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If your shower has started running lukewarm no matter how far you turn the tap, or scalding hot for no reason, there is a small brass valve near your hot water unit that is almost certainly the cause. It is called a tempering valve, and most Sydney homeowners have never heard of it until it fails.
A tempering valve is a legal requirement on almost every domestic hot water system in Australia, and when it stops working it affects every hot tap in the house. Here is what it does, why it matters, and how to tell when yours needs replacing.
Your hot water system stores water hot, but the water that reaches your bathroom taps has to be cooler. A tempering valve sits on the outlet of your hot water unit and blends a measured amount of cold water into the hot supply, so the water leaving for your bathroom never exceeds a safe temperature.
It does two jobs at once. It protects your household from scalding, particularly young children and older people whose skin burns faster. And it lets the cylinder itself stay hot enough to kill dangerous bacteria. Without a tempering valve you are forced to choose between safe taps and a safe tank. You cannot have both.
The rules sit in the national plumbing standard, AS/NZS 3500.4, and they are not optional. Stored hot water has to be kept at 60 degrees or above so that Legionella bacteria cannot multiply in the tank. At the same time, water delivered to any fixture used for personal hygiene, your shower, bath and bathroom basin, must not exceed 50 degrees. The gap between those two numbers is exactly what a tempering valve manages. You can read the temperature rules in the regulator's explainer on AS/NZS 3500.4 heated water services.
The 50 degree ceiling drops to 45 degrees in higher-risk settings such as early childhood centres, schools, aged care and healthcare facilities, where a thermostatic mixing valve is used instead. For a standard Sydney home, 50 degrees at the bathroom outlet is the number that applies.
A tempering valve has moving parts and a temperature-sensitive element inside it, and it wears out. The symptoms are easy to miss because people blame the hot water system itself. Watch for water that has gone lukewarm at every tap even though the unit is heating normally, water that suddenly runs much hotter than it used to, hot water that fluctuates between hot and cold during a single shower, or a noticeable drop in how much usable hot water you get.
If you have ruled out a flat-out hot water failure and the temperature is simply wrong or unstable, the valve is the most likely culprit. It is a far cheaper fix than a new cylinder, which is one reason it pays to diagnose it correctly rather than replacing the whole system.
These two get confused constantly. A tempering valve is the standard fitting for domestic homes. It blends to a set temperature and is accurate enough for residential use. A thermostatic mixing valve, or TMV, is a more precise device that reacts faster to changes in supply and is required in the higher-risk settings mentioned above. A TMV also needs annual servicing by a licensed person, whereas a domestic tempering valve does not. For the vast majority of Sydney houses, a tempering valve is the correct and compliant choice.
A tempering valve is an inexpensive part, but it is fitted with a licensed plumber's labour and often replaced at the same time as other valves on the unit. As a guide, supply and install typically lands somewhere in the $200 to $400 range for a straightforward swap, though access and the condition of the surrounding pipework can change that. Plumberoo quotes a fixed price before any work starts, so you are never surprised by the invoice.
Expect a tempering valve to last around five years before it drifts out of calibration. Many manufacturers warrant them for a similar period, which is a useful prompt: if your unit is more than five years old and the temperature has gone strange, the valve is the first thing to check.
No. Work on a hot water system is licensed plumbing work in New South Wales, and a tempering valve that is fitted or set incorrectly can deliver scalding water to a child's bath. It is exactly the kind of small job where the consequences of getting it wrong are serious. Plumberoo is fully licensed (NSW Fair Trading licence 289252c), and a valve replacement is usually a same-visit job. If we do not arrive within two hours of an emergency call, the call-out fee is free.
Look at the pipework on top of or beside your hot water unit. A tempering valve is a small brass body, usually with a coloured cap or adjustment screw, sitting where the hot outlet meets a cold inlet. If your home was built or had its hot water replaced in the last couple of decades, you almost certainly have one.
No. Lowering the tank below 60 degrees creates a Legionella risk inside the cylinder. The correct approach is to keep the tank hot and let the tempering valve bring the delivered temperature down to a safe level.
A tempering valve that has failed open lets too much cold water through, so every hot tap runs lukewarm even though the unit is working perfectly. This is one of the most common signs of a worn valve.
A standard domestic tempering valve does not require scheduled servicing, but it should be replaced when it drifts out of range. A thermostatic mixing valve in a commercial or high-risk setting is different and does require annual servicing by a licensed person.
Lukewarm or scalding water at the taps? It is usually a five-minute diagnosis. Call Plumberoo on (02) 9191 8787. We are licensed (289252c), available 24/7 across Sydney, and quote a fixed price before we start.