Hydronic Injection-Mixing Loop Flow
The injection flow that feeds a lower-temperature secondary (radiant or reset) loop from a hotter primary.
Example
You enter
- Secondary loop flow (gpm) 10
- Secondary supply temp (°F) 110
- Secondary return temp (°F) 90
- Primary supply temp (°F) 180
You get
- Injection flow 2.222
- Injection (% of secondary) 22.22
Details, formula, and sources
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.
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.
The injection-flow energy balance is public first-principles (the injection water carries the secondary loop's load); the flows and temperatures come from the loop design and the heat loss.
Estimate. AHJ and licensed professional govern.
Field names used by the API: secondary_gpm, secondary_supply_f, secondary_return_f, primary_supply_f, injection_gpm, injection_pct_of_secondary
- Injection energy balance injection gpm = secondary gpm x (Tss - Tsr)/(Tps - Tsr); the injected hot water carries the secondary loadSiegenthaler / hydronic practice
- Flow sizing only sizes the injection flow; the pump/valve Cv, boiler protection, and the control strategy govern the design; unreachable if primary is not warmer than the required secondary supplycontrol design practice