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Cauton: The prmary desgn prncple behnd a dranback system s that the water n the collectors s
able to dran back when the dfferental control turns the pumps off. In order to accomplsh ths, the
collectors must be nclned 1/4” per foot towards the nlet lne. In addton, all the ppng must also be
angled to facilitate the flow of water towards the drainback reservoir. If you violate this principle, you
jeopardze rupturng your ppes to/from the collector or the absorber n the collector tself should t freeze
wth water stll n t.
There are several different styles of drainback systems out there. Double-pumped external heat
exchange, double-pumped with an integral heat exchange in the drainback reservoir, and single-
pumped with an integral heat exchanger in the storage tank. The argument for the last is generally that
the pump requrements of that system are the lowest so the parastc losses of runnng the pumps are
mnmzed. Ths s not necessarly the case. Of the two pumps n a dranback system, the larger one
s the one that runs the water through the collectors. That pump wll run as long as t takes to brng the
temperature dfferental between the storage tank bottom and the collectors wthn a preset lmt. If you
make your heat exchange between the storage loop and the collector loop worse, you will require the
large pump to run for a longer perod of tme to put the heat n storage. So the argument that you have
less pump horsepower is correct although you may be significantly increasing the running time of the
pump and thereby eliminating or even reversing the “benefit” of having only one pump. While having
a single pump system will reduce pump maintenance, the added cost for the special heat exchange
tank and ts mantenance make a double pump system a better approach than a sngle pump dranback
system.
I recommend an externally mounted heat exchanger. Either shell and tube or brazed plate heat
exchangers work well with their own unique advantages. I prefer the brazed plate heat exchanger
up to the collectors. Once the water reaches the hghest pont on the collectors, the water then falls
back to the reservoir where it starts its journey all over again. Since water finds its own level, when the
pumps turn off, f all the ppng s angled from the collectors towards the storage tank, water wll dran
out of the collectors, pass through the pump and heat exchanger and then ultimately lift the level in the
dranback tank (hence the name).
The Beckett SolarHot Advantage:
Our system uses a double-pumped efficient design
as mentoned above. It comes wth the two closed loops already pre-assembled n the
SolVelox package in order to reduce both your installation effort and potential assembly
ssues. As a result, ths should reduce the overall cost of the system.
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