Heat migration: On secondary loops that extend vertically to a load that is above the
primary loop, steps must be taken such as fabricating a thermal trap in the return piping -
minimum 18" (46 cm) drop - to prevent thermal siphoning and heat migration to the load
when there is no demand for heat to that loop. Alternatively, use check valves on both
supply and return of secondary piping.
Figure 30 Important Primary/Secondary piping details with closely-spaced tees
DHW indirect tank
Air handler
Low temperature radiant protected by a mixing valve for simultaneous heating during higher
temperature air handler operation. Boiler can return to lower supply temperature when
radiant alone is operating.
Figure 31 Primary/Secondary piping concept with simultaneous heating calls
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