Other controls may have a switch position marked "run" instead of "auto". In non-
freezing climates, the control may turn on the collector loop pump to keep the water
in the collectors from freezing. The control may open the collector loop control valve
to drain the collectors. In either case, a light marked "freeze" or "FRZ" may be
supplied to let users know the collectors are being protected from freezing. (Figure
Most differential thermostats include a storage temperature high limit function. When
the storage tank reaches a pre-set absolute temperature (typically 160F-180F),
the collection pumps are turned off. The control may have a sensor specifically for
this high limit function, or it may simply use the one also used for differential
measurement. See Table 2-3 for more information.
A Typical Differential
High Limit Adjustment
Dial (left) and Switch
(right). Note Indicator
Lights Below Dial and
Another specialized function of differential thermostats is freeze detection. On
draindown systems, when the collector temperature begins to approach freezing,
the control valve is turned off, and the collectors and exposed piping are drained.
Freeze sensors are usually snap switches which are either completely closed or
completely open. (Figure 2-50)
In some systems, one or more freeze snap switches are installed in series or
parallel with the collector sensor. In others, they are part of a separate circuit.
2.7 COMPONENT OPERATION