Hot water systems: a complete guide for Australian homes

Hot water is worth more attention than it usually gets. It runs continuously for ten to fifteen years, it is typically the largest or second-largest energy use in the house, and the decision is normally made in a hurry because the old one has failed. Reading this before that happens is the single most useful thing you can do about it.

The four system types

The temperature rule, and why it matters

Storage systems hold water at around 60°C because that is what suppresses Legionella. That water would scald, so the plumbing standard requires delivery to outlets used for personal hygiene at no more than 50°C, and a tempering valve blends cold water in to achieve it.

Sizing

Sizing is about peak simultaneous demand rather than household size alone. Four people showering consecutively in the morning is a harder problem than four people spread across the day. Undersizing produces the classic complaint of running out mid-shower; oversizing on a storage system means paying to keep water hot that nobody uses. For continuous flow, the constraint is flow rate rather than volume.

What drives replacement cost

• Changing fuel type. The biggest single driver, because it means new connections.
• Moving the unit's location.
• Access, particularly roof-space or elevated installations.
• Compliance work the old installation predates, most often a tempering valve.
• Gas line capacity for continuous flow units.
• A dedicated electrical circuit for heat pumps and some electric units.

Rebates

Victoria publishes a hot water rebate of up to $1,000, or up to $1,400 for eligible locally made products, structured as a 50% rebate on the product purchase price. Eligibility requires combined household taxable income under $150,000, a property valued under $3 million, and an existing system at least three years old. It applies to heat pumps and electric-boosted solar, which can change which system is cheapest overall. Other states run their own schemes or none.

Signs yours is near the end

• Rusty or discoloured hot water, which usually means the tank is corroding internally.
• Water pooling under or around the unit.
• Running out of hot water noticeably sooner than it used to.
• Age beyond about ten to fifteen years for a storage unit.
• Repeated relief valve discharge, which is a symptom rather than a nuisance.

A failing unit gives warning more often than not. Replacing on your own timetable rather than in an emergency is worth real money, because emergency replacement removes your ability to compare quotes or consider a different system type.