The short answer
When a home moves from a tank-fed (vented) system to a combi boiler, siliphos treatment almost never carries over unchanged. Either the dosing point is removed with the tank and cylinder, or the remaining dose is now in the wrong place, at the wrong flow rate, for an appliance that is far less tolerant of scale. In practice the treatment has to be relocated to the cold mains inlet upstream of the combi, re-sized for the flow the boiler needs, and re-checked for compliance if treated water still reaches a drinking tap. Siliphos crystals left in a loft that no longer holds a tank do nothing at all.
Why a combi conversion breaks the old treatment
The dosing point disappears with the tank
In a vented installation, polyphosphate media is typically dosed in one of three ways: loose in the cold water storage tank, inside a cartridge fitted on the feed to the hot water cylinder, or on a dedicated branch that only serves the cylinder. All three arrangements depend on a tank and a cylinder that a combi conversion removes. When the loft tank and cylinder are taken out, the media bed goes with them — and anything still sitting in abandoned pipework is in a dead leg, where water stagnates and no longer passes through the appliance.
The appliance behaves differently
A combi produces domestic hot water on demand inside a plate heat exchanger with narrow waterways and a high heat flux. Scale deposits form faster in that environment than in a cylinder, and the consequences are different: a scaled plate exchanger reduces hot water output and can eventually block, so scale control becomes an appliance-protection issue rather than only a comfort issue. Existing scale in an old cylinder was largely a storage problem; in a combi it is a performance and repair problem.
Mains pressure and flow now set the rules
A combi is fed directly from the mains, and everything installed upstream sits in the path of the full flow needed to fire the boiler and hold a stable hot water temperature. A dosing cartridge or housing that was adequate on a gravity branch may add unacceptable pressure loss on a mains-fed combi, particularly at peak draw-off when a shower and a tap run together. Flow rate and pressure drop therefore become primary selection criteria, not afterthoughts.
What siliphos actually does — and what it does not
The term “siliphos” is used loosely in the trade, and it is worth being precise before deciding whether it is still the right answer after a combi conversion. Siliphos-type media are slow-dissolving polyphosphate-based materials, often blended with silicate. They release a small amount of phosphate into the water as it passes through the bed. The phosphate interferes with the growth of calcium carbonate crystals, so hardness minerals tend to stay suspended rather than depositing as hard scale. This is scale inhibition, not water softening.
| Function | Does siliphos-type treatment provide it? | Notes |
|---|---|---|
| Scale inhibition | Yes — this is the core function | Works on the tendency of calcium carbonate to deposit; effectiveness depends on dose, contact and water chemistry |
| Water softening (hardness removal) | No | Calcium and magnesium remain in the water; only ion exchange or similar processes remove them |
| Removal of existing scale | No | Inhibition slows new deposits; it does not dissolve what is already on a heat exchanger |
| Corrosion inhibition | Depends on the blend | Some products include silicate for film-forming corrosion protection; confirm the specific grade |
| Sediment or chlorine removal | No | Requires separate filtration media |
| Disinfection | No | Requires a separate microbiological treatment step |
If the previous installation was chosen mainly to stop scale in a cylinder, the same logic still applies to a combi — but if the water is genuinely hard, inhibition may need to be reviewed alongside softening rather than treated as a like-for-like swap.
A changeover review checklist
Before ordering replacement media, work through the installation as it now stands rather than as it used to be.
- Locate the old dosing point. Was the media in the loft tank, in a cartridge, or on a cylinder branch? If the answer is the tank or the cylinder, that treatment no longer exists in the system.
- Confirm what still receives treated water. After conversion, the kitchen cold tap is usually fed straight from the mains. Decide whether it should be dosed, bypassed, or left untreated, based on local rules and the product’s approvals.
- Measure the flow the combi needs. Establish the design flow at peak demand and the available dynamic mains pressure, then check the pressure drop the proposed housing and media bed will add.
- Check for dead legs. Old pipework left in place after a tank removal can hold stagnant water; if a cartridge is connected to it, the media is effectively offline.
- Review the water analysis. Hardness, alkalinity, pH, temperature and any other relevant parameters determine whether inhibition alone is appropriate.
- Verify approvals for the exact product. A media described as food grade is not automatically approved for drinking water in every market; the specific grade, application and destination market must be checked.
- Plan maintenance access. A point-of-entry housing on a mains-fed combi needs to be reachable for media replacement without disturbing the boiler.
Selecting the replacement dosing arrangement
Point-of-entry dosing on the cold mains
The most common answer after a combi conversion is a refillable cartridge or housing installed on the incoming cold main, upstream of the boiler and upstream of any branch that should be treated. This keeps dosing proportional to the water actually used by the appliance instead of relying on a tank that no longer exists. The main selection variables are the media grade, the housing size needed to give adequate contact without excessive pressure loss, and the flow range over which the installation will operate.
When inhibition alone is not enough
In harder water areas, installers increasingly combine scale inhibition with a physical water softener on the cold main, or use a softener for the whole house and reserve phosphate dosing for specific branches. This is a design decision, not a product preference: inhibition changes how scale behaves, softening removes the hardness minerals that form it. Siliphos-type media should not be presented as a substitute for softening where the objective is genuinely to reduce hardness.
Approvals, the kitchen tap and hot water temperature
If treated water reaches a drinking water outlet, the exact product must be approved for that use in that market, and the local regulator’s limits on phosphate addition apply. Separately, polyphosphate behaviour is temperature-dependent: dosing is normally carried out on the cold side for this reason, and any proposal to dose heated water should be checked against the media supplier’s own guidance rather than assumed.
Specification details to confirm before ordering media
| Item | Why it matters after a combi conversion |
|---|---|
| Media type and blend | Determines whether the product provides scale inhibition only or also corrosion inhibition |
| Dissolution behaviour and recommended dose | Must be supplied by the manufacturer for the specific grade; it cannot be transferred from another product |
| Flow range and pressure drop | The combi needs its design flow at the outlet; an oversized or undersized housing both cause problems |
| Contact volume per unit of flow | Sets the minimum housing size for the installation |
| Water chemistry limits | Hardness, pH and alkalinity ranges must be confirmed for the grade in question |
| Regulatory status | Potable-water suitability depends on the exact product, application and destination market |
| Replacement indicator | Establish how media exhaustion will be judged in service — by throughput, by inspection, or otherwise |
| Disposal and handling | Affects site practice and maintenance planning |
Five mistakes to avoid during the changeover
- Assuming the old cartridge still works. If it is no longer in the water path, it is not treating anything.
- Sizing on house size instead of flow. A combi’s demand is set by its hot water output, not by the number of bedrooms.
- Confusing inhibition with softening. Selling siliphos as a softener leads to disappointed customers in hard water areas.
- Forgetting the kitchen tap. Decide deliberately whether it is dosed or bypassed, and make sure approvals match that decision.
- Ignoring existing scale. If the old cylinder was heavily scaled, the plate exchanger may already be affected; treatment prevents new deposits but does not clean the appliance.
FAQ
Can I just drop siliphos crystals into the new combi’s cold feed?
No. Loose media needs a controlled housing or cartridge so that water passes through the bed at a defined rate and the media does not migrate into the appliance. A refillable point-of-entry housing on the cold mains is the usual arrangement.
Does siliphos soften hard water?
No. It inhibits scale formation but leaves calcium and magnesium in the water. If the objective is to reduce hardness, an ion exchange softener or another removal process is required.
Will siliphos treatment clean scale that is already inside the combi?
No. Inhibition slows new deposit formation. Existing scale on a plate heat exchanger normally needs descaling or component replacement, and the appropriate route should be confirmed with the appliance manufacturer.
Is siliphos-treated water safe to drink?
That depends on the exact product and the market. Food-grade status alone does not establish drinking-water suitability; the specific grade must be approved for potable use under the applicable local rules, and phosphate addition limits may apply.
How long will the media last after conversion?
Service life depends on the media grade, the volume of water treated and the water chemistry. Because a combi conversion changes both the flow pattern and the volume of treated water, the previous replacement interval is no longer a reliable guide — the supplier should state the basis for the new installation.
What to send when requesting media or a review
If you are specifying siliphos or a comparable polyphosphate media for a post-conversion combi installation, the useful starting point is the installation itself rather than a product name. Send the source-water analysis (hardness, alkalinity, pH), the combi model and its required design flow, the available dynamic mains pressure, the proposed housing or cartridge type, the outlets that should receive treated water, the target objective — scale inhibition, corrosion protection or both — the quantity required, and the destination market so that the correct approval status can be confirmed. ENVO CHEMICAL can review those details against the relevant media grade and advise on the specification points that still need to be verified for your application.
