How to Flush a Hot Water System in Australia

Here’s the short version: turn off power or gas, shut the cold-water supply to the heater, connect a hose to the drain valve, open a hot tap, then open the drain valve (and the relief valve lever) until water runs clear. Close everything, refill, bleed air, then restore power or relight. That’s the essence of how to flush hot water system Australia homeowners can do for basic maintenance. Below is the full, safer way to do it for storage tanks and continuous-flow units.

Safety first (and what to turn off)

Hot water inside a storage tank is typically 60°C or hotter. That can cause instant scalds. Always isolate energy and water before you start, and give the tank time to cool if you’re doing a full drain.

• Electric storage or heat pump: switch off at the circuit breaker and the local isolator (if fitted).
• Gas storage: turn the gas isolation cock to OFF and set the control to PILOT or OFF per the label.
• Continuous flow/instantaneous gas: turn off gas and unplug or switch off power.
• Solar hot water with booster: turn off the booster power and the cold supply to the storage tank (do not drain roof collectors).
• On rainwater or a pressure pump: switch off the pump.

Allow the system to cool if practical, or at least run a hot tap into a bath to bring the temperature down before draining. If you’re in a unit or strata, check by-laws and ensure you can direct discharge water safely to a drain.

What you’ll need

• Standard garden hose long enough to reach a drain or safe discharge point
• Bucket and old towels (for drips)
• Screwdriver or spanner (some drain cocks need a tool)
• Pliers (for stubborn valve caps)
• Protective gloves and eye protection

How to flush a storage tank hot water system

This applies to most electric, gas storage, heat pump tanks and the storage part of solar hot water. Adjust steps slightly for your model, but the principles are the same.

1. Turn off power or gas. Confirm the electric breaker is off or the gas control is off. For heat pumps, use the outdoor isolator as well.
2. Shut the cold water supply to the tank. This is usually a valve on the cold inlet pipe near the tempering valve. Turn clockwise to close.
3. Connect a hose to the tank’s drain valve (often a brass drain cock at the base). Run the hose to a floor drain, gully trap, or outside onto gravel — not onto lawns if water is very hot, as it can scald plants and you.
4. Open a hot tap inside the house (e.g., laundry). This prevents vacuum lock and helps the tank drain smoothly.
5. Lift the PTR valve lever (pressure-temperature relief valve) for a few seconds to break any vacuum and check it operates. The PTR is usually on the top or side of the tank with a drain line to outside. Then release it.
6. Open the drain valve at the tank. Water should start flowing through the hose. If it only trickles, gently lift the PTR lever again to let more air in. Be cautious: discharge may be hot.
7. Partial flush or full drain? For routine maintenance, let it run until water looks clear and no gritty sediment is coming out (often 5–10 minutes). For a deep clean, drain the tank completely.
8. Pulse rinse (optional but effective): Close the drain valve, briefly open the cold supply for 5–10 seconds to stir sediment, then stop and re-open the drain. Repeat a few times until the water runs clear.
9. Shut the drain valve and lower the PTR lever. Remove the hose. Check the drain valve for drips and snug it gently if needed — don’t overtighten.
10. Refill the tank: Keep a hot tap open indoors. Open the cold inlet valve fully. Air will spit and hiss from the hot tap; leave it open until you get a steady stream of water (no spluttering). This ensures the tank is full and air-free.
11. Restore power or gas: Close the hot tap. Turn the breaker back on or relight per the lighting instructions. For gas, follow the label for lighting the pilot or ignition. For heat pumps, restore power and allow the unit to start its cycle.
12. Final checks: Inspect around the drain cock, PTR outlet and pipe joints for leaks while the tank heats. Over the next day, glance at the PTR drain line — occasional drip during heat-up is normal, a constant stream is not.

If it won’t drain

• No flow at all: Ensure a hot tap is open and the PTR lever has been lifted briefly. If still blocked, sediment may have clogged the drain valve — close everything and call a licensed plumber.
• Drain valve stuck or cracked: Do not force it. Old plastic valves can snap. Get a plumber to replace the valve.
• PTR won’t reseat and keeps dripping: Sometimes lifting the lever flushes debris. If it still drips, have the valve assessed; PTR valves are safety devices and must be functional.

Descaling a continuous-flow (instantaneous) gas unit

These units don’t have a storage tank to “flush”; instead, their heat exchanger can scale up, especially with hard water. Descaling usually needs isolation valves and a small pump to circulate a vinegar/descaler solution. If your unit lacks service valves or you’re unsure, get a licensed plumber. Basic outline:

1. Isolate power and gas.
2. Close the unit’s cold and hot isolation valves. Connect hoses to the service ports.
3. Set up a small recirculation pump with a bucket containing a suitable descaling solution (often food-grade vinegar or a commercial descaler diluted per instructions).
4. Circulate the solution through the heat exchanger for the recommended time (commonly 30–45 minutes), then flush with clean water until clear.
5. Return valves to normal, restore gas and power, and check operation.

If you’d like more detail, see our guide to continuous flow maintenance. In hard-water areas, annual descaling helps keep efficiency and flow rates up.

How often to flush (and signs you need to)

For storage tanks, a quick flush every 6–12 months is a good rule of thumb. In soft-water cities you might stretch to every 12–18 months; in hard-water areas (parts of SA, WA and QLD), aim for 6–12 months. Continuous-flow units should be descaled as needed — often annually if you notice symptoms.

• Slow or noisy heating: Rumbling or kettling from a tank suggests sediment on the bottom.
• Cloudy or rusty hot water: Short bursts of brown water at hot taps point to sediment; persistent rust may mean an anode or tank issue.
• Reduced hot water flow: Scale can narrow pipes or a heat exchanger.
• Higher bills: Sediment and scale reduce efficiency, so the system runs longer to reach temperature.

Extra maintenance while you’re there

• Test the PTR valve: Lift and release the lever briefly every 6 months to ensure it moves freely. If it sticks or won’t stop dripping, replace it.
• Check the tempering valve: This mixes hot and cold to deliver safe 50°C water to bathrooms. If temperature swings or drops, have it checked.
• Inspect the sacrificial anode (storage tanks): If your tank has a removable anode, checking or replacing it every 3–5 years can extend tank life — see our anode replacement guide.
• Look for leaks and corrosion: Stains, wet patches, or rust on fittings should be addressed early.
• Solar hot water note: Only flush the ground-level storage tank. Do not drain the roof collectors; leave that to a solar specialist — see our solar hot water maintenance checklist.

Aftercare and common pitfalls

• Air in lines: Sputtering or milky water after refill is just air. Run each hot tap for 30–60 seconds to clear.
• No hot water after refill: Check that power/gas is restored and the thermostat hasn’t been turned down. For gas, confirm the pilot is lit.
• Leak from the drain cock: Try closing it another quarter-turn. If it still weeps, the washer may be worn — get it replaced.
• Low pressure on hot only: Sediment may have lodged in aerators or the tempering valve. Clean tap aerators; if still low, see our guide to low hot water pressure or call a plumber.
• Disposal: Let discharged water cool before directing onto gardens. Avoid scalding risk to people, pets and plants.

Done carefully, a simple flush keeps your hot water reliable, quieter and more efficient. If valves are seized, you’re unsure about gas relighting, or your system shows signs of serious corrosion, it’s safer and cheaper in the long run to book a licensed plumber for a service.