Hard water and your boiler in north and east London

The water in north and east London is hard. That is not a fault with the supply. It is the geology. Thames Water says it takes 65% of its water from rivers and 35% from underground sources, and the water passes through chalk and limestone on the way. That is why it describes all the water in its region as hard.

For a gas boiler, hard water means limescale. Limescale is the chalky white crust you see in a kettle. Inside a boiler it forms on the hottest surfaces, where water is heated fast. This page sets out the local figures, what scale does, and what you can do about it.

How hard is the water where you live?

Thames Water publishes a report for each water supply zone. The figures below are the average total hardness for 1 January to 31 December 2025, taken from the zone reports Thames Water released in 2026. Hardness is measured in milligrams of calcium carbonate per litre (mg/l).

Thames Water zone	Average hardness (mg/l)	Highest sample (mg/l)
Hackney West	268	290
Hackney South	278	309
Highbury	272	286
Kentish Town	276	285
Bethnal Green	276	284
Highams Park	276	298
Tottenham South	273	288
Stratford East	263	289
Enfield Town	266	294
Stamford Hill	260	271

You can read the Hackney West report here. Thames Water’s scale calls 200 to 300 mg/l hard and anything over 300 very hard. So every zone in the table is hard, near the top of the band, and some single samples went over 300.

Not every part of north and east London is supplied by Thames Water. Essex & Suffolk Water supplies much of Redbridge, including Ilford, and Affinity Water supplies parts of Barnet and Enfield. Your water bill tells you which company you have, and each one has a postcode lookup.

What limescale does inside a boiler

Most homes in Hackney, Dalston and Clapton flats have a combi boiler. A combi heats tap water on demand through a small plate heat exchanger. This is a stack of thin metal plates with narrow channels between them. Hot water from the heating side passes one way and your tap water passes the other.

Those narrow channels are where scale settles. As it builds up:

• hot water at the tap gets weaker, or runs hot then cold
• the boiler takes longer to heat water
• the boiler may make a rumbling or kettle noise, known as kettling
• the boiler can overheat and lock out, which means it shuts itself off and shows a fault

The heating side of a sealed system is a closed loop, so it does not take in fresh hard water all the time. Scale there comes mostly from repeated topping up. If a system loses pressure and is refilled often, each refill brings in new minerals. That is one reason a leak that keeps dropping the pressure is worth fixing. Our guide on why boiler pressure keeps dropping explains the usual causes.

The guidance on hard water and new boilers

Industry guidance treats 200 parts per million as the line. One mg/l is the same as one part per million. The Domestic Building Services Compliance Guide says that where mains hardness is over 200 ppm, provision should be made to treat the water feeding water heaters and the hot water circuit. BS 7593, the British Standard for treating water in heating systems, says a scale reducer should be fitted in areas of 200 ppm and above, as Fernox summarises here.

Every Thames Water zone in our table is well above 200. In practice this means a new combi boiler in north or east London should normally be fitted with some form of scale protection on the cold water feed. If yours was not, it is worth asking about at the next service.

Ways to protect a boiler from hard water

Option	What it does	Notes
Scale reducer on the cold feed	Reduces scale forming in the boiler’s hot water side	Usual choice for combis in hard water areas
Chemical inhibitor in the heating water	Protects the closed heating loop from corrosion and scale	Should be checked at each service
Magnetic filter on the heating return	Catches iron sludge before it reaches the boiler	Cleaned out during a service
Lower hot water temperature	Scale forms faster at higher temperatures	Thames Water suggests about 60 degrees for hot water
Whole house water softener	Removes hardness from the supply	Keep a separate hard water tap for drinking, per Thames Water

A softener is a bigger job and is your decision. Thames Water advises that softened water is not used for drinking or cooking, and that softening units should be inspected regularly.

What an annual service checks

A boiler service is the time to spot scale before it causes a breakdown. During a boiler service, a Gas Safe registered engineer checks that the boiler burns gas safely and tests the flue. They also look at hot water performance, the heating water condition and any filter. If hot water flow or temperature is poor, scale in the plate heat exchanger is one of the first things to rule out.

Gas work in the UK must be done by a Gas Safe registered engineer. Ask to see their ID card and check it on the Gas Safe Register before they start. Our guide on checking an engineer on the Gas Safe Register shows what to look for.

Hackney homes and hard water

Hackney has a lot of Victorian terraces split into flats, plus large council estates. Many flats have a combi in a kitchen cupboard, used by one household for heating and every hot tap. That means heavy use of the plate heat exchanger in water at 268 to 278 mg/l. If you live in E8, E9 or E5 and your hot water has slowly got worse, scale is a likely cause. See our Hackney area page for more on local homes.

If the boiler has started making a kettle noise, read why boilers kettle before it gets worse.

Signs it is time to book

Book a check if you notice any of these:

• hot water that is weaker than it used to be
• a boiler that bangs, gurgles or rumbles when heating water
• a boiler that locks out with an overheat fault
• white crust on taps and shower heads building up fast

None of these mean the boiler is unsafe on their own. They do mean it is working harder than it should. A look at the job tells you whether a clean, a part or a scale reducer is the right fix, and the price is confirmed after that look.