What is a Water Heater Filter and How Does It Work?

Posted by
John Woodard on August 30, 2023

A water heater filter protects your appliances from scale by retaining minerals in a solution so they cannot accumulate and damage your water heater. Water heater filters don't actually filter out scale but use a special blend to keep minerals dissolved in water. Our Master Water Specialist, John Woodard, explains how water heater filters work and how to maintain them. 

What is a water heater filter?

First of all, the word filter is a bit of a misnomer. The primary purpose of a water heater filter is to protect the water heater from scale. Scale destroys traditional or tankless water heaters. Water heater filters are really anti-scale products.

Learn more: How to prevent limescale buildup in your home

How does a water heater filter work?

Water heater filters are scale inhibitors that sequester or retain hardness minerals in solution so they don't come out of solution and scale up on the heating elements or on the bottom of the heater. In a tankless water heater, the scale inhibitor protects from scale accumulating in all of the orifices inside.

What are water heater filters used for?

A water heater filter protects your water heater. When hard water enters the heater and heats up, all the scaling minerals come out of solution and create rock inside the heater. This build-up reduces the efficiency of the water heater until it finally dies. The purpose of the heater filter is to keep those minerals from coming out of solution.

water heater limescale

Do tankless water heaters need a filter?

Yes, a water heater filter is essential to preventing the buildup of limescale in tankless water heaters. When the heating element becomes encrusted with limescale, it must heat the element in addition to the scale. This creates inefficiency within the system and shortens the lifespan of a tankless water heater. Additionally, the water coming from your heater will take longer to heat, possibly not reaching the desired temperature at all.

Learn more: Do I need a tankless water heater filter?

How do you install a water heater filter?

Install a water heater filter system on the cold water line that feeds the heater to eliminate or sequester these minerals before they get into the heater and create scale. Water heater filters are contained in a filter housing. The TAC or scale-net filters use a Big Blue housing that installs on the cold water line to feed the water heater.

What does a water heater filter out?

Water heater filters keep minerals from coming out of solution becoming scale, so there is really nothing filtered out. If dirt and debris get into the heater, you may want to install a filter at the point of entry to protect the whole house from dirt and debris in the water.

When do you need to replace the water heater filter?

Most water heater filters need to be replaced once per year.

The phosphate or HydroBlend inside slowly dissolves as water passes through the filter. Eventually, the water heater filter needs replaced because the material dissolves to fight against scale. Most water heater filters need replacing annually. A scale-net or TAC product could last three to five years depending on how much water goes through it and how much hardness you have in the water supply.

What causes scale in a water heater filter?

Water hardness. Hard water contains a lot of minerals like calcium and magnesium that want to come out of solution, especially in the hot environment of a water heater. When heated, these minerals come out quickly and attach to the electric heating elements. They clog the pathways of a tankless heater and settle on the bottom. Water heaters become less efficient because they have to first heat the scale and rock before they can heat your water supply.

The best way to protect your water heater is to remove the mineral with a water softener, like an ion exchange water softener. But if all you want to do is protect your heater, scale inhibitors like the HydroBlend or scale net are your best defense.

Learn more: What is a water softener?

What causes sediment to accumulate in a water heater?

If you use well water or a city water supply that contains excessive debris, you might want to think about putting a filter at the point of entry to capture the sediment. Sediment not only gets in your water heater, but it also causes damage all of your fixtures, including your dishwater, shower heads, and toilets. All water heaters have a drain at the bottom. Periodically, you should turn the water supply off and drain it to try to remove as much debris as possible.

How do you flush your water heater and how often should you flush it?

Your water heater will not need to be flushed often if it is maintained properly. Protect it from sediment and scale with either a water softer or an anti-scale product. But if debris still accumulates, turn off the incoming water supply, open the drain at the bottom of the heater, and let the water drain out. You can usually hook a garden hose up to the heater and run it to a slop sink or garage, depending upon where it is located. How frequently you flush the filter depends on how much sediment accumulates in the water tank. While you're draining it, you might want to turn the inlet on a couple of times stir the water so everything flows out the drain. 

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