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Drain-back or closed-loop

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Posted by Vincent on July 23, 2004, 9:44 pm
Hi, Everyone

For a solar water heating system, is a drain-back system better then a
non-drain-back pressurized system?


Posted by Anthony Matonak on July 24, 2004, 1:23 am
Vincent wrote:

Both have advantages and disadvantages so neither one is better based
on this factor alone.


Posted by nicksanspam on July 24, 2004, 11:47 am

Unpressurized drainback systems (are there pressurized versions?) need
heat exchangers. If a pressurized system circulates tap water up through
collectors to prevent freezing, what happens when power fails?

I'd vote for the drainback system, since it is simpler and more fail-safe
and doesn't require heat and electrical energy for freeze-protection.
Drainback systems seem to need higher head pumps, unless the collectors
are vacuum-tolerant. Then again, we might attach half of a UK shower pump
(2 pumps on a single shaft) to the downgoing pipe while using the other
half to pump water up, or find a way to make one pump with up- and down-
going valves and diaphragms.


Posted by Gary on July 24, 2004, 10:38 pm
 Vincent wrote:

One thing to be careful of with drain back systems is that you can put
in plumbing that actually allows the fluid in the collector and outdoor
plumbing to drain correctly.  Piping to/from the collector should be 3/4
inch minimum and have a 10 degree slope.  This can be difficult to
achieve on some houses.  The collectors must have large enough diameter
risers so that the fluid drains from them when the pump shuts down.

The Tom Lane book "Solar Hot Water Systems 1977 to Today -- Lessons
Learned" (which seems like quite a good reference to me)likes drain back
systems, and thinks they are more reliable than non-drain back systems
when properly installed.  He also provides quite a bit of advice on
pumps, collectors, controls etc. for both types of systems.


Posted by Steve Spence on July 27, 2004, 9:46 pm
 Drain back works better for us when temps drop down below freezing. pv
powered pump only fills the system when the sun shines.

Steve Spence

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