Hot Water Relief Valve Leaking: Normal Drip or a Fault?

The relief valve on a hot water tank is a safety valve, and letting out some water is part of its job. On a mains pressure storage tank it's the valve near the top of the cylinder with a small lever on it and a drain line running down from it. Rheem's owner's guide for its domestic electric range describes it as essential to the heater's safe operation.

Its full name, temperature pressure relief valve, says what it watches. Each valve has a set rating (1000 kPa on the larger tanks in that Rheem range), and its job is to release water through the drain line rather than let the cylinder hold more than it was built for.

That's why the one thing you must never do to a dripping relief valve is stop it. Rheem's guide says never to block the outlet of either valve or their drain lines for any reason, and the drain has to stay open to the air and run continuously downward. A capped, plugged or tied-off relief valve turns a nuisance drip into a pressure vessel with no way to let go.

Before deciding anything, work out which valve the water is coming from, because some systems have two. The temperature pressure relief valve sits on the tank itself, near the top. An expansion control valve, where one is fitted, sits on the cold water pipe feeding the heater and has its own drain line.

According to Rheem's guide, the expansion control valve has a lower pressure rating than the relief valve, so it releases the expansion water instead. It's mandatory in some areas, including places with scaling water, and it has a side benefit: the water it lets go is cooler, so less heated water goes down the drain.

So a drip from a pipe on the cold side of the heater doesn't on its own mean the tank valve has failed. It may be the expansion control valve doing the same job a step earlier. Note which pipe is wet and take a photo of it. It's useful detail for a plumber, and it costs you nothing to find out.

Some water from the relief valve while the tank heats is normal, and Rheem puts a number on it. Its guide says the valve may release water through the drain line during each heating period, roughly 1/50 of the hot water used, because water expands by about that much when it's heated. A household using 100 litres of hot water a day could see around two litres come out of the drain line over the heating cycle, and that's within the yardstick.

The same guide draws the line where normal stops: if the valve discharges more than a bucket full of water in 24 hours, there may be another problem.

That makes a safe check possible without touching anything. Watch when the drain line runs. A drip that comes while the system is heating, then stops once the tank is up to temperature, fits the normal pattern. A drip that carries on around the clock, or would fill a bucket in a day, doesn't. The table below sets out the common patterns and what each one usually means.

Each leak pattern points somewhere different, and Rheem's guide deals with them one by one.

A constant dribble that never quite stops can be a small particle of grit sitting in the valve. The guide suggests gently raising the easing lever for a few seconds to dislodge it. If that doesn't stop it, or you'd rather not handle a valve that releases hot water, leave it for a plumber.

A steady flow for long periods, often at night, may mean the mains pressure sometimes rises above what the heater is designed for. The fix Rheem names is a pressure limiting valve fitted by a plumber, covered in the next section.

Heavy flows of hot water from the relief valve until the tank runs cold, then stopping until it reheats, is the pattern to act on today. Rheem's instruction for its electric heaters is to switch the water heater off at the switchboard and have it inspected. On a gas or solar system, follow the shutdown steps in your owner's booklet and call a licensed plumber rather than adjusting anything.

Water coming from the tank body, a seam or the base, rather than out of the drain line, is a leak from the heater itself, not a relief valve drip. Rheem's guide turns an electric heater off by switching it off at the switchboard and closing the cold water isolation valve at the inlet. Do that, then call a plumber.

High mains pressure is the cause on this list a homeowner can actually do something about. Every mains pressure heater has a maximum supply pressure. In Rheem's domestic electric range, the larger tanks carry a 1000 kPa relief valve and allow a maximum mains supply of 800 kPa without an expansion control valve, or 680 kPa with one. Above that, the guide says a pressure limiting valve is required.

Across the Sunshine Coast, Noosa and Moreton Bay council areas the water supplier is Unitywater. It says it changes pressure across its network from time to time, and commits to a minimum supply pressure of 210 kPa. It also notes that pressure is usually lower in the morning and evening peaks, which fits with a valve that runs hardest overnight when demand drops.

Unitywater says a house built after 2006 most likely already has a limiting device, generally in the meter box or on the hot water system inlet, and that keeping it maintained is the owner's responsibility. Where pressure seems too high, it recommends a private pressure limiting device fitted at the meter by a private plumber. Fitting, adjusting or replacing one is licensed plumbing work.

Rheem's guide asks owners to operate the relief valve's easing lever every six months, raising and lowering it gently, to clear deposits and confirm the valve and its drain line aren't blocked. It warns that skipping this may lead to the cylinder failing. Other brands may set a different routine, so check the booklet that came with your system.

Two warnings in the same guide matter more than the routine. The water that comes out of the drain line is hot, so stand well clear of where it discharges. And if water doesn't flow freely when the lever is lifted, the heater must be checked.

That's where the owner's part ends. A lever that's stiff, a valve that won't stop running after it's been eased, or one that hasn't been touched in years should go to a licensed plumber, ideally as part of a service. Don't force the lever and don't take anything apart. Rheem also says the relief valve must not be tampered with or removed, and its warranty may not cover faults where safety devices have been.

A relief valve has its own replacement interval, separate from the tank, and that interval depends on the model. Rheem's guide for its 481, 491 and 492 domestic electric tanks says the relief valve should be replaced at intervals not exceeding six years. Its heavy duty electric guide says five. Both say sooner in areas where water leaves heavy deposits. Same brand, two numbers, which is why the booklet for your own system is the one to go by.

The rating of the replacement matters. Rheem's guide says never to replace the relief valve with one of a higher pressure rating. A higher rated valve might stop a drip only by letting the cylinder sit at a higher pressure before it opens, which is the very thing the valve is there to prevent.

In Queensland this isn't owner work. The Plumbing and Drainage Regulation 2019 lists the plumbing an unlicensed person may do, such as changing a shower head or a tap washer, and valves on a hot water system aren't on that list. The Queensland Government suggests checking a plumber's licence online or with the QBCC on 139 333.

A few details make the first visit quicker and can save a second trip. None of them need you to touch the valve.

Which valve is running: the one on the tank, or one on the cold pipe going in. When it runs: only while the system heats, around the clock, or mostly overnight. Roughly how much: a few drips, or more than a bucket in a day. Whether the water coming out is hot or cold.

The brand, model and age from the label on the tank, or a photo of the label. In Unitywater's area, whether the house was built after 2006, since Unitywater says those homes most likely already have a limiting device.

If the drip is small and only happens while the tank heats, it can wait for a booked service. If the valve is running hard, releasing hot water, or the tank itself is wet, call for a repair.