Field guide · Hydronic controls

Injection mixing: GPM → Cv

A high-temperature boiler and a low-temperature loop meet at an injection point. The design question is how much hot water has to cross that point at design load — then which mixing-valve Cv sits closest to that flow so the valve can actually modulate.

Estimate only

Starting point from published sizing procedure, not a substitute for a licensed hydronic design or the valve manufacturer's pressure-drop curves. Glycol constants are approximations — verify against your fluid supplier before a submittal.

When injection mixing is the tool

Hot boiler side, cooler mixed loop, controlled crossover.

Use injection mixing when the boiler (or primary) runs hotter than the emitters want — radiant floors, low-temp panels, or a mixed zone off a high-temp plant. Hot water is injected into the cooler loop; the injection flow carries the load across the temperature difference between Ti (boiler / injection supply) and Tr (mixed-system return).

Injection mixing is not the same job as outdoor reset. Reset sets the target supply vs outdoor temperature; injection sizes the crossover that can deliver that mixed temperature under load. Many jobs need both — curve first, then valve flow.

From load and ΔT to injection GPM

Design injection flow is the load divided by the fluid constant times the injection ΔT:

injection GPM = load ÷ (constant × (Ti − Tr))

For water the constant is 500 (60 min/hr × 8.33 lb/gal × 1.0 BTU/lb-°F) — the figure Taco uses in Eq. 1. Glycol drops the constant because the mix carries less heat per gallon-degree. A wide Ti−Tr means a small injection flow and usually a smaller valve — which is why a 180 °F boiler feeding a 100 °F return needs surprisingly little crossover pipe.

This is not the mixed-loop flow. The radiant (or mixed) loop circulates its own, larger flow on its own ΔT. Sizing the injection circuit off the loop pump is the usual way this gets built wrong.

What injection GPM does your design load and Ti−Tr actually require? Run the injection mixing calculator.

Reading GPM into valve Cv

Cv is a flow coefficient — the GPM the valve passes at 1 psi drop — not a pipe size. Taco's published rule for iSeries 2-way bodies: pick the valve whose Cv is closest to the calculated injection flow rate. Do not size solely on the pipe you happen to be cutting into.

Matching Cv to design flow puts the valve in the middle of its control range, where it can modulate instead of swinging between barely-open and wide-open. After the closest-Cv pick, still read the manufacturer pressure-drop curve at your calculated GPM against the pump you have.

Need the closest 2-way Cv pick (and candidates) for your injection GPM? Open the injection mixing calculator — GPM → Cv.

Protect the boiler and the fluid

Mixing on the supply side does not by itself keep a cast-iron boiler out of condensing territory — you still need return protection when the loop runs cold. If the loop is glycol, size pumps and emitters on manufacturer data, not a water case with a fudge factor. Outdoor reset curve setpoints are covered in the outdoor reset curve guide.

Next step

What injection GPM does the load and Ti−Tr produce, and which iSeries Cv sits closest to that flow? Enter load, Ti, Tr, and fluid in the calculator.

Open injection mixing calculator

Sources

Flow equation and closest-Cv selection rule are quoted from Taco's iSeries sizing procedure; Cv tables are transcribed from the same documents. This guide does not invent GPM or Cv figures — run the load and temperatures in the calculator.