Hard water and your hot water system

When water with a high calcium and magnesium content is heated, those minerals are more likely to come out of solution and form scale. The Water Corporation notes that hard water can affect hot water systems, and it advises a simple monthly check of the pressure valves. How much this matters in Perth depends on where you live. Official 2024–25 typical hardness ranged from 29 mg/L in Dwellingup to 228 mg/L in Two Rocks. Most Perth supply localities were still inside the Australian good-quality band of 60 to 200 mg/L. The far-northern supplies, especially Two Rocks, sit higher.

This is not a product guide, and it is not a reason to replace a working system on a hardness figure alone. Use your locality reading, the hardness scale, and the servicing instructions that came with your unit. Anode intervals, in particular, vary by brand and model. A licensed plumber should do work inside the cylinder.

Suburb pages on WaterWatch still show the 2023–24 report. The Perth figures in this guide are from the 2024–25 tables. Look up your suburb for the figure the site is currently publishing, and use this guide for the newer ranking.

Why hard water and hot water do not mix well

Hardness is dissolved calcium and magnesium, reported as mg/L of calcium carbonate. The Australian Drinking Water Guidelines say readings from 200 to 500 mg/L will increase scale formation, and readings above 500 mg/L will cause high-level scaling. Scale can coat heating elements and heat exchangers, settle in a storage tank, and interfere with valves. The Water Corporation also mentions spots on glass and the extra soap needed for a lather, which are the same minerals at work on the rest of the house.

In 2024–25, Two Rocks was the only Perth locality with a typical hardness above 200 mg/L (228 mg/L, with a range of 220 to 240). Yanchep's typical reading was 198 mg/L, with a peak of 210. Neerabup's typical reading was 190 mg/L, with a peak of 200. Everywhere else in the Perth tables had a typical reading under 130 mg/L, and the softest group was under 60 mg/L. If you are in a softer supply, scale from hardness is less likely to be the main issue. If you are in Two Rocks, Yanchep or Neerabup, it is worth reading your system's manual more closely. Look up your exact locality on Does Perth have hard water? and on the map.

The Integrated Water Supply Scheme that serves most of Perth blends groundwater, desalinated seawater and dam water, and that mix changes through the year. Two Rocks is a separate groundwater scheme. A typical reading is a yearly mean from a small number of scheme samples, not a live reading inside your tank.

What Water Corporation advises

On its hard water advice page, Water Corporation says that to help prevent scale build-up in a hot water system, you should release the pressure valves for a few seconds each month. That is the official, Perth-relevant instruction we can cite. Follow the method in your owner's guide as well, because valves differ between systems, and a lever snapped back too hard can damage a seat. If a valve weeps, will not close, or does not discharge, stop and call a licensed plumber. Do not block the drain line.

The same page suggests bicarbonate soda and vinegar-based cleaning products for domestic appliances, used according to the label and rinsed thoroughly, and a slice of lemon brought to the boil for a kettle, then a refill before the next use. For a new dishwasher, it suggests checking whether your area has hard water and choosing a machine and detergent accordingly.

Anodes and servicing: follow the manual

Many storage tanks include a sacrificial anode, a rod that is designed to wear away so the cylinder lasts longer. Continuous-flow (instantaneous) units do not store a tank of water, so they do not have that anode, though their heat exchangers can still collect scale.

There is no single replacement interval that applies to every system in Perth. Manufacturer guides set their own schedules, and they often tighten the inspection period where the water supply is variable, desalinated, softened, or drawn from more than one source. Perth's blended scheme fits that "variable supply" description in several manufacturers' wordings.

One widely used Australian example, a current Rheem owner's guide, asks for anode inspection by a qualified person, with earlier inspection where the supply is variable, and a latest replacement point if the anode was not changed at a major service. The same family of guides also warns that the anode type should match the water's total dissolved solids, and that an incorrect anode can become overactive. Most Perth localities had a 2024–25 typical TDS above 150 mg/L, which is a threshold some of those guides treat as significant.

Use the owner's guide for your exact model, found from the rating plate. If the guide schedules anode work, have a licensed plumber do it. Do not treat a blog interval, including this one, as a substitute for that document. Servicing is usually not included in a standard warranty, and manufacturers often specify genuine parts.

What this does not mean

It does not mean every Perth home needs a water softener. Softening is one way to reduce hardness, but it is a household choice, and WaterWatch does not sell or recommend brands. A carbon filter can improve chlorine taste. It does not remove calcium and magnesium.

It also does not tell you which hot water system to buy. Warranty conditions, energy source, household size and the manual's water-quality limits should drive that decision, with advice from a licensed plumber if you need it.