Waking up to no hot water is one of those household problems that demands immediate attention. Whether your hot water cylinder has given up overnight or your continuous flow unit has stopped mid-shower, the cause is often straightforward — once you know where to look. This guide walks you through diagnosing the problem step by step, explains what you can safely check yourself, and makes clear when you need a licensed plumber.
Why Hot Water Systems Stop Working
New Zealand homes typically use one of three hot water systems: an electric hot water cylinder, a gas hot water cylinder, or a continuous flow (instantaneous) gas unit. Heat pump hot water systems are also becoming more common. Each has its own failure points, but the most common culprits across all types are:
- A tripped circuit breaker or blown fuse (electric systems)
- A faulty or failed heating element (electric cylinders)
- A pilot light that has gone out or a gas supply issue (gas systems)
- A failed thermostat
- A faulty tempering valve or pressure relief valve (PRV)
- Sediment build-up reducing efficiency
- The cylinder simply reaching the end of its life (typically 10–15 years)
Step-by-Step Diagnosis
Work through these checks in order before calling anyone out.
Step 1: Check Your Electrical Panel
Head to your switchboard and look for any tripped breakers. A tripped breaker will sit in the middle position between ON and OFF. Reset it by switching it fully OFF, then back ON. If it trips again immediately, stop — there is likely a fault in the element or wiring that needs a licensed electrician.
⚠️ Safety note: Do not repeatedly reset a tripping breaker. Repeated tripping indicates a fault. Electrical work on hot water cylinders is restricted work under the Electricity Act and must be carried out by a registered electrician. Check the Electrical Workers Registration Board (EWRB) for a licensed professional.
Step 2: Check the Hot Water Cylinder Itself
Locate your cylinder (usually in a cupboard, garage, or roof space). Look for:
- Water pooling underneath — could indicate a leaking PRV or a failing cylinder
- The thermostat dial — it should typically be set between 60°C and 65°C (as recommended to prevent Legionella growth)
- The element indicator light (if fitted) — no light may mean a blown element
- Unusual noises — loud popping or rumbling often means sediment build-up
Step 3: Check Your Gas Supply (Gas Systems)
If you have a gas hot water system:
- Check that other gas appliances (hob, heater) are working. If nothing gas-powered works, contact your gas retailer — there may be a supply interruption or your LPG bottle may be empty.
- For continuous flow units, check the unit's display panel for an error code. Most modern units (Rinnai, Rheem, Bosch) display fault codes — look up the code in your manual or on the manufacturer's website.
- Check that the pilot light is lit on older gas cylinders. Relighting a pilot light is something most homeowners can do safely by following the instructions on the unit's label.
⚠️ Gas safety: If you smell gas, do not attempt to relight anything. Ventilate the area, leave the house, and call your gas supplier or 111. Gas work beyond relighting a pilot light is restricted work and must be done by a licensed gasfitter registered with the Plumbers, Gasfitters and Drainlayers Board (PGDB).
Step 4: Check the Tempering Valve
A tempering valve (also called a thermostatic mixing valve) blends cold water into the hot supply to deliver a safe temperature at your taps — typically around 55°C for most outlets. If this valve fails or sticks, you may get lukewarm or cold water even though the cylinder is heating normally. Tempering valves are inexpensive but their replacement is restricted plumbing work — a licensed plumber must do this.
Step 5: Check the Pressure Relief Valve
The PRV is a safety device on your cylinder. If it is constantly dripping or has released a large amount of water, it may be doing its job (relieving excess pressure) or it may be faulty. Either way, a dripping or weeping PRV needs a plumber to assess — do not cap it off or ignore it.
What You Can Safely Fix Yourself
| Issue | DIY Safe? | Notes |
|---|---|---|
| Reset a tripped breaker (once) | ✅ Yes | Do not reset repeatedly |
| Relight a pilot light | ✅ Yes | Follow label instructions carefully |
| Check LPG bottle level | ✅ Yes | Swap bottle if empty |
| Adjust thermostat dial | ✅ Yes | Set to 60–65°C |
| Replace element or wiring | ❌ No | Restricted electrical work |
| Replace tempering valve | ❌ No | Restricted plumbing work |
| Replace PRV | ❌ No | Restricted plumbing work |
| Gas pipe or valve work | ❌ No | Restricted gasfitting work |
How to Prevent Hot Water Problems Recurring
- Annual servicing: Have a licensed plumber inspect your cylinder, PRV, and tempering valve every one to two years. BRANZ recommends regular maintenance to extend cylinder life.
- Anode rod replacement: Electric and gas cylinders have a sacrificial anode rod that protects the tank from corrosion. It should be checked every three to five years and replaced when worn.
- Flush sediment: In areas with harder water, sediment can build up at the base of the cylinder. A plumber can flush this during a service.
- Know your cylinder's age: Most cylinders last 10–15 years. If yours is approaching that age and causing problems, replacement is often more cost-effective than repeated repairs.
- Insulate your pipes: In colder South Island or high-altitude areas, uninsulated pipes can slow hot water delivery and increase energy use. Energywise has good guidance on home insulation and hot water efficiency.
When to Call a Professional
Call a licensed plumber if you have:
- A leaking cylinder or pooling water
- A constantly dripping or weeping PRV
- Lukewarm water despite the cylinder appearing to work (likely tempering valve)
- A cylinder older than 10–12 years that is underperforming
- Any work involving valves, pipework, or the cylinder itself
Call a registered electrician if:
- The breaker keeps tripping after one reset
- You suspect a failed heating element
- There is any sign of scorching or burning near the element cover
Call a licensed gasfitter if:
- You smell gas at any point
- Your continuous flow unit shows a persistent fault code after basic checks
- Any gas valve, pipe, or fitting needs attention
All three trades are legally required to be licensed in New Zealand. You can find qualified, local plumbers and other tradespeople on tradedirectory.co.nz — always ask to see their licence or check the PGDB register before work begins.
Sources & Further Reading
- Plumbers, Gasfitters and Drainlayers Board (PGDB) — Licensing & Restricted Work
- Electrical Workers Registration Board (EWRB) — Find a Licensed Electrician
- MBIE Building Performance — Hot Water Systems & the NZ Building Code
- Energywise — Hot Water Efficiency Tips
- BRANZ — Home Maintenance Guidance
- Tenancy Services — Healthy Homes Hot Water Standards
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