A scale reducer is recommended by boiler manufacturers rather than required by law, and the trigger is a hardness figure above 200 ppm in your mains supply. Worcester Bosch, Ideal, Glow-worm and Baxi all ask for a proprietary device in the cold feed to the boiler in a hard water area. Your water company publishes the hardness figure for your postcode, and that figure decides the question for your home.
When does a boiler manufacturer ask for a scale reducer?
Boiler manufacturers set the threshold at 200 ppm of hardness in the mains supply. Worcester Bosch recommends a proprietary scale reducer in the cold mains connection to the appliance where temporary hardness is above that figure. The installation instructions add two conditions. The device is fitted to suit the local water company's requirements, and an isolating valve is provided so it can be serviced.
- Worcester Bosch: a proprietary scale reducer in the cold mains connection where temporary hardness is above 200 ppm, with an isolating valve for servicing.
- Ideal Heating: where water hardness exceeds 200 mg/litre (200 ppm), a proprietary scale reducing device is recommended in the cold feed to the boiler.
- Glow-worm: a scale reducer above 200 mg/l, with a double check valve assembly upstream and the position taken from the manual.
- Baxi: in a hard water area, a suitable device should be fitted to treat the mains water supply to the boiler.
The answer is written down at commissioning. The Worcester Bosch checklist asks the installer whether the property is in a hard water area above 200 ppm. It then asks whether a scale reducer was fitted and what type it was. That record stays with the installation paperwork for the appliance.
How do you find out your water hardness before a new boiler is fitted?
The Drinking Water Inspectorate sets out the hardness classes, and your water company publishes the figure for your postcode. Hardness is measured in milligrams per litre of calcium carbonate. Up to 100 mg/l is soft, 100 to 150 slightly hard, 150 to 200 moderately hard, 200 to 300 hard, and above 300 very hard. The minerals come from the local geology, as calcium and magnesium dissolve when water passes through rock.
There is no legal limit for hardness in the drinking water regulations. The Drinking Water Inspectorate points instead to World Health Organization guidance from 2017, which places the point where limescale begins to form at around 200 mg/l of calcium carbonate. That is the same number the boiler manuals use, which is why one figure settles the whole question.
The survey before the work is where the figure belongs. Our page on new boiler installation in London describes the free home survey and the written fixed price that follows it. Manufacturers expect hardness to be established by reference to the local water company, not estimated on site.
Scale reducer or water softener: which one does a new boiler call for?
Both devices are sold against hard water, but only one of them is what a boiler manual asks for. A scale reducer treats the cold feed to the appliance, while a softener changes the water throughout the house.
| Criterion | Scale reducer at the appliance inlet | Whole-house water softener |
|---|---|---|
| What it treats | Only the cold mains feed to the boiler or cylinder | All water in the house, unless a separate unsoftened supply is kept |
| How it works | Magnetic or electrolytic types hold back or alter the scale so it does not stick to the pipe wall | Ion exchange at resin beads removes calcium and magnesium from the water |
| What the boiler manual asks for | A proprietary scale reducer above 200 ppm, of a type the manufacturer accepts | Manuals do not ask for a softener, so it stays a household choice |
| Drinking water rules in England | The drinking supply is unchanged, so Approved Document G is not engaged | Approved Document G: softened water not in place of wholesome water at a drinking tap or food preparation sink |
| Installation detail | Fitted in the cold feed with isolation for servicing; Glow-worm requires a double check valve upstream | Needs plumbing changes and a separate unsoftened supply for drinking and cooking |
Viessmann describes the two scale reducer types plainly. A magnetic device holds back the minerals that start the deposit, and an electrolytic device changes the structure of the scale so it does not cling to the inside of the pipe. Either way, the installation is work for a suitably qualified engineer.
What limescale does to a combi boiler and to a hot water cylinder
Limescale settles on the hottest surfaces first, which in a combi boiler means the heat exchanger. Ideal Heating calls the result kettling: the deposit narrows the waterway, the flow slows and churns, and water boils in pockets against the metal. You hear it as popping, banging or a whistle while the boiler heats water. Putting it right means removing and cleaning the heat exchanger, or replacing it.
Sludge from corrosion makes a similar noise, so the cause has to be identified rather than assumed. Our guide on a boiler banging or kettling when it heats water sets the two apart.
In a hot water cylinder, the scale collects on the immersion heater element. Heatrae Sadia treats anything above 200 ppm as a hard water area for its Megaflo cylinders and advises considering water treatment above that figure. The same manual asks for shorter service intervals in hard water, so build-up on the elements is spotted early. An engineer drains the cylinder to descale the elements, and an annual service is recommended.
Your own checks stop at boiler pressure, thermostat and programmer settings, the reset button and bleeding radiators. Opening a boiler casing or working on the flue is work for a Gas Safe registered engineer under the Gas Safety (Installation and Use) Regulations 1998 as amended, which apply in England. Descaling a cylinder element means draining the unit, which is also work for a competent person.
Does a boiler warranty still cover damage caused by limescale?
A boiler warranty generally does not cover damage caused by limescale. The Baxi warranty terms and conditions exclude damage from hard water scale deposits. They also exclude damage from sludge caused by corrosion in the system. Those are the two things a hard water area and an old system are most likely to produce.
The same terms set conditions on the installation itself. They require the boiler to be installed strictly in line with the installation instructions, including cleaning the system and adding an inhibitor to BS 7593. They also require an annual service by a Gas Safe registered engineer. Other manufacturers set their own conditions, so the terms supplied with your appliance are the ones that count.
Read together, the scale reducer stops looking like an optional extra. It is one of the conditions you can meet before the appliance is even commissioned. Cleaning the existing pipework belongs in the same conversation, and our guide on whether you need a power flush with a new boiler covers what manufacturers expect at that stage.
Does a magnetic filter protect a boiler against limescale as well?
A magnetic filter does not deal with limescale. The filter sits in the central heating circuit and collects magnetite, the black iron oxide sludge produced by corrosion inside radiators and pipework. Limescale forms elsewhere, in the mains water running through the domestic hot water side of a combi or into a cylinder. Two different waters, two different deposits, two different devices.
The heating water carries its own hardness condition, and the number is different. Viessmann's installation instructions require fill and top-up water above 300 ppm to be softened, and ask for heating water of drinking water quality. Unsuitable fill water increases deposits and corrosion in the circuit. The 200 ppm figure in the boiler manuals applies to the mains water on the hot water side, not to what goes into the radiators.
Both measures can sit in the same house without either replacing the other. Our guide on whether a magnetic filter is worth it on a central heating system explains what the filter actually catches.
Does turning down the hot water temperature reduce scale in a combi or a cylinder?
A lower hot water temperature slows scale formation, but a combi boiler and a stored cylinder are different cases. Worcester Bosch names a lower domestic hot water setting of around 45 °C as an alternative to a scale reducer. Glow-worm asks for hot water at 50 °C or below once its hardness threshold is passed. Both figures describe water heated on demand, with nothing standing in a vessel.
A cylinder stores the water, which changes the temperature question completely. HSE guidance on hot and cold water systems states that legionella bacteria multiply between 20 °C and 45 °C. The same guidance states that they do not survive above 60 °C, and sets stored water above 60 °C with distribution above 50 °C. It is written for employers and landlords under the Approved Code of Practice L8.
Heatrae Sadia approaches the same cylinder from the scale side. In very hard areas it suggests limiting the operating temperature to between 60 °C and 65 °C rather than 80 °C. The setting at the cylinder is made by your engineer, not adjusted as a household fix. Our guide on what temperature a hot water cylinder should be set to goes through both sides of it.
Is a scale reducer for a new boiler required by law in England?
No rule in England requires a scale reducer by name. Approved Document L is the guidance supporting the Building Regulations 2010 in England. It says that in hard water areas, suitable measures should be taken to treat the feed water to water heaters and to the hot water circuit of combination boilers. The aim stated is to reduce limescale accumulation, and the same guidance expects heating and primary circuits to be cleaned and flushed at commissioning with a suitable inhibitor added.
Two further rules shape how any such device is installed. The Water Supply (Water Fittings) Regulations 1999 apply in England and Wales. They require fittings of appropriate quality and suitability, installed in a workmanlike manner, and prohibit installations likely to cause waste, misuse, undue consumption, contamination or erroneous measurement. Approved Document G in England allows softened wholesome water at a basin, bidet, bath or shower, but not in place of wholesome water at a drinking tap or at a sink where food is prepared.
If your hot water comes from an unvented cylinder, the hardness figure belongs in the specification rather than in the first service visit. Our page on Megaflo and unvented cylinders in London explains how that work is surveyed and priced in writing before anything starts.
Sources
- Drinking Water Inspectorate: Water Hardness / Hard Water
- Worcester Bosch: Installation and Servicing Instructions R 30/35/40 HE plus combi
- Ideal Heating: Exclusive 2 Installation and Servicing guide
- Ideal Heating: What is kettling in a boiler and is it dangerous?
- Glow-worm: Easicom 3 combi installation and maintenance instructions
- Baxi: Customer Support Warranty Terms and Conditions
- Heatrae Sadia: Megaflo Eco installation manual
- Viessmann: How to prevent limescale in boilers
- Viessmann: Vitodens 050-W installation and service instructions
- Approved Document L, Conservation of fuel and power, Volume 1: dwellings
- Approved Document G with 2024 amendments: sanitation, hot water safety and water efficiency
- The Water Supply (Water Fittings) Regulations 1999
- HSE: Legionella, hot and cold water systems
- HSE: Gas safety, home owners