PE7 8SB Water Filtration
15mm Mains — Fresh Research

A 15mm supply already carries less flow than 22mm. Adding any inline device with restriction — especially TAC cartridge units designed for 22mm — will further reduce pressure to the boiler and taps. Any unit you install on 15mm must have a low pressure drop, ideally <0.1 bar at typical domestic flow (<15 L/min).

Many UK plumbers do install 22mm TAC units on 15mm supplies using a 22mm × 15mm compression reducer on each end. The reducers cost £3–6 each. It works hydraulically — the unit is essentially a slightly wider section in the pipe — but you're paying for fittings you don't need if a native 15mm unit exists, and there's a small additional pressure drop. Ask your plumber: they'll often just do this automatically if you hand them a 22mm product, but it's worth knowing.

The good news: there are good 15mm native options, and most of the major brands (Fernox, Adey, BWT, Combimate) sell 15mm versions. You don't need to bodge it.

Price: £64.98 at Screwfix · £81.44 at Plumb Warehouse · widely available

Type: Polyphosphate dosing — releases a minute trace of food-grade polyphosphate into the cold water feed, which sequesters calcium ions before they reach the heat exchanger and coats internal surfaces with a microscopic protective layer.

Connection: Native 15mm compression — fits directly, no reducers.

Maintenance: Cartridge replacement annually (typically ~£15–20 at Screwfix/Toolstation).

WRAS Approved: Yes. Meets Part L Building Regulations requirements for combi boilers.

Max working pressure: 5 bar. Max water temp: 30°C (inlet only — it's on cold feed).

Price: £212.40 (device + Combiphos starter pack) · direct from combimate.co.uk

Type: Polyphosphate dosing (Combiphos balls) — same principle as BWT Combi Care but higher-capacity reservoir and more granular dosing control. Popular with British Gas and Vaillant-approved installers.

Connection: Native 15mm or 22mm compression — specify at order.

Maintenance: Top up Combiphos balls every 6–12 months (~£10–15).

Certification: ISO 9001:2015, WRAS Approved. 3-year manufacturer warranty.

Price: £54.99 at Screwfix (495KG) · ~£47.55 at BES.co.uk

Type: Electrolytic — uses a sacrificial zinc anode to alter the crystal structure of calcium carbonate. No power, no chemicals, no cartridges. Claimed 10-year lifespan.

Connection: Native 15mm compression.

Evidence at 280 mg/L: Electrolytic units are independently verified but work best in moderate hardness. At 280 mg/L they will reduce scale significantly but may not fully protect the heat exchanger on their own in the first 2–3 years before the protective film builds. Good as a secondary layer; acceptable as primary if budget is the constraint.

Price: ~£46.96 (MyTub) · ~£54.59 ex-VAT (trade accounts)

Type: Electrolytic, same principle as Fernox. Adey is a premium heating brand — gas engineers know and trust this product. Claimed 10-year lifespan. No power, no chemicals.

Connection: Native 15mm compression.

Note: Also available in 15mm push-fit (Plumbing Stocks) if your plumber prefers speed of install.

Price: ~£150–180 on Amazon UK (B0CCW7L18V)

Type: Electronic — clamps around the pipe, no fittings, no plumbing. Sends a 150kHz signal through the water to prevent crystal adhesion. Fits pipes up to 1.5" OD, so works on 15mm, 22mm, or anything in between without any fittings at all.

Evidence: Mixed. HydroFLOW have genuine academic research (Hydropath Technology) and anecdotal success in moderate hard water. But a 2010 Arizona State University study found electromagnetic treatment reduced scale by 42% vs 96% for TAC — well below the 80% threshold DVGW requires for certification. At 280 mg/L, this alone is not a reliable boiler protector. Some combi manufacturers will reject warranty claims if only an electronic descaler was used.

Price: Eddy ~£55–75 (Amazon UK) · Flomasta Magnetic 15mm £20.49 (Screwfix) · Fernox Magnetic 15mm £42.99 (Screwfix)

Real-world forum evidence (Screwfix Community, UK Plumbers Forums, PistonHeads) is consistently mixed-to-negative for magnetic and basic electronic units in genuinely hard areas. The Flomasta at £20.49 is popular for washing machines and dishwashers where it helps prevent deposits on elements, but for combi heat exchangers at 280 mg/L the consensus among Gas Safe installers is: magnetic alone doesn't cut it. The Eddy has a money-back guarantee but also has a notable number of "did nothing" reviews from UK hard water users.

Verdict: Fine as a supplementary protection on individual appliances (washing machine, dishwasher). Not sufficient as sole combi boiler protection at Peterborough hardness levels.

Price: £105.99 (frizzlife.co.uk) — use code JUK20 for £20 off, bringing it to ~£85.99 before any other promotions.

How it connects: Direct-connect inline to the cold supply under sink. No separate filter tap needed — your Peppermint tap stays as-is. This is the main reason SK99 was picked over competitors.

What it does: 3-stage compound activated carbon block. Removes chlorine, chloramine, lead, VOCs, heavy metals, cysts. NSF/ANSI 42 and 53 certified.

Remineralisation note: The SK99 does not strip minerals — it keeps calcium and magnesium in the water (unlike RO). At 280 mg/L you'll still have mineral-rich water, just without the chlorine taste and any heavy metal contamination. There's no remineralisation stage because it doesn't remove minerals in the first place. This is correct behaviour for a carbon-block filter.

Filter life and replacement cost: Stage 1 every 3–6 months (M3001), Stage 2 every 8–12 months (M3002, £18.99), Stage 3 every 12–18 months (M3003, £25.99). Annual running cost ~£35–55 depending on water usage.

Caveat: M3002 and M3003 replacement filters are currently showing as backordered on the UK store. Order the system now but check filter stock — or buy the first set from Amazon UK as a backup.

~£300+ upfront, requires a dedicated filter tap (not your Peppermint), produces waste water, and removes the minerals from water that are actually fine for drinking. At £300+ it's significantly outside the brief. Recommended if you specifically want near-zero TDS water or cook with hard water and want scale-free cooking. Not the call here.

~£110 first year, ~£50/yr ongoing. Has an anti-scale stage in addition to carbon filtration. Good for very hard water. Requires connecting to cold water supply but does need a separate faucet. Since you specifically don't want a second tap (Peppermint is already there), the SK99's direct-connect design wins.