How to Use a Hot Tub as Thermal Storage

Adding water thermal storage to your greenhouse helps keep the greenhouse temperatures stable through the long winter months. Generally, greenhouse enthusiasts use barrels filled with water and painted black. When the sun shines on the barrels, they absorb the heat, preventing the greenhouse from overheating. At night, the barrels radiate a small amount of heat, helping the greenhouse stay warmer than the outside temperatures. By adding a hot tub to the array of barrels, you are providing extra thermal storage while you enjoy a long soak after a hard day of work.

Tape measure
Shovel
Crushed gravel
Plate compacter
Rake
Sand
Spirit level
Hot tub with cover
55-gallon barrels with lids, painted black

Instructions

- 1
  Measure the space for the hot tub and barrels in the greenhouse. Select a location where the sun will shine for most of the day.
- 2
  Remove the soil to a depth of 6 inches.
- 3
  Fill the excavation with 2 inches of crushed gravel. Sprinkle with water and compact with a plate compacter. Add another 2 inches of gravel and compact again.
- 4
  Rake 2 inches of coarse sand over the compacted gravel. Sprinkle with water and compact firmly. Check the sand's surface with a spirit level and add more sand if necessary to make the surface perfectly level.
- 5
  Place the hot tub in the center of the compacted gravel and sand. Arrange the barrels on each side of the hot tub where they receive the maximum amount of sunlight.
- 6
  Fill the hot tub and all the barrels with water. When full, place the lids on the barrels.
- 7
  Plug the hot tub into a GFCI-protected outlet. While the solar gain will keep the hot tub warm, generally a hot tub is heated to 104 degrees Fahrenheit. The excess heat from the hot tub will help heat the greenhouse.
- 8
  Cover the hot tub when not in use. This will serve a dual purpose: first, for safety, as any uncovered water is a drowning hazard for children and pets, and second, it will keep the water warmer than in an uncovered hot tub.

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