Hard water and your hot water system: scale, heat pumps and fixes

1. Heat is what turns hardness into scale, so hot water systems take the hit first.
2. Above 200 mg/L the guidelines expect rapid scale build-up in hot water pipes and fittings.
3. A whole house filter alone doesn't stop scale. It needs a scale stage.
4. Check your hot water system's manual: some warranties exclude harsh water.

Why heating water makes scale

Hardness is the dissolved calcium and magnesium in water, measured as calcium carbonate. Cold, it stays in solution. Heat it, and some of it comes out as a hard, chalky deposit: scale. That's why you see it first in kettles, and why the inside of a hot water system sees more of it than any tap in the house.

The Australian Drinking Water Guidelines set an aesthetic value of 200 mg/L for hardness to limit scale in hot water systems, and warn that above it water can cause a rapid build-up of undesirable deposits, or scale, in hot water pipes and fittings. Removal of these deposits can be costly.

Hard water is a scale problem, not a safety one. Water Corporation is clear that its supply meets all health guidelines and is safe to drink whatever its hardness.

How hard is the water in Adelaide, Perth and Brisbane?

Perth varies more than almost any capital, because its supply blends dams, groundwater and desalination. Water Corporation's 2024–25 data for the Perth region shows average total hardness ranging from 29 mg/L in Dwellingup to 228 mg/L in Two Rocks, where elevated hardness is characteristic of the source supplying this locality.

Source: Water Corporation, Drinking Water Quality Annual Report data, Perth region, 1 July 2024 to 30 June 2025. Whole Home Water doesn’t install in Perth yet; this table is for information.

In south-east Queensland, Seqwater’s results for the 12 months to August 2026 average about 118 mg/L in the Brisbane zone (moderate) and about 40 mg/L in the Gold Coast zone (soft) (Brisbane, Gold Coast, PDF reports).

In Adelaide, SA Water publishes hardness for every postcode through its drinking water profile. SA Water counts more than 200 mg/L as hard and notes hard water can lead to scaling problems on your appliances and taps. Check your own postcode, or see your water by city and our guide to hard water in Australia.

What scale does to a hot water system or heat pump

Hot water is a big part of your energy bill. The Australian Government's Your Home guide notes that over a quarter of the average household's energy is used to heat water. Scale works against that in a few ways:

• Electric storage tanks. Scale settles in the bottom of the tank and coats the element.
• Heat pumps. Scale can build up on the heat-transfer surfaces and in valves, which matters more as heat pumps replace older systems.
• Valves. Water Corporation suggests releasing your hot water system's pressure valve for a few seconds each month to help prevent build-up.
• Tank life. Your Home notes sacrificial anodes in enamel tanks need replacing every few years, more frequently where water quality is poor.

Manufacturers know this. One major brand's terms say its warranty may not apply where water is scaling with a Saturation Index above +0.8, or where total dissolved solids exceed 2,500 mg/L. SA Water's advice is simply to read your appliance manuals for anything that mentions water hardness. It's worth five minutes before you buy a new system.

Signs scale is building up

You'll usually see scale around the house before it shows up as a problem inside the hot water system. Common signs:

• Kettle and showerhead. White, chalky crust inside the kettle and blocked showerhead nozzles that spray sideways.
• Glass and taps. Spots and film on shower screens, glasses out of the dishwasher and chrome tapware that won't wipe clean.
• Hot water system noises. Popping, crackling or rumbling as water heats can point to sediment and scale in the bottom of a tank.
• Relief valve trouble. A pressure relief valve that drips constantly or seizes when you test it.
• Slower hot water. Taking longer to recover after showers, or running out sooner than it used to.

None of these prove scale on their own, and a plumber should check the system itself. But if you have two or three of them and your postcode's hardness is high, scale is the likely culprit.

Scale conditioners vs softeners vs doing nothing

Results from salt-free conditioners depend on the product and your water, so ask for independent test data, not just a brochure. For most mains-water homes in hard-water areas, a filter plus a salt-free scale stage covers sediment, chlorine and scale without salt top-ups. Very hard bore water can justify a softener. The longer version is in water softener vs whole house filter.

How we handle it

At your Home Water Check we measure hardness at your tap, look at your hot water system's type and age, and ask about what you're seeing: kettle scale, spotty glasses, a noisy tank. We compare that with your utility's published figures for your postcode. If it's high enough to matter for your hot water system, your proposal will include our optional salt-free scale conditioner (priced in your quote after your free water test). If it isn't, we won't recommend it. It's never pre-ticked.

Questions

Plumbing and installation details reviewed by a licensed plumber. See our editorial policy.