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XIII. TWO OR MORE COLLECTOR SYSTEMS
Households with more than six members or with a large hot water demand should consider
having a two-unit system. Light commercial applications such as restaurants, car washes,
housing with central hot water systems, bathhouses, and process hot water can be served with
multiple collector systems.
Two unit systems must be plumbed together in parallel to ensure that both collectors drain
properly. The piping flow paths must be of equal length so each collector receives equal flow
pressure. Make sure all piping slopes ¼” per foot so the collectors and all piping drain
completely.
A two unit system could be plumbed in series, but only in non-freezing areas because the
second collector cannot be easily drained. If a series configuration is necessary, a boiler drain
must be installed at the inlet and the outlet of the second collector or else the supply line and
the return line have to be disconnected from the collector in order for it to drain. The
performance of tow unit systems is equal whether plumbed in parallel or series so the ability to
drain and the ease of performing this function are the main considerations when choosing the
flow pattern.
As show in
Drawing M
the recommended configuration has both units installed in the normal
horizontal position with the inlet/outlet pipe nipples on the right hand side of the collector. This
configuration has a considerable length of piping exposed on the roof. Be sure to insulate this
piping well and to run it as close to the collectors as possible to limit any possible negative
aesthetics. If the collectors are tilted up, the piping can run behind them, making a much
cleaner looking installation.
The Alternative Configuration has improved aesthetics by “flipping” the second collector around
so the pipe nipples are on the left hand side of the collector facing the pipe nipples of the first
collector, which is installed in the normal horizontal position. The inlet on the second collector
now becomes the outlet and the outlet becomes the inlet. This configuration is acceptable, but
there are three factors that could affect performance. In-house testing has shown that a flipped
collector will have slightly reduced performance, some air entrapment, and will only partially
drain. The contractor will have to decide the requirements of each installation when choosing
which configuration to use.
Proper positioning of a two unit system is an important consideration because of potential
excessive roof loading. On south facing roofs the units must be installed side by side, as in
Drawing N.
Never install two units one above the other on the same rafters. Be sure to
properly distribute the weight of the two units and to install all necessary bracing to the roof.
Another acceptable method of installing a two unit system on an east or west facing roof is to
face one unit due east and one unit due west within 15” of the roof crown. The units should be
installed in the normal horizontal position and plumbed in parallel. Do not “flip” the east facing
unit. Plumb the supply line to the inlet connection of both units and the return line to the outlet
connections of both collectors
(see Drawing O)
.
Systems that require multiple collectors should be designed to operate in parallel. Each unit
should be plumbed independently into the supply and return lines or as parallel pairs so they will
drain properly.
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