I have located an old cold frame 30'w x 12'h x 96' L. I need some thoughs on this. I plan to plant in ground citrus about 8'x8' spacing under the frame and cover with greenhouse film the months of dec, jan, and feb. take it off in march, use about 20, 55 gal. drums filled with water for heat and have the covered ends of the house hung so I can open to venilate on warm days and close when a cold front is coming. I live in zone 8, our lowest temp in 2010 was 16 and 09 was 18. we usually have about two or three days of cold weather followed by a warm up. I plan to put in some cold hardy citrus but most will not be. Will this work? any suggestions or comments welcome.



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Contact Hershell. He is in the greenhouse business, both building & using.

A 55-gallon drum filled with water contains 556-pounds of water (filled completely to the very top). 20 X 55-gallon drum will have a total of 11,120 pounds of water. By definition, it takes 1-BTU of heat to raise 1-pound of water 1 degree F. Conversely, when one pound of water looses 1 degree of heat it gives off 1 BTU of heat. The 20 drums will, of course, heat up during the day and cool down during the night. In the case of your 20 drums, for every degree F, that the drums cool down during the night, they will give off 11,120 BTUs. that is not close to enough to maintain much heat in a greenhouse the size of your cold frame. Your cold frame could freeze on the cold nights that you state in your post. My greenhouse is 30 X 11 X 72 feet. I have 100 X 55-gallon drums which gives off 55,600 BTUs per every 1 degree drop in the water temperature. During a normal winter the temperature drop of the water in the drums during the night is 5-6 degrees F. giving 333,600 BTUs of additional heat to the greenhouse. This free heat certainly helps with the heating bill, but my greenhouse still requires a 250,000 BTU over head propane heater. I also have a second 200,000 BTU over head back up heater in case of emergencies. Unless you put in some commercial heating source, your cold frame will just act as a season extender. - Millet (471-)

Thanks for the replies, I will start looking for a commercial gas heater.

In addition to the heat Millet mentioned for every degree of cooling water, it gives off 80 cal/g when it freezes, (I do not know BTU units off the top of my head, but since Millet gave the value of 1 BTU per degree per pound, I do know the heat of cooling in calories is 1 cal/g/degree, so that should equal 80 BTU/# when water freezes).

I went to a seminar at Auburn Extension service a couple yrs ago where they looked at the economics of cold protection for satsumas in So. AL (zone . They experience a killing freeze about every 20 yrs. With current prices for satsumas, the spray mist system was the best choice economically--saving the trees in the case of a killing freeze, but loosing the next yr production. The High Tunnel system paid for itself if the wholesale price of satsumas went over 80 cents per pound because of the one yr of extra production every 20 yrs. Their high tunnel system did not have any extra heat --not even passive--and they only covered the trees when a freeze was expected. They expected the residual ground heat and the solar heating in the day to be sufficient. They did have the ground covered with black weed block.

I beleive you need something to reduce heat lost due to night radiatoin, even a huge blue tarp may help a lot