Hilary thesane

I have family that sits on a cold porch in winter. They use heat lights and propane and electric heaters to stay warm on a cold porch in winter. The porch floor is cement covered with an outdoor carpet and it is screened in. IF I try to build a RMH on this porch, I encounter several problems: I can't go lower unless I bust up the concrete floor. The porch is set up with tables and a walk way that would be disrupted if I try to make a cob bench on top of it. I am concerned that any build in top of the concrete floor with heat will crack the concrete. And I don't want to melt the carpet. The porch itself is roughly 9 feet x 44 feet.

I believe that ANYTHING you do, even if folks call it 'wrong', is better than doing nothing at all. The 'right way vs. wrong way' factors of building a greenhouse are fairly straight forward and uncomplicated. However, you still must know what those factors are and understand how they effect your final build if you don't want to rely on blind luck helping you create a functioning green house for your situation and location and needs. I have 3 or 4 books that explain these factors that date from the 1970's. I haven't looked at any reference books more current than that, but I presume outside of new building materials, the basic concepts are the same. I would hope that anyone who builds a greenhouse puts in at least some time researching the particulars. Outside of what I have read in greenhouse building books, my personal experience with my home built greenhouse has taught me a few things.

First, if you can dig a foundation for your greenhouse down to the frost line for your area, then by all means do it! If you insulate the outside of a cinder block foundation down to below the frost line, you are essentially getting free heat from the ground. This reduces how much heat you have to provide. Also, if your greenhouse is not getting enough light, either in terms of candles per square foot or length of day, then you are better off operating at a cooler temperature. Warm plants in low light conditions get leggy. You are better off slowing their growth with cooler temps. Do NOT make your greenhouse so freaking tall (like I did) that you can't employ night time insulation! Covering your glass at night will save a LOT of heat loss. I cannot do this because my greenhouse has three angles for the glazing (to catch both direct and sky dome) and it extends 20 feet into the air. I tried making a hinged insulating cover but it turned into a big sail in the wind. You could probably get away with clear plastic or similar over glass IF you are able to cover it with night-time insulation. And finally, STORE THAT HEAT! My greenhouse let in about half a million BTUs of energy on a clear sunny day in the middle of winter. But it would get so hot that I had to vent. My solution was to install water tanks, a fan and a heat exchanger (truck radiator) to store all that heat. While this violates the 'passive' test, I feel it is 100% worth it. You are not supplying heat to keep the greenhouse warm. You are using a minimal amount of electric to run the blower fan and the pump to store the free heat provided by the sun. It's either store it or waste it.

If we learn by trial and error, we make errors but at least we learn.

The biggest issue I have had with my greenhouse is condensation.

It has south facing glazing and couple of west facing windows set into the wall. The north side and all non-glass areas of wall are insulated. It has water circulating through the ground, into tanks on the north wall and through a radiator hooked to a ducted air system drawing the hottest air from the peak. I have grown tomatoes through the winter relying on the thermal mass to provide heat at night and on cloudy days. But it got pretty cold at night due to the phase change of water from gas to liquid on the interior of the glass. (The phase change gives up 1000 times the energy needed to raise that amount of water 1 degree C) The west windows are from sliding glass doors and water does not condense on them. While I planned on having a removable night-time outside insulation/solar reflector, the size of the structure made that a very difficult build. Removable insulation would add a labor factor and a cost factor but would solve my condensation problem, which mostly occurs at night. I think the cost of double glazing is well worth the fighting and fussing with condensation issues.

Whatever your greenhouse shape, design and material used, the cheapest way that I can see to add raw thermal mass, is to insulate the perimeter to two times the depth of your frostline. If your frost line is 18 inches, then insulate to 36 inches. This turns the ground under your greenhouse into thermal mass.

Now, the idea is to HEAT that thermal mass, and I believe a rocket Mass stove can do that. (You may add additional thermal mass as you wish. Water as part of an enclosed system requires a very small wattage pump to circulate it. And it can be used to transfer heat from your heat source to your ground.) The problem is where to put the RMH?

The two main designs are 1) Full wall glazing. All the walls allow sunlight in. These work great if you get mostly skydome light. 2) South facing glazing. The northern wall is insulated (and maybe some or all of the east and/or western walls. These work the best for areas that have direct sun light, presumably from a low in the sky sun to the south. In the latter design, the heating system would go against the north facing wall.

The issue that I see with using a RMH in a greenhouse is that people don't live in greenhouses. There isn't going to be someone there to feed the standard RMH little sticks of wood all day long. So either you need to design some continuous fuel feeding system or come up with a way to have your fuel burn by itself for several hours at a time. MY idea is to do the latter, and to do it with a batch charcoal making stove.

The retort design for making batches of charcoal seems like the answer to me. You fill it with wood that you want to turn into charcoal, you surround that with some kindling to get the process started, and you stand back and let the thing go. The gasses that are released by the wood heated without oxygen exit the inner chamber and burn at a very high temperature when they hit air. They continue to off-gas for several hours. And when there is nothing left but carbon, the process stops. There are several videos of these stoves on youtube. I will not bother posting any in this thread. But check them out on your own time.

Now the trick is incorporating a charcoal making retort into a RMH design. This may not be straightforward. The batch method presents some logistical issues. you need to open the thing up and remove your inner drum, barrel or whatever, and allow it to cool before opening. One other issue, a charge of wood turning into charcoal puts off a LOT of heat. Seriously. You would need a fast heat exchanging system to cool all that exhaust fast enough for the gravity draw system of a RMH to work properly. AND you don't want to cook your plants in the process. Charcoal retort stoves tend to have a minimum size or else they will not off-gas long enough to be self sustaining. I think a 25-30 gallon inner drum is about as small as you should go. Smaller than that, and they will not put out enough off-gasses.