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Descaling Hot Water System

Why is descaling important?

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 be chemically treated to reduce the rate of limescale accumulation.

What should be done?

Limescale prevention
An effective method of treating water to avoid the build-up of limescale is to install a Fry’s 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 horizontal or vertical pipe work using compression fittings.


Limescale removal
Where limescale has already built up it can be removed from a hot water system, including the storage cylinder, immersion heater, pipe work 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 1 kg per 2.2 gallons (10 liters) 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, 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 half to one pint 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.