None of the manufacturer or code sources we checked sets a replacement age for a tempering valve. Makers such as Zurn and Watts call for yearly inspection and cleaning. Replace the valve, or its cartridge, when cleaning no longer gives a steady, correct temperature. Hard water scale is a common cause of wear.
What the manufacturers say
Zurn's instructions for its ZW1017XL tempering valve, the kind that mounts at a water heater, say: "Annual inspection and maintenance is required of all plumbing system components. To ensure proper performance and maximum life, this product must be subject to regular inspection, testing and cleaning."
Watts is just as direct for its LFUSG-CS under-sink mixing valve, which is a commercial point-of-use model: "Periodic inspection and yearly maintenance by a licensed contractor is required." It adds that "Frequency of cleaning and inspection depends upon local water conditions."
So the pattern is a yearly check, with water quality deciding whether you need to look more often. A fixed lifespan is not in any of those documents.
Why valves wear out
Hard water is the wear factor Resideo names. Resideo's product data for its AM-1 series says: "Hard water conditions may result in scale deposits causing binding of internal parts in extreme cases. Cleaning the internal parts will usually restore the valve operating conditions. In some cases it may be necessary to replace the lower assembly."
That tells you the usual order of events. Scale builds up, parts stick, and cleaning often fixes it. When it doesn't, you replace the working part, not necessarily the whole body. Watts builds its valve the same way, with a replaceable cartridge. Its troubleshooting chart lists "Replace cartridge" for blocked strainers or check valves, and those blockages show up as swinging temperatures or reduced flow. If you're curious how fast scale builds in your water, our page on whether hard water ruins a water heater covers the same minerals.
Signs it is time to service or replace it
You can watch for these without touching the valve:
- The hot water is noticeably hotter or cooler than it used to be at the same setting.
- The temperature drifts or swings while you run a faucet.
- Flow drops, or one side of a fixture seems to get only hot or only cold water.
- The valve is old, corroded, or has never been cleaned.
Watts lists "Mixed water temperature is unstable or changes over time" and "Reduced or fluctuating flow rate" as symptoms in its own troubleshooting chart. If the valve isn't doing what it used to, find out why.
Why the temperature matters: Zurn's instructions warn that "Water Temperature in Excess of 122°F(50C) Is Dangerous and Will Cause Scalding, Severe Injury or Death!" The same sheet says the installer "must use a thermometer at the outlet to verify the temperature." A tempering valve that has drifted hot is a scald risk, so a hot-running valve is not something to leave for next year.
What this means for a home
Put it on the same calendar as your water heater maintenance. When you flush the water heater, have the valve checked: read the outlet temperature with a thermometer, clean the internals if the maker's instructions call for it, and replace the cartridge or valve if it can't hold the setting. That's the yearly check.
For commercial buildings, the CDC adds a second reason. Its Legionella toolkit says to "Clean and maintain water system components, such as thermostatic mixing valves, aerators, showerheads, hoses, filters, and storage tanks, regularly." Our page on why commercial buildings use thermostatic mixing valves goes into that.
Two quick notes so you don't chase the wrong part. A tempering valve is not the same as the temperature and pressure relief valve on top of the tank; if that one is dripping, see why a water heater pressure relief valve leaks. And if your shower temperature is the problem, the cause may be the shower valve instead, which we cover in pressure balancing versus thermostatic shower valves.
Who should do it
Zurn says installation of these products "must be performed by qualified, licensed personnel," and Watts calls for maintenance by a licensed contractor. Cleaning or swapping a mixing valve means shutting off water, working on a device that controls scald protection, and then proving the outlet temperature. That is worth doing right.
If you'd like a valve checked, HQ Plumbing is licensed and insured, and we give free estimates and upfront pricing before work starts. If you are wondering whether the whole system is nearing its end, our page on signs a water heater is about to fail is the next stop.