[quote]R2 as we know it in America is quite low for house insulation. I used that number to allow for our test greenhouse to have 6mm twinwall on the south and south roof and some insulation on the North walls etc.[/quote] Heh, perhaps I'm just weird then, but I have metric R2 on my north wall (+parts of east and west, +underground) and about metric R 1.4 for the glazing. Metric R values are about 0.17 times US R values. [quote]I am sorry but many here with small greenhouses are infact coming as close to an hermetically sealed structure as possible. 10-20 tubes of calking... on a 6x8ft? [/quote] Well, it depends on what the caulk is being used for. I caulked my insulation on. Let me tell you, that takes a lot of caulk. Even if you seal up all of the cracks, I would be surprised if the small amount of circulation couldn't occur through things like: * The drainage system * Between panes of closed vents * Through the zipper or doorframe. Sure, a vent is a good idea. But we're talking about needing a very small amount of air indeed. [quote]Also in this thread nobody has mentioned the water. Unvented heater, sealed up as best you can, no air exchange. Adding CO2 by burning propane makes it wet in there too.[/quote] That is true. The moisture beads on the glazing and runs down to the floor; it's like a jungle in there (when I wear glasses, it's instant fogging when I walk in). I haven't grown any plants that are sensitive to moisture-related diseases before, but I've got some in the seedling stage right now, so we'll see if they complain. They're doing quite beautifully so far (I could get pictures if you like), but moisture-loving diseases like powdery mildew take some time to get established. I'll get back to you about them in a month or so and let you know how they compare to the equivalent plants grown outside this summer. [quote]stressbaby, Gladly. Greenhouse cucumber operation in Calif. with newly installed unvented heaters. After a cold winter period, severe damage was noted on the crop. Air samples analyzed showed elevated levels of ethylene and mercaptan.[/quote] Elevated *mercaptan* levels? Wow, that heater must have been doing an awful job. Mercaptans are only in PPM quantities to begin with, so if you're having measurable amounts *after* combustion, something is seriously wrong there. [quote]economic losses to greenhouse growers from ethylene pollution are reported each year[/quote] You do realize, I presume, that ethylene pollution can happen even when no heater is used. Decaying plant matter is an especially bad ethylene producer, but even healthy plants produce it (some species more than others). [quote]For an interesting thread from about a year ago, complete with more examples of problems with unvented heaters, use the search function, type "carbon dioxide" and read the posts on "plants suffering from propane". Some noted problems in other "real-life" situations.[/quote] Well, for your information, with Thanksgiving dinner, my main course was a delicious vegetarian lasagne made with a heaping handful of basil from my "suffering" greenhouse basil plants. Last night for dinner, I had pasta primavera made using some of my "suffering" tomatoes. My point? A proper installation doesn't have problems. An improper installation does. Notice how you haven't gotten into the risks of the other common hobby greenhouse heating system, electric. You know, a box of resistors with large amounts of current flowing through it, in a moist environment that usually has a metal frame, and plenty of combustables within, possibly even standing water? I read the "suffering from propane" thread before I got my heater. I read enough from elsewhere to realize that this person had an improper installation. Notice -- it only took a few days of use of their heater for problems to occur. Mine has been running for two months this winter. Note what was mentioned in the thread: "Sometimes the smell is very strong." Huge red flag right there. Properly combusted propane/natural gas has no smell. The instant they smelled it, they should have disconnected the heater -- it sounds like a leak. Note what others said in the comments trail, such as Orchiddude and trigger_m (who also provided pictures). [quote]One factor missed in this thread was ethylene.[/quote] Ethylene was already mentioned several times in this thread. Use your browser's search function; you'll find it.

jostus, it sounds so logical doesn't it? Roots take in nutrients as ions. When salts like ammonium sulfate, potassium chloride etc. dissolve in the soil water, they break up into ions. The mineral ions in the soil water displace (hydrogen) ions on the root hairs of the plant and these are assimilated into the Xylem via the root tissues. Sucrose (cane sugar) does not disassociate into ions. Molecules of the solid sugar bond with molecules of water and cannot be used directly by the plant. It is similar to the idea that the application of lime to citrus trees will make the fruit less acid (sour). After all, doesn't lime reduce acidity? Disclaimer: It has been many years since I had to recall this sort of thing. I am acutely aware that my knowledge may be out of date. I welcome enlightenment.

The roots need oxygen, and you can easily aerate the water a couple ways. That will raise the dissolved oxygen levels in the water. But as far as Co2, that's another story. I know there are products that manufactures claim by adding to the water will increase the Co2 absorbed by the plant. But as far as I have seen there is nothing by any real research to substantiate that claim. Though I do believe the roots can absorb some Co2, but not in any significant amounts. it's not the job of the roots to uptake Co2. And even if you add Co2 to the water, 99% or more of the Co2 will be absorbed through the plants leaves. If you are growing in a well ventilated greenhouse, you wont need to worry about the plants getting enough Co2. The plants willstill deplete the Co2 from the air. But by ventilating the greenhouse with fresh air, you will be continually replacing the Co2 in the air the plants have useed. Thus, you wont actually be depleting the co2 at all, as long as it's well ventilated. Also, I just thought I would mention that just by increasing Co2 levels, that wont necessarily make the plants absorb it. The plants Co2 consumption is also related to environmental conditions like light levels, humidity, temperature etc..

While a number of interesting efforts are ongoing in the carbon capture and storage domain, there are some really novel concepts in the works as well. One such I came across is the use of biomimetics in CCS. The overall objective of the research is to develop a system resembling a CO2 scrubber that can be used to reduce CO2 emissions from, for example, fossil-fuel-burning power plants. In the system envisaged, an enzyme serves to catalyze the rate of CO2 hydration for subsequent fixation into stable mineral carbonates, the counterions for which may be supplied from such sources as brines from saline aquifers, waste brines from desalination operations, or seawater. The enzyme, carbonic anhydrase, is the biological catalyst responsible for the interconversion of carbon dioxide and bicarbonate in living organisms. The resulting biomimetic sequestration system would offer several advantages, according to the paper. What do you think? Do you feel that such distinct approaches are pretty much in the lab/research stages, or do you see these technologies coming into the market soon? Here is a link that might be useful: Use of Biomimetics in co2 capture