"D"
System - Differential Control
This is the type of system most common in the Southern
United States and Tropical Climates. As illustrated
in Figure 1, the system consists of solar collector(s)
installed on the roof and a hot water storage tank usually
located in the garage or utility room. An alternating
current (A/C) pump circulates the water from the tank
up to the collector and back. This system is referred
to a direct because the sun’s heat is transferred
through the collector directly to the potable water.
No antifreeze solution or heat exchanger is required.
The “D” System utilizes a differential
control to sense the temperature differences between
water leaving the collector and the coldest water in
the bottom of the storage tank. The control turns the
pump on when the water in the collector is about 20F
warmer than the water in the tank. Similarly, the pump
is turned off when the temperature variation is approximately
5F. This process ensures that the water is always being
heated while the pump is operating.
A thermally operated valve is in-stalled at the collector
to provide freeze protection where required. This valve
will open to let warm water flow through the collector
whenever temperatures approach freezing. As an alternative,
the collector can be manually drained by closing the
isolation valves and opening the drain valves (located
above the storage tank). |
