The Fix
One
problem with the original design is that the supply temperature sensor for the
multiple boiler controller is installed on the outlet side of the hydraulic
separator. Most boiler controllers only have a single supply temperature sensor
that has to provide feedback to the controller in both the space heating and
domestic water heating modes. The sensor cannot provide accurate feedback
without flow passing through the piping to which it is attached (as would be
the case with the original design).
The cost
of a large motorized valve is probably as much if not more than a separate
circulator for the indirect water heater. The latter also ensures good flow
through the tank’s heat exchanger regardless of flow through the boiler(s).
Maintaining
flow through the hydraulic separator in both operating modes also allows the
air and dirt separating functions of the hydraulic separator to remain in
effect. Heat loss from the hydraulic separator is reduced during both operating
modes by its insulating shell û this shell is an absolute must for any
hydraulic separator.
The
reverse return piping on the boilers, while not necessarily an error, is
usually not necessary provided the header is sized for a very low pressure
drop. This is almost always the case when the parallel piping paths connected
to the headers each contains a circulator.
Finally,
purging valves, an expansion tank, and a make-up water assembly have been added
to the sub-circuits in the fix drawing.
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