The Fix
1. The boiler circulator should be positioned
to push flow into a modulating/condensing boiler with a compact heat exchanger.
Such boilers typically generate significantly higher flow resistance relative
to traditional boilers with cast-iron heat exchangers. The position of the
boiler circulator in the glitch drawing decreases boiler pressure whenever the
circulator is operating. If the water temperature in the boiler is high enough,
steam flashing may occur.
2. The supply temperature sensor placement
shown in the glitch drawing will not detect the water temperature supplied to
the distribution system. If the flow rate on the right side of the hydraulic
separator is higher than the flow rate on the left, mixing will occur within
the separator. This will reduce the supply temperature to the distribution
system. Ideally, boiler modulation should be based on the load side supply
temperature and, thus, the temperature sensor should be moved to the position
shown on the fix drawing.
3. There is no need of a “primary loop” on the
load side of the hydraulic separator. The details shown in the glitch drawing
not only use an extra circulator, they also induce a differential pressure
between the top and bottom portions of the loop (due to head loss across the
partially closed valve). This pressure differential will attempt to push flow
through both load circuits whenever the primary circulator is operating. The
fix is to eliminate the primary loop and simply use short, generously sized
headers as shown.
4. Add a purging valve on the boiler circuit
as shown in the Fix drawing to provide efficient air removal from the boiler at
start up.
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