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AGS-Annualised Geo Solar…anyone having success with AGS

Richard Clark | Posted in Mechanicals on

WE are planning a build in zone 6, SE British Columbia. We are looking at pulling heated air from a 2 story sunspace and/ or an air space behind the metal roof, directing it down beneath the Bsmnt slab to heat the earth.
The idea looks good, but wonder if others have tried it and what kind of results are you getting.

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  1. GBA Editor
    Martin Holladay | | #1

    I disagree with you conclusion. I think both sides of your idea -- collecting hot air, and blowing hot air under you basement slab -- sound like they will cost too much to set up and will yield too little heat to justify the expense.

    Moreover, I think such a system would introduce several ways that would allow your house to lose heat in ways that are counterproductive.

  2. Richard Clark | | #2

    Thanks for your reply. I must admit that I am on a design comittee for a 'group effort' and am playing devils advocate presenting several design ideas affecting the Energy Efficiency of the proposed bldg , that I believe are questionable.
    --I think the necessary duct work (to ceiling level of 2nd floor and into attic under roof)is too long to sustain significant heat (several 90degree turns also)
    --Inline fans and their associated dampers will be tricky to adjust properly (air flow and cycling)
    --Ducts and dampers will be source of heat loss
    --I really don't know if enough heat can pumped into bsmnt. floor to make a worthwhile contribution.
    --This under the bsmnt floor pipe system (4' big O) will likely mean a comprimised insulation layer

    Are there other points I have missed?
    Really thankful for yor input.

  3. Aaron Vander Meulen | | #3

    Am I missing something or wouldn't it be easier to use the same idea but with radiant pipes instead? I can't see how doing it with ducts could possibly be effective. I still don't know if the added costs and hassle would be worth the gain, but to some degree I guess that's a bit subjective.

  4. Richard Clark | | #4

    Well the idea is that the heated air gives it's heat to the earth and the slab, the heat then rises to heat the house. It is simply collected at source (the roof panels or ceiling level in a sunspace) then drawn w/ inline fan to the sub slab ducts.
    I agree that it will likely prove an inefficient system so I will have to argue my point at our next meeting.
    When you say radiant pipes what heat source are you suggesting?

  5. Aaron Vander Meulen | | #5

    Couldn't you use the same air space that you are using to heat the air to heat liquid. I realize it takes more energy to heat the fluid, which is why i wonder how well it would work. I can't remember where I heard it, but I remember hearing in passing a similar idea running radiant pipes behind corrugated metal siding. Another question, wouldn't the most efficient way to do what you are describing be to "attract" solar radiation (dark colors and low reflectivity) which would increase cooling load? Is the trade off worth it?

  6. Eric Tollefson | | #6

    Water is a far better medium to transfer heat than air. It can absorb more heat than air due to higher heat capacity and its density means it's far easier to transport. 3/4" PEX tubing or similar would be a lot easier to work with than 4" duct. Using air would be a really silly design choice.

    A heat source for water is easy - solar thermal panels are widely available. You can even DIY if you're really cost sensitive. Many design ideas are on the web for coupling solar thermal and radiant slab.

  7. Kevin Dickson, MSME | | #7

    Air-based active solar systems were declared hopelessly inefficient by the late '80s. Some folks who apparently didn't get the memo, keep trying to revive them. Even small, simple ones and passive (thermosyphon) ones usually can't compete with basic direct gain systems, which are just south facing windows.

    AGS, as defined by Don Stephens: is pretty intriguing until you start thinking about the hundreds of linear feet of earth tubes required. Earth tubes have also fallen from favor.

    I also have never heard of a single successful AGS system for a single family residence that uses water, even though it's been tried. The Drake Landing project works well for a whole neighborhood, but uses water source heat pumps. This isn't catching on either.

    There are a few Passive Annual Heat Storage (PAHS) homes that work really well, but they are impractical caves requiring way too much concrete.

    There is a way to store summertime solar energy for wintertime use, however, and that's a grid-tied, net-metered solar photovoltaic system. The storage medium is dollars.

  8. GBA Editor
    Martin Holladay | | #8

    A good wrap-up! I agree.

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