Drain Down Solar Water Heating Systems
With Preheat Tank


Drain Down Systems are suitable for single application systems, particularly domestic hot water, in moderate climates. One or two tanks can be used in various configurations and they retrofit easily into existing systems.


NOTE: The information presented in this page is for guidance only - no part of this may be used for any agreement, whether express or implied, or to form any contract. THERMO TECHNOLOGIES reserves the right to change specifications and prices without prior notice.


The drain down valve provides system freeze protection. Without power, the valve closes the supply and return lines from the tank and the collector water drains away, preventing freeze-ups.

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Connecting an Extra Tank in a Series - a Preheat Tank
1. Collector Supply 4. Tempering Valve 7. New DHW Tank           
2. Collector Return 5. Hot Water to Taps 8. Existing DHW Tank  
3. Hot Out to Cold In      6. Cold Feed

Connecting an Extra Tank in a Series - a Preheat Tank

In some situations, particularly integrating with an existing system, a two tank system is necessary. The traditional arrangement is to have a solar preheat tank, in which the tanks are plumbed in series. If the tanks can be sited close together connecting tanks in parallel has some advantages. Both these arrangements provide adequate backup.
  • Easy to retrofit to existing tank with minimum disturbance of existing piping
  • Versatile system - the preheat tank can be located away from the conventional tank.
  • Useful if the conventional tank is gas or oil fired. Many gas fired DHW tanks have high standby heat losses and will waste solar energy.
  • The "pretank" is usually cold in the morning and ready to receive solar energy.
  • Good control over the system and easily monitored performance.
  • Efficient if the house is occupied through the day. Solar heated water will frequently be drawn into the conventional tank, helping offset standby losses.
  • A well insulated standard DHW electric tanks is mass produced and reasonably priced.
  • This solar system can be disassembled easily, leaving the existing system intact, and moved to another location.
  • Solar energy is only heating half of the available storage. If no-one is home through the day the solar heat remains in the preheat tank.
  • A larger pre-heat tank will be necessary than if a tank was added in parallel.
  • Stand-by losses are more than twice those of a single tank system.

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