Direct answer
For an indirect hot water cylinder, Siliphos or a similar polyphosphate crystal should be installed on the domestic water side, normally in the cold water feed to the cylinder and upstream of the heat exchanger. It should not be dosed into the primary heating circuit. In simple terms, treat the water that becomes hot water at the tap, not the closed loop that carries heat from the boiler.
Why the domestic side is the correct side
An indirect cylinder separates two water circuits:
- The primary circuit carries heated water or a glycol mix from the boiler, heat pump, or other heat source. It is normally a closed loop and is not consumed at the tap.
- The domestic or secondary circuit is the water that enters the cylinder, is heated by the heat exchanger, and then leaves as hot water for showers, taps, and appliances.
Scale forms when hard water is heated. In an indirect cylinder, the domestic water is the side that is heated and the side where calcium carbonate can deposit on the heat exchanger surfaces. The primary side is usually treated as a closed heating system with corrosion and scale inhibitors designed for that duty. Siliphos is a polyphosphate material used for scale inhibition in domestic water, so its place is on the domestic cold inlet, before the water reaches the heat exchanger.
Siliphos and similar polyphosphate media are not water softeners. They do not remove calcium and magnesium. They work by interfering with the formation of scale crystals and, depending on the formulation, by forming a thin film that can help reduce new deposits. That is a different job from softening, filtration, descaling, or disinfection.
Where exactly to place the unit
In most indirect cylinder installations, the dosing unit goes on the cold water supply pipe feeding the cylinder. If the cylinder has a dedicated cold feed, that is the preferred line. If the cylinder is fed from a shared cold main, the decision is whether to treat the whole house or only the branch to the cylinder.
Treating only the cylinder feed can be appropriate when the objective is to protect the cylinder and its heat exchanger. Treating the whole main protects downstream hot and cold outlets, but the media is then exposed to all water use and must be suitable for the intended use, including drinking water where required.
Upstream or downstream of filters and valves?
A sediment filter is often placed upstream of a polyphosphate dosing unit to reduce fouling of the media. The exact position relative to pressure-reducing valves, combination valves, check valves, and expansion relief valves should follow the cylinder manufacturer and the dosing-unit manufacturer’s instructions. The essential point is that the media sits in the cold domestic feed before the water enters the heat exchanger. Do not place it after the cylinder on the hot outlet if the goal is to protect the heat exchanger.
What about vented cylinders with a cold water storage cistern?
If the indirect cylinder is fed from a cold water storage cistern, the dosing unit is normally installed on the cold feed from the cistern to the cylinder, upstream of the cylinder. The same principle applies: treat the domestic water before heating. If the cistern also supplies cold drinking water, confirm that the treated water is approved for that use.
Placement options compared
| Installation point | Is it correct for Siliphos? | Why |
|---|---|---|
| Domestic cold inlet to the cylinder, upstream of the heat exchanger | Yes, for scale inhibition on the domestic side | Protects the heat exchanger, cylinder, and hot water pipework. The media usually sees cold water, which is generally preferred for polyphosphate dissolution. |
| Primary heating circuit | No, not for standard Siliphos use | The primary side is a closed loop. Polyphosphate would not reach the domestic hot water. The primary circuit should use a treatment intended for closed heating systems. |
| Hot water outlet after the cylinder | Usually not recommended | It does not protect the heat exchanger upstream. The media may also be exposed to higher temperatures that can affect its performance. |
| Whole-house cold main | Can be appropriate if all water is to be treated | Must be sized for total flow and approved for potable water where required. Media consumption is higher than cylinder-only treatment. |
What Siliphos can and cannot do in an indirect cylinder
| Function | Does Siliphos do it? | Notes |
|---|---|---|
| Scale inhibition | Yes, within product limits | Helps prevent new limescale on the domestic side of the heat exchanger when correctly sized and maintained. |
| Water softening | No | Does not remove calcium and magnesium. It alters scale behavior rather than removing hardness. |
| Scale removal | No | Does not dissolve existing scale. Deposits that are already reducing heat transfer may need separate cleaning. |
| Filtration | No | Does not remove sediment, chlorine, or microorganisms. |
| Disinfection | No | Not a disinfectant and not a substitute for microbiological control. |
| Corrosion inhibition | Formulation-dependent | Some polyphosphates can form protective films, but this is not the same as a dedicated corrosion inhibitor unless the exact product is specified and approved for that duty. |
Selection and sizing factors for an indirect cylinder
- Peak flow rate and pipe size: the unit must handle the highest domestic flow without excessive pressure drop.
- Water hardness and alkalinity: harder water generally needs more attention to sizing, media volume, and replacement frequency.
- Water temperature at the dosing point: polyphosphate media are usually installed on cold water, but the exact temperature limit must come from the product supplier.
- Pressure and pressure drop: confirm the unit is suitable for the system pressure and does not interfere with cylinder safety devices.
- Media form and chemistry: Siliphos crystal, polyphosphate cartridge, and other phosphate media are not automatically interchangeable. Check the chemistry, dissolution behavior, and approvals.
- Service and replacement: plan access for media replacement and a method to monitor depletion. Do not rely on a guessed calendar interval.
- Regulatory approval: if the treated water is for human consumption, verify the exact product’s potable-water suitability for the destination market.
- Cylinder warranty: some cylinder manufacturers have requirements for water treatment. Confirm that the proposed dosing method does not affect the warranty.
Common installation mistakes
- Installing Siliphos on the primary boiler side instead of the domestic cold inlet.
- Fitting the unit after the cylinder on the hot outlet and expecting heat exchanger protection.
- Assuming Siliphos softens water or removes existing scale.
- Ignoring the media’s maximum temperature or pressure rating.
- Treating drinking water without confirming potable-water approval.
- Under-sizing the unit for peak flow, causing pressure loss or poor contact.
- Omitting upstream sediment filtration where debris could foul the media.
FAQs
Can Siliphos be installed on the primary side of an indirect cylinder?
Generally no. The primary side is a closed heating circuit and is not the water that scales on the domestic side of the heat exchanger. If the primary circuit needs scale or corrosion control, use a treatment intended for that circuit. Adding a polyphosphate crystal to the primary loop is not a substitute for a proper heating-system inhibitor.
Will Siliphos stop existing scale from reducing hot water performance?
No. It is a scale inhibitor, not a descaler. Existing scale may need mechanical or chemical cleaning if it is already affecting heat transfer or flow. Treating the feed water can help prevent further build-up, but it will not remove deposits that are already present.
Does Siliphos soften hard water for an indirect cylinder?
No. Softening means removing hardness ions, usually with an ion-exchange softener. Siliphos does not remove calcium and magnesium. It changes how scale forms. Sites with very hard water may need a softener, blending, or another approved scale-control approach, sometimes alongside polyphosphate treatment.
Is food-grade Siliphos automatically suitable for drinking water?
No. Food-grade describes one aspect of material quality. Drinking-water suitability depends on the exact product, dosage, application conditions, and local regulations. Verify potable-water approval for the specific product and destination market before use on drinking water.
Where should the unit go on an unvented indirect cylinder?
On the cold water supply to the cylinder, upstream of the domestic-side heat exchanger. The exact position relative to the combination valve, pressure-reducing valve, and expansion relief arrangement must follow the cylinder manufacturer and local plumbing code. Do not guess; the installation sequence can affect pressure and safety devices.
What to confirm before buying
- The circuit to be treated: domestic secondary side, not the primary loop.
- Cylinder type and feed arrangement: vented with cistern or unvented from mains.
- Water analysis: hardness, pH, alkalinity, and any relevant water quality data.
- Peak domestic flow rate and cold inlet pipe size.
- Dosing-point temperature and system pressure.
- Required product form, media chemistry, and potable-water approval.
- Maintenance plan, access, and replacement method.
- Compatibility with cylinder warranty and local plumbing requirements.
Inquiry invitation
If you are selecting Siliphos crystal or a related polyphosphate material for an indirect cylinder, contact ENVO CHEMICAL with the application details: cylinder type (vented or unvented), domestic water source and hardness, peak flow rate, cold inlet pipe size, dosing-point temperature and pressure, primary and secondary circuit arrangement, required drinking-water approval, and the specific objective such as scale inhibition only or combined treatment. These details are enough to discuss the technical fit without guessing at specifications.
