A couple in Oran Park called me about a hot water system that kept going lukewarm in the ensuite. The kitchen tap was scalding. The shower would not get past tepid. They had already paid someone to replace the element and thermostat in the tank, which changed nothing, because the tank was fine. The problem was the tempering valve on the pipe beside it, stuck partly closed on the hot side after nine years. Twenty minutes and a new valve, and they had a hot shower.
Almost every home in South West Sydney has a tempering valve bolted to the hot water system, and when it fails the symptoms look exactly like a dying hot water system. This post covers what a tempering valve and a thermostatic mixing valve actually do, what the NSW rules say, how to tell yours has failed, and what replacing one involves. If the shower is cold right now, we are a hot water plumber, no call-out fee, across Liverpool, Campbelltown and Camden.
A tempering valve mixes cold water into the 60°C water leaving your tank so your bathroom taps get no more than 50°C, as NSW plumbing rules require. A thermostatic mixing valve does the same job more precisely and is required in aged care, childcare and healthcare. If the kitchen tap is hot but the shower is lukewarm, or the bathroom water is suddenly scalding, the valve has failed and needs a licensed plumber to replace it.
What a tempering valve does
Your hot water system stores water at 60°C or more. That is not a manufacturer's preference, it is a requirement, because Legionella bacteria grow in warm water and storing at 60°C keeps them under control. But 60°C water scalds skin in seconds, and children and older people scald faster. So between the tank and your bathroom sits a tempering valve: a brass fitting with a thermostatic element inside that blends cold water into the hot and sends it on at 50°C.
Under AS/NZS 3500.4, the plumbing standard applied through the National Construction Code, heated water delivered to any fixture used for personal hygiene (basins, showers, baths) must not exceed 50°C, and the limit drops to 45°C in early childhood centres, schools, nursing homes and aged care. The kitchen sink and laundry tub are not personal hygiene fixtures, so they are allowed the full 60°C-plus, which is why your kitchen tap is hotter than your shower. That difference is also your best diagnostic tool, and I will come back to it.
Tempering valve vs thermostatic mixing valve (TMV)
People use the names interchangeably. They are not the same thing.
A tempering valve is the standard domestic fitting. It holds the outlet temperature to within about 3°C of its setting, reacts to changes over a few seconds, and is a sealed unit: when it fails, it gets replaced. A thermostatic mixing valve does the same job with a faster, more precise element (within about 2°C), shuts off the hot side if the cold supply fails, and is built to be serviced rather than thrown away. TMVs are required where the users cannot protect themselves: hospitals, aged care, childcare centres, schools and disability accommodation, where the 45°C limit applies. Under AS 4032.3 a TMV has to be field tested at least every 12 months and have its thermostatic element and seals replaced at least every five years, by a plumber trained to do it.
For a house in Gregory Hills or Leppington, a tempering valve is the right call unless there is a specific reason for a TMV: a family member with a disability, an in-home care arrangement, or a home business that has to meet the stricter limit. A TMV costs more to buy and more to keep serviced, and it does nothing extra for a healthy household.
| Feature | Tempering valve | Thermostatic mixing valve (TMV) |
|---|---|---|
| Where it is used | Houses, units, most small businesses | Hospitals, aged care, childcare, schools, disability housing |
| Delivery temperature | 50°C | 45°C, or lower where specified |
| Accuracy | About 3°C either way | About 2°C either way |
| If the cold supply fails | Hot can pass through | Shuts off the hot side |
| Servicing | Not serviceable, replace when it fails | Test every 12 months, major service every 5 years (AS 4032.3) |
| Typical working life | Five to eight years (AS 4032.3 says replace at five) | Longer, if serviced on schedule |
Signs your tempering valve has failed
Tempering valves fail in two directions, and both are common once they get past five years. Scale, grit from the mains and tired springs are the usual reasons.
Lukewarm hot water in the bathroom, hot in the kitchen
This is the classic. The element has stuck and is letting too much cold through, or the hot port is partly blocked. Run the kitchen hot tap for a minute. If it gets properly hot and the shower will not, the tank is doing its job and the valve is not. If everything in the house is lukewarm, including the kitchen, look at the tank first: an element, thermostat or gas problem. My post on what to check when there is no hot water covers that side.
Hot water too hot
Steam off the basin, a shower you cannot stand under without the cold on, a bath that hits 60°C. The valve has failed open. This is the dangerous failure, especially with kids or grandparents in the house, and it should be fixed straight away. Until it is, keep children away from the hot taps.
Temperature that wanders
A shower that is fine, then cold, then hot again with nobody else using water is a valve that is sticking as it tries to adjust. It is on its way out.
Low flow on the hot side only
If the hot runs at a dribble while the cold is fine, the valve strainer or inlet is scaled up. Sometimes a clean sorts it. More often the valve is due.
What a tempering valve does not cause: no hot water at all, hot water that runs out quickly, or a tank leaking from the bottom. Those are tank problems, and I have a separate post on a leaking hot water system, including the valve itself weeping at its unions.
The 50 degree hot water rule in plain English
Three things worth knowing. First, when a hot water system is replaced in NSW the installer has to make sure the water to the bathrooms is tempered, so if your old tank never had a valve, the new one will come with one. Second, fitting, replacing or adjusting a tempering valve is plumbing work, which in NSW means a licensed plumber. Third, every tempering valve has an adjustment under a cap, and winding it up to get a hotter shower is exactly what the rule exists to prevent. Nobody showers at 50°C; if your shower feels lukewarm, the valve is not delivering 50°C, and that is a fault, not a setting.
The valve also has to be reachable for testing. If yours is buried behind the tank or under a deck, mention it when you book.
Tempering valve replacement: what it involves and what it costs
A straightforward swap takes well under an hour. The water is shut at the tank's isolation valve, the old valve comes out, the new one goes in with new unions and strainers, then the outlet temperature is checked with a thermometer at the nearest bathroom tap and set to 50°C. There is no line for tempering valves in the Sydney price guides, so I quote it on inspection. What moves the price is the valve type (a TMV costs more than a tempering valve) and access. For a sense of scale, the Sydney 2026 price guides put a plumber's call-out at $69 to $170 and hourly rates at $92 to $230, most around $140, and a standard valve swap sits comfortably inside an hour. A lukewarm shower is not an emergency, so book it for business hours and skip the night call-out.
If the tank itself is eight or ten years old, it is worth asking whether the valve is the only thing on its way out. A new hot water system includes the valve, and my post on hot water system installation costs has the numbers. The same goes for a bathroom renovation: the valve gets checked at rough-in, and it is the cheapest time to replace it.
Thermostatic mixing valves: where NSW requires them
A tempering valve delivering no more than 50°C to bathroom outlets is the rule for homes. A thermostatic mixing valve, which holds temperature far more precisely and fails safe, is required where the occupants are more vulnerable: aged care, early childhood centres, schools, hospitals and disability accommodation, delivering 45°C. Both need a licensed plumber to install and both wear out; no hot water or lukewarm water at the shower with a hot kitchen tap is the classic sign the valve is done.
When to call a plumber
Call the same day if the hot water is too hot. That is a scald risk, not something to live with. Call this week if the bathroom is lukewarm while the kitchen is hot, if the temperature wanders, or if the hot flow has dropped off; none of those need a night call-out. And if you are buying a place in Campbelltown or Camden that is old enough to predate tempering valves, get the hot water checked at a bathroom tap with a thermometer as part of the inspection. It takes two minutes and tells you whether there is a valve at all.
Done right. Done once. Monarch covers South West Sydney and the Macarthur region for tempering valve replacement and hot water repairs and replacements. We do not do commercial TMV testing, which needs an accredited TMV plumber. If you need a plumber in Oran Park or anywhere around it, ring 0455 222 292 and I will give you a price before I start.
Related guides: no hot water.
Need it sorted? Ring 0455 222 292 any hour or book online. Monarch Civil & Plumbing is a hot water plumber, no call-out fee: repairs where they are worth it, same-day replacement where we can, and the NSW rebate sorted.
No hot water in Campbelltown, Liverpool or Camden?
Campbelltown has its own page: hot water service Campbelltown. For Liverpool, Camden and the rest of South West Sydney start with hot water systems, then the suburb pages plumber Liverpool and plumber Camden. Same-day replacement for common electric and gas sizes where stock allows, no call-out fee, NSW rebate sorted for heat pumps.
Frequently asked
How do I know if my tempering valve has failed?
Run the kitchen hot tap for a minute, then the bathroom basin. The kitchen is usually untempered, so if it gets properly hot while the basin and shower stay lukewarm, the tank is fine and the valve is stuck. The other failure is the reverse: bathroom water that is suddenly scalding. Wandering temperature in the shower and a weak hot flow with a strong cold flow are the other signs. If everything in the house is lukewarm, suspect the tank first.
How much does a tempering valve replacement cost?
It is quoted on inspection, because there is no fixed line for it in the Sydney price guides. A standard swap is well under an hour of labour plus the valve, and the guides put call out fees at $69 to $170 and hourly rates at $92 to $230. A thermostatic mixing valve costs more than a tempering valve, and awkward access adds to it. Book it in business hours; a lukewarm shower is not an emergency.
Can I adjust my tempering valve myself?
I would not. Fitting, replacing and adjusting a tempering valve is plumbing work in NSW, which means a licensed plumber, and the adjustment exists to set the valve to 50°C with a thermometer, not to make the shower hotter. Nobody showers at 50°C. If yours feels lukewarm, the valve is failing rather than set too low, and winding it up just delays the replacement while risking a scald when it finally lets go.
How long does a tempering valve last?
AS 4032.3 says a tempering valve should be replaced every five years, because unlike a TMV its element cannot be swapped, and in practice I see them last five to eight. Hard water, high mains pressure and grit from the supply shorten that. A thermostatic mixing valve lasts longer because it is designed to be serviced, but it has to be tested every 12 months and have its element and seals replaced every five years under AS 4032.3 to stay compliant.
Does my house legally need a tempering valve in NSW?
If the hot water system was installed or replaced in the last couple of decades, yes, the bathrooms must be tempered to 50°C under AS/NZS 3500.4, and the installer had to make sure of it. An older home with an original tank may have nothing fitted, which is legal until the system is replaced, at which point the valve goes in. Either way, 60°C water at a basin is a scald risk worth fixing regardless of the paperwork.
What is the difference between a tempering valve and a thermostatic mixing valve?
Both blend cold water into hot to limit the temperature at the tap. A tempering valve is the standard household unit: accurate to about 3°C, sealed, and replaced when it fails. A thermostatic mixing valve is accurate to about 2°C, responds faster, shuts off the hot side if the cold supply drops out, and can be serviced. TMVs are required in hospitals, aged care, childcare and schools where the limit is 45°C.
Licensed plumber and drainer based in Austral, 13+ years in the trade, working across Liverpool, Fairfield and the Macarthur region. About Paul