern demand for absolute comfort and convenience. For those who are willing to be actively involved in the energy management of their homes and are content with floor and indoor temperatures that vary with the ambient conditions (as humans always were up until very recent times), long-term thermal storage systems are certainly worth experimenting with.

    Though it's complex, I appreciate what DayCreek as been able to accomplish with their solar thermal system by valving the hot water flow to any combination of DHW tank, slab, and sub-slab sand bed storage. http://www.daycreek.com/dc/html/journal050509.html

  22. user-716970 | | #22

    It will be interesting to see how the Isabella Eco Home's mass storage and heating system works for them...they really went all out on this one. 5700 cubic feet of sand and taconite surrounded by 16 inches of EPS foam on all six sides....potential storage of over 13,000,000 BTU's!!! http://isabellaecohome.blogspot.com/

  23. Robert Riversong | | #23

    It will be interesting, particulary given that the website description of the operation of the system is full of inconsistencies and assumptions (not the least of them being claimed -60 temperatures in a region with a coldest recorded temperature of -45 (I used to live - and camp - north of there in Ely).

    But this PH/Net Zero home is typical of the extreme, price-is-no-object approach to "green" that is becoming all-too-common in the US. An enormous PV system, imported windows, a state-of-the-art living roof, 18" walls and 24" roof, and a very complex thermal collection, storage and distribution system that has almost 300 million Btus of embodied energy in the XPS alone.

  24. Rick | | #24

    Garth,

    Your original post was taken off into tangent world, but I guess that's part of learning. You were asking about solar thermal storage, specifically using sand. Then came the following, in brackets:

    [Another, lower-tech approach to sand-bed storage is to incorporate a passive solar thermal mass and mechanically-heated radiant slab (whether solar or otherwise heated) with a shallow, frost-protected (SFP) foundation.]

    That certainly is an alternate approach, but has relatively little to do with heat storage, and will not maximize your solar fraction in most areas, which goal you were clearly pursuing. The stated SFP approach is more like dabbling in Passive Annual Heat Storage (which see), but PAHS usually takes things much farther (such as insulated, mostly underground, living, in order to achieve high solar fractions. PAHS encapsulates with insulation a much larger earth mass than SFP. The SFP appoach simply will not provide serious, on demand BTU to get you through a run of cloudy days (days, not weeks, as is needed in some areas). The SFP approach is marketable by contractors. The "average" house buyer is not ready for mass heat storage, but you clearly are. So you encapsulate by insulation a large mass and store heat with it. Sand is cheap and relatively easy, starting from scratch. I prefer water storage, not only because I am retrofitting, but because it has a real nice heat capacity, is more responsive, and can be readily used for both dhw and space heat.

    If you don't want water storage, you should get back to your sand box and taconite and just use XPS insulation (closed cell, less water absorbing than EPS). How much insulation? You can get as many diverse answers to that as there are noses. Your HDD value is not harsh, as I remember, something like here in MD, 4500 or so. So the "right" answer is probably somewhere between 2 and 6 inches, unless you are looking for some sort of formal certification, such as Pasivhaus, or unless you want to calculate payback to the dime, or unless you are a contractor for someone else (dimes get involved, again). Insulation levels are not the only variable. Insulation is nothing more than a time buffer for heat loss, to include with considering other rates of heat loss and to balance with rates of heat gain. To perform that balancing act, you might try the Pasivhaus software (I have not used it), but you need not necessarily take things to Pasivhaus performance. The free Energy Plus software will do it too, but it's not for the casual user.

    How much sand? The Ramlow book gives the basics for that, and you will need to do some BTU accounting (heat in and heat out) based on your heating load and how often you can expect the sun to show up in the winter. The sand box is a closer approach to PAHS than the SPF foundation, but without the underground living of PAHS.

  25. Robert Riversong | | #25

    Rick,

    You and Martin both responded to an argument which was never made: that a solar thermal slab with SFP foundation serves as a heat source.

    But my comment was hardly a tangent, since the purpose (and title) of the original question was "Higher Solar Heating Fractions???". I simply pointed out that another, simpler and lower-cost approach to increase the Solar Contribution is to use passive solar strategies and reduce the envelope heat loss. Once clever way to reduce envelope heat loss is to temper the sub-slab soil. Thus what starts as a necessity to maintain a frost-free environment for a shallow foundation also serves to decrease envelope losses without additional insulation. And the no-cost reduction in envelope heat loss increases the relative percentage of solar contribution to the heat load, without resorting to difficult to engineer and costly active solar systems.

    Now if your goal is to approach 100% solar heat, then (like the PH strategy) you'll have no choice but to use extreme measures. But if it's a more reasonable 50+% solar contribution, then that's easy to achieve with low-tech passive approaches.

  26. Robert Riversong | | #26

    Correction to above: A solar thermal floor IS a heat source (or heat storage medium that can return heat to the indoor environment). The contention that the warmed sub-slab soil contributes heat to the living space, is a straw man. That argument was never presented.

  27. Robert Swinburne | | #27

    simple is good
    good discussion. I would pint out that there are a lot of people here in VT living with just wood heat, good insulation and windows and a bit of passive solar. (banks and insurance companies don't like this) As an architect, I rather like a system with water storage such as a Tarm storage tank and solar hot water and/or solar hot water with either gas or electric boost right on the tank. I have also seen several systems with a Tarm wood boiler and solar hot water with a large tank. The tank is reasonably priced but the boilers are very expensive but will last as long as several conventional boilers.

  28. GBA Editor
    Martin Holladay | | #28

    Robert,
    For 29 years I've been heating my house with a used woodstove (welded steel, homemade) that I bought for $100. It's got a stainless-steel coil in it to make hot water -- the coil cost more than the stove.

    I think when the stove finally gets too warped to use -- maybe in another 29 years -- I might buy another used stove. Hard to justify the price of a Tarm boiler.

  29. Robert Riversong | | #29

    Martin,

    Who would have guessed that you're such a Luddite!

  30. GBA Editor
    Martin Holladay | | #30

    Robert,
    Simple is good.

  31. Rick | | #31

    Robert,

    You write alot here, so it's relatively important that you write right, but you have not.
    You said: [You and Martin both responded to an argument which was never made: that a solar thermal slab with SFP foundation serves as a heat source.]

    Martin seemed to imply that he was thinking that is what you were saying, at first, but he in fact concluded that the homeowner wasn't getting any of your SPF heat back--I knew it 10 years ago. What I said was that the SPF approach was "more like dabbling in PAHS." However, I should have said it is dabbling so very poorly in PAHS that you don't get any heat storage at all.

    But since you are not getting any heat back, the SPF approach is just insulation, Robert, just like other insulation. The SPF slab is just a wall facing the ground. Insulation slows down the conduction of heat, but it is still escaping through the ground, around your SPF, and to the air. You said [Once clever way to reduce envelope heat loss is to temper the sub-slab soil. Thus what starts as a necessity to maintain a frost-free environment for a shallow foundation also serves to decrease envelope losses without additional insulation.] It is true that the SPF approach heats up the ground enough so that you don't heave the slab, but the homeowner is paying for that heat, albeit the load is reduced by the slab insulation not unlike as with any other slab insulation. You are not decreasing any envelope losses in any way more clever than if it were wall or roof insulation. You may as well go around to potential customers and tell them you intend to build their house by reducing envelope heat loss by tempering their inner wall and sub-roof continuum, at the same time you are tempering their soil.

    Your statement, "steady-state heat loss to the soil, unlike to the air around the house, diminishes as the heating season progresses, the ground temperature rises" pretends that some envelope heating advantage is being gained. Heat conduction is relatively slow in the earth, but the heat just keeps leaking out around the SPF all winter until the air begins to warm again in the Spring. It doesn't diminish as the heating season progresses. It keeps conducting in response to the delta T between the house and the air outside, and only begins to diminish and reverse after things start warming up toward summer. The ground below the house may heat up some to a point as the season progresses, but the heat is still headed out. The deep ground temperature you speak of is a direct result of the outside air temperature, but because of the time needed for conduction to that depth (10-12 feet or more), and the time lag associated with the yearly reversal of conduction, the temperature averages between winter and summer temperatures. I doubt if the deep ground temperature is higher than the yearly average air temperature, as you said, but it has very little to do with SPF, anyway.

  32. Robert Riversong | | #32

    Rick,

    You stated (#24), "The SFP approach simply will not provide serious, on demand BTU to get you through a run of cloudy days." No one claimed it would, so you were attacking a straw man.

    And it's not true that "The SPF slab is just a wall facing the ground." The R-10 insulated slab I described, poured over 50 tons of dry sand inside 15 tons of R-10 insulated concrete grade beam with R-10 wing insulation extending outward 12" is a completely different thing than a monolithic slab-on-grade, which IS "a wall facing the ground".

    This SPF system is, in fact, a wall facing a very large mass of tempered earth. The subslab sand fill will approach room temperature over the course of the heating season (measured soil isotherms have demonstrated this), by the heat contribution of both the mechanically-heated radiant slab and the solar contribution to slab thermal storage. The well-drained gravel subsoil on which this system is built (6' deep native gravel) is a poor conductor and, once the winter snow cover is in place, even the soil surrounding the insulated mass is moderately insulated (snow ≈ R-1/in).

    Assuming that the subslab sand and insulated concrete perimeter are raised just 10° over initial temperature, then there will be a 230,000 BTUs stored in the media (it's more likely double that). Since the storage temperature will never exceed indoor setpoint temperature, those BTUs can not become a heat source, but the enormous thermal mass reduces subsequent heat loss from the slab because the delta-T is reduced as the storage media warms.

    Wall or roof insulation does nothing to decrease the delta-T across the thermal boundary, so is reliant solely on the decreased thermal conductivity. Though you sarcastically say this is similar to "reducing envelope heat loss by tempering their inner wall and sub-roof continuum", that is exactly how the Envelope House works - it has a solar-heated air plenum between the inner and outer walls and roofs which reduces envelope heat loss. That tempered volume is also heated, like my SPF sub-slab media, by both internal heat loss and insolation. The primary difference is, in the Envelope House, there is not enough thermal mass in the envelope to significantly slow the envelope heat loss, whereas in the SPF slab system the gargantuan thermal mass offers a very large thermal inertia.

    And, back to the point of this thread, any reduction in envelope heat loss increases the percentage of solar contribution without increasing the solar glazing area or requiring expensive and technically-sophisticated active storage systems.

  33. user-716970 | | #33

    Robert
    If you are using a mechanically heated radiant floor to contribute to the 230,000 plus BTUs of heat stored in the media, and if those BTUs cannot become a heat source, that energy has essentially become lost, has it not?

  34. Rick | | #34

    Robert,
    I wasn't attacking a straw man (although I might wash him in clay water and mash him into a wall). I was just trying to help a Sproule man with something he actually asked about, which was giving thoughts on whether or not "most or even all of a home's space and hot water heating can...be provided by solar." I don't think that telling him that the SPF will not provide what he is asking about represents an attack at all. While the subject of his investigation may not have been initially clear to you, it became even more clear after the #4 (a good, but already somewhat divergent response) and #5 entries, and also clear that he already knew quite a bit about passive techniques and the limits of their contribution. I have gotten a lot of good information from this site, including from you.

  35. Robert Riversong | | #35

    Garth,

    All high-exergy energy used in a home (or any building) becomes "lost" to entropy. Low-exergy energy, such as solar radiation, is merely returned to the environment.

    Any building which is in any way earth-coupled (having a foundation other than piers) "loses" energy to the ground. The ground isotherms under a concrete basement look very similar, though more dispersed, to those under an SPF slab.

    My point is that it's advantageous to direct building heat loss to an earth reservoir which can temper subsequent heat loss by reducing the delta-T, than to the air where it's truly and completely lost to the structure. And, because this is a low-cost, passive approach to reducing building heat loss, it can increase the SSF at no expense.

    There are two methods to reduce heat loss to the environment: insulation and thermal mass. The soil under a SFP slab is, in essence, "free" thermal mass.

    Interestingly, the shallow, frost-protected foundation system was developed in cold northern Europe and then migrated to Canada and Alaska. In 50 years, more than a million such structures have been built in Norway, Sweden and Finland, with at least 5,000 now in the US. Frank Lloyd Wright used them in the 30s and 40s. It was developed as a way to reduce both foundation costs and energy consumption.

  36. GBA Editor
    Martin Holladay | | #36

    [I am posting the following comment from Robert Starr at his request:]

    I will chime in on Garth’s original posting about high solar heating fractions. I am Robert Starr, President of the Radiantec Company and co author of the DOE Report that Garth cites. It is indeed possible to achieve a high solar heating fraction in Northern Vermont we can prove it.

    The DOE study is not biased as my friend and former eco warrior, Martin Holliday claims. I wrote the report, but many others who were involved in the research were not obligated to agree.
    The Department of Mechanical Engineering at UMASS did not have to agree, but they did.
    The DOE did not have to agree, but they did and they published it.

    The American Solar Energy Society (ASES) did not have to accept the paper after peer review, but they did.

    Rodale did not have to write an article and put it on the cover of NEW SHELTER (March 1983), but they did.

    Most importantly, this report can speak for itself. The science, the mathematics and the language are all quite simple. All you have to do is read it.
    http://www.radiantsolar.com/pdf/DOEREPORT.pdf
    http://www.radiantsolar.com/pdf/rodales.pdf
    http://www.radiantsolar.com/pdf/ases_paper.pdf

    With all due respect, some of the “advice” coming out of GBA on the subject of the potential of solar energy is flawed.

    This is a matter of some importance because if a very simple solar heating system can achieve a high solar heating fraction in Northeastern Vermont, it will work anywhere in the country and GBA should not be trashing it.

    I would invite anyone interested in solar thermal to read the report and make their own decision. It is also possible to make a site visit at several facilities that use this design.

    Robert Starr
    Radiantec Company

  37. Garth Sproule | | #37

    Martin
    Thanks for posting Starr's comments....curious as to why he did not just post them himself. Sounds like maybe you two have a "history"...

  38. GBA Editor
    Martin Holladay | | #38

    Robert lives in nearby Lyndonville. He has a business selling radiant floor heating supplies and solar thermal equipment. He sold me two thermal collectors several years ago.

    Robert was encountering a technical problem posting his message, and I am happy to help him out.

  39. Garth Sproule | | #39

    Robert
    A question for you. Would it be an advantage to include a solar powered circulating pump to the radiant floor heating system? My thinking here is that if this pump activates when the sun shines, that it would significantly improve how the passive solar is stored by distributing the stored heat throughout the whole slab. This assumes that the slab is all on one zone.

    Your thoughts??

  40. Robert Starr | | #40

    Thank you for challenging the assumption that solar does not work well. Solar can work very well when it is done right. I am not sure that I understand your question, but, if the radiant heating system is not a solar powered system, then a solar powered pump would override the thermostat and circulate heat from an area that might be overheated from passive and send the excess heat to areas of the building that are not well heated and result in more even heat and greater usefullness of the passive. Good luck !!

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