Hydronic Injection-Mixing Loop Flow
The injection flow that feeds a lower-temperature secondary (radiant or reset) loop from a hotter primary, by an energy balance: injection gpm = secondary gpm x (secondary supply - secondary return) / (primary supply - secondary return). A 10 gpm secondary at 110/90 F off a 180 F primary needs only 2.2 gpm of injection (22% of the secondary flow); the rest is recirculated secondary water. A cooler primary needs MORE injection for the same heat -- drop it to 140 F and the injection climbs to 4.0 gpm. If the primary supply is not warmer than the required secondary supply, the target is unreachable (flagged). Sizes the injection FLOW; a variable-speed injection pump or modulating valve, boiler protection, and the heat loss govern the design.
Formula and source
injection_gpm = secondary_gpm x (secondary_supply_f - secondary_return_f) / (primary_supply_f - secondary_return_f); injection_pct = 100 x injection_gpm / secondary_gpm; reachable when primary_supply_f >= secondary_supply_f.
Hydronic injection-mixing loop flow (primary/secondary energy balance; Siegenthaler, Modern Hydronic Heating), by name; a variable-speed injection pump or modulating valve and the control strategy govern the design.
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