London hard water and your combi boiler

If you live in London you already know about limescale. It is the white crust on your kettle, the cloudy film on the shower screen and the build up around taps. The same thing happens inside your boiler, and combis feel it most.

How hard is London water?

Thames Water, which supplies most of London, says it takes 65% of its water from rivers and 35% from underground sources. That water passes through soft chalky limestone many times, which is why, in Thames Water’s words, all the water in its region is hard.

Thames Water uses this scale, the same one the Drinking Water Inspectorate uses:

Hardness	mg/l calcium carbonate
Soft	up to 100
Slightly hard	100 to 150
Moderately hard	150 to 200
Hard	200 to 300
Very hard	more than 300

Parts of the southern edge of London, from Morden and South Croydon southwards, are supplied by SES Water from chalk wells. SES publishes hardness for each of its supply zones through a postcode lookup. Your water bill tells you which company you are with.

Why combis are hit hardest

A combi boiler heats your radiators and your tap water, but in two different ways.

The heating side is a closed loop. The same water goes round the radiators and back to the boiler. Once its minerals have dropped out, it does not pick up many more.

The hot water side heats fresh mains water every time you run a tap. That water passes through a small plate heat exchanger, a stack of thin metal plates with narrow gaps between them. When the water heats up, some of the dissolved minerals come out as scale and stick to the plates. Every shower brings a fresh supply.

Over months and years, the gaps narrow. Less water gets through and the boiler finds it harder to control the temperature.

Signs your combi is scaling up

• Hot water runs hot, then cold, then hot again in the shower
• Hot water flow is weaker than cold flow from the same tap
• The boiler cuts out when you turn a hot tap down low
• A rumbling or kettle like noise when hot water is running
• It takes longer than it used to for hot water to arrive

These signs can also come from other faults, such as a sticking diverter valve or a failing flow sensor. That is why an engineer tests before deciding. Our post on boiler kettling explains that noise in more detail.

Converted flats work their combis hard

In Brixton, Clapham and similar areas, many Victorian houses have been split into flats, each with a combi. In a flat share, one combi might run several showers a day. More hot water through the plate heat exchanger means more scale. Our Brixton area page covers the conversions there.

What a service checks

At a combi boiler service, as well as the standard gas, flue and combustion checks, the engineer:

1. Runs a hot tap and checks flow rate and temperature
2. Compares the results with what the boiler should deliver
3. Checks any scale reducer on the cold supply
4. Notes results so next year’s figures can be compared

Scale builds slowly, so a trend over a couple of years tells you more than a single reading.

What can be done

Option	What it involves
Lower hot water temperature	Thames Water suggests about 60 degrees to reduce scale. Many combis let you set this on the panel
Scale reducer on the cold feed	Many London combis already have one. Some need a cartridge replaced
Clean the plate heat exchanger	Removing and descaling it. Gas work, priced after a look
Replace the plate heat exchanger	Often simpler on modern boilers. Priced after a look
Water softener	Thames Water does not recommend softened water for drinking and suggests a separate drinking tap. Check boiler maker rules

Building guidance for new installations expects scale protection on the water feed to combis in hard water areas, which is why many London combis already have a scale reducer. Our post on combi limescale filters explains the types.

Hot water temperature

Turning hot water down a little can reduce scale. But do not set it so low that it no longer suits your household, and remember stored hot water in cylinders needs to be kept hot enough to control bacteria. Combis do not store hot water, so this mainly matters for system and regular boilers.

Do not try to descale it yourself

Some online guides suggest running descaler through a combi. Getting to the plate heat exchanger means opening the boiler. That is gas work and must be done by a Gas Safe registered engineer. Check the ID card and the Gas Safe Register.

Summary

Hard water is part of life in London. You cannot change it, but you can slow its effect on your combi and catch problems early. A yearly service with a hot water test is the simplest way.