As I have mentioned before, I use 10 black plastic 55-gallon drums lined in a row as greenhouse benches. The drums are filled with water which totals 551-lbs. of water per each drum. The water heats up during the day, and cools at night. It takes one BTU of heat to raise 1-pound of water one degree, or reversibly, when 1-pound of water loses one degree of heat it gives off 1-BTU of heat to the greenhouse. The 10 barrels used to make a greenhouse bench, contain 5,510-lbs. of water. Lets say when night comes the water in the barrels is at 66 degrees, over night the water has cooled down to 57 degrees. That means the watered cooled by 9-degrees. Therefore, the total weight of water in the 10 barrels (5,510-lbs.) gave of 59,590 BTU's of FREE heat into the greenhouse. I have one hundred 55-gal. drums in my greenhouse. Six benches, plus the sides and ends are lined with barrels. One added note, plants setting on top of the warm drums grow extra well through the long cold low light winter season. - Millet