Water vs Air Geothermal Systems for Greenhouses

All the on-site geothermal systems currently used for greenhouse heating fall into one of the following categories:

  • Air to water heating; that uses a heat pump to move heat from underground air to water for loop or radiator distribution
  • Water-to-water heat pump; uses a body of water with a stable temperature to a boiler
  • Direct water heating; with distribution of pre-heated water or direct air heating from buried pipes and wells
  • Ground to air heating; relying on a heat pump, dry wells, and blowers for forced air distribution.

Source of Heat

Air geothermal systems can use both dry wells and underground lakes and springs to supply heat, as do water systems. You’re not limited to the type of heat source available to determine the best way to store and distribute that energy. The various combinations of air and water vary in their efficiency, with direct water or air heating requiring the least energy and transferring the most heat. Air-based heat pump or direct systems can be built almost anywhere, while water-powered geothermal systems must be located in an area with an appropriate water table or large lake.

Heat Pump Use

If you want to avoid the use of heat pumps to cut electrical requirements or build greenhouses in remote and off-grid areas, you’ll have to choose a direct transfer system based on air or water. Water direct heating systems are difficult to locate because of the limited availability of a hot enough supply. Direct air circulation systems still function best if you can power blowers and fans with solar power or generators, but they can be designed for complete passive circulation when necessary.

Limitations in Cold Climates

Since geothermal heat pumps are drawing on a buried source of air or water that’s a constant temperature, these heating units come with fewer cold climate limitations than above-ground pumps. Water circulation systems are generally filled with anti-freeze instead so that there’s no chance of freezing or pipe ruptures when the system’s off for a short time. Air-based geothermal systems don’t suffer from the same problems, especially if they’re ground-to-air designs that rely on heat pumps and blowers alone.

Open Loop and Closed Loop for Fluids

Even after settling on a water-based distribution system for your geothermal heating unit, you’ll still need to choose between open and closed loop designs. Unless you’re directly circulating a source of very hot water through metal radiators, you’re likely planning to bury loops of pipe to distribute the heat slowly and evenly throughout the greenhouse. Open loops simply discharge the water or fluid somewhere outside of the greenhouse after it’s traveled through and released its heat. Even if the water’s captured in a man-made pond or lake, it’s usually not reused for heating but may serve as irrigation water. Closed loop systems recirculate the water or anti-freeze, reducing the costs of refilling the system to minimal top-ups every few weeks or months.

Greenhouse Design

It’s far easier to send water over long distances to remote greenhouses than to deliver air the same distance. You’ll need to design smaller and more clustered layouts to maximize the value of a forced air or passive air system, while long and scattered structures are easily served with a branched water distribution design. The water will still lose some heat as it travels long distances, but this is easily prevented with wrap insulation or by shallowly burying the plumbing. This is why most water systems rely on buried loops to heat the soil and gradually release heat rather than exposed metal radiators.

Soil Heating

If you’re not concerned very much with air temperatures in your greenhouses, but need consistent soil temperatures for germination or root growth, you can design geothermal heating for this purpose alone. These systems are almost exclusively designed around water loops, either closed or open, that are buried below the beds or rows. Direct and forced air heating do little to heat the soil, especially when you’re planting in the ground for row crops or bed cultivation rather on raised tables or aquaponics racks. You may need both a buried water loop for soil warming and a separate water or air heating system from the geothermal wells to ensure the entire greenhouse stays warm from bottom to top.

An engineer, or greenhouse designer, is the best source of advice on whether your designs and climate area call for an air-based or water-based geothermal. Regardless of which type of earth-based heating you choose, you’ll find the system is much more efficient when paired with the right greenhouse film. Here at BTL Liners, we can help you find the right film to insulate your greenhouse and maximize the success of your new heating system.


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