Switch it off now if
You see steam, hear boiling or loud bangs from the cylinder, or water is pouring out. Turn the cylinder off at the switchboard, and the gas too for a gas heater, keep people clear of hot water, and ring for help. Never block, cap or tie down a valve that's letting water out, because those valves are there for safety.
Why cylinders have valves that make noise
The Building Code's water clause, G12, insists that cylinders carry safety features. They have to let off excessive pressure, whether the cylinder is working normally or not, and keep temperatures in check so a rupture doesn't produce flash steam.
How your cylinder does that depends on its set-up. An open vented low pressure system has an open vent, while a valve vented one has a pressure relief valve in its place. Mains pressure cylinders have valves too; Rheem's All Pressure guide, for example, calls for a cold water expansion valve on the cold line.
So a little water from a valve can be normal. A valve that runs constantly, or a vent giving off steam, is a fault to get looked at.
What the noise might mean
| Likely source | What to do | |
|---|---|---|
| Hissing or a trickle | A valve letting out water or pressure, or a leaking fitting | Find where it's coming from. A constant trickle needs a plumber. |
| Dripping into the safe tray or onto the floor | A valve discharging, or a leak | Check the tray and the floor. Water under the cylinder means a call today. |
| Banging in the pipes when a tap shuts | Could be the pipework rather than the cylinder | Note which tap sets it off. See water pressure. |
| Rumbling, popping or boiling sounds | Treat it as a fault until a plumber says otherwise | Switch it off and ring a plumber. |
| Noises only when the fire is lit | The wetback side of the system | Tell the plumber, because the fire and cylinder layout matters. |
| A hum from a unit outside | A heat pump water heater's outdoor unit | Ask the installer what's normal for that model. |
What you can safely check
Find the source
Stand near the cylinder and the pipes when the noise happens. Is it the cylinder, a valve, pipes in the wall, or something outside?
Note the timing
While it heats? When a tap closes? Only with the fire going? Write it down, because the pattern helps whoever comes to look.
Look for water
Check the safe tray, the floor, the cupboard and the outside pipe where the tray or valves drain.
Leave the valves alone
Don't lift, wedge or tighten them. They're safety parts, and tinkering can hide a fault.
Record it
A short video with the sound is easier to describe than any noise you try to imitate.
Dripping is worth fixing, even when it's quiet
In its owner's guide, Rheem tells people to have a plumber fix leaking pipework, warning that a small leak can waste a surprising volume of hot water and the energy that heated it.
Water around a cylinder can damage floors and walls as well. Rheem's warranty exclusions include failing to install a properly drained safe tray where its guide requires one, so ask whether yours has one and where it drains.
The Building Code expects water systems to be installed so they avoid leaks, with reasonable access to the parts that need maintenance. See leaking cylinders if water is already escaping.
Who should look at it
Rheem says a repair that needs covers or casing removed with tools is for a suitably qualified plumber, gasfitter or electrician. Valves and pipework on a cylinder are plumbing, and the person doing it must be authorised under the Plumbers, Gasfitters, and Drainlayers Act 2006. On a gas heater, the gas side is for an authorised gasfitter.
Questions to ask the plumber
- Which valve is making the noise, and is it doing its job or failing?
- Is the pressure coming into the cylinder right for its type?
- Is there a drained safe tray, and where does it drain to?
- Could the noise be the pipes rather than the cylinder?
- If a valve needs replacing, is the new one the type the maker specifies?
What we would do
Don't live with a new noise. Note what you hear and when, take a short video if you can, and show it to whoever comes. An authorised tradie who specialises in hot water cylinders can tell you whether a valve is doing its job or failing, and a steady drip dealt with early saves water, energy and flooring.