How to Drain and Flush a Hot Water System
A few years ago I was called to a place in The Gap where the hot water unit had died at 7 years old — about half the lifespan it should have got. The owner thought it was just bad luck. I pulled the element and it was caked in two centimetres of calcium carbonate scale, the kind you only get when nobody’s flushed the tank for a decade. The element had been running hotter and hotter to push heat through the scale into the water, and finally it burnt out. He’d never been told the 5-yearly flush was homeowner-doable. Most installers don’t mention it because it’s not a chargeable job for them. So nobody does it. Then the unit dies young, gets replaced, and the cycle continues. I’ve seen this go wrong more times than I can count, and the fix is genuinely a Saturday morning with a garden hose.
Quick legal note up front. Draining and flushing an ELECTRIC storage hot water system is homeowner-legal in every Australian state under AS/NZS 3500. Operating the PTR (pressure-temperature relief) valve is not just legal — it’s a regulatory requirement under AS 3498, annually. What is NOT homeowner-legal: anything involving the gas connection on a gas storage unit (licensed gas fitter only, no exceptions), any work on the cold-water inlet valve, the expansion control valve, or the tempering valve (licensed plumber only). This article covers homeowner-doable maintenance on an electric storage tank. Gas units, ring me.
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 and bucket
- Safety glasses — mandatory, discharged hot water spits and the splash is real
- Long-sleeved shirt and closed shoes
- Torch
- The brand-name service guide for your unit (Rheem, Dux, Aquamax, Vulcan — all online as PDF)
Step 1: Identify your unit and confirm it’s electric storage
Find the data plate (usually on the side or top of the unit). Confirm it’s an electric storage unit — there’ll be an element kW rating (1.8 kW, 3.6 kW or 4.8 kW). If it says “Continuous Flow” or “Instantaneous”, this article doesn’t apply — those have no tank to flush. 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, or tape over it with a note. 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. Heres where the legal line sits: the electrical isolation must happen at the board. If you cant identify the right breaker, an electrician sorts it in 10 minutes.
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 typically a quarter-turn ball valve — turn the handle 90° so it’s perpendicular to the pipe. This stops fresh cold water entering as you drain. Note: this is operating the valve, not replacing or modifying it. Replacing the cold inlet valve is licensed work.
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. If water keeps flowing from the hot tap, your cold inlet valve isn’t fully closed and you’ll be draining mains pressure forever. Stop and recheck.
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 firmly. 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 is for — and conversely, never discharge hot water onto bitumen because it’ll lift the surface. Grass or stormwater only.
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 and you don’t want that on your forearms. 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 the drain hose. 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 — usually after 3 cycles, occasionally more on a heavily scaled tank.
Step 8: Test the PTR valve — 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 out the side of the house. 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 its replaced — that’s licensed plumber work. Annual operation of the lever is what stops the seat seizing in the first place, and AS 3498 mandates it. Document the date somewhere on the unit or in your home maintenance log. For related licensed-DIY plumbing, see our replace leaking tap washer and replace toilet inlet valve guides.
Step 9: Close the drain valve, refill the tank
Close the drain valve firmly 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 with 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 — the element will burn out instantly without water to absorb the heat, and that’s a $400 element replacement on what should have been a free flush. 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 — permanent marker on the data plate cover is fine — so you know when the next 5-year flush is due. Five years from now you’ll thank yourself.
When to call a tradie
Heres where the legal line sits clearly. Homeowner DIY: drain and flush (electric storage only), annual PTR operation per AS 3498, exterior cleaning. Licensed plumber only: anything on a gas unit, cold inlet valve replacement, PTR replacement, tempering valve adjustment or replacement, anode replacement, element replacement, thermostat reset (in most states), and any pipework modification. Cracks in the tank or visible water at the base of the unit means stop everything and ring a licensed plumber — a failed tank is a flooding risk. The missus made me put a moisture sensor under ours after a colleague had one go in his roof cavity. Cheaper to do once.
Common screw-ups
- Power on with the tank empty. Element burns out in seconds. Always confirm refill complete before flipping the breaker.
- Hot tap not open during refill. Airlock, spluttering, occured on plenty of jobs Ive been called to. Always vent.
- Boiling hot discharge onto the lawn or driveway. Kills the grass, lifts the bitumen. Stormwater or a tolerated garden bed only.
- Skipping the PTR test. AS 3498 mandates annual operation. Seized PTR is a tank-explosion risk.
- Flushing a 12-year-old unit. Element often fails right after from thermal shock. Plan replacement first.
Cost & time
Free if you own a garden hose and a shifter. Cost of doing it: about 90 minutes elapsed plus the 2–3 hour reheat. Compare with a plumber’s annual service: $250–$400 in southeast QLD. Save yourself that, doubled across the 12-year life of a tank, and the maintenance pays for replacing the unit when it’s due.
The Tomo rule — wrap
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, regardless of what YouTube tells you. Got a tank making kettle noises, or a PTR that wont reseat after testing? Send us a write-up and well help you sort whether its DIY or a callout.
