Why a Changing Cold-Water Fraction Breaks Your Siliphos Dosing Calculation
When a thermostatic or tempering mixing valve reduces the cold-water fraction in your supply line, the polyphosphate concentration delivered to the point of use drops in direct proportion. If you originally dosed Siliphos crystal for 100 % cold water, a 50 % cold mix delivers roughly half the active dose—often below the minimum threshold needed for reliable scale inhibition. The fix is to recalculate your feed rate based on the actual cold-water fraction at the valve’s set point, or to dose downstream of the mixing point where the full flow is treated.
Understanding the Mechanics of a Mixing Valve
What a mixing valve actually does to your water chemistry
A mixing valve (sometimes called a tempering valve or blend valve) combines hot and cold water to achieve a safer or more comfortable delivery temperature. In scale-inhibition applications this is important because the hot leg typically bypasses the polyphosphate feeder entirely. Only the cold leg receives the metered Siliphos crystal dose.
The result is simple dilution math: if the valve is set to deliver 60 % cold water and 40 % hot water, the treated water constitutes only 60 % of the total flow. Every molecule of dissolved polyphosphate from the crystal feeder is now spread across a larger volume.
- Cold fraction = treated fraction. The cold leg is the only one that carries polyphosphate.
- Hot fraction = untreated dilution. No polyphosphate is added to the hot leg unless you install a second feeder.
- Outlet concentration = cold fraction × feeder concentration. This linear relationship holds as long as the feeder is sized for steady-state operation.
Why this matters specifically for polyphosphate media
Polyphosphate products such as Siliphos crystal work by releasing soluble orthophosphate and long-chain polyphosphate species into the water. These species interfere with calcium carbonate crystal growth, keeping scale in a suspended, non-adherent state. This is scale inhibition, not water softening—the hardness ions remain in solution, but they do not deposit on pipe walls and heating elements.
Because inhibition depends on maintaining a minimum residual polyphosphate concentration (typically in the low single-digit mg/L range for many systems), any dilution that pushes the residual below that threshold means scale protection is lost even though the feeder is still operating. You may not notice the problem until scale builds up in a boiler, dishwasher, or heat exchanger downstream.
How to Reassess Your Dose When the Cold-Water Fraction Changes
Step 1: Identify the actual cold-water fraction at the valve
Most tempering valves have a dial or digital setting, but the real cold fraction depends on inlet temperatures and the target outlet temperature. A practical method is to measure flow from each leg at the valve’s current setting using a simple bucket-and-stopwatch test, or to read the valve’s technical data sheet for the rated mixing ratio at your specific inlet conditions.
If you do not have access to the valve’s performance curve, treat the cold fraction as a variable that you must verify rather than assume. An incorrect assumption here is the most common cause of under-dosing in mixed-supply systems.
Step 2: Calculate the corrected feed rate
Use the following relationship:
Required feed rate = (original dose for 100 % cold) ÷ (actual cold fraction)
For example, if your system was originally sized to feed 200 g of Siliphos crystal per 1 000 L of 100 % cold water, and the mixing valve delivers 70 % cold water, the new target is roughly 285 g per 1 000 L of total mixed flow. The feeder must supply more crystal per unit of total output to compensate for the hot-water dilution.
This calculation assumes the feeder’s dissolution rate can keep up. Verify that the contact chamber or vessel is large enough to dissolve the higher mass of crystal within the available contact time. Undissolved crystal passing through the system wastes product and may cause downstream issues.
Step 3: Check the residual polyphosphate concentration
After adjusting the feeder, test the water at the point of use—ideally just downstream of the mixing valve—for orthophosphate or total phosphate residual. The target residual depends on your water hardness, pH, temperature, and the specific protection goal (e.g., boiler, domestic hot-water system, commercial dishwasher). Without a residual test, you are adjusting dose based on theory alone.
Note that polyphosphate slowly hydrolyzes to orthophosphate over time and at higher temperatures. If the mixing valve raises the water temperature significantly, the hydrolysis rate increases, which can further reduce the effective polyphosphate residual. This is another reason why dose must be verified by testing rather than purely calculated.
Design Options When You Cannot Simply Increase the Dose
Sometimes raising the feed rate is impractical. The feeder may be at capacity, the contact vessel may be too small, or local regulations may limit phosphate discharge. In those cases consider one of the following approaches.
| Approach | How it works | Key consideration |
|---|---|---|
| Dose downstream of the mixing valve | Install a second polyphosphate feeder on the mixed line so the entire flow receives treatment. | Requires a new contact vessel sized for the total flow; simplest way to eliminate dilution error. |
| Treat both hot and cold legs | Add a feeder to the hot leg so both streams carry polyphosphate before mixing. | More complex plumbing; hot-water temperatures accelerate hydrolysis, so dose may need to be higher on the hot side. |
| Replace the mixing valve with a point-of-use tempering device | Keep the main supply fully treated and temper only at individual taps. | Shifts the problem to each fixture; may not be practical for central systems. |
| Switch to a different scale-control method | Use electronic descaling, magnetic treatment, or a physical water softener where polyphosphate is unsuitable. | Each alternative has different performance characteristics; evaluate based on your specific water and equipment. |
Common Mistakes to Avoid
- Assuming the valve setting equals the cold fraction. Valve labels are approximate. Always verify with measurement or manufacturer data.
- Ignoring hydrolysis at elevated temperatures. If the mixed water is above 50 °C, polyphosphate breaks down faster, and your calculated dose may still be too low.
- Confusing scale inhibition with softening. Polyphosphate does not remove hardness. If your application requires actual hardness reduction (e.g., for soap efficiency), you need a different technology.
- Treating phosphate dose as a one-time set-and-forget value. Source-water hardness, temperature, and flow all vary seasonally. Re-test residuals periodically and adjust as needed.
- Overlooking discharge regulations. Some jurisdictions restrict phosphate in wastewater. Confirm that the increased dose required by a mixed system remains within permitted limits for your location.
When to Reach Out for Product-Specific Guidance
The principles above apply to polyphosphate dosing in general, but the exact crystal formulation, dissolution rate, and recommended dose range depend on the specific Siliphos product you are using. If you need to confirm whether a particular ENVO CHEMICAL polyphosphate crystal grade can handle the increased feed rate that a mixed system demands—or if you want help sizing a second feeder for downstream dosing—providing your source-water hardness, target outlet temperature, total flow rate, and the type of equipment being protected will allow a precise recommendation.
Send those details to our technical team and we will help you work out the correct crystal specification and dosing setup for your mixing-valve configuration.
