Hot Water Cylinder Leaking? Causes, Quick Fixes and When to Call

A hot water cylinder leaking into the cupboard is one of those jobs that never picks a convenient moment.

Some leaks are minor and normal, some need a plumber today, and a few mean the cylinder has reached the end of its life. This guide explains how to tell which is which, what to do in the first ten minutes, and the usual causes behind a leaking hot water cylinder NZ plumbers get called out for. It's written for Auckland homeowners and landlords staring at a damp floor and wondering how worried to be.

First, make it safe.

Before you work out the cause, limit the damage. If water is pooling, spreading or coming out faster than a slow drip, do this:



• Turn off the power: switch off the hot water circuit at the switchboard. For gas, turn the unit off at its controls and close the gas valve.
• Shut off the cold water supply: find the valve on the cold pipe going into the cylinder and turn it off. If you can't find it, turn off the water at the toby (the main valve near your boundary).
• Protect the floor: get towels or a bucket down and move anything that might be damaged.
• Don't drain it yourself: a cylinder full of hot water is heavy and can scald. Leave draining to a plumber.
• Call for help: if it's a steady leak, call a plumber now rather than waiting for business hours.

Once the water's under control you've got time to look for where it's coming from.

Where is the leak coming from?

Hot water cylinder leaking from the bottom

If water is collecting around the base, check exactly where it starts. A wet patch around the base can come from three places:



• The element or thermostat cover: a seal behind the access panel near the bottom can perish with age. This is often repairable.
• The drain valve: some cylinders have a valve near the base that can drip if it's worn or wasn't fully closed.
• The tank itself: if water is seeping from the body of the cylinder, or there's rust staining down the side, the tank has corroded through.

That last one is the bad news. A tank that has rusted through can't be patched, and it can fail quickly once it starts. Plan for a replacement, and don't leave it running unattended.

Leaking from the valves or pipes from the top

Mains pressure cylinders have several valves, usually on or near the top. A loose fitting or worn valve can drip down the outside of the cylinder and look like a tank leak, so check here before assuming the worst. These are usually straightforward repairs.

Some dripping is normal. The BRANZ Level guide to controlling pressure in storage cylinders notes that the expansion control valve on a mains pressure system normally discharges about 5 litres a day while the water heats. That water should run away through a drain pipe, not onto your floor.

Hot water cylinder leaking from roof

If you've spotted a hot water cylinder leaking from roof pipework, it's usually water coming out of an overflow or relief pipe that runs from the cylinder up through the roof or out an exterior wall. It's doing its job, but it's a sign something isn't right:



• A constant flow from the relief pipe can mean the temperature or pressure inside the cylinder is too high, often a thermostat fault.
• On low pressure systems, the header tank in the ceiling has its own overflow. If its float valve sticks, water keeps topping up and spills out.
• Steam or very hot water from the overflow is a warning sign. Turn the power off and call a plumber.

A genuine roof leak from rain can also drip into a cylinder cupboard, so check the cylinder is actually the source before assuming it is.

How to stop hot water cylinder from overflowing

Searching how to stop a hot water cylinder from overflowing usually means water is running out of the overflow pipe non-stop. The fix depends on the system:



• Mains pressure: a failing valve or thermostat is the usual cause. Turn the power off to stop the water heating further and call a plumber to test the valves.
• Low pressure: lift the lid on the header tank (carefully, it's in the ceiling) and check whether the float valve is stuck open. Freeing it can stop the overflow, but a worn valve needs replacing.

Don't block an overflow or relief pipe to stop the water. Those pipes are a safety feature and the cylinder needs them to release pressure.

Repair or replace?

Get your leak sorted

Pipe Fix's licensed plumbers are available 24 hours, 7 days a week for [emergency hot water cylinder repairs] across Auckland. Most replacements take around half a day, and Rinnai installs come with a 10-year tank warranty and a 12-month workmanship guarantee.

Call 021 217 2782 now if water is still coming out. If it's a slow drip and you'd like a second opinion first, email a photo of where the water is coming from to info@pipefixplumbing.co.nz and the team will tell you how urgent it looks.

Frequently asked questions