In loops filled with solar fluid, the fill/drain assembly may be all that is required for
complete system draining. If the piping must be routed in such a way that local low
points are created, each low point should be equipped with a drain.
Boiler drains may not be acceptable for this purpose, because their seats and
packings may not be compatible with the solar fluid. The simplest way to provide an
inexpensive drain is the use of an all-metal "coin vent." These manual air vents
have no plastic or rubber seals and are relatively inexpensive. If the low spot is at
an elbow, a baseboard tee can be used instead of an elbow. These tees include an
1/8" female threaded port designed for the coin vents. (Figure 2-37)
Whenever a drain is being used, air must be introduced into the top of the loop to
speed up the draining process. The following section on air vents provides some
information on this.
Automatic (float type) air vents, as shown in Figure 2-38, are probably the most
incorrectly applied component in solar systems. Very few, if any automatic air vents
are capable of withstanding the pressures, temperatures, solar fluids and other
conditions they are exposed to in solar applications.
Automatic air vents can be used only in piping loops containing water. If used with
solar fluids, an automatic air vent would eventually vent enough fluid vapor or leak
enough to render the system inoperative. Automatic air vents shall not be installed
in piping loops containing solar fluid.
2.7 COMPONENT OPERATION