Short answer: In hard water areas like London, a combi boiler should have some form of scale protection on its cold water feed. It slows limescale building up in the hot water heat exchanger. Many London combis already have one fitted under the boiler. Types include polyphosphate cartridges, magnetic and electrolytic units. Some need parts replacing, so check yours at the service.
Croydon is a good place to start this question, because it sits across two water companies. North and central Croydon are supplied by Thames Water. From South Croydon southwards, the supply comes from SES Water. Both supply hard water drawn from chalk. If you live anywhere in London, your combi is dealing with hard water.
Why scale protection matters on combis
A combi heats mains water on demand through a plate heat exchanger. When hard water is heated, some of its minerals form limescale on the plates. Over time the gaps narrow, flow drops and hot water becomes unreliable. Our post on London hard water and your combi explains what happens inside.
A scale reducer sits on the cold water pipe into the boiler. It treats the water before it reaches the heat exchanger, so less scale forms.
What the guidance says
Building guidance for new boiler installations in England expects scale protection on the water feed to combis and other water heaters in hard water areas. That is why many London combis fitted in recent years have a scale reducer already. Boiler makers’ installation manuals usually say the same, and set the water hardness level above which protection is needed.
Thames Water itself suggests scale reducers as one way to reduce the effects of hard water.
Croydon’s two water companies
| Area | Supplier | Notes |
|---|---|---|
| North and central Croydon, Thornton Heath, Norbury | Thames Water | Says all water in its region is hard |
| South Croydon, Purley, Coulsdon, Sanderstead and south | SES Water | Supply area runs from Morden and South Croydon southwards. Hardness by zone in its postcode lookup |
SES Water publishes average hardness for each of its 22 zones through a postcode lookup. Thames Water has its own lookup on its website. Check your bill to see which applies. Our Croydon area page covers the housing in each part of the borough.
The main types
| Type | How it works | Upkeep |
|---|---|---|
| Polyphosphate | A cartridge slowly releases a food safe chemical that stops scale sticking | Cartridge needs replacing, often yearly. Check maker guidance |
| Magnetic | Magnets on or around the pipe change how scale forms | Little upkeep |
| Electrolytic | A small electrical current from metal plates affects how scale forms | Usually little upkeep, some have a lifespan |
| Electronic | A powered coil around the pipe | Needs power. Check it is switched on |
Makers claim different results for each. What matters most for your combi is that the scale reducer fitted is one the boiler maker accepts, that it is fitted correctly, and that any cartridge is in date.
How to find yours
Look under the boiler at the pipes. There are usually several: gas, heating flow and return, hot water out and cold water in. The cold water in pipe often has a small isolating valve. A scale reducer, if fitted, sits on that cold pipe, often just below the boiler. Some are clear canisters with a visible cartridge. Others are small metal or plastic cylinders.
If you cannot tell, take a photo of the pipes under the boiler when you book a combi boiler service. The engineer can identify it.
What a scale reducer cannot do
A scale reducer slows new scale. It does not remove scale already inside the boiler. If your combi already shows signs of a scaled plate heat exchanger, such as swinging hot water temperatures, a scale reducer alone will not fix it. The plate heat exchanger may need cleaning or replacing.
It also does nothing for the heating side. A magnetic system filter on the heating return catches black sludge from radiators, which is a different problem. See our magnetic filter clean and health check page.
Should you add one to an older combi?
If your combi has no scale protection and you are on hard water, adding a scale reducer is often worthwhile. It is a small job on the cold water pipe. Ask at the service. If the combi is already heavily scaled, deal with that first.
Hot water temperature
Thames Water also suggests keeping hot water at around 60 degrees to reduce scale. On many combis the hot water temperature is set on the front panel. If yours is set to maximum, turning it down a little can help.
Who can fit or service a scale reducer?
Fitting a scale reducer on the cold water pipe is plumbing work. Anything inside the boiler is gas work and must be done by a Gas Safe registered engineer. At a service, the engineer can check the scale reducer and the boiler together. Check the engineer’s ID card and the Gas Safe Register.
Summary
| Question | Answer |
|---|---|
| Is London water hard? | Yes, across Thames Water and SES Water areas |
| Do combis need scale protection here? | Guidance for new installs expects it in hard water areas |
| Do I already have one? | Possibly. Look on the cold pipe under the boiler |
| Does it need upkeep? | Polyphosphate cartridges do. Others less so |
| Will it fix existing scale? | No |
Quick questions
How do I know if I have a scale reducer?
Look at the pipes under the boiler. A scale reducer is a small inline unit, often a cylinder or canister, on the cold water pipe going into the boiler.
Does a magnetic filter stop limescale?
No. A magnetic system filter catches sludge in the heating water. A scale reducer treats the cold water going to your taps. They are different parts.
Do I need one if I have a water softener?
A softener removes hardness, so a scale reducer may not be needed, but check your boiler maker's rules on softened water.
Last updated 23 October 2026. Written for homes in London. Rules can change, so check the linked sources for the latest position.