Hot water cylinder replacement: what do you need to know?

Should you repair or replace your cylinder?

Repair the parts, replace the tank. If the cylinder itself is leaking, it usually needs replacing. If a part has failed, such as an element, thermostat, valve or anode, it can often be repaired.

MBIE’s guidance on consent exemptions lists repairing valves, repairing a leaking storage heater and replacing sacrificial anodes as repair work, done with comparable parts.

Signs it may be time to replace include water leaking from the tank body, rusty or discoloured hot water, running out of hot water more often, and repeated repairs. EECA’s installer guide also lists three common reasons people replace a system: it is near the end of its life, it keeps causing problems, or the owner wants to switch from low pressure to mains pressure.

Low pressure or mains pressure?

Mains pressure gives stronger showers, while low pressure is simpler to replace like for like in an older home. EECA says low-pressure systems have been common in New Zealand for decades, and mains-pressure systems are now common in new builds.

EECA notes that mains pressure gives better pressure and flow but may use more hot water unless you fit an efficient shower head. Fitting mains pressure into a home built for low pressure can need a pressure-reducing valve or upgraded pipes and fittings, and pipes may need securing to stop knocking. Low pressure suits a like-for-like swap in an older home, but flow at the taps can be weak.

Electric, gas continuous flow or heat pump?

Each type suits a different home. An electric cylinder stores hot water and heats it with an element. It is simple and familiar, and it can run on cheaper off-peak power.

A gas continuous flow unit heats water only as it flows, so there is no tank to run out. It needs a gas supply and suitable flue or outdoor position, and the gas work must be done by certified gasfitters. You get certification paperwork for the gas work when the job is done.

A heat pump water heater moves heat from the air into the water. EECA says these units are much more energy efficient than electric storage or gas water heaters, but cost more to buy and install. EECA also says they usually take longer to reheat a cylinder, up to several hours after it is used up, so busy households may need a bigger tank. They make some noise, so place the outdoor unit away from bedrooms.

If you ever smell gas at home, do not use light switches, appliances or flames. Leave the building, and call your gas supplier or 111 from outside.

What size cylinder does your household need?

Size the cylinder to how many people use hot water and when they use it. EECA’s market report says 200 litres is generally enough for a small household, 200 to 300 litres for a medium household, and more than 300 litres for a larger one.

As a rough guide, MBIE’s solar water heater Acceptable Solution says most houses use 40 to 60 litres of hot water per person per day when stored at 60°C. Think about how many showers run back to back in the morning, whether there is a bath, and whether more people will live in the home later.

EECA also warns that a cylinder that is too big for the household loses more heat, and that the hot water cupboard may limit the size that fits.

Do you need a building consent to replace a hot water cylinder?

Usually not. Exemption 38 in Schedule 1 of the Building Act 2004 covers replacing or moving any water heater, if every heat source connected to it is controlled. MBIE explains that a controlled heat source keeps the stored water at no more than 90°C.

MBIE’s examples of work that is exempt include swapping electric for electric, gas for gas, electric for gas or gas for electric, replacing an open-vented (low-pressure) cylinder with a valve-vented (mains-pressure) one that has a controlled heat source, replacing a cylinder with a heat pump water heater, and replacing a storage heater with a gas instantaneous unit.

The exemption does not apply when a heat source is uncontrolled, such as a wetback on a fire or a solar thermosiphon system. Replacing an open-vented cylinder on a wetback or solar panels is covered by a separate exemption, but only if the new cylinder is also open-vented, in the same position and on the same pipework. Adding a wetback, solar collector or split heat pump that was not there before needs a consent.

Two conditions always apply. The work must be done by an authorised person, which means a registered certifying plumber or a plumber working under one. Exempt work must still comply with the Building Code. If you are unsure, ask Auckland Council about a discretionary exemption or apply for a consent.

What temperature rules apply to hot water?

Store hot water at 60°C or more and deliver it at no more than 50°C to bathroom fixtures in a home. The Building Code requires hot water systems to control legionella bacteria and to deliver water used for personal hygiene at a temperature that avoids scalding.

The Acceptable Solution G12/AS1 turns that into numbers. The cylinder thermostat must be set to at least 60°C to stop legionella growing. Hot water at showers, baths, basins and bidets must not exceed 45°C in early childhood centres, schools, rest homes and hospitals, and 50°C in all other buildings, including houses. A tempering valve or thermostatic mixing valve is used to achieve this.

This limit changed recently. Amendment 13 of G12/AS1, effective 2 November 2023, lowered the limit for other buildings from 55°C to 50°C, and the older version could be used until 1 November 2024. The current version is Amendment 14, effective 2 November 2024. Some older advice still quotes 55°C.

What affects the cost of a new hot water cylinder?

The type of system and how much has to change around it drive the cost. A like-for-like swap in the same spot costs the least. The cost rises when you:

1. Change type, such as electric to gas or to a heat pump, which can add a gas line, a flue, an outdoor slab or new wiring by a registered electrician.

2. Change from low pressure to mains pressure, which can mean new valves, fittings or pipe supports.

3. Move the cylinder, or install it where access is hard, such as a ceiling space or upstairs cupboard.

4. Choose a larger size, or a heat pump, which EECA says costs more to buy and install but less to run.

5. Need a tempering valve, seismic restraints or removal of the old cylinder.

Stately Plumbing replaces electric, gas and continuous flow hot water systems across Auckland, and our certified gasfitters handle electric to gas conversions. We can check your current setup, explain the options that fit and give you a clear quote. Call 022 159 2013.