Solar System Design Guide
67
6020 457 - 01
Systems for central heating backup can be configured
several different ways. The simple method utilizes a
dual coil as a DHW preheat tank prior to the auxiliary
DHW tank, and uses the top coil to inject excess solar
heat into the space heating circuit. A single dual coil tank
is suitable for most small applications, but two tanks in
parallel can be combined together for larger systems.
Another common space heating supplement configuration
is to use a dedicated space heating buffer tank in addition
to a solar DHW tank. A solar control with two-tank logic
is required.
For sizing a system for DHW heating and central heating
backup, the seasonal efficiency of the entire heating
system must be taken into consideration. The summer
heat demand is always decisive. This is a combination
of the heat demand for DHW heating and other project
specific consumers. The collector area must be sized
for this demand. The calculated collector area for DHW
heating is multiplied by a factor of 2 to 2.5. The result is
the range within which the collector area should be for
solar central heating backup. The precise determination
is then made taking into consideration the building
conditions and the planning of an operationally reliable
collector array. The storage tank capacity must be
sufficiently sized to minimize overheating. At least 1.7
to 2.5 gallons of storage is required per ft
2
of collector
aperture area (70 to 100 L/m
2
). A heat rejection circuit
is recommended for all DHW/space heating systems to
dissipate excess solar energy during the summer months.
If the ThermProtect switching absorber collectors are to
be used, then the heat rejection circuit may be exempt or
removed.
DHW + Central Heating Support Example:
■
House occupants:....................................................4
■
Average daily DHW usage:..................60 USG (225 L)
■
Aperture area:.................................Vitosol-FM panels
4 panels =100 ft
2
(9.3 m
2
)
5 panels =125 ft
2
(11.6 m
2
)
■
Storage volume required: Vitosol-FM panels
4 panels = 170 - 250 USG (640 - 950 L)
5 panels = 212 - 312 USG (800 - 1200 L)
System with single coil heating buffer tank
and a dual coil DHW tank
System with dual coil heating buffer tank
and a direct fire DHW tank
System for DHW Heating and Central Heating Backup
Examples
: For further detailed piping layout examples,
see the "Sample Piping Layout Drawings”
manual, available for download from the
from the Viessmann website.
Design and Operation
