The water that will be used as domestic hot water is circulated directly
into the collectors from the storage tank (typically a hot water heater
which will back up the solar heating).
There are two types of direct systems - draindown and recirculating.
In both systems, a controller will activate a pump when the temperature
in the collectors is higher than the temperature in the storage tank.
The draindown system includes a valve that will purge the water in
the collectors when the outdoor temperature reaches 38 degrees. When
the temperature is higher than 38 degrees and the collectors are hotter
than the storage tank, the valve allows the system collectors to refill
and the heating operation resumes.
The recirculating system will pump heated water from the storage tank
through the collectors when the temperature drops to 38 degrees.
These two systems have serious drawbacks. The draindown valves can
fail in a draindown system and the result can be the expensive breakage
of the solar collectors. The draindown valve will typically sit unused
for a very long time and then will need to work the first time without
failing. The cycling of air and water in a draindown system collectors
as a result of periodically draining down (thereby emptying the collectors)
can cause a buildup of mineral deposits in the collectors and reduce
their efficiency. The recirculating system circulates buildup from potable
water heated from the storage tank through collectors during potential
freeze conditions and effectively cools the water (wasting energy).