so this is what have here on long island, a 20x48 double poly covered hoophouse, on any given sunny day, it will get well over 100 degrees inside, even if its below freezing outside, even with a 24 inch fan, its quite difficult to keep the GH withhin 10 degrees of outside, the way around that Im told from a friend with a GH building is shadecloth, as it stands, on any given morning, by about 10:30 or 11am, the fan kicks on, and starts exhausting excess heat, (I have it set at 80 degrees) I have a coal stoker stove for heat that idles all day long, and starts kickin up after the sun goes down, Im told the max the stove puts out is 90k BTU, tonight its mild out, probably in the 50s, the GH is 65, however much coal I burn, times the BTU value is the heat demand for that given night. once the GH temp starts falling below 65 degrees, Im buring coal to keep it at 65, the longer it takes to fall below that temp, the further into the night I can go before Im paying to heat the house. on the flip side, the longer into the morning I can go before the fans kick on, the longer I can go before Im paying to cool it, the actual numbers dont really matter, fact is, I pay to heat it at night, I pay to cool it during the day, the longer into the night or day I can go before I have to do one or the other, the less I spend to do it, so with a heat-bank, I could absorb some of the surplus heat during the day, which would release its heat value as the GH cooled off in the evening at a slower rate. my plan is to build a heat storage container running the length of the greenhouse, exact numbers I dont know, but Im figuring on laying down on the floor 2 inch styro insulation, on top of which Ill frame a box maybe 16 inches high, running the length of the GH, inside that box, Ill lay 4 inch ducting, and some sort of poly pipe for water to flow through, and fill it in with sand. on top of which, I will place my benchtops, so maybe an 8ft wide box running 38 feet in the center of the greenhouse. in the morning, as the GH starts warming up, Ill use duct fans to blow the warm GH air through the pipes in the insulated sand bed, this small fan will continue doing that for as long as the GH temp is warmer than the sand temp, the exhaust fans will continue to work as they already do. the water loop will be plumbed through the coal stove to extract the stray heat created during the day, and store in the sand as well. in the evening, this heat bank wll start to radiate heat as the surrounding temp drops below the temp of the bank, although it probably wont provide anywhere near the heat required, it will provide an additional heat source, as well as a means of harvesting some of that exhausted heat from the daytime sun. I have no idea how much heat I can store that way, or how long it will take for the temp of the heatbank to drop to the ambient temp, what I do know is that it has to be of some help, so Im gonna give it a shot, if nothing else, Ill have an elevated structure to support my benchtops, heres hoping.

My Greenhouse is 10Wx12Dx10h. I currently have 36 Gallons in Covered/painted black Rubbermaid storage containers, under each of my plant benches, with 8 benches, that is just 288 Gallons of water. I have also been building a Wall of blank painted two liter bottles around the outside edges of my benches (Except on the potting bench where I need the low angle light. I also have one painted black steel 55 gallon drum in one corner I learned a few things right away and one of them by accident. The Water Storage being right under the plants is maintaining a very Constant temperature for my soil. I learned this by placing my temperature probe between the water containers and the plants to keep the probe out of direct sunlight. Anyhow, on to the Computer results. My Computer gathers the outside temperature and the greenhouse temperature every 15 seconds. I keep a running graph of the temps displayed on my computer 24 hours a day, and then store the data. The Data Cleary showed a Few different results. 1. Prior to putting the water in, the Greenhouse daytime Temps were easily breaking 110F to 120F 2. Prior to putting the water in, At Sundown, Greenhouse temperatures would quickly drop down to and match outside temperatures. 3. After putting in the water, Daytime temps in the greenhouse have not even exceeded 100F, even without any venting. But here comes the real magic 4. After putting in the water, at sundown the Greenhouse temps will drop down close to outside temps (Around 65F.... but then the magic comes. As the outside temps continue to decrease, the gap between greenhouse temps and outside temps continue to increase, 5 degrees separation at 60, 10 degrees of separation at 50, and 15-18 degrees of separation at 40 (Our lowest temps so far this year have only been 39f), but I look forward to see the results this winter, It would be nice to see 20 to 25 degrees of temperature difference when we go below freezing. All this with less than 350 Gallons of Water, that most of, was easily stored in the unused space under my benches. I am continuously adding two litre soda bottles, and 1 gallon milk and water containers from my house and my neighborÂs house, I am hoping to get enough water in that I will not have to vent heat during the day... Heck if I get enough water in there perhaps I will not have to move my plants out in the summer, if I use some shade cloth. I was thinking about playing with some of the Clear bottles and only painting the side the faces the greenhouse walls black, and leaving the inward facing side clear,(would that allow the heat to escape better at night?

One kilogram of water holds 1.16 watt-hours of heat energy per degree difference in Celsius. That tells you nothing about how fast that heat will be released though, just how much heat energy the water can store when it's at its warmest point. So for example, if the daytime and nighttime temperatures are 30 and 10 degrees Celsius, 1 kilogram of water should be able to hold 2.32 watt-hours of heat that it can release later. That assumes that the water is actually able to heat up to 30 degrees Celsius. If you have a lot of water in there, the temperature of the water may never be able to rise to the ambient temperature inside the greenhouse. The other issue is that as the inside of the greenhouse begins to cool, the water is already going to be releasing energy even when you don't need it. Suppose the temperature inside the greenhouse reaches 32 C and then is wanting to cool down to 25 C. The water is already going to be releasing heat during that time to prevent the inside of the greenhouse from going down to 26. Now suppose the temperature of the water has reached 25 and the temperature inside the greenhouse is reaching towards 17.5 C. The water is still going to be trying to keep the temperature inside the greenhouse from going down to 17.5. That is heat energy that's not going to be there much later in the night when it could be going down to 2 degrees. Depending on the rate of heat transfer, it might not flatten out daytime-nighttime temperatures as much as delay the temperature drop by two or three hours. That might mean if it has dropped down to 3 degrees C by 11pm, the warmest the water is going to be able to keep the greenhouse by 2am (the coldest part of the night) is 3 degrees.

Air exchange heat pumps are only good, ( operational) to -10c. They start to bite at +5c, but are as efficient, ( the newer models) as geothermal for cooling, and for heat, within their designed operating range. Unfortunately, they use an electric heater as back up for when the temps fall below their operating range, and due to its small size and the inherent design, it is much less efficent than even proper electric baseboard heat at those temps. Geothermal are better in colder weather than heat pumps, because rather than trying to draw heat out of cold air, are drawing heat out of cool, (56 degree) soil. Other than the medium, (air vs. earth), there is little difference between geothermal and air exchange in the operating equiptment, ( compressors, exchange units, etc). The motors as yet, are major power consuming AC units, not the ultra efficient DC motors, but due to the source of the heat, geothermal is the most efficient source of electrically powered heating. With geothermal, you either need a lot of space to run a surface grid, or pay the costs of deep drilling to sink the coils in wells. Personally, the best method of heating, IMHO, is evacuated tube solar hot water radiant heat, backed up in colder climes with a demand hot water heater like a Bosch, with the thermostat set on cool, with passive solar for the day heat- evening gains. We face west, and being off grid, use evacuated tube solar hot water radiant heat, cycled through a electric hot water heater, converted to 24dc, that burns the excess solar and wind production, circulated by a efficient solar powered DC pump on its own panel. Once I added in an automatic switch to control the pump, ( the solar panel in the winter, still produces power when the solar hot water tubes are no longer producing heat), we find that this system produces all our heat and domestic hot water from April to November. We get some passive solar assist in the afternoon. The system stops providing enough heat when the outside temps reach -15c or 5F so it is backed up with a demand propane hot water heater (a Bosch). The system still provides some heat, so from December to April, the solar tubes basically pre-heat the water for the Bosch. All told, I spent $3800 for the Sunda tubes, the solar panel and DC pump, and the pump control. The DC element for the electric hot water heater was another $400, but I had to "burn off" that excess solar power somehow, and using seemed better than dumping it to a light bar. The Bosch cost another $1500, and all told, the plumbing cost about $800. I also built a Russian mass heater, ( a huge airtight fireplace that uses a quick fire, like a rocket heater, to heat a large mass of brick and stone), which we use in the winter, to boost the temps above the thermostat setting of 65F. First however, before I built any of this, we installed new windows and doors, rock wool insulation, (R32 in the walls, R50 in the ceiling), radiant heat barrier, and resheathed the exterior over R10 foam board to provide a thermal break between the studs and the sheathing. I might actually have R42 or higher in the walls! BTW, earlier a poster suggested closing off some attic vents. Dont! The attic vents keep the air moving above the insulation and prevent moisture buildup in the attic. Close off your vents and you will trap heat in the summer, baking you out of house and home, and will trap moisture in the winter, reducing the effectiveness of your insulation, and of course, growing mold. Older homes, if anything, dont have enough ventilation in the attic. If you reinsulate an older home, first make sure that you have soffit vents and good ventilation. When you reinsulate or add insulation, make sure you keep your soffit vents free and clear. For a final trick, when all your insulation is in place, top it off with a radiant heat barrier like a mylar film over the insulation. This wont do much to keep you warm in winter, but will make a huge difference in keeping you cool in summer.