What Is a Tempering Valve?

A tempering valve takes two supplies, hot from the water heater and cold from the mains, and mixes them into one outlet at a set temperature. It is set on site when it is installed: the plumber runs the water, measures the mixed temperature with a thermometer and adjusts the valve until it delivers the temperature wanted, and from then on the valve holds the mix close to that setting.

The point is the gap between two safe temperatures. Water in the tank needs to be hot enough to stop bacteria growing, and that is hot enough to scald skin. The tempering valve lets the tank stay hot while the shower, basin and bath get water that is safer to wash in.

RMC, an Australian valve maker, specifies its tempering valves to hold the set temperature within 3°C either side when tested to the Australian Standard AS 4032.2 between 40°C and 50°C. The delivery limit in the National Construction Code applies to installations for personal hygiene (basins, baths and showers), so kitchen and laundry taps may be plumbed from the untempered supply.

Queensland homes have a tempering valve because two rules pull in opposite directions. Business Queensland states that hot water in residential storage systems must be heated to at least 60°C to prevent the growth of bacteria such as Legionella, and that a temperature control device is required so hot water is delivered to taps at a maximum of 50°C. Queensland Health makes the same point from the other side: stored water at 60°C or more may cause scalding, so water for washing and bathing should be delivered cooler through a temperature controller.

The requirement applies to new work. Business Queensland says a temperature control device must be fitted to all new and replacement hot water systems, and points to a separate state guideline for the detailed circumstances. An older home whose system has never been replaced may still have no valve at all, and the requirement applies when that system is replaced.

Queensland Health also advises against turning a hot water system down to make the tap water cooler. Lowering the storage temperature undoes the bacteria control the 60°C rule is there for.

A tempering valve and a thermostatic mixing valve (TMV) both blend hot and cold water to a set temperature, and the difference that matters for a homeowner is where each is allowed. The National Construction Code limits delivery at bathroom fixtures to 45°C in aged care residential areas, patient care areas of health care buildings, early childhood centres and parts of schools used by children, and in some accessible facilities. Everywhere else, which covers an ordinary house, the limit is 50°C.

For the 45°C settings the code accepts only a thermostatic mixing valve or a thermostatically controlled tap. For the 50°C case it also accepts a tempering valve or a temperature limited water heater. So an ordinary house may use a tempering valve, while a childcare centre or aged care home must use a thermostatic mixing valve or a thermostatically controlled tap.

The code's explanatory notes add that where the people living in a house are older adults or young children, a delivery temperature lower than 50°C should be considered. That is a setting a licensed plumber makes at commissioning, not a change to the tank.

Look first on the hot water pipe close to the water heater. RMC's installation manual recommends fitting it as close as possible to the hot water system, although it may be fitted anywhere on the hot supply pipe. If it is fitted near the point of use instead, RMC requires at least one metre of pipe between the valve and the outlet.

A tempering valve is a compact valve body with three pipe connections, shown on RMC's diagrams as hot, cold and mixed outlet. It is a different part from the temperature and pressure relief valve, which RMC's diagrams show separately, fitted on the water heater itself.

RMC's instructions also ask the installer to record the installation details on a sticker fixed near the system. That sticker matters later: RMC's maintenance instructions refer back to it for the outlet used at commissioning, and the temperature at that outlet is what a plumber checks at the next service.

RMC's troubleshooting guide describes tempering valve faults by what happens at the outlet, so the signs to watch for are changes in temperature or flow at the bathroom taps. The signs, matched to that guide:

Water that is lukewarm when it used to be hot, with the kitchen tap still running properly hot (RMC's diagrams show the kitchen and laundry on the untempered supply). RMC lists debris in the valve and in the strainers among the causes when the valve cannot deliver its set temperature.

Temperature that drifts or wanders while the shower is running, or changes from season to season. RMC links an unstable mix to fluctuating supply pressure, debris in the strainers and changes in the incoming water temperature.

Water that runs fully hot or fully cold, or hot water appearing at a cold tap. RMC's guide ties these to fouled non-return valves or hot and cold connections that have been reversed.

Reduced or fluctuating flow from the hot taps, or a valve that is noisy. RMC links reduced flow to debris in the inlets or fluctuating supply pressure, and noise to water moving through the valve too fast.

If the bathroom water is suddenly much hotter than it used to be, treat it as a scald risk: stop using the hot tap for bathing and call a licensed plumber.

On solar and heat pump systems, lukewarm water at the shower is not always a failed tempering valve, because the valve can only cool water down, never heat it. A tempering valve needs the incoming hot water to be well above the temperature it delivers. RMC's specification for its valves lists a minimum difference of 10°C or 15°C, depending on the model, between the hot supply and the outlet, and expects a hot supply of 60°C or more.

If the tank is not reaching temperature, perhaps because a solar booster is off or a heat pump is struggling, the valve has less hot water to work with and the tap runs cool even though the valve itself is fine. Replacing the valve would not fix that.

Valves are also chosen to suit the system. RMC's selection chart sorts its models by system type, separating solar and continuous flow from standard mains pressure storage, and it makes a dedicated solar model. A plumber checks that the fitted valve is rated for the system before blaming its performance.

You shouldn't adjust a tempering valve yourself: treat it as a job for a licensed plumber. The QBCC says most plumbing work undertaken in Queensland is regulated and requires a QBCC licensed plumber or drainer. Installing, replacing or removing a temperature control device in an existing building is one of the QBCC's categories of notifiable work, which a licensed plumber registers with the QBCC. The QBCC's examples of unregulated work an owner may do cover jobs such as replacing a shower head, a filter cartridge or a tap washer. Valves on a hot water system are not among them.

There is also a practical reason. Setting a tempering valve means measuring the delivered temperature at a known outlet and confirming it stays within the delivery limit, and a valve that is set wrongly or has a fault can send water that scalds to a shower. A plumber records the result so it can be checked again later.

Queensland Health's advice for owners points the same way: maintain hot water systems using a registered plumber and the manufacturer's instructions. What you can do safely is note the symptoms, which taps are affected and since when, and photograph the installation sticker for the plumber.

A tempering valve should be checked once a year. RMC recommends an annual check to confirm the valve is still working, and says more frequent checks may be needed where water quality is poor or unknown.

The check compares the delivered temperature at the same outlet used at commissioning with the setting recorded on the installation sticker. Under RMC's instructions, a reading more than 2°C away from the commissioning temperature, or outside the AS/NZS 3500.4 requirements, sends the plumber to the troubleshooting steps.

Some faults are cleared by cleaning, since RMC's design gives access to the strainers and non-return valves through the union connections. The valve body is a different matter. RMC states that its valve itself cannot be serviced and that a failed valve must be replaced. Other manufacturers' instructions may differ, which is why the plumber works from the instructions for the valve actually fitted.

The manufacturer documents checked for this guide give no lifespan in years, so treat any fixed figure with caution. A valve that keeps drifting is due for attention whatever its age.

When a plumber looks at a tempering valve complaint, the first job is to work out whether the valve or the water heater is the problem. That means checking the tank is reaching its storage temperature, then measuring the delivered temperature at the commissioning outlet and comparing it with the sticker.

From there the checks follow the manufacturer's troubleshooting: debris in the strainers, fouled non-return valves, hot and cold connections the right way round, and supply pressures within the valve's limits. The plumber also confirms the valve model suits the system, which matters most on solar and heat pump installations.

If the valve has failed, it is replaced and the new one is set and recorded. If the system is old and the valve is one of several faults, that is the point to weigh repair against replacement. Business Queensland says a temperature control device must be fitted to a replacement system too.