Save with Solar Hot Water
(Page 4 of 6)
In cold climates where freezing is a concern, solar
contractor Tom Lane, owner of Energy Conservation Services
of North Florida, Inc. in Gainsville. Florida. recommends
consumers choose from one of two closed-loop, active
systems: unpressurized drainback systems or pressurized
glycol systems.
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"The most important criteria for both is that they won't
freeze and burst."
An unpressurized drainback system uses a
closed- loop of unpressurized water separate from domestic
pressurized water. When solar heat can be collected, a
differential control, which measures the difference in
temperature between two sensors and turns equipment on and
off, turns the AC circulating pump on, and water circulates
through the solar collectors. When the pump stops, the
collectors and pipes drain automatically into a drainback
tank. Hot water from the drainback tank is pumped through a
heat exchanger located in (or sometimes outside) the main
potable water storage tank. Almost all drainback systems
use a differential control and sensors to automatically
drain the water from the collector when the temperature
drops to freezing. In very cold climates, a propylene
glycol solution can be added to the unpressurized solar
loop for added freeze protection. Drainback systems
typically have the least maintenance and are the highest
performing, closedloop solar hot water systems available.
Closed-loop glycol systems are the preferred option for extremely cold climates,
but can be installed anywhere. Instead of using a drainback
system, closed-loop, pressurized glycol systems rely on a
continuous supply of antifreeze that remains in the pipes
and collector. whether the circulating pump is operating or
not. Closedloop glycol systems often rely on a differential
controller, sensors and an AC pump. A small PV panel can be
used to operate a DC circulating pump, which runs while the
sun shines and automatically shuts itself off when night
falls or cloudy weather prevails.
Double-walled heat exchangers prevent contamination of
household water with antifreeze or other heat-exchange
fluids. Heat exchangers can be mounted outside the storage
tank or located inside the tank. Because antifreeze tends
to degrade over time, however, these systems need regular
inspections to ensure that the antifreeze solution is still
viable.
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