Solar Thermal Information

23
5. DRAINBACK SOLAR COMBISYSTEMS
Drainback systems are the most common alternative to
antifreeze-based solar thermal systems. There are many
ways to configure drainback combisystems, depending
on the heating distribution system used, the percentage
of the load to be supplied by solar energy and the type of
auxiliary heat source used.
Whenever they are not operating, drainback systems
allow water from the collector array and piping outside
heated space to drain back to a tank located within
heated space. This action requires only gravity and
properly sloping piping components. It does not rely on
devices such as solenoid valves or vacuum breakers, and
as such is highly reliable.
This section discusses several systems that show a wide
variety of concepts. The systems are assumed to be for
residential applications. However, most can bescaled up
for commercial installations.
ADVANTAGES OF DRAINBACK SYSTEMS:
Perhaps the most apparent advantage of a drainback
system is the elimination of antifreeze fluids. This, in turn,
implies several other benefits, including:
• Antifreeze solutions require a heat exchanger between
the collector subsystem and the water in the remainder
of the system. Beside the fact that heat exchangers can
add significant cost to the system, they also induce a
performance penalty. All heat exchangers require a
temperature differential to drive heat from their “hot”
side to their “cool” side. In solar collection subsystems,
the temperature differential shows up as an increase
in collector inlet temperature relative to the water
temperature near the bottom of the storage tank. This
decreases collector efficiency. The magnitude of this
performance penalty depends on the size of the heat
exchanger. Larger, more expensive heat exchangers
reduce the penalty, but never completely eliminate it.
Appendix B gives information for estimating the thermal
performance penalty associated with a heat exchanger
between the collector array and storage tank.
• Eliminating antifreeze also eliminates the annual fluid
testing, and thus reduces system owning cost.
• The collectors in a drainback combisystem do not
contain fluid when they stagnate. A power outage,
control failure or other condition that stops the collector
loop circulator results in the water draining back from
the collectors and exposed piping. This minimizes
internal stresses on the collector relative to systems in
air vent
w/ shut off
valve
solar collector array
solar
circulation
station
check
valve
DHW
CW
solar storage tank
(dual coil)
relay
mod/con boiler
make-up water
AB
3-way 
diverter 
valve
A B
(P1)
closely spaced tees
outdoor
temperature
sensor
(P2)
pressure-
regulated
circulator
space-heating circuits
(highly zoned)
heat source
circulator

anti-scald
tempering 
valve
relay
manifold valve actuator
thermostat
end switch leads
3-way
motorized
mixing
valve
end switch leads from actuators
A
B
AB
spring loaded
check valve
sensor
S3
pool pump
filter
bypass valve
(normally closed)
pool
pool heat
exchanger
flow
switch
figure 4-9