How to Replace Gas Hot Water in NZ: Step-by-Step Guide

Gas hot water systems rarely die without warning. Spotting the signs early is what turns a panicked emergency swap into a planned upgrade.

• Rusty or discoloured hot water: internal tank corrosion. Once the tank itself is rusting, replacement is close.
• Rumbling or banging: sediment build-up creating hot spots in the tank. It accelerates failure.
• Hot-cold-hot temperature swings: a failing thermostat or gas valve, often an early sign of broader wear.
• Pilot light that keeps going out: repairable in isolation, but recurring failures on a unit over 8 years old usually signal the end approaching.
• Water pooling around the base: a leaking tank is not repairable. Start arranging the replacement now.
• Age over 10 years: most gas storage systems fail between 8 and 12 years. Past 10, every winter is borrowed time.
• Creeping gas bills: declining efficiency from sediment and burner wear, on top of NZ gas prices that are rising anyway as supply tightens.

Any of these on a system 8+ years old means it is time to get quotes, while you still have the luxury of choosing.

The difference is bigger than it looks:

Planned replacement:

• Compare 2-3 quotes and system options
• Choose the right cylinder size for your household, not whatever is on the truck
• Arrange green-loan financing (0-1%) in advance if you want it
• Schedule installation for a day that suits, ideally in mild weather

Emergency replacement (the system has died):

• No hot water today, so the first available installer wins
• Selection limited to what is in stock; like-for-like gas is the path of least resistance
• Urgent call-out and short-notice labour add cost
• No time to sort financing or compare anything

The emergency path is precisely how North Island households end up recommitted to gas for another 10-15 years, at the exact moment the gas supply picture argues against it. If your system is old, decide your replacement now and keep a current quote on file.

A typical gas-to-heat-pump changeover runs like this:

Step 1: Old system removal. The old gas unit is drained, disconnected and taken away. The gas line is capped and made safe at the isolation point, work for a licensed gasfitter.

Step 2: Plumbing. The new cylinder is positioned, hot and cold connections are made, and the tempering valve is fitted so taps deliver water at a safe temperature. For split systems, refrigerant lines are run between the outdoor unit and the cylinder. In New Zealand this is sanitary plumbing that must be carried out and certified by a certifying plumber.

Step 3: Electrical. An EWRB-registered electrician installs a dedicated circuit from your switchboard, wires the unit, and provides the required electrical certification. Timer or controller settings are configured, including any day/night plan or solar timing strategy.

Step 4: Commissioning. The cylinder fills, connections are leak-checked, the system is run up, and you get a walkthrough of the controls.

Expect to be without hot water for the working day, plus the first full heat-up of the new cylinder. Book a morning start and you will shower hot that night.

Replacing gas hot water raises a bigger question: what happens to the gas connection itself?

If you still use gas for cooking or heating: the line to the old hot water unit is simply capped, and the rest of your gas service carries on. Be aware you are still paying the daily fixed charge for the connection, and hot water was probably the majority of your gas use, so the fixed charge is now spread over much less actual usage.

If hot water was your last gas appliance: seriously consider full disconnection. An idle connection still bills its daily fixed charge every single day, for nothing. Contact your gas retailer to cancel supply and ask about meter removal; allow some weeks for the network side of the process.

With the gas network's costs being shared across a shrinking customer base, fixed charges are expected to keep climbing. Exiting entirely is the only way to stop paying them. Some households keep a single gas cooktop out of preference; just go in knowing the daily charge that hob is really costing.

This is the area that surprises gas households most: your gas system needed little or no electrical supply, while a hot water heat pump needs a proper dedicated circuit.

• Dedicated circuit: the heat pump gets its own circuit from the switchboard, sized to the unit. It cannot share with other appliances.
• Protection: modern RCD protection on the new circuit, installed and certified by an EWRB-registered electrician.
• Switchboard capacity: older switchboards, especially those still on ceramic fuses, may need upgrading before a new circuit can be added.

A good installer checks the switchboard as part of quoting and includes any upgrade in the written quote. If a quote arrives without anyone having looked at your switchboard (in person or via photos), ask the question explicitly. You do not want that conversation for the first time on installation day.

Controlled supply note: many NZ homes have ripple control on the old cylinder circuit. Ask your electrician and retailer how your new heat pump should be wired relative to controlled and uncontrolled supply on your plan, because it changes the per-kWh price you pay for hot water.

A gas-to-heat-pump replacement touches three licensed trades, usually coordinated by one installer business:

Certifying plumber: handles the cylinder, water connections and tempering valve, and certifies the sanitary plumbing work.

Licensed gasfitter: disconnects and caps the gas line safely. Often the same business as the plumber.

EWRB-registered electrician: installs the dedicated circuit and certifies the electrical work. You should receive the appropriate electrical certification for the new circuit; keep it with your house records.

Consents: a like-for-like hot water system replacement is generally exempt building work in NZ, but rules have edges (relocations, structural changes). Your installer should confirm what applies; if in doubt, ask your council.

When you request quotes, expect a single price covering all trades. A quote that quietly excludes the electrical work or the gas capping is not comparable to one that includes them.

1. Get at least 3 quotes. Installed prices for comparable systems vary meaningfully between installers. PumpSwap's free quote service exists for exactly this.
2. Use green-loan financing, not the credit card. ANZ, ASB and BNZ at about 1% for 3 years, Westpac at 0% over 5 years. Details and catches in our financing guide.
3. Replace proactively. Emergency replacements cost more and default to gas. A planned switch on your schedule is the single biggest money-saver on this list.
4. Right-size, do not over-size wildly. Size up one step for cold climates or solar storage, but a needlessly huge cylinder is money in steel you will not use. Our sizing guide has the method.
5. Bundle electrical work. If the switchboard needs attention anyway, doing other planned electrical jobs in the same visit saves call-out fees.
6. Cancel the gas connection if it is now idle. Ending the daily fixed charge is a permanent saving that starts immediately.