Legacy Steam + Hydronic Controls

Free tool · Hydronic

Mixing valve calculator

Size a 2-way injection mixing valve for a radiant or low-temperature loop from three numbers: the loop's design load, the boiler supply temperature, and the mixed return.

Direct answer

Injection GPM = load ÷ (500 × (Ti − Tr)) for water (Taco Eq. 1). 40,000 BTU/hr with 180°F supply and 100°F return is 1.0 GPM. Pick the Taco iSeries 2-way body whose published Cv is closest to that GPM — not the one that matches the pipe size.

Inputs

Taco Eq. 1 — load, boiler supply (Ti), mixed return (Tr).

60 min/hr x 8.33 lb/gal x 1.0 BTU/lb-F. The constant Taco uses in Eq. 1.

Injection flow
1.00GPM
2-way iSeries pick
1/2"Cv 4.9

How it was picked

GPM = 40000 ÷ (500 × 80°F). Taco step 5: pick the 2-way body whose published Cv is closest to that GPM — 1/2" Cv 4.9 (Δ 3.90) · 3/4" Cv 10.3 (Δ 9.30) · 1" Cv 8.9 (Δ 7.90).

The judgment call

An injection valve moves only the hot water that crosses into the mixed loop, so the flows are small — often one or two GPM for a whole radiant zone. That is why Taco tells you not to size the 2-way body from pipe size: a 1" valve on a 1" line with 1 GPM through it has almost no authority and hunts. The wider the Ti − Tr gap, the smaller the injection flow and the smaller the valve.

Note the printed Cv table is not monotonic (3/4" is 10.3, 1" is 8.9); this tool reproduces it as published. For 3-way and 4-way bodies, step size, pressure drop, and glycol corrections, use the full injection mixing calculator and the GPM → Cv guide. Setting the reset curve that drives the valve: outdoor reset calculator.

Estimate only

Not a substitute for licensed HVAC design. Glycol constants (490 / 450) are approximations; check the fluid maker's data.

FAQ

How do you size a mixing valve for radiant heat?

Compute the design injection flow: GPM = load ÷ (500 × (boiler supply − mixed return)) for water. Then choose the Taco iSeries 2-way body whose published Cv is closest to that GPM (Taco PA04/PA06 step 5).

Should the mixing valve match the pipe size?

No. Taco states not to size the 2-way iSeries valve based solely on pipe size. Injection flows are small, so the right body is often smaller than the line.

What constant do I use for glycol?

Water uses 500 (60 × 8.33 × 1.0). Common rules of thumb are about 490 for 30% propylene glycol and 450 for 50%; verify with the glycol manufacturer’s specific heat and density.

What are the Taco iSeries 2-way Cv values?

As published: 1/2" Cv 4.9, 3/4" Cv 10.3, 1" Cv 8.9 (Taco PA04 / PA06).

Sources

More free tools

Need the full worksheet? Open the Injection mixing calculator.