The Fix
For high-capacity domestic water
heating, it’s essential not to create “bottlenecks” to flow or heat transfer.
Instead, the circulator and piping for the indirect water heater should be able
to convey the full output of the boiler plant while operating at a reasonable
temperature drop.
In this case that’s 400,000 Btu/hr.
Assuming a nominal 20 ºF temperature drop across the heat exchanger in the
tank, this requires a flow of 40 gem, far beyond the capacity limits of a 1/25
hp zone circulator and ¾-inch piping.
The relatively small internal coil in
the indirect tank also creates a bottleneck to heat transfer. The tank can only
accept a fraction of the heat output of the boiler system.
The solution shown is a high-capacity
indirect with large internal heat exchanger. Large enough to dissipate the full
output of both boilers without need for an inlet temperature above 180 ºF. The
heat exchanger in the tank also needs to be piped for counterflow (hot water
enters at top of coil). If a high capacity indirect is not available, an alternative
solution is to use an external brazed plate heat exchanger.
The use of a series primary/secondary
distribution system creates a temperature drop from one secondary circuit to
another. Since the space heating loads are all low temperature, this will
likely result in inadequate heat delivery to the downstream secondary circuits.
The fix is to put those low temperature circuits in parallel. This is done with
a header system coming off the hydraulic separator. The combination of
generously sized “short & fat” headers along with the hydraulic separator
provides the same hydraulic separation capability of primary/secondary piping
without the need for a primary circulator.
Finally,
the system supply temperature sensor for a multiple boiler controller should
always be mounted downstream of the point that hydraulically separates the
boiler plant from the distribution system. This ensures constant flow past the
sensor regardless of which loads are operating.
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