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Solar-Air-Heated Slab

lance_p | Posted in Green Building Techniques on

Here’s something I haven’t seen before; a solar air heated slab on grade build.

They are also using what looks like a resistance electric heater to heat the floor’s air circuit when the sun isn’t shining on the side of the house.

Has anyone seen this done before?  Quebec has very inexpensive electricity, so heating with resistance heat isn’t unheard of, but spreading it through the floor slab with plastic air ducts sure seems different.

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  1. onslow | | #1

    Mr. Peters,

    This idea is basically not entirely new, though the execution is a bit different. I considered something similar when first foraying into alternative heating designs.

    A gentleman in the northeast, maybe Vermont was promoting a solar floor air system that used common cement blocks on their sides to create a modern version of the Roman hypocaust floor heaters. I believe that Korean and Chinese versions of the same idea have existed over the millennia. I think he built three or four homes with the block system. I even own his book, but it is somewhere in deep storage so I can't give you his name.

    There was also a company selling pre-stamped metal forms to create the same sort of network of air channels. Each form looked like the vaulted intersections in a medieval church. I tossed out those references some years back. I concluded the main issue with either idea was the low energy density of warm air in northern enviroments. That combined with difficulty getting the air to move according to the magic arrows (homage to Martin). The piping in this case at least forces the air around where directed. One solar heated air system that has been embraced at a commercial level is the SolarWall. I think its main virtue is acting as a pre-heater for ventilation air in large buildings. I also considered trying to use similar thinking for heating my garage, but it was not to be.

    Cement slabs are slow to take in and distribute heat, as has been found by people experimenting with Trombe-type wall masses. I recall that someone determined that most of the heat value resided in the first two inches or so. The cycle time of input to output means that walls over 4" thick were largely pointless. I could go on to add more about true adobe and wattle/daub cottages, but not right now. I think the biggest problem facing this current iteration will be collecting enough air during the months most needed. What is the intended capacity for collecting of heated air.

    The delta rise from ambient will only serve to make the supply air drier which reduces the heat capacity. It will perhaps save him from growing a mold farm under the floor. Overall the tube layout also looks as though it will have a great deal of flow resistance, so the power devoted to pumping the air through may be significant. The effort to minimize overheating near the point of entry is laudable if minimal in appearance. The loss of temperature for the far reaches may not be too severe over the run, given the slow input capacity of the slab, so maybe I will be proven off base. A more immediate issue I see is the total thickness of the slab. I also think the mesh is way too low in the slab and there may be a lot of cracking over the piping where the slab will be the thinnest.

    As the video mentioned there are lots of DIY attempts that have been put forth like using cans with both ends cut off and strung together, as well as simply putting glass or plexi across ribbed paneling to create air channels. There was a version that cleverly utilized ones double hung windows, but stopping the air cycling at night was problematic. Getting some warm air out of the panel is relatively easy, getting the warm air to where it is needed proves to be a bit more tricky.

    I do wish the person luck. If the rest of the house is built to high insulation and low air exchange standards it may be workable. The most interesting part of the video to me was the floating slab being well isolated from ground losses. Still, it would be nice to see some numbers on the heat load planned for as well as the collector panels construction, placement and theoretical output.

  2. GBA Editor
    Martin Holladay | | #2

    Roger has given a good summary of the problems with hypocausts. (By the way, the "gentleman in the Northeast" that Roger refers to is James Kachadorian, and his 1997 book was titled The Passive Solar House Book.)

    Here is an excerpt from a long article I wrote on solar air systems and hypocausts in the December 2004 issue of Energy Design Update:

    "Although rock bins work well, they appear to be losing favor in Europe, where solar air systems are more likely to use hypocausts than rock bins for thermal storage. Hypocausts — massive floors with embedded ductwork or flues — have been used in Korea for hundreds of years, and were used in some public buildings in ancient Rome. These days most hypocausts are made of concrete. When warm, solar-heated air is blown through a network of ducts embedded in a concrete floor, the entire floor system warms up, and, over the next few hours, radiates the heat back to the room.

    "In the US, James Kachadorian popularized one version of the hypocaust in his 1997 book, The Passive Solar House Book (see EDU, December 1997). Kachadorian recommended using a depressurized under-slab plenum as part of an HVAC system’s return-air duct system. Kachadorian’s details — which require the hypocaust to be installed under a slab on grade — make some experts nervous. “The main issues I have with the ‘Kachadorian approach’ is the use of the under floor block-bed as an HVAC return,” says Bion Howard, the president of Building Environmental Science and Technology in Edgewater, Maryland. “This is bad practice in my opinion for the following reasons: A return air plenum creates negative pressures under the floor slab, which is questionable in any building since the foundation system is in earth-contact. There is a potential for soil gas — radon, moisture, termiticide, biological agents — to enter the breathing zone, since the block-bed air is then mixed with indoor conditioned air via an air-handler (furnace, heat pump, etc.).”

    "Unlike Kachadorian’s hypocausts, the European hypocausts studied by the Task 19 researchers are generally elevated above grade and pressurized rather than depressurized, rendering Howard’s concerns moot. These European hypocausts are used exclusively for heating. “In-floor heating is okay as long as you don’t need air conditioning,” says Beckman, reflecting a concern that cool slabs might provide a condensing surface in humid weather. However, some US builders have successfully used under-slab plenums in homes with air conditioning. A company called Aero Systems in Wheeling, Illinois, promotes the AirFloor system — essentially an HVAC plenum installed in a concrete floor. The AirFloor system uses hollow interlocking metal forms and air registers that are installed on a subfloor and covered with concrete. Gary Watrous (, a Louisville, Kentucky, architect, has been specifying residential AirFloor systems for ten years. “In the spring and fall, in humid weather, it is conceivable, if the floor were very cold, and the windows were left open, there could be moisture forming on the floor, but it’s never happened to me,” says Watrous. “The floor just never gets that cold — maybe 65ºF, but not much below that — generally only 5ºF or 6ºF below room temperature.”

    "US solar experts sometimes cite another disadvantage of hypocausts: they generally cost more than a rock bin of similar heat-storage capacity. “If you pour a 4-inch slab on grade, and then install a layer of light-gauge corrugated steel decking, and then pour another slab on top of that, you can create a ducted plenum,” says Joshua Plaisted, the lead design engineer at SunEarth, a solar equipment manufacturer in Fontana, California. “You end up with an 8-inch hybrid slab, and in fact that’s done to a degree in Europe. The problem is it’s capital-intensive. Do you really want to spend $2,500 on a huge elaborate hypocaust?”

    "...The International Energy Agency hopes that the findings of the Task 19 researchers will stimulate architects and engineers to reconsider the viability of solar air systems. In the US, however, where cost-effectiveness is king, such systems face an uphill battle. “I’ve never seen such systems make sense for new construction,” says Jay Burch, a senior scientist at the National Renewable Energy Laboratory in Golden, Colorado. “I’m a skeptic.”

    "...In spite of Reid’s experience, most solar experts figure that solar air systems are not currently cost-effective. “My views on this are a little bit traitorous to the solar field,” says George Löf. “I have to feel that until energy prices get higher than they are now, solar isn’t going to go very far. Cutting heat loss is the more economical and the smarter way to do it.” Hittle, like Löf, believes that shell improvements should always precede an investment in an active solar system. “The cheapest and most effective way to save utility dollars is to build a well-insulated, well-sealed house with very good windows, and with some passive solar features,” says Hittle. “...An auxiliary solar system for space heating would probably not be cost-effective in today’s marketplace.”"

  3. Aedi | | #3

    Great responses Roger and Martin. I had actually stumbled across a system like this online a couple months and was pretty dumbfounded. I simply don't understand what advantage these systems are supposed to have over hydronic solutions in the first place. Water is way better at moving heat after all, and hydronic systems are well understood. Those traditional hypocaust systems weren't exactly efficient in the first place...

    1. DanielZ | | #5
      I think the major benefits are reliability and simplicity. It will last the lifetime of the building and installation seems fool-proof. If it achieves the purpose of warming the slab, I think it's worth considering for some, depending on their priorities. I am definitely going to consider it for my new home. Given equal functional outcomes, simplicity and reliability are paramount, to me.

  4. krom | | #4

    These guys do air heated fpsf in Canada

  5. DFVellone | | #6

    I have a friend who built a passive solar home utilizing an air-duct heated monolitic salb on grade and it performed beautifully. Location is zone 6, west-central Adirondacks - not a lot of winter sun, and cold enough. The house has a central stack which draws warm air off the vaulted ceiling and circulates it through the slab to the perimeter floor vents. Woodstove in the center is the heat source. The house is around 1500 sq' and requires around 3 face cords of wood per year. Construction is a wrap and strap modified timber frame. Beautiful functioning design.

    1. DanielZ | | #7

      That's great to hear, thanks Daniel. Until something performs in the real world, you never know. I like the idea of air returns in the ceiling to get the warm air back to the slab. Interesting. I'm not planning on a wood stove, just an air source heat pump with backup electric resistance (should not be needed much), and high velocity mini ducts, so I'm not sure what the best HVAC layout/design is. I'll definitely consult some experts.

  6. Yupster | | #8

    As someone who has been a part of HVAC designs where the Legalett system was specced, I have one short thing to say: They do everything a hydronic floor warming system does with one caveat. They do it worse.

  7. Expert Member
    NICK KEENAN | | #9

    I built my first solar collector 45 years ago, it's something I've been interested in almost my entire life.

    Maybe I'm just Contrarian, but I now realize that the reason thermal solar never takes off is that the science -- the physics, the astronomy, the meteorology and the economics -- are all against you. Every few years someone comes up with a new design, but it doesn't address the underlying problems: too much heat when you don't need it, not enough when you do. The reason we need heat in the winter is that there is less sunlight.

    I look at this system and I don't see anything fundamentally different. The reason that thermal solar hasn't been successful is not the choice of water as the heat transfer medium over air.

    I've converted to build tight, insulate well, and put PV panels on the roof.

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