A tempering valve fitted at the hot water storage outlet
Every stored hot water system has a conflict built into it. The tank has to run hot enough to kill bacteria, and the tap has to run cool enough not to scald anyone. Those two requirements do not overlap, and the component that resolves them is the tempering valve.
It is a small brass fitting on the outlet of the cylinder. It costs less than a service call. And it is the reason a household can store water at 60 °C (140 °F) and still deliver 48 °C (118 °F) to the bathroom basin.
This page covers what the valve does, the temperature figures and the standards behind them, how to match one to your heat source, and the failure mode that makes an old valve dangerous without anyone noticing.
Why the Tank Runs Hot and the Tap Does Not
Two separate risks set the two temperatures, and they pull in opposite directions.
Bacteria Set the Storage Temperature
Legionella pneumophila multiplies in water between roughly 20 °C and 45 °C (68 °F to 113 °F). Below 20 °C it goes dormant. Above 50 °C it begins dying, and the rate matters enormously: around two hours to reduce the population by 90 percent at 50 °C, about two minutes at 60 °C, and a couple of seconds at 70 °C.
That is why plumbing codes worldwide require stored hot water to sit at or above 60 °C (140 °F). Some jurisdictions require more, up to around 70 °C (160 °F) in specific applications. A tank held at 45 °C to save energy is not an efficient tank, it is a culture vessel.
Worth knowing how the infection actually happens, because it explains where the risk sits. Legionnaires’ disease and the milder Pontiac fever are contracted by inhaling aerosolised droplets, not by drinking the water. A shower is the single most effective aerosol generator in a house, which is precisely why a shower fed from an under-heated tank is the scenario the codes are written against.
Scalding Sets the Outlet Temperature
Now the opposite pressure. At 60 °C, a child’s skin sustains a full thickness burn in roughly one second. At 50 °C the same injury takes around five minutes, which is long enough to react and move away.
Ten degrees is the difference between a shower and a hospital admission. And the people on the wrong side of that line are the ones least able to react: young children, older adults, and anyone with reduced mobility or reduced sensation in the skin.
So the system stores hot and delivers warm. The tempering valve is what sits between the two.
What the Valve Does, and What It Does Not
Worth stating plainly, because this gets described backwards often.
A tempering valve blends cold water into the hot supply leaving the storage vessel, so the water reaching the taps is at a safe temperature. Inside it is a thermostatic element, usually a wax element, that expands and contracts with temperature and moves a piston to change the hot and cold mix ratio continuously.
What it does not do is heat anything. It has no role whatsoever in getting the tank up to 60 °C. That is the thermostat on the gas burner, the electric element or the heat pump. The valve only ever reduces temperature.
This matters practically. If your hot water is not hot enough, the tempering valve is almost never the cause and adjusting it will not fix it. If your hot water is too hot at the tap, the valve is the first thing to check.
It also does nothing for pressure safety. That is a different device, covered further down.
Outlet Temperature: The Numbers and the Standards
The target depends on the market and on who uses the outlet.
In Australia and New Zealand, AS/NZS 3500.4 caps heated water delivered to sanitary fixtures used for personal hygiene, meaning baths, showers and basins, at 50 °C (122 °F) in residential premises.
The limit tightens where the occupants are more vulnerable. 45 °C (113 °F) applies to early childhood centres, primary and secondary schools, nursing homes and aged care facilities, and facilities for people with disabilities.
Kitchen sinks and laundry outlets are generally excluded, because those need hotter water and are not personal hygiene fixtures. That is why the kitchen branch is normally taken off the pipework before the tempering valve.
Other markets land in the same region by different routes. UK guidance under Building Regulations Part G caps bath outlets at 48 °C (118 °F). European practice references EN 1111 and EN 15092 for the valve itself. Whatever your market, confirm the local figure before specifying, because a valve set correctly to the wrong number is still a compliance failure.
A practical setting note: aim a little below the limit rather than exactly at it. A valve set at 50 °C with a ±3 °C tolerance can legitimately deliver 53 °C. Set the target around 46 °C to 48 °C and the whole tolerance band stays compliant.
Tempering Valve or Thermostatic Mixing Valve?
These two get used as synonyms and they are different products with different approvals. The distinction is not response speed, which is how it is usually explained. It is tolerance and failure behaviour.
A tempering valve, covered by AS 4032.3, holds the outlet within a wider band, commonly around ±3 °C. It is approved for the 50 °C residential duty and it is the standard fitting on a domestic storage heater.
A thermostatic mixing valve, covered by AS 4032.1, holds a tighter tolerance and adds a fail-safe: if the cold supply fails, the valve shuts down rather than passing full-temperature water to the outlet. That fail-safe is the reason TMVs are mandatory for the 45 °C high-risk applications. In a nursing home you cannot accept a valve that passes 60 °C water when a pump trips.
So the rule is simple. Residential storage heater, tempering valve. Healthcare, aged care, childcare, schools, disability facilities, thermostatic mixing valve to AS 4032.1. One is not an upgrade of the other, they are specified for different duties.
If a TMV is what your application needs, we cover it separately in why we need thermostatic mixing valves, comfort and safety with a thermostatic mixing valve and thermostatic mixing valve installation. The scald protection side is covered in how scald protection valves work and anti-scald valves.
One more thing that matters for selling into Australia: the product needs WaterMark certification, which is a separate approval from standards compliance and is not optional for plumbing products placed on that market.
Matching the Valve to the Heat Source
Tempering valves are sold for gas storage, electric storage, solar and heat pump systems, and the difference between them is real rather than marketing. It is the maximum inlet temperature rating.
Gas or electric storage. A thermostat cuts the input at setpoint, so the valve sees a fairly narrow and predictable inlet range for its whole service life. A standard valve rated to 65 °C or 70 °C is adequate.
Solar. This is where a standard valve gets specified wrongly most often. On a clear summer day with little draw-off, a solar store can climb well past 80 °C, and flat plate collectors reaching stagnation run absorber temperatures far above that. Evacuated tubes go higher still. The valve’s hot inlet is therefore both hotter and far more variable than on a conventional system, and a valve rated to 70 °C is being asked to work outside its envelope every summer. Solar-specific valves carry a higher maximum inlet rating, commonly up to 99 °C, with an element selected for stability across that span. See our note on the solar water heater mixing valve.
Heat pump. The opposite problem. Many heat pump water heaters store at 55 °C or lower to protect their own efficiency, which weakens Legionella control, so most run a periodic sterilisation cycle using an electric element to push the tank to 60 °C or 65 °C. That cycle often runs overnight when nobody is watching, and it is exactly when the tempering valve earns its keep.
Installing It Properly
The valve goes on the hot outlet of the storage vessel, downstream of the tank and upstream of the hygiene fixtures. Kitchen and laundry branches are taken off before it.
Six details decide whether it performs as specified:
- Accessibility. It needs to be reachable for commissioning, verification and replacement. Buried in a wall cavity or above a fixed ceiling, a fifteen minute job becomes a demolition job.
- Isolation valves and strainers on both inlets. Debris on the seat is the most common cause of temperature drift, and a strainer prevents most of it for almost no cost.
- Non-return valves on both inlets. Without them, hot water migrates into the cold line. That wastes energy and produces erratic outlet temperatures at other fixtures, including warm water at the cold tap.
- Balanced inlet pressures. A large imbalance between hot and cold supply pressure pushes the valve off target. Where the hot side is fed from a header tank and the cold from mains, you need a pressure-balancing arrangement, not just a valve.
- Correct size, so the minimum flow rate is respected. Every thermostatic valve has a lower flow limit, often around 4 to 5 L/min, below which control becomes unstable and the outlet temperature wanders. Undersizing the valve restricts flow and reduces pressure; oversizing it means low-flow outlets fall below its control range.
- Full bore pipework to match. A valve body smaller than the hot and cold pipes it connects creates a restriction. Size on the flow rate the outlets actually demand, not on the pipe you happen to have.
In Australia and in parts of the UK, this work is restricted to licensed plumbers. That is a jurisdictional requirement rather than a universal one, so check your local rules.
Commissioning
Commissioning is one measurement and one record, and skipping it is why nobody can tell later whether the valve has drifted.
Measure the delivered temperature at the fixture, with the water running, not at the valve body. Let it stabilise for a minute. Adjust to your target, allowing headroom below the regulatory limit for the tolerance band.
Then write down the figure and the date, and leave that record with the system. In healthcare and aged care it is a legal requirement. In a house it is the only way anyone will know, in four years’ time, whether something has changed.
Service Life and the Failure Mode That Creeps
Tempering valves wear out, and they fail quietly.
In Australia, five years is the commonly recommended replacement interval for a residential tempering valve. Thermostatic mixing valves in healthcare require annual verification at minimum, with more frequent service depending on the facility’s risk assessment. Hard water shortens every one of those intervals.
The failure modes, in rough order of frequency:
- Scale on the thermostatic element. Response slows, then the outlet temperature creeps upward over months. This is the dangerous one, because it develops gradually. Nobody notices until the shower feels hotter than it used to, and by then the margin that protected a child has already gone.
- Debris on the seat. Outlet temperature becomes erratic rather than steadily wrong. Often fixed by cleaning the inlet strainers.
- Element fatigue. The wax thermostatic element loses travel after long service, usually drifting cold as the valve loses authority.
- Failed non-return valve. Shows up as warm water at the cold tap and unstable mixing.
To be clear about something the older advice gets wrong: scale forms because of water hardness, not because of poor installation. A perfectly installed valve in hard water will still scale. Good installation prevents debris problems and pressure problems; it does not prevent scale, which is why the service interval exists at all.
The only reliable test is a thermometer at the outlet compared against the commissioning figure. If the delivered temperature has risen more than a couple of degrees, replace the valve rather than adjusting it. An adjustment masks a degrading element instead of fixing it, and the drift will simply continue.
It Is Not a Relief Valve
Both sit on a hot water cylinder and both respond to temperature, so the confusion is understandable and the consequence is not trivial.
A temperature and pressure relief valve, the T&P or TPR valve, is a safety device. It stays shut and does nothing until temperature or pressure exceeds a dangerous threshold, at which point it discharges to prevent the vessel rupturing. It may never operate across the life of the tank.
A tempering valve is a control device. It works continuously, every time someone opens a tap. It does nothing to protect the vessel.
Both are required, and neither substitutes for the other. The safety side is covered in seven facts about the T&P valve and T&P valve replacement cost, with the wider family in pressure valves. For the full component picture, see water heater components and their functions.
One further distinction worth drawing, since the product looks similar again: a blending valve on an underfloor heating circuit does the same arithmetic but carries no potable water approvals, because heating circuit water is never drunk. It will thread onto a hot water outlet perfectly well, which is exactly the problem. Do not substitute one for the other in either direction.
Legom Tempering and Mixing Valves
Jiaxing Legom Technology Co., Ltd. manufactures thermostatic valves for hot water and heating systems from our facility in Jiaxing, Zhejiang, supplying more than 90 countries.
Across the duties described above our range covers the 910021NT for general domestic service, the 910075NT horizontal body where the pipe run dictates orientation, the 910064CC electronic version for applications needing remote setpoint and monitoring, and the 910018CC solar valve rated for the high and variable inlet temperature a solar store produces, with the matching 910024CC connection kit. The full HVAC valve range covers the rest of the hot water assembly.
We manufacture our own wax thermostatic elements rather than buying them in. That is what lets us hold element response consistent across a production run, and it means we can develop an element to a specific temperature curve rather than selecting the nearest one from a catalogue.
Material matters as much as function on a potable valve, and the grade is often what decides compliance. Our note on HVAC valve materials covers the brass grades, including the dezincification resistant alloy some markets require for potable applications.
For brands and distributors specifying to their own range, our OEM and ODM service covers body configuration, connection type, setpoint range, element specification, branded packaging and the documentation pack for your market. Tell us the destination market and the heat source alongside the specification, and we will confirm the approval position before quoting.
Frequently Asked Questions
What does a tempering valve do?
It blends cold water into the hot supply leaving a storage heater so the water reaching baths, showers and basins stays at a safe temperature, typically 50 °C (122 °F) or below, while the tank continues to store at 60 °C (140 °F) or above for bacterial control. It does not heat water and has no role in reaching storage temperature.
What temperature should a tempering valve be set to?
Below the regulatory limit for your market, with headroom for the valve’s tolerance. Where the limit is 50 °C, set the target around 46 °C to 48 °C so a ±3 °C band stays compliant. High-risk premises such as aged care, childcare and schools require 45 °C and a thermostatic mixing valve rather than a tempering valve.
Is a tempering valve the same as a thermostatic mixing valve?
No. A tempering valve to AS 4032.3 holds a wider tolerance and suits residential duty. A thermostatic mixing valve to AS 4032.1 holds a tighter tolerance and shuts down if the cold supply fails, which is why it is required in healthcare, aged care, childcare and disability facilities. They are specified for different duties, not as upgrades of each other.
Why can’t I just set my water heater to 50 °C and skip the valve?
Because 50 °C storage does not control Legionella. The bacterium needs sustained temperature above 60 °C to be reliably suppressed, and it is contracted by inhaling aerosols, which a shower generates very effectively. Lowering the tank trades a burn risk for an infection risk, so codes require both the hot store and the tempered outlet.
How often should a tempering valve be replaced?
Five years is the commonly recommended interval for a residential valve, sooner in hard water. Thermostatic mixing valves in healthcare require annual verification at minimum. Check the delivered temperature against the commissioning figure rather than waiting for a complaint.
Does a solar hot water system need a different tempering valve?
Usually yes. A solar store can exceed 80 °C in summer, so the valve needs a higher maximum inlet rating, commonly up to 99 °C, and an element selected for stability across that range. A standard valve rated to 65 °C or 70 °C is under-specified for the duty.
My hot water is not hot enough. Is the tempering valve the problem?
Almost certainly not. A tempering valve can only reduce temperature, never raise it. Insufficient hot water points to the heat source, the thermostat setting, or the cylinder’s recovery capacity. The valve is the first suspect only when the water is too hot at the tap.
Why is the cold tap running warm?
Usually a failed non-return valve on the tempering valve inlet, letting hot water migrate into the cold line. It wastes energy and destabilises the mixed outlet temperature elsewhere in the house. Check the non-return valves on both inlets.
“The tempering valve is the cheapest component in a hot water system and the one that decides whether it is safe. What concerns us is how it fails, because it does not fail loudly. The outlet temperature creeps up over a couple of years as scale builds on the element, and by the time anyone notices the shower is hotter, the margin that protected a child has already gone. Measure the delivered temperature against the commissioning record, and replace rather than adjust.”
— Maggie Shen, Director of Legom
Reviewed by Maggie Shen, Director at Legom, on October 9, 2026. This article was reviewed for technical accuracy, including the Legionella thermal inactivation rates and aerosol transmission route, the AS/NZS 3500.4 delivery temperature limits in both unit systems, the distinction between AS 4032.1 and AS 4032.3 valve classes, maximum inlet temperature ratings for solar and heat pump sources, the installation and minimum flow rate requirements, and the scale-driven upward temperature drift that defines the service interval.