Chalk Lines: Protecting a Glow-worm Combi in Britain's Hard-Water Belt

There's a particular kind of grey-white crust that builds up around a tap in Kent or Essex, and most of us scrub it off without a second thought. Inside your boiler, that same crust is forming on metal that costs a few hundred pounds to replace and which you'll never see until something starts whistling.

This isn't a scare piece. Plenty of Glow-worm combis live long, happy lives in hard-water towns. But if you're buying a new boiler in 2026 — or wondering why the hot water in your current one has become oddly temperamental — it's worth understanding what those dissolved minerals are actually doing, because the fix is usually cheap when it's chosen at quote stage and annoying when it's retrofitted later.

What hardness actually is (in one paragraph)

Rainwater is soft. As it filters through chalk and limestone — which is most of the geology under southern and eastern England — it picks up calcium and magnesium ions. That's it. That's hardness. It's measured in milligrams per litre of calcium carbonate, or in that older, charmingly British unit, degrees Clarke. Broadly: under 100 mg/l is soft, 100–200 is moderately hard, and anything north of 250 is the sort of water that leaves a visible ring in a glass within a day.

A rough rule of thumb for the UK: Scotland, Wales, Cornwall, Devon and much of the North West sit in the soft camp. London, Kent, Surrey, Sussex, Hampshire, Essex, Suffolk, Norfolk, Lincolnshire and swathes of the Midlands are hard to very hard. If you're on the border, your water company's website will give you a figure for your postcode — it takes about ninety seconds and it's the single most useful bit of admin you can do before you ring an installer.

Why the combi cares more than the system boiler

A conventional or system boiler heats water in a cylinder and doesn't much mind a bit of scale in the primary circuit. A combi is different in one specific way: it heats your hot water on demand by passing mains water through a plate heat exchanger — a stack of thin stainless steel plates with narrow gaps between them, maybe two or three millimetres wide.

Narrow gaps plus hot water plus dissolved calcium equals scale deposition. The plates don't block solid, they fur up gradually, and the effect compounds. The waterway narrows. Flow drops. The boiler has to work harder to raise the same tapful of water through a restricted channel, and your hot water can start to feel less powerful, less consistent, or noticeably cooler in winter when the incoming mains is colder anyway.

None of this is dramatic. It's a slow drift, which is exactly why it goes unnoticed for three or four years and then gets blamed on the boiler being 'past it'.

Signs worth taking seriously

• Hot water temperature that wanders. Warm, then cool, then warm again on a shower that used to be rock steady.
• Reduced flow at hot taps compared with cold, especially at the kitchen tap where you can compare directly.
• Kettling or a low rumble during hot water draw-off — scale on the primary side acts as a hot spot and encourages local boiling.
• A showerhead that clogs quarterly, which is the cheapest early-warning device you own.
• Gas use creeping up without a change in habits. Restricted waterways mean longer burns for the same amount of hot water.

If two or more of those sound familiar and you know you're in a hard area, it's worth having the plate exchanger looked at before you conclude the whole boiler needs replacing.

The filter misunderstanding

Here's where a lot of quotes get muddled, sometimes accidentally and occasionally not.

A magnetic filter — the canister on the return pipe with a magnet inside — captures black iron oxide sludge. That's magnetite, the corrosion product from the radiators and pipework. It protects the central heating side of the boiler and it's a genuinely good idea on almost any installation.

It does nothing for limescale. Scale is calcium carbonate, which is not magnetic and doesn't respond to a magnet. If someone tells you a magnetic filter will sort out your hard water, they've either misunderstood the product or they're hoping you have.

The three options that actually address scale

1. A scale reducer (phosphate dosing). A small in-line cartridge, often called a Combimate-style device, fitted on the cold mains feed to the boiler. It doses a tiny amount of food-grade polyphosphate into the water, which doesn't soften it but does interfere with scale crystallisation — so the minerals stay in suspension and pass through rather than sticking. Cheap to buy, cheap to fit, cartridge replaced roughly annually. Ideal for moderate to hard water and a sensible default on a combi swap in a chalky postcode.

2. A proper ion-exchange water softener. This genuinely removes calcium and magnesium and swaps them for sodium. It works, it's effective, and it's a whole-house appliance costing considerably more than a boiler accessory, with salt to carry and a footprint to find. Note that many boiler manufacturers — Glow-worm included — have specific guidance about softened water and certain heat exchanger materials, so raise it explicitly with your installer rather than assuming it's fine.

3. Nothing, but with a maintenance plan. In moderately hard areas on a boiler with a decent stainless exchanger and a household that doesn't use much hot water, an annual service that includes checking the plate exchanger can be a perfectly rational choice. It's a judgement call, not a moral failing.

Putting it on the quote properly

When you get your written quote, look for three things. First, whether the hardness of your area has been acknowledged at all — if the installer hasn't asked, that's a small flag. Second, whether the scale reducer is listed as a line item with a model name, not lumped into 'extras'. Third, whether the service regime you're being offered includes descaling the plate heat exchanger or just a generic annual visit.

Ask directly: 'What's the hardness figure for my postcode, and what would you fit on a combi here as standard?' A good installer will answer without hesitation. A great one will tell you when the answer is 'nothing'.

Hard water isn't a defect in your supply and it isn't a reason to avoid a combi. It's just a known local condition — like knowing your postcode gets frost earlier than most. Plan for it at quote stage and your Glow-worm will spend the next decade doing exactly what it says on the box.