Common Hot Water Terms Explained
Some hot water terms can be confusing ā here's a plain-English guide to the systems, valves and safety devices we work with every day.
System Types
The main kinds of hot water system you'll find in Sydney homes.
Storage Hot Water System
A storage system heats and stores water in an insulated tank until it's needed. Common in Sydney homes and available in electric, gas and solar. Tank size (typically 50Lā400L) determines how much hot water you have before it reheats. Available in single or twin (double) element.
Continuous Flow / Instantaneous
A continuous flow (instantaneous) heater heats water only when a tap is turned on, so you never run out. Compact and energy-efficient, they're rated by how much water they heat per minute ā e.g. 26 L/min ā and suit households with high hot water demand.
Electric Hot Water
An electric system uses one or two heating elements inside the tank to warm the water. Reliable, simple to install and ideal where gas isn't available. Running on an off-peak tariff can significantly reduce running costs. Available in a range of tank sizes.
Gas Hot Water
A gas system heats water with a gas burner ā available as both storage tanks and continuous flow units. Gas heats water faster than electric and isn't affected by electricity tariffs, making it a popular, cost-effective choice for homes on natural gas or LPG.
Solar Hot Water
Solar systems use heat from the sun to warm your water. Two main types: Lowline (tank on the ground, panels on the roof) and Highline (all components on the roof). Most include a gas or electric booster for when solar gain isn't enough.
Heat Pump Hot Water
A heat pump works like a reverse fridge ā it draws warmth from the surrounding air to heat your water, using up to 60ā70% less electricity than a standard electric system. One of the most energy-efficient options, and may qualify for government rebates.
Valves & Safety Devices
The valves and protection devices that keep your system safe and compliant.
Tempering Valve
A tempering valve mixes hot water from the tank with cold water to deliver it to your taps at no more than 50Ā°C, reducing the risk of scalding. Required under AS/NZS 3500 and must be fitted to any tank replacement.
PTR Valve (Pressure & Temperature Relief)
The main safety device on a storage tank. It releases water if pressure or temperature climbs too high, preventing the tank from over-pressurising. Occasional dripping is normal ā but constant discharge means the valve or system pressure should be checked.
Why Is Water Leaking From My Relief Valve?
Water discharging from the pressure/temperature relief valve usually means the system is releasing excess pressure or heat ā occasional dripping can be normal. Constant leaking, however, may point to a faulty valve or a pressure problem and should be checked by a licensed plumber.
Expansion Control Valve (ECV)
Fitted to mains-pressure systems, the ECV relieves the extra pressure created as water heats and expands. It takes the strain off the tank and PTR valve, helping the system last longer. In many areas it's required to keep your installation compliant.
Safety Tray
Under AS/NZS 3500, water heaters installed indoors in concealed or non-wet areas (no floor drain) must sit in a safety drip tray connected to an approved drain ā catching leaks before they can damage floors, walls or ceilings below.
Leak-Protection Valve
Where a safety tray can't drain to a suitable point, an approved leak-protection / automatic shut-off valve is fitted instead. We install devices such as the Mildred shut-off valve, the AVG eFlow stop valve and the Thompson leak-detection valve ā each detects water in the tray and automatically shuts off the supply.
Duo Valve
A duo valve combines two functions in one: a stop tap (to shut off water flowing into the heater) and a non-return valve (to stop hot water flowing back down the cold pipe). It lets you quickly isolate the hot water supply in an emergency or for plumbing work.
Parts, Maintenance & Performance
The components and terms that affect how your system runs and lasts.
Sacrificial Anode
An anode is a metal rod inside the tank that corrodes instead of the steel tank ā "sacrificing" itself to prevent rust. Over years it wears away, and once it's gone the tank starts to corrode. Replacing the anode periodically can significantly extend a storage system's life.
Pilot Light
On older gas systems, the pilot light is a small constant flame that ignites the main burner when hot water is needed. If it goes out, you'll have no hot water. Many modern gas units use electronic ignition instead, removing the pilot light altogether.
Element & Thermostat
The element heats the water (like a giant kettle element) and the thermostat controls the temperature and switches the element on and off. These are the two most commonly replaced parts on an electric system ā a frequent cause of "no hot water".
Hard Water & Sediment
Hard water carries dissolved minerals that build up as sediment and scale inside the tank and on the element over time. This reduces efficiency, causes rumbling noises and can shorten the system's life. Periodic flushing helps keep things running well.
Recovery Rate
Recovery rate is how quickly a system reheats a tank of water after it's been used. A faster recovery rate means less waiting for hot water to come back. Gas systems generally recover faster than electric, which is worth considering for busy households.
Star Rating & kW
The energy star rating shows how efficient a system is ā more stars means lower running costs. kW (kilowatts) measures how much power a unit draws or how much heat it produces. Higher-efficiency systems like heat pumps cost less to run over time.
Mains Pressure vs Low Pressure
Mains-pressure systems deliver hot water at the same strong pressure as your cold taps ā the most common type today. Constant/low-pressure (gravity-fed) systems run at reduced pressure and are usually found in older homes. The type affects flow, valves required and which replacement units suit your home.
Still Unsure About a Term?
Our team explains everything in plain English ā no jargon, no pressure. Give us a call.