Water has a volumetric heat capacity more than twice that of concrete, three times that of bricks and readily absorbs and releases its heat to the surrounding atmosphere. So we can absorb either twice as much heat as in the same volume of concrete or the same amount in only half the volume.

There are a number of examples of water tanks being built into interiors of homes to act as a buffer to the daily swings in temperature and this is certainly an easy way to create varying quantities of thermal mass into houses without the need to transport large quantities of heavy weight materials to site and pay for placement and installation.


Add to this the fact that it can be easily moved around in systems that are well understood and readily available and that, due to the thermosiphon effect of hot liquids rising, it can even provide/remove heat from our interior environment without the use of electrical energy.

We already see this type of system in solar hot water collectors with linked tanks. Where space is available on sloping sites collector panels can be mounted at low level with insulated pipe work  supplying hot water to a system inside the house for overnight warmth.

In summer, simply reversing the flow at night will cause flushing of the heat through the radiant panels open to the night sky, cooling the liquid thermal mass ready for the next day’s heat build up and reducing the temperature in the interior.

This can be improved where a below ground water source (Bore or well) is available to provide a heat sink and simply cycling the thermal mass water through a closed loop pipe system immersed in the ground water in a similar but more effective way as is used in ground source heat pumps. With thermal mass in large quantities high temperature differences are not required when linked to efficient levels of insulation in the exterior envelope of the home.

Water contained in low cost, non-pressurized cylinders has proven to be the most practical and effective approach to the capture and storage of thermal energy for space and hot water heating.   Water stores 3 to 4 times as many BTU’s per lb. as rock or masonry.  It also releases this heat energy faster when you really need it. Water Tubes are the most efficient and cost effective way to store solar thermal energy and keep building cooler in the summer and warmer in the winter.

Water Tubes work by absorbing the excess heat energy generated in your greenhouse or sunpace during the day and releasing it at night to effectively control the large temperature swings between day and night.  Think of them as heat sinks.  The cooler you can keep your greenhouse or sunpace during the day the less heat you lose through your glazing or window areas.  The stored heat in the tubes through simple thermo-dynamics will automatically release itself into the space when the air temperature drops below the water temperature in the tubes.  No moving parts.

The general rule of thumb is 2 to 3 gallons of water per each sq. ft. of south facing glazing. An example is if your greenhouse has a south facing wall that is 12′ long x 8′ high you would need somewhere between 192 to 288 gallons of water.  In this example four to six 12″ diameter x 8′ tall tubes would work, or you could use any combination of the sizes.

Benefits of Water Tank Thermal Storage

  • Low cost containers designed to be the most economical per volume unit of temperature storage;
  • Containers transmit natural daylight;
  • Wide selection of diameters and heights available to compliment any design;
  • Corrosion free fiberglass construction;
  • Faster usable B.T.U. gain in 60% less space and 80% less weight than with rock or masonry;
  • Self supporting containers require only flat, level floor;
  • Easy to install or remove; largest tubes weigh less than 20 lbs. empty!


Designs based on these concepts have been proven to provide up to 100% of space heating and hot water needs depending upon climate and site conditions. Installed adjacent to a solar window, the tubes absorb solar energy, store the energy, and transmit natural daylight. The tubes temper both the heat and light from the sun by removing the wide temperature fluctuations common to other direct gain systems, and by reducing glare from the solar window to workable levels. The tubes may be filled with dyed water to increase temperature absorption efficiency and to provide design versatility.