Why Hot Water Is Costing You More: A Plumber’s Diagnostic Guide

Most Melbourne homeowners’ first move when hot water costs climb is to assume the system is dying and start pricing replacements. About a third of the time that’s right. The other two thirds, the system has a fixable problem, and the fix is cheaper than replacement by an order of magnitude.

Worth noting up front: there’s no “hot water bill” as a separate line item. The cost of running hot water splits across two bills. The energy bill (gas or electricity) covers the heating, usually the bigger share. The water bill covers the actual water you use, hot or cold, cold water comes in, gets heated, and goes out a tap. A leaking or wasteful hot water system can push both bills up: more water through, and more energy to heat it.

This is the diagnostic walk a licensed plumber goes through before recommending replacement. I’ll cover what each cause looks like, what you can check yourself, and what the typical fix costs.

I’m Alister at Prime Plumbing & Gasfitting. BPC Licence No. 103414.

The seven causes I check first, in order of frequency

1. Sediment buildup at the bottom of the tank (storage units only)

Melbourne’s water is relatively soft compared to Adelaide or Perth, but it still leaves mineral deposits over 5-10 years. Sediment settles at the base of the tank, sits between the heating element (if electric) or the burner (if gas) and the water. Heat now has to travel through a layer of mineral grit before it reaches the water. The unit runs longer to heat the same volume.

How to tell: the tank rumbles, pops, or bangs when heating. Hot water takes longer to come through. Recovery time after a long shower is noticeably worse than it used to be.

Fix: licensed plumber flushes the tank. About $180, $300 typically. Worth doing every 3-5 years on storage units in Melbourne. After the first 8-10 years, the efficiency lift is sometimes marginal because the sediment has fused; at that point replacement starts to make sense.

2. Anode rod past its life (storage units only)

Inside every storage hot water tank there’s a sacrificial magnesium or aluminium rod. Its job is to corrode instead of the steel tank. Useful life: typically 5-7 years for magnesium in Melbourne’s water, 7-10 for aluminium. Once the rod is consumed, the tank itself starts corroding from the inside, and a perforated tank is the failure that makes the energy bill spike, because heated water is leaking through the corrosion before the thermostat shuts off.

How to tell: rusty or discoloured hot water for the first 30 seconds when the tap is opened (especially after the tank’s been idle overnight). White or rust-coloured staining around the anode access port on top of the unit. Unit is 6+ years old and you’ve never had the anode checked.

Fix: anode replacement, $180, $280 including parts. Buys another 5-7 years of tank life. If you skip it, the tank typically perforates at year 10-12 and you’re replacing the entire unit ($1,500, $3,500 for gas storage, more for heat pump).

3. Aging element or thermostat (electric units only)

Electric storage units have an immersion element (the part that does the heating) and a thermostat (the part that decides when to heat). Both wear. A failing element draws more current to deliver the same heat. A miscalibrated thermostat overshoots the set temperature, then re-heats sooner than it needs to.

How to tell: energy bill has climbed noticeably (e.g. 20%+) on what you know is the same hot water usage. Element age is over 8 years. Some indication that hot water is being heated more often than expected (you can sometimes hear the unit cycling).

Fix: licensed plumber tests element resistance and thermostat calibration. Element replacement is around $250, $450. If both need replacing on a unit that’s already 8+ years old, replacement may be the better economics.

4. Visible leaks (any system)

The dramatic ones are obvious: water on the floor, the discharge pipe dripping continuously. The quiet ones are easy to miss: a damp patch under the tank base, mineral staining where there used to be none, a slight hissing from a fitting.

How to tell: walk around the unit with a torch. Look at the floor, the wall behind, the discharge pipe outlet (small drip = pressure relief valve venting, see below), and every brass or copper fitting connection.

Fix: depends entirely on the leak. A fitting that needs tightening or a washer replaced is a $150, $300 callout. A tank that’s perforated is a replacement, not a repair.

5. Invisible leaks (slab or wall), the meter test

If your bills (energy and water) have both crept up but the unit looks fine, the leak might not be at the unit. It might be in the hot-water pipework running through the slab or wall to a tap somewhere. The water bill goes up because water is flowing out of the leak; the energy bill goes up because the hot water unit keeps re-heating what’s being lost. Slab leaks are uncommon in Melbourne but they happen, especially in homes built between 1960 and 1990 with copper laid directly in concrete.

How to tell, DIY: turn off every tap in the house at night. Check your water meter reading. Don’t use any water overnight (no toilet flushes, no ice maker). Check the reading first thing in the morning. If it’s moved at all, there’s a leak somewhere in the property, could be a running toilet (more common) or could be a slab leak (rarer but expensive to find).

Fix: licensed plumber locates the leak (thermal imaging, acoustic detection, or systematic isolation). Slab leak repair ranges $1,500, $5,000 depending on access; the fix itself can be quick but cutting and patching the concrete is the costly part.

6. Pressure relief valve venting (storage units only)

The pressure relief valve (PRV) is the safety device that lets water vent if tank pressure climbs too high. It should be still 99% of the time. A PRV that’s constantly dribbling out the discharge pipe is venting hot water continuously, every dribble is heated water you’ve paid to heat, leaving the system.

Three causes: the valve itself is worn out and seeping (replace), the supply pressure is too high (need a pressure-limiting valve at the meter), or there’s no expansion vessel and the tank has nowhere to expand into.

How to tell: look at the PRV discharge pipe outlet (usually exits the wall outside near the unit). Continuous drip or trickle of warm water = the valve is venting. Should be dry between rare expansion events.

Fix: $200, $500 depending on whether it’s valve replacement, pressure limiter install, or expansion vessel install.

7. Undersized system for current demand

The cheapest cause to diagnose, often the most expensive to fix: the system is fine, you just have more demand than it’s rated for. Family of 5 with two teenagers using a 135L gas storage that was sized for the original family of 3. The unit runs at near-full capacity all day, recovery time is poor, and the bill reflects the constant heating cycle.

How to tell: hot water runs out partway through the second consecutive shower. The unit recovers more slowly than it used to. Family or household size has increased noticeably since the system was installed.

Fix: replacement with a larger or different-type system. Heat pump or continuous flow are often the right answer here rather than a bigger gas storage. Quote varies $2,500, $6,000 depending on the conversion and whether existing connections suit.

What you can do before calling a plumber

• Note the unit’s age, manufacture date on the dataplate. Over 10 years old changes the repair-vs-replace economics.
• Listen for rumbling or banging when the unit is heating, tells you sediment.
• Walk around the unit with a torch, check for visible leaks or damp.
• Check the PRV discharge outlet, should be dry.
• Run the overnight meter test, rules in or out an invisible leak.
• Compare recent bills, quantify the climb. A 10% rise is normal seasonal variation; a 30% rise is something happening.

Repair vs replace, the rough rule

The honest rule of thumb most plumbers use: if the repair cost is more than 50% of replacement cost and the unit is over 8 years old, replace. Under 8 years, repair almost always makes sense. Over 12 years, almost always replace regardless of the failure mode, you’re likely paying for the same diagnostic visit again within two years.

FAQs

How much should running hot water actually cost in Melbourne?

For a 3-bedroom home, the hot water portion of your energy bill (gas or electric, whichever powers the unit) is typically: gas storage $150, $300 per quarter, electric storage $200, $450 per quarter on the controlled-load tariff, heat pump $80, $180 per quarter. That’s just the heating energy, not the water itself. If yours is significantly above these for a comparable household, something on this list is likely the cause.

How long should a hot water system last?

Gas storage: 8-12 years typical, 15 with proactive maintenance. Electric storage: 10-15 years. Heat pump: 10-15 years (depends heavily on installation conditions). Continuous flow (gas): 12-20 years. Solar with electric or gas boost: panels 20+ years, tank 10-15.

Is heat pump worth it for Melbourne?

Generally yes if the unit can be sited where it gets reasonable airflow and isn’t in a very cold pocket of the property. Running costs are roughly a third of gas, half of electric storage. The Victorian Energy Upgrades program also has rebates for heat-pump installations in many circumstances. The catch: heat pumps don’t perform as well below 5°C and they’re slower to recover than gas instantaneous. Pure-resale-value answer for Melbourne: probably yes.

My PRV drips occasionally after a hot shower. Is that normal?

A brief drip during or just after a heavy hot water draw, then the valve sealing again, can be normal thermal expansion venting. Continuous drip or any drip that doesn’t stop after the unit has finished its post-shower heating cycle is not normal, book a plumber.

Should I service a hot water unit every year?

Gas storage and gas continuous flow benefit from a service every 2 years (combustion check, sediment flush, anode inspection on storage). Electric storage doesn’t need annual servicing but the anode should be inspected at year 5. Heat pump units have manufacturer-specified service intervals, usually annual or biennial, that protect the compressor.

Book a diagnostic

If your hot water running costs have climbed and you’d like the cause identified before pricing replacement, ring 0475 407 670 or contact Prime Plumbing & Gasfitting. The diagnostic visit is $150, $220 typically and tells you definitively which of the seven causes you’re looking at. From there, you can decide whether to fix or replace with real numbers in front of you.

, Alister Williams, Prime Plumbing & Gasfitting
BPC Licence No. 103414 · ABN 12 721 359 467 · Serving Melbourne since 2014

Related reading

• Hot water system maintenance tips to avoid emergencies
• When to replace a hot water system
• Hot water system replacement Melbourne: costs, types & 2026 guide