How to Drain and Flush a Hot Water System

By Tomo — licensed plumber, Brisbane QLD.

Legal note up front: draining and flushing an ELECTRIC storage hot water system is homeowner-legal in every Australian state. Operating the PTR (pressure-temperature relief) valve is not just legal — it’s a regulatory requirement under AS 3498. What is NOT homeowner-legal: anything involving the gas connection on a gas storage unit (licensed gas fitter only), and any work on the cold-water inlet valve, expansion control valve, or tempering valve (licensed plumber only). This article covers the homeowner-doable maintenance flush on an electric storage tank. Gas units, you call me.

Sediment in the bottom of the tank halves the lifespan of an Aussie hot water unit. Mineral scale (mostly calcium carbonate from hard water) builds up on the element, the element runs hotter to compensate, and the element burns out 5–7 years in instead of the 12–15 you should get. The 5-yearly flush is meant to be part of normal maintenance, but most installers never tell their customers it’s homeowner-doable. So nobody does it. Then the unit dies young and gets replaced, and the cycle continues.

The other thing that’s regulatorily required and almost universally ignored: the PTR valve must be operated annually. Operate, in this context, means lift the lever and let it discharge water for 5 seconds. AS 3498 mandates this. If your PTR valve has never been operated, the seat may be seized with mineral scale, meaning if the tank ever over-pressurised, the safety relief wouldn’t function. That’s a tank-explosion risk, and it’s why the standard exists.

What you’ll need

• Garden hose long enough to reach from the tank’s drain valve to a stormwater drain or grass area
• Adjustable shifter (250 mm)
• Rubber gloves
• Old towel
• Bucket
• Safety glasses (mandatory — discharged hot water spits)
• Long-sleeved shirt and closed shoes
• Torch
• The brand-name service guide for your unit (Rheem, Dux, Aquamax, Vulcan — they’re all online)

Step 1: Identify your unit and confirm it’s electric storage

Find the data plate (usually on the side of the unit). Confirm it’s an electric storage unit — there’ll be an element kW rating (1.8 kW, 3.6 kW, 4.8 kW). If it says “Continuous Flow” or “Instantaneous”, this article doesn’t apply. If it says “Gas Storage”, stop and call a licensed gas fitter. Note the model number and year of manufacture — anything older than 12 years is at end of life and you should consider replacement before flushing, because old elements often fail right after a flush from thermal shock.

Step 2: Turn off the power

Find your electrical switchboard. There’s a circuit breaker labelled “Hot Water” or “HWS” — flip it off. Lock it off if you have a lockout tag. Wait at least 30 minutes for residual heat in the element to dissipate before you start. Don’t trust just turning the unit off at any switch on the unit itself — kill it at the board.

Step 3: Turn off the cold water inlet

The cold-water inlet valve is on the cold pipe entering the top (or side) of the tank. It’s a quarter-turn ball valve — turn the handle 90 degrees so it’s perpendicular to the pipe. This stops fresh cold water entering as you drain.

Step 4: Open a hot tap somewhere in the house

Open the kitchen hot tap (or any hot tap) and leave it open. This breaks the vacuum so the tank can drain, and it confirms the cold inlet is properly shut (no water should come out of the hot tap once the tank starts draining).

Step 5: Connect the garden hose to the drain valve

The drain valve is at the bottom of the tank — usually a brass nipple with a square or hex head, or sometimes a small lever. Screw the garden hose onto it. Run the other end of the hose to a stormwater drain, a garden bed, or a grassed area where 250 litres of hot water won’t damage anything. NOT to a sewer overflow — that’s not what stormwater drains are for, and conversely, hot water will lift bitumen if you discharge it onto a driveway.

Step 6: Open the drain valve and run until clear

Open the drain valve. Hot water will start flowing through the hose. Stay back — if there’s a leak in the hose connection it’ll spray hot water. Let the tank drain completely. The first 30 seconds of discharge will look brown and rusty; that’s the sediment you’re flushing out. Keep it running until the discharge runs clear, which is usually 5–10 minutes after the brown phase ends.

Step 7: Brief refill-and-flush cycle

With the drain valve still open, briefly open the cold inlet for 30 seconds, then close it. This sends a slug of fresh cold water through the bottom of the tank, agitating any remaining sediment and flushing it out. Repeat 2–3 times. You’ll see another small puff of brown water each time. Stop when the discharge stays clear after the agitation slug.

Step 8: Test the PTR valve (this is the AS 3498 bit)

Look at the top of the tank for the PTR valve — it has a small lever, usually red or yellow plastic. With the tank still empty (or partially empty) and the power still off, lift the PTR lever and hold it for 5 seconds. Water should discharge from the PTR drain pipe. If nothing comes out, or if it sticks open after you release it, the PTR is failed and the tank shouldn’t be returned to service until it’s replaced (this is licensed plumber work — call me). Annual operation is what stops the seat seizing in the first place. Document the date you operated it, somewhere on the unit or in your home maintenance log.

Step 9: Close the drain valve, refill the tank

Close the drain valve and disconnect the hose. Open the cold inlet valve fully. The tank will start refilling — this takes 15–30 minutes for a 250-315 L tank. During refill, leave the hot tap in the house OPEN so air can escape from the system. Once water flows steadily from that hot tap (no spluttering), the tank is full. Close the hot tap.

Step 10: Restore power and verify

Switch the HWS circuit breaker back on at the board. Don’t turn the power on with the tank empty — element will burn out instantly without water to absorb the heat. Wait 2–3 hours for the element to bring the tank up to temperature, then test a hot tap. Note the date of service somewhere on the unit (a permanent marker on the data plate cover is fine) so you know when the next 5-year flush is due.

Common things that go wrong

What I see when call-outs follow a homeowner flush attempt:

• No hot water after refill and power-on. Most likely cause: element burnt out from being powered with the tank empty. If you’re sure power was off during draining and it’s still cold, the thermostat may have tripped — there’s usually a reset button under the access panel, but reaching that is licensed plumber/electrician territory under most state codes.
• PTR drip continues after testing. The valve seat hasn’t reseated properly because grit is caught under it. Lift the lever again briefly to flush, then release. If still dripping, the PTR needs replacement (licensed plumber).
• Drain valve won’t seal after closing. Mineral scale on the drain valve seat. Usually clears with a half-open / full-close cycle. If it keeps weeping, the drain valve seat is worn — plumber replacement.
• Tank takes hours to come up to temperature after flush. Normal for a 250–315 L tank — full reheat takes 3–4 hours on a 3.6 kW element. Don’t panic-call a plumber until 6 hours have passed.
• Sediment didn’t flush out (water stays brown for the whole drain). Tank is heavily scaled — common in hard-water areas or when no flush has been done in 10+ years. May need a chemical descale by a plumber, or it may be time for tank replacement.
• Cold water is now discoloured at all taps. The flush stirred up sediment in the cold lines. Run cold taps for 5–10 minutes around the house to clear it.
• Sacrificial anode check. While your warranty paperwork is out, find out when the sacrificial anode is due for replacement (usually 5 years). Anode replacement is licensed plumber, but knowing it’s due saves the tank from corroding through.

The Tomo rule

The 5-yearly flush is owner-doable on electric storage units, and the annual PTR valve test is a regulatory requirement under AS 3498 — not optional maintenance. Treat them as a pair: flush every 5 years, test the PTR every year. That single discipline doubles the lifespan of your hot water system. And anything to do with gas, the cold inlet valve replacement, or the tempering valve — that’s licensed-only, no exceptions.

Got a tank that’s making noises like a kettle, or a PTR that won’t reseat after testing? Send us a write-up.