Central Heating System Cleansing and Inhibitors: BS 7593, Sentinel vs Fernox, MagnaCleanse and Commissioning

Quick Answer: BS 7593:2019 is the UK code of practice for cleaning, flushing, dosing and the ongoing maintenance of domestic central heating systems. It requires a system clean before a new boiler is fitted, a corrosion inhibitor dosed to the manufacturer's concentration, an in-line magnetic filter, and the inhibitor concentration to be verified by test (kit or laboratory) and re-checked annually. Failure to clean, dose and document under BS 7593 invalidates almost every UK boiler warranty (Benchmark).

Summary

Magnetite sludge — black iron-oxide corrosion debris — is the single biggest killer of UK heating systems. It blocks radiator bottoms, jams pump impellers and zone valves, scours the narrow waterways of modern condensing heat exchangers, and steadily strangles efficiency. BS 7593:2019 exists to stop it: clean the system, protect it with a corrosion inhibitor, catch what's left with a magnetic filter, and keep checking. This article covers what each step involves, the difference between a chemical clean, a power flush and a MagnaCleanse, the leading inhibitor brands, and how to verify and document the dose so the warranty stands.

Every Gas Safe engineer fitting a boiler is contractually bound to this. The Benchmark scheme — backed by virtually all UK boiler manufacturers — makes cleaning and inhibitor dosing to BS 7593 a warranty condition. An engineer who fits a new boiler onto an old, un-cleaned, un-inhibited system has installed a warranty claim waiting to be rejected. The most common misconception is that "the system looked OK" is good enough — it isn't; BS 7593 is a documented, verified process, not a judgement call.

The 2019 revision tightened things significantly over the 2006 version: it made the in-line magnetic filter a clear recommendation, introduced a structured approach to verifying inhibitor concentration (you must test, not assume), and set out the ongoing maintenance regime — re-test inhibitor at the annual service and re-dose as required, with a full re-clean and re-dose typically on a defined interval. This article is the cleaning-and-protection companion to system flush, powerflush and magnetic filters.

Key Facts

• BS 7593:2019 — Code of practice for the preparation, commissioning and maintenance of domestic central heating systems; the authoritative UK standard for cleaning, flushing, dosing and ongoing maintenance
• Three-step BS 7593 process — (1) clean/flush to remove debris, (2) dose corrosion inhibitor to the correct concentration, (3) fit an in-line filter and verify/maintain
• Inhibitor dose — typically 1 litre per ~100 litres of system water, or one 500 ml dose per ~10 single-panel radiators; under-dosing leaves the system unprotected
• Concentration verification — BS 7593:2019 requires the inhibitor level to be confirmed by test kit on site or by laboratory water sample, not estimated; record the result
• Annual re-test — inhibitor concentration must be checked at every annual service and topped up if below the protective threshold; a documented inhibitor test is part of the service
• Magnetic filter — BS 7593:2019 recommends an in-line filter (magnetic for ferrous magnetite); fitted on the return to the boiler, typically in 22 mm
• Power flush — high-velocity, reversing-flow mechanical clean using a dedicated pump; best for systems already laden with sludge or with cold-spot radiators
• Chemical clean (cleanse) — a cleaner dosed into the system, circulated hot (often for hours to days), then flushed out; lighter intervention than a power flush
• MagnaCleanse — Fernox's rapid magnetic flushing service using a high-capacity magnetic filter (the MagnaCleanse rig) plus agitation; captures magnetite faster than dilution flushing alone
• Biocide — sealed systems can grow bacteria/biofilm causing black water and hydrogen gassing; a biocide is dosed where biological contamination is identified
• Cleaner contact time — a chemical cleaner is circulated hot; aggressive cleaners are typically dosed for hours, milder ones can be left for up to a week before flushing (follow the product MI)
• Aluminium heat exchangers — some boilers use aluminium HEs; the inhibitor and cleaner must be compatible (correct pH range) — using the wrong chemical can attack aluminium
• pH range — a healthy inhibited system sits around pH 6.5–8.5; very low or very high pH indicates a problem
• New build vs existing — a brand-new system still needs cleaning (flux residue, swarf, jointing compound) before dosing; "new" is not "clean"
• Documentation — record the clean method, inhibitor product and batch, dose volume, verified concentration, and filter fitted on the Benchmark checklist — this is the warranty evidence
• Re-clean interval — BS 7593:2019 anticipates ongoing maintenance with periodic re-checking; a full re-clean and re-dose is commonly carried out on a defined cycle

Quick Reference Table

Detailed Guidance

What BS 7593:2019 actually requires

BS 7593 sets out a documented sequence, not a "have a look and decide" approach:

1. Pre-clean assessment — check system water for colour/clarity, take an inhibitor reading on an existing system, inspect for sludge (cold radiator bottoms, dirty water at the filter).
2. Clean — remove debris and corrosion products by the appropriate method (chemical clean, power flush or MagnaCleanse), matched to the system condition.
3. Flush — remove the cleaner and loosened debris with fresh water until the water runs clear.
4. Dose inhibitor — add a corrosion inhibitor to the manufacturer's concentration for the actual system volume.
5. Verify concentration — confirm the inhibitor level with a test kit on site, or send a water sample to the manufacturer's lab. Record the result.
6. Fit/confirm a magnetic filter — an in-line filter on the boiler return.
7. Document — record everything on the Benchmark commissioning checklist.
8. Maintain — re-test inhibitor annually, clean the filter, top up or re-clean as the test dictates.

The verification step is what most engineers miss. "I put a bottle of inhibitor in" is not BS 7593 compliance — you have to prove the concentration is correct, because a too-large system or a partial drain-down can dilute the dose below protection level.

Chemical clean vs power flush vs MagnaCleanse

These are not interchangeable — match the method to the system condition:

• Chemical clean suits a relatively clean or new system. A cleaner (e.g. Sentinel X400, Fernox F3) is dosed in and circulated hot through the boiler and radiators, then the whole system is drained and refilled with fresh water until clear. It loosens light deposits and removes installation residues (flux, swarf, jointing paste). It will not shift heavy, settled magnetite from radiator bottoms on its own.
• Power flush is the heavy-duty option for sludged systems — cold radiator bottoms, slow warm-up, jammed valves. A dedicated high-flow pump pushes water at velocity, reversing direction and isolating radiators one at a time, with a magnetic capture in the flush rig. It physically dislodges and removes settled magnetite. See powerflush for the full procedure and when not to flush (microbore, very old/fragile pipework where a flush can spring leaks).
• MagnaCleanse (Fernox) is a faster, lower-water-use magnetic clean: a large-capacity magnetic filter and agitator are connected, the system is circulated, and magnetite is captured on the magnet rather than diluted out through the drain. Effective at pulling iron oxide quickly, often used alongside a chemical cleaner.

A common pairing: chemical clean to loosen deposits, then a power flush or MagnaCleanse to remove them, then flush clear and dose.

Choosing and dosing the inhibitor

The leading UK inhibitors — Sentinel X100, Fernox F1 Protector, Adey MC1+ — are all multi-functional: they raise corrosion resistance, buffer pH, and suppress scale and gassing. Key points:

• Dose by system volume. The label gives a dose per litre of system water or per radiator. A typical figure is 1 litre per ~100 litres, or one 500 ml dose per ~10 single radiators. Estimate the volume properly — a large house with cast-iron rads holds far more than a flat with panel rads.
• Dose into a clean, flushed system. Inhibitor dropped into a dirty system is partly consumed reacting with existing corrosion and offers reduced protection.
• Verify the concentration. Use the manufacturer's on-site test kit (a quick colour/strip test) or send a sample to their lab. BS 7593:2019 wants a confirmed result, recorded.
• Check compatibility. Aluminium heat-exchanger boilers (and some specialist systems) need a compatible inhibitor and cleaner within the right pH window. Using a product that attacks aluminium can wreck the heat exchanger — always cross-check the boiler MI.

Biocide and biological contamination

Sealed systems can develop bacterial growth and biofilm, especially where the system runs at modest temperatures (heat pumps, underfloor heating) or where oxygen has been ingressing. The signs are smelly black water, hydrogen gassing (radiators repeatedly needing bleeding), and a rotten-egg odour. Where biological contamination is identified, a biocide is dosed in addition to the corrosion inhibitor. Low-temperature systems (heat pump and UFH) are particularly prone and should be assessed for biocide as part of commissioning — relevant to heat pump cylinders and cylinder selection installs.

Magnetic filter — the third leg of the stool

BS 7593:2019 recommends an in-line filter, and for ferrous magnetite a magnetic filter is the standard. It is fitted on the return pipe to the boiler (the manufacturer's preferred position), usually in 22 mm, and continuously captures the iron oxide that corrosion inevitably produces over time even in an inhibited system. The filter is serviced annually — emptied of captured sludge and the magnet wiped down. A clean inhibited system with a filter is the BS 7593 belt-and-braces approach; the filter alone is not a substitute for cleaning and inhibitor. Full detail in magnetic filters.

Commissioning documentation and warranty

The whole point of BS 7593 from a commercial standpoint is the warranty. The Benchmark Commissioning Checklist (in the back of the boiler manual) has a dedicated water-treatment section. Record:

• Clean method used (chemical / power flush / MagnaCleanse)
• Inhibitor product name and batch/lot
• Volume dosed and estimated system volume
• Verified inhibitor concentration (test result)
• Filter make/model fitted

If a heat exchanger fails under warranty, the manufacturer will ask for this evidence. No documented BS 7593 water treatment, no warranty. This ties directly into central heating commissioning and commissioning procedure.

Frequently Asked Questions

Do I really need to clean a brand-new system before dosing?

Yes. A new system contains installation residues — soldering flux, copper swarf from cutting pipe, jointing compounds, oils and grease from manufacture. These are corrosive and will start magnetite production immediately. BS 7593:2019 requires a clean of new systems too; "new" does not mean "clean". A cleanser circulated and flushed out, then inhibitor dosed, is the correct sequence.

Sentinel or Fernox — does it matter which I use?

Both Sentinel X100 and Fernox F1 are well-proven, BS 7593-compliant corrosion inhibitors, as is Adey MC1+. What matters far more is that you (a) clean first, (b) dose to the correct concentration for the system volume, (c) verify by test, and (d) use a product compatible with the boiler's heat exchanger material. Don't mix brands without checking compatibility, and stick to one product family per system for predictable test readings.

How do I know if the inhibitor level is still OK?

Test it. The manufacturer sells an on-site test kit (a quick strip or colour test) and offers a free or low-cost laboratory water analysis. BS 7593:2019 requires the level to be verified at every annual service, not assumed. If the test shows the concentration has fallen below the protective threshold — common after a leak repair, a drain-down, or radiator changes — top it back up and record it.

Can a magnetic filter replace cleaning and inhibitor?

No. A magnetic filter captures magnetite that has already formed, but it does not prevent corrosion and it does not remove non-magnetic deposits or limescale. BS 7593:2019 treats cleaning, inhibitor and filter as three complementary steps. The inhibitor stops corrosion happening; the filter mops up what slips through; the clean gives them a fair start. Relying on the filter alone leaves the system corroding.

How often should a system be re-cleaned?

The inhibitor is tested and topped up annually. A full re-clean and re-dose is carried out when testing or symptoms indicate (low inhibitor that won't hold, returning cold spots, dirty filter catches) and on the periodic cycle anticipated by BS 7593:2019. After any major work that drains the system — boiler swap, multiple radiator changes — re-clean and re-dose, then re-verify.

Regulations & Standards

• BS 7593:2019 — Code of practice for the preparation, commissioning and maintenance of domestic central heating systems (cleaning, flushing, dosing, filtration, ongoing maintenance)

• Benchmark Commissioning & Service Record (HHIC) — water-treatment section is a boiler warranty condition

• Manufacturer's Installation & Servicing Instructions — heat-exchanger material compatibility, dose rates, filter requirements

• BS EN 14868 / BS EN 14336 — corrosion protection and commissioning of water-based heating systems (referenced background)

• Gas Safety (Installation and Use) Regulations 1998 — overarching duty for safe, properly commissioned gas appliances

• BSI — BS 7593:2019 — Code of practice for preparation, commissioning and maintenance of domestic central heating systems

• HHIC Benchmark — commissioning checklist water-treatment requirements

• Sentinel — Best Practice / BS 7593 guidance — cleaning, dosing and verification guidance

• Fernox — Technical resources — F1/F3 and MagnaCleanse technical documentation

• Adey — MagnaClean and water treatment — magnetic filtration and dosing best practice

• system flush — chemical clean vs power flush comparison and step-by-step procedures

• powerflush — when to power flush, procedure, and when not to flush

• magnetic filters — in-line magnetic filter selection, installation and annual cleaning

• central heating commissioning — full commissioning sequence including water treatment documentation