I've heard, granted mostly from the person that came up with the following
idea, that water and other normal thermal masses aren't very effective
in a green house environment.

With that said, has anyone here tried sunny john's subterranean heating
and cooling system for a greenhouse with orchids?
http://www.sunnyjohn.com/indexpages/...reenhouses.htm

I may be setting one up for someone that needs to propagate grapes,
and will collect environmental data to see if it's compatible with
anything I grow.

But if someone else has done it, it'll save me a lot of trouble :-).



Depends on what you mean by effective. You need to have an active
component to a solar hot water system to get real efficiency. Pumps,
heat exchangers... Passive heat transfer is slow. So you will never
be able to rely on jugs or barrels of water for your primary heating.
At least in a tropical greenhouse. It is possible to design such a
system, but it is fairly expensive (I consulted an engineer about the
possibility, and decided on waiting for a while...).

But, the upside to the slow heat transfer is just that, slow heat
transfer. If you have enough water mass in a structure, it will absorb
heat relatively quickly (slow from the air, but direct sunlight will
heat more quickly), and release heat slowly. This has pretty much the
same effect as a chemical buffer. It will take more heat to raise the
temperature of the greenouse, and more cold (technically more 'absence
of heat') to lower it.

Putting in a dark colored rock floor (slate is good) is another way to
capture heat and release it slowly. Of course in that case it helps to
have a bit of insulation between your rock and the ground, otherwise
most of the heat goes the wrong way.... If you are installing a new
greenhouse, consider spending the extra 100 dollars or so to put a layer
of high density foam insulation under your greenhouse floor. Regardless
of what your flooring material is. I wish I had done that.

Practically, the air in the greenhouse will warm up much faster than the
water in the greenhouse, which makes relying on water to keep your
greenhouse cool suicide. You need vents for sunny days. And similarly,
relying on water to keep your greenhouse tropical requires a furnace of
some sort. But, water mass provides moderation, and you will require
less heat to warm the air to the right temperature, and less 'coolth' to
get it down. And if you have enough water mass in your greenhouse, if
the furnace goes out on a cold evening your greenhouse might not freeze,
or at least not freeze as fast. It might get darn cold... but warmer
than the surrounding air. Think of it as insulation you install on the
inside of the greenhouse. It won't do the job by itself, but the more
you have the better.

Of course that is just my opinion... To me, water is cheap and easy,
and helps me sleep at night.

--
Rob's Rules: http://littlefrogfarm.com
1) There is always room for one more orchid
2) There is always room for two more orchids
2a) See rule 1
3) When one has insufficient credit to obtain more
orchids, obtain more credit