How to descale a hot water system manually - Fernox UK

July 3, 2026

How to descale a hot water system manually

The damaging effects that limescale can have on the lifespan of a boiler and the energy efficiency of a central heating system are significant.

Part L of the Building Regulations stipulates that where mains hard water exceeds 200 ppm, feed water to water heaters and the hot water circuit of combination boilers must have a scale reducing device fitted.

Why is descaling important

Limescale prevention
An effective method of treating water to avoid the build-up of limescale is to install an in-line Fernox Scale Reducer. An Electrolytic Scale Reducer will provide whole house protection, while a Magnetic Scale Reducer is suitable for single appliances. Both products can be easily connected directly onto 15 mm or 22mm horizontal or vertical pipe work using compression fittings or 15mm horizontal or vertical pipework using 15mm pushfit fittings.

Limescale removal
Where limescale has already formed it can be removed from a hot water system, including the storage cylinder, immersion heater, pipework and taps using Fernox DS-3 Cleaner.

After turning off the boiler and/or immersion heater, tie-up the ball valve or isolate the mains water supply to the cold water storage tank. Then open all hot water taps before draining the entire system, and close the taps again afterwards.

Having dissolved the Fernox DS-3 in warm water at the maximum concentration of 1kg per 10 litres of water, introduce the solution into the cold water storage tank either via a funnel connected to rubber or plastic tubing or directly, if the cold water storage tank serves the hot water cylinder only. Refill the system until the water level in the cold water storage tank just covers the cylinder feed connection. After this is done, turn on the boiler and/or immersion heater and heat the acid solution in the hot water cylinder to 50-60¬∞C. Maintain this temperature for at least four hours, opening all hot water taps briefly every 10-15 minutes to allow a half to one pint (284 to 568ml) of acid solution to run into a plastic container. Finally, turn off the boiler and immersion heater, and drain the hot water system completely, bailing out any residual water from the cold water storage tank.

Release the ball valve of the cold water storage tank and thoroughly rinse the entire hot water supply system, opening all hot water taps. Where applicable the cold water system should also be rinsed, as some acid may have entered via the cold water storage tank. Any waste solutions of DS-3 Cleaner should be neutralised with Fernox System Neutraliser before discharging to the drain.

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How to repair system leaks

Leaks in water systems can result from external frost damage, dry solder joints, accidental perforation during maintenance or decoration, or internal corrosion. Sealing internal leaks (central heating systems only) Small leaks, including pressure drops on combination boilers and other sealed systems, can be repaired using Fernox Leak Sealer F4. Each Leak Sealer F4 product treats 100 litres or up to

Does Fernox Cleaner F3 need to be flushed out of the system?

Fernox Cleaner F3 should ideally circulate through the system for at least one hour, or up to a week. Afterward, it should be drained, and the system flushed and cleaned before adding Fernox Protector F1 or Protector + Filter Fluid, together with a Fernox TF1 Filter, to protect the system from further corrosion and scale formation.

Can I overdose with Fernox Protector F1 or Protector + Filter Fluid?

No, overdosing is not detrimental to the system. It is important to ensure that sufficient Protector is present in the system. There is a test kit and a postal testing service to check the Protector concentration. Refer to Protector Test Kit product data sheet for further guidance.