Solar Thermal Information

20
The speed of circulator (P3) could be varied based
on maintaining a fixed supply temperature to the
distribution system. It could also be varied based on
outdoor reset logic.
The ability to control the speed of circulator (P3) also
prevents what could be very hot water in the solar storage
tank from flowing directly to a low-temperature distribution.
As such, it eliminates the need for a motorized mixing
valve in most applications. However, if the distribution
system operates at very low water temperatures relative
to the minimum temperature maintained in the top of
the storage tank (for delivery of domestic hot water),
the space heating distribution circulator (P2) should be
temporarily turned off while the boiler is heating the top
of the storage tank. Circulator (P3) should also operate
at full speed during this mode to maximize heat transfer
across the heat exchanger. When the top of the storage
tank has reached the preset water temperature, variable-
speed operation of circulator (P3) can resume.
If circulator (P3) is operated in an on/off manner, a mixing
valve is required in the distribution system. Circulator (P3)
would turn on whenever the boiler is operating as well as
when there is a call for space heating.
One advantage of either of these approaches is that the
external heat exchanger is serviceable or replaceable if
ever necessary. Another advantage is that the thermal
mass of the water in the storage tank will buffer a highly
zoned space heating distribution system, and thus
reduce the potential for boiler short cycling.
ANTIFREEZE-BASED COMBISYSTEM #4:
There are many homes and commercial buildings with
forced-air distribution systems for heating. It is possible
to build a solar combisystem using hydronic hardware
to collect and convey solar energy, and then pass this
energy to a forced-air distribution system. One approach
is shown in figure 4-7.
VENT
water-to-air
heat exchanger 
coil
blower
air vent
w/ shut off
valve
solar collector array
solar storage tank
(dual coil)
cold water
hot water
diverter
valve
(modulating)
water heater
anti-scald
tempering
valve
motorized mixing valve
w/ controller
(P1)
check
valve
temperature
controller
solar
circulation
station
P&T
figure 4-7