A polyphosphate filter that performs correctly at average flow can still under-dose during a bath fill. The reason is that a bath filler is a peak-flow device: water moves through the media bed quickly, contact time collapses, and the media has only a short window to release the phosphate it is supposed to deliver. Checking a short burst of high demand therefore means checking the worst case — the highest flow the filler will ever see, at the temperature it will actually be used at — rather than the daily average consumption of the building.
This article is written for filter manufacturers, water-treatment equipment suppliers, contractors and specification engineers who need to confirm that a polyphosphate doser (sometimes supplied as Siliphos crystal or a comparable phosphate glass medium) will still do its job when a bathtub or spa is filled at full flow.
Why a bath fill is a different duty from a basin tap
A basin tap is used in short, low-volume draws. A bath filler is used less often but at a much higher flow rate and for a longer continuous period — often several minutes of uninterrupted draw. That combination creates three effects at the same time:
- Flow velocity across the bed rises sharply. The same housing that behaves calmly at low flow may see a very different hydraulic regime during a fill.
- Contact time falls. Residence time in the media bed is roughly bed volume divided by flow rate, so an increase in flow reduces contact time in direct proportion.
- The media is consumed faster than the average-flow calculation predicts. Whatever the release mechanism, more water passing through the bed removes more material. Burst demand pulls a large share of the day’s treated volume in a few minutes.
There is also a plumbing question that is easy to overlook: is the bath filler actually downstream of the doser? In many installations the doser sits on the cold inlet to a water heater, so hot-water outlets are treated while cold outlets are not. A bath filled from a blended hot/cold supply may receive partly treated water, and the effective dose at the outlet will be lower than the dose measured at the doser.
What polyphosphate media do and do not do
Before sizing anything, it is worth being precise about the function, because polyphosphate dosing is frequently confused with other water treatments.
- Scale inhibition (threshold treatment): small concentrations of polyphosphate interfere with the growth of calcium carbonate crystals so that hardness salts tend to stay in suspension instead of depositing on hot surfaces. This is the main reason a doser is fitted ahead of a water heater or bath circuit.
- Corrosion inhibition: phosphate-based treatments can contribute to a thin protective layer on metal surfaces. Formulations differ, and some products combine phosphate with silicate or other components. The corrosion-control performance of a specific product must be verified for the specific metal and water chemistry.
- Not water softening: softening removes calcium and magnesium by ion exchange. Polyphosphate dosing does not remove hardness; it changes how that hardness behaves.
- Not scale removal: existing limescale deposits are not dissolved by a polyphosphate bed. Removing scale requires a dedicated descaling treatment.
- Not filtration in the particulate sense: some housings include a mechanical filter element or a screen, but the phosphate medium itself is a dissolving bed, not a sediment filter.
- Not disinfection: polyphosphate media have no biocidal function.
Terminology matters here as well. “Siliphos” is a trade name associated with a particular type of phosphate glass medium, and “polyphosphate crystal,” “phosphate crystal” and similar terms are used loosely in the market. Different products have different dissolution behaviour, temperature limits and water-chemistry tolerances. Specifications should not be assumed to transfer from one product to another.
Checking a short burst of high demand: a practical procedure
Work through the following steps before selecting or approving a medium for a bath-filler duty.
- Define the burst. Record the flow rate at the bath filler during a full-open fill, the duration of a typical fill, the number of fills per day, and the water temperature delivered at the outlet.
- Confirm the treated path. Trace the pipework from the doser to the bath filler. Note whether the outlet is fed hot, cold or blended, and whether any part of the draw bypasses the doser.
- Convert bursts into a treated volume. Multiply fill volume by fills per day and add all other treated draws. This figure, not average hourly flow, is the basis for media consumption.
- Check contact time at peak flow. Compare the residence time during a full-open fill with the contact time assumed in the media supplier’s data. A medium validated at low flow is not automatically validated at high flow.
- Check hydraulic behaviour. Confirm the pressure drop across the housing at peak flow, the bed depth, the flow direction, and whether the medium can fluidise, compact or channel at the maximum rate.
- Check the temperature at the doser. Phosphate glasses do not all behave the same way in hot water. Confirm the maximum temperature the medium is rated for and where in the system it will actually sit.
- Check water chemistry. Hardness, pH, alkalinity, chlorine or chloramine residual and dissolved solids all influence how a phosphate medium performs. Ask the supplier which of these parameters their data is based on.
- Measure, don’t assume. Test the phosphate residual at the bath filler mid-fill, not from a first-draw sample. Water that has been standing in the bed can pick up more phosphate than fast-moving water, so a first-draw reading is a poor proxy for burst performance.
What to record during the check
| Check item | Why average-flow sizing misses it | Evidence to collect |
|---|---|---|
| Peak flow at the bath filler | Average flow hides the short period of maximum demand | Measured fill rate in litres per minute at normal supply pressure |
| Duration and frequency of fills | Total daily volume can be dominated by a few large draws | Litres per fill, fills per day, seasonal variation |
| Treated path | Blended or untreaated outlets dilute the dose | Simple schematic from doser to outlet |
| Contact time at peak flow | Residence time falls as flow rises | Bed volume, void volume, peak flow rate |
| Pressure drop at peak flow | Low-flow pressure data does not describe peak conditions | Differential pressure across the housing at full open |
| Temperature at the media | Hot-water duty can exceed a medium’s rating | Measured temperature at the doser, not at the tank |
| Residual during the burst | Static or low-flow samples overstate performance | Phosphate test result taken mid-fill at the outlet |
Common failure modes during bursts
When a polyphosphate bed underperforms specifically during bath filling, the cause is usually one of the following:
- Bypass. Part of the flow routes around the medium through a poorly seated cartridge, an oversized housing, or a parallel untreated line.
- Channeling. Water finds a preferential path through the bed and much of the medium never contacts the flow, so the effective dose drops at higher velocities.
- Media movement or loss. High flow can lift or fluidise a bed if the housing is not designed for that rate, and fines or whole beads may be carried downstream.
- Exhaustion ahead of schedule. Burst demand consumes media faster than a daily-average calculation suggests, so replacement intervals based on average flow are too long.
- Temperature effects. A medium installed on a hot line beyond its rated temperature may dissolve faster, deform, or lose mechanical integrity.
- Water chemistry mismatch. Hardness, pH or oxidant levels outside the range assumed by the supplier can shift performance in either direction.
Specification questions for a media supplier
Ask these before committing to a product for a bath-filler circuit:
- What is the maximum continuous and peak flow rate per unit of media, and how was that figure determined?
- Is there release-rate data as a function of flow, or only a single rated service volume?
- What temperature range is the medium rated for, and does that rating apply to continuous hot-water exposure?
- How does the product behave at the hardness, pH and alkalinity of the actual supply water?
- What is the recommended replacement criterion — volume treated, elapsed time, or a measured residual?
- Is the product intended for drinking-water contact in the destination market, and what documentation supports that?
On that last point, “food grade” and “suitable for drinking water” are not the same statement. Drinking-water suitability depends on the exact product, the dose applied, the contact materials and the regulations of the destination market. It should be confirmed for each of those conditions rather than inferred from a general material description.
Frequently asked questions
Does a polyphosphate filter soften water for a bath filler?
No. It inhibits scale formation; it does not remove calcium and magnesium. If hardness reduction is the objective, ion exchange softening is the relevant technology, and polyphosphate dosing may be used alongside it rather than instead of it.
Will it remove existing scale from a heater or bath surface?
No. Polyphosphate dosing is a preventative treatment. Existing deposits need a separate descaling procedure.
Can a doser be fitted on the hot-water side?
Only if the specific medium is rated for that temperature and the housing is suitable for hot water. Many installations place the doser on the cold feed ahead of the heater instead. The correct arrangement depends on the product and the system layout.
How do I know whether the dose is adequate during a bath fill?
Measure the phosphate residual at the outlet while the bath is filling, and compare it with the target agreed with the media supplier. A mid-fill sample is far more informative than a first-draw sample from a tap that has been idle.
How often should the media be replaced?
Replacement is normally triggered by treated volume, elapsed time, or a falling measured residual. Because burst demand can consume media faster than average-flow estimates predict, the replacement interval should be set from the burst-based volume calculation and confirmed by monitoring.
Getting the material right for your duty
The outcome of a bath-filler installation usually comes down to a handful of details: the true peak flow, the treated path, the temperature at the media, the contact time during a fill, and the actual water chemistry. When those are known, selecting a phosphate medium becomes a straightforward comparison rather than a guess.
If you are evaluating polyphosphate or Siliphos-type crystal media for a bath-filler or hot-water circuit, send ENVO CHEMICAL the application details — outlet type and expected peak flow, fill duration and frequency, water temperature at the doser, source-water analysis (hardness, pH, alkalinity, oxidants), housing or equipment type, required specification, target objective and quantity, plus the destination market and any documentation you need. That information is enough to check whether a given medium fits the duty and to identify what still needs verification.
