Hydronic controls · P2
Injection mixing
A high-temperature boiler and a low-temperature loop meet at an injection point, and the only question that matters is how much hot water has to cross it at design. Enter the load and the two temperatures and you get the flow rate, then the valve Cv that matches it.
Design conditions
The three numbers in Taco's Eq. 1, plus the fluid.
The heat the radiant or low-temperature loop has to deliver at design, not the boiler's input rating.
Boiler supply temperature feeding the injection point.
Return temperature of the loop being mixed.
60 min/hr x 8.33 lb/gal x 1.0 BTU/lb-F. The constant Taco uses in Eq. 1.
The arithmetic
injection GPM = load ÷ (constant × (Ti − Tr))
= 60,000 ÷ (500 × 80.0)
= 1.50 GPM
500 is the water constant — 60 min/hr × 8.33 lb/gal × 1.0 BTU/lb·°F. It is exact in the sense that Taco prints it; it assumes water near 60 °F and does not get adjusted for temperature in this procedure.
Results
That is the flow the injection circuit has to deliver at design — not the flow in the mixed loop itself, which is set separately by the emitters and their own ΔT.
2-way iSeries valve — Cv selection
Taco's rule: pick the valve whose Cv is closest to the injection flow rate.
| Valve size | Published Cv | |Cv − GPM| | Selection |
|---|---|---|---|
| 1/2" | 4.9 | 3.40 | closest Cv |
| 3/4" | 10.3 | 8.80 | |
| 1" | 8.9 | 7.40 |
Closest published Cv: 1/2"
Do not size the valve by pipe size
The 3/4" Cv sitting above the 1" Cv is reproduced as printed in Taco's table; it is not a transcription slip.
3-way and 4-way Cv, for reference
Published values only. Taco documents the closest-Cv rule for the 2-way valve, so we do not apply it to these.
| Body | Size | Cv |
|---|---|---|
| 3-way | 1/2" | 1.5 |
| 3/4" | 3.3 | |
| 1" | 3.0 | |
| 4-way | 3/4" | 7.0 |
| 1" | 9.3 | |
| 1-1/4" | 17.5 |
What the numbers mean
Ti is the boiler side, Tr is the loop side. The injection flow carries the full load across the difference between them. A wide ΔT means a small injection flow and a small valve — which is why a 180 °F boiler feeding a 100 °F return needs surprisingly little pipe.
This is not the mixed-loop flow. The radiant 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.
Cv is a flow coefficient, not a pipe size. It is the GPM the valve passes at 1 psi drop. Matching Cv to the design flow puts the valve in the middle of its control range, where it can actually modulate instead of swinging between barely-open and wide-open.
Before you order
Check the pressure drop. The closest-Cv pick still has to work against the pump you have. Taco publishes pressure-drop curves for each body in the same documents cited below — read the drop at your calculated GPM.
Protect the boiler. A cast-iron boiler feeding a low-temperature loop needs return protection. Mixing on the supply side does not by itself keep the boiler out of condensing territory.
Glycol changes everything downstream. Lower constant, more flow, more head, less heat transfer at the emitter. If the loop is glycol, size the pump and the emitters on the manufacturer's data, not on the water case with a fudge factor.
Sources
- Taco — OVERVIEW iSeries-R (Outdoor Reset) Mixing Valve (PA04)
- Taco — Radiant Made Easy Application Guide, iSeries-S Mixing Valve (PA06)
The flow equation and the closest-Cv selection rule are quoted from Taco's iSeries sizing procedure; the Cv tables are transcribed from the same documents. The glycol constants are widely published hydronic approximations, not Taco figures — verify them against your fluid supplier's specific heat and density before a submittal.