Drain Down Solar Water Heating Systems
Extra Parallel Tanks

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.
The draindown 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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Top Feed Tanks in Parallel

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Bottom Feed Tanks in Parallel
Connection an Extra Tank in Parallel 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.
With an extra circuit breaker for the new tank, the two top heating elements can provide an adequate backup. Both bottom elements can be turned off. In the bottom feed tanks the solar heat will thermosyphon between the tanks.
Advantages of an Extra Tank in Parallel
  • The existing tank is also used to store the solar energy so the extra tank can be smaller.
  • Minimum disturbance to the existing tank.
  • When possible solar energy takes care of the standby losses
Disadvantages of an Extra Tank in Parallel
  • The tanks must be sited close together.
  • Takeoffs to the valve or hot water must be central so that each tank is drawn from equally.
  • Stand-by heat losses are more than twice those of a single tank system.

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