14 June 2002
or short circuits. Since a non-functioning solar system can go undetected by
maintenance personnel due to the presence of the backup heating system, some
means for determining if the system is not operating or has not functioned for a given
amount of time is helpful. The most commonly used method provides a visual
indication at the control panel when the pump(s) are energized, although this indication
is only instantaneous and does not provide any history. Some controllers indicate the
elapsed time that the controller has signaled the pumps to switch on, but this is not
necessarily an indication of whether the pumps have in fact been operational. The
elapsed time indicator required on the pump showing cumulative running time of the
system provides a check of system operation, if maintenance personnel choose to
inspect and record it.
Temperature Sensors and Locations. There are two temperature sensors
that the DTC relies upon to determine when to activate the collector loop pump and
storage loop pump. It is important that these sensors be reliable and accurate, as they
can have a significant impact on system performance. Platinum resistance temperature
detectors (RTD's) are most commonly used and are recommended, although 10 K-ohm
thermistors are also sometimes used for this application.
Collector Temperature Sensor. One sensor is required on the collector
array to determine when sufficient energy is available for collection. This sensor is
typically located in the fluid stream or is fastened directly to the absorber plate. When
specifying a location in the fluid stream, the sensor should be located on a nearby
collector bank and in the top internal manifold piping between two collectors. This
location allows the sensor to be heated by the heat transfer fluid by natural convection.
To minimize the length of sensor wiring, mount the sensor between two collectors on
the bank closest to the roof penetration whenever possible. Most sensor manufacturers
provide threaded wells to allow insertion of sensors into pipe flows. These wells should
not consist of ferrous materials due to material compatibility with the propylene glycol
heat transfer fluid. The sensor assembly should also be covered with a weatherproof
junction box to shield connections from moisture while allowing room for the insulation
around the manifold. The collector temperature sensor may be attached to the
absorber plate of a collector only if the collector manufacturer provides this service at
the factory. Sensors located in wells are easy to replace but may leak, whereas those
located on the absorber plate are usually quite difficult to repair.
Storage Tank Sensor. The storage tank temperature sensor is intended to
measure the temperature of the coolest part of the storage tank. This is the fluid that
will be delivered to the heat exchanger. Ideally, this sensor should be located within a
well protruding into the storage tank near the outlet to the heat exchanger. If desired,
auxiliary sensors may be added in the top half of the tank to check for stratification and
in the bottom of the tank to provide backup.
Sensor Wiring. Wiring from the controller to the collector and storage
sensors should be located within metal conduit. It is recommended that spare
conductors be provided in the conduit for future maintenance or expansion needs.
Color-coding should be consistent from the controller to the sensor, and junctions or