Tempering Valves: What They Do and When to Replace One

A tempering valve is a small brass valve fitted near your hot water system that blends hot and cold water so the water reaching your bathroom is no hotter than 50°C. It exists because the tank has to store water at 60°C or more to stop legionella growing, and water at that temperature can scald a child in about a second.

It is one of the most overlooked parts of a hot water setup, and one of the most common reasons a perfectly good heater gets blamed for lukewarm showers.

What does a tempering valve do?

Your hot water system is doing two jobs that pull in opposite directions.

The tank needs to hold water hot enough to control legionella, which is why storage systems are set to at least 60°C. But the plumbing code, AS/NZS 3500, requires hot water delivered to bathrooms and ensuites in new and replacement installations to be no more than 50°C, because hotter water causes serious burns very quickly.

The valve reconciles the two. The tank stays hot, and it mixes in cold water on the way out so the basin, shower and bath get water at a safe temperature.

Where you’ll find it

It’s usually on the hot outlet pipe within a metre or so of the hot water system, often with a cold pipe teed into its side. On gas storage units it’s typically on the wall beside the tank. On continuous flow and heat pump installs it may be tucked under a cover or near the unit’s pipework.

In most homes the valve only serves the bathroom branch. The kitchen and laundry often get full-temperature water, which is why the kitchen tap can be scalding while the shower feels merely warm.

How does a tempering valve work?

Inside the valve is a temperature-sensitive element, usually a wax or bimetallic capsule. It expands as the mixed water gets hotter and contracts as it cools, sliding a small piston that opens the cold port and closes the hot port, or the reverse.

That movement is constant. When someone turns on the kitchen tap and the pressure shifts, the element reacts and rebalances so the shower temperature stays roughly where it was set.

A household valve is not the same as a thermostatic mixing valve (TMV). TMVs are more precise, react faster and are used in hospitals, aged care and childcare centres, where the delivered temperature is set lower again. In a house, the simpler valve is the standard fitting.

Signs the valve is failing

The valve works hard in a small space, and Sydney water leaves mineral deposits on the element and seats over time. When it starts to go, the symptoms are fairly recognisable.

• Lukewarm showers while the kitchen tap still runs properly hot
• Temperature that swings hot and cold mid-shower, especially when another tap is used
• Hot water that takes much longer than it used to reach full warmth in the bathroom
• Water dripping from the valve body or its fittings
• Very hot water at the bathroom taps, which means the valve has failed open and is no longer tempering at all

That last one is the dangerous failure. If your bathroom hot tap is suddenly hotter than it has ever been, don’t let children use it and get it looked at promptly.

Why it gets misdiagnosed

We see this regularly across the Hills District. In the newer estates around Kellyville, The Ponds and Rouse Hill, the original builder-grade hot water units are reaching the age where people start expecting problems, so lukewarm showers get put down to a dying heater.

The quick test is the kitchen tap. If the kitchen runs hot and the bathroom doesn’t, the heater is almost certainly fine and the valve is the likely culprit. If everything is lukewarm, start with our guide to no hot water instead.

Are tempering valves mandatory in NSW?

For new hot water installations, and when an existing system is replaced, yes. The plumbing code requires bathroom hot water to be delivered at no more than 50°C, and a mixing valve at the heater is how that’s normally achieved in a home.

If your existing system was installed before the rule and has never been replaced, you may not have one. There’s no need to panic, but it’s worth adding one next time the system is changed, and sooner if there are young children or elderly people in the house. When you replace a hot water system, a licensed plumber will fit a compliant valve as part of the job and issue a compliance certificate for the work.

Some systems, particularly solar and some heat pumps, need a high-temperature rated valve because the water coming off the collectors can get much hotter than a standard tank.

How do you adjust a tempering valve?

Most valves have an adjustment screw or knob under a cap, and it takes a hex key or screwdriver to turn. That’s where the simplicity ends.

Adjusting one is a licensed job in NSW, and for good reason. The setting has to be verified with a thermometer at the outlet after the water has stabilised, and a valve that’s been wound up to fix a lukewarm shower can end up delivering water well over 50°C. That is a scald risk, and it leaves the installation non-compliant.

The other problem is that a lukewarm shower is usually a symptom of a worn or scaled element rather than a wrong setting. Turning the screw might mask the fault for a week, then the valve drifts again or fails open.

What you can safely do yourself

• Compare the kitchen and bathroom hot taps to work out whether the valve is the likely cause
• Check the valve and its fittings for drips or corrosion, and note where they are
• Make sure the isolation valves near the hot water system are fully open
• Run a basin tap on full hot for a minute and see whether the temperature is steady or swings

What needs a licensed plumber

• Any adjustment of the set temperature
• Servicing or replacing the internal cartridge, strainers or check valves
• Replacing the valve itself
• Fitting a valve where one has never been installed
• Anything involving the pressure and temperature relief valve on the tank

If the temperature swings badly when other taps run, uneven hot and cold pressure can be part of the problem. A plumber may suggest a pressure limiting valve to balance the supply so the valve has something steady to work with.

How long do tempering valves last?

It varies with water quality and use, but manufacturers generally recommend the valve is checked and serviced about every five years. A service usually means cleaning or replacing the internal cartridge and strainers, testing the non-return valves and confirming the outlet temperature.

A valve that’s never been touched will often keep working, just not accurately. Many are replaced rather than serviced once they’re past the ten-year mark, because a new valve is a quick job and removes the guesswork.

A sensible habit is to have it checked whenever the hot water system is serviced. If your unit is getting on, it’s also worth reading how long hot water systems last so you know whether a valve replacement is money well spent or whether the whole system is close to due.

Tempering valve replacement: what’s involved

A straightforward tempering valve replacement on an accessible gas or electric storage unit is usually one of the quicker hot water jobs we do. The water is isolated, the old valve is cut out, the new one is fitted with its strainers and check valves, and the outlet temperature is tested at the bathroom tap before we leave.

What moves the time and price is access (a unit in a tight side passage or ceiling space is slower), the condition of the surrounding pipework, and whether the system needs a high-temperature rated valve. We’ll tell you the fixed price before we start, with no call-out fee and no charge to quote. You can see how that works on how we price.

Getting it checked

If your showers have gone lukewarm or unpredictable, or the bathroom tap is suddenly scalding, it’s worth having the valve looked at before you replace a hot water system that may have years left in it. We work across the Hills District, Hornsby Shire and the Upper North Shore, and we’ll test it, tell you plainly whether it’s the valve or the heater, and fix whichever it is. Have a look at our hot water system repairs and installation for what’s involved.