How Does a Water Heater Filter Work?


There are several types of water heaters. Some heat with gas or electricity, while older versions use a fine oil spray to ignite a flame instead. No matter the type of heater, water heater filters are used to prevent one thing--scale buildup. This scale forms as deposits in the pipes and tanks of the water heater.

Water Heater Scale

  • When scale builds up in a water heater without an effective filtering system, it can cause several problems. If the scale develops enough, it can clog pipes and impede water flow. Most scale problems start out smaller and coat the bottom of water heater tanks, causing them to make loud popping and groaning sounds. As this scale gets worse, the water heater may misread temperatures and shut down unexpectedly.

Hardness Definition

  • Scale deposits are caused by minerals that are dissolved into water and released again as the water is heated. These minerals are most often calcium and magnesium that have been absorbed into water supplies. Water with high concentrations of these minerals is called hard water, and is responsible for scale buildup. Water heater filters work to filter out these dissolved minerals before they can form scale.


  • Microfilters are simple fine filters, often made of carbon, that strain out particles from the water. These filters can be effective at absorbing some dissolved minerals from the water, but sometimes the dissolved particles bond with the water molecules and pass by these mechanical filters completely. These filters can also help remove odors and unwanted chemicals from water before it is heated

Ion Exchange

  • Ion exchange filters are the most common method of softening water, and are whole-house filters that soften water before it is used for any application. These filters use an electrochemical process that replaces the ions of the minerals with sodium ions, rendering them inert. These filters need a constant supply of salt to keep working.

Anode Rods

  • The anode rod inside the water heater is designed to stop corrosion by filtering unwanted molecules floating in the water. Slight hard water problems can often be solved by a good anode rod, and replacing an older anode rod for a new one can solve some problems before they begin. More heavy-duty rods, such as those made from aluminum, are more effective at keeping scale from building up.

Chemical Treatments

  • For a system that experiences frequent scale problems, chemical flushes are used to clear out recent layers of scale. This is not a filter, but a treatment using acidic, vinegar-like compounds that break up scale that has recently been deposited. This is used on tankless heater systems that have scale problems.

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  • Photo Credit plumbing image by Inger Anne Hulbækdal from
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