Hot water too hot or scalding? Your tempering valve explained

What a tempering valve does

A tempering valve is a small brass valve fitted on the hot water line just after the system. Its job is simple but important: it blends a little cold water into the hot before it reaches your tap, so water leaving bathroom outlets never exceeds 50°C. Storage tanks hold water at around 60°C to kill bacteria like legionella, but 60°C water can cause a serious burn in a few seconds, especially on kids and older people. The tempering valve is the safety device that keeps the tap safe. In NSW it's mandatory on bathroom outlets in homes.

Signs the tempering valve has failed

These valves have moving parts and a temperature-sensitive element inside, and they wear out, usually somewhere around 5 to 10 years. When one fails, it tends to stick one way or the other.

• Water suddenly too hot: if the valve stops blending cold in, full-temperature tank water reaches the tap. That's the scalding risk, and it needs attention quickly.
• Water never gets hot enough: stuck the other way, it blends in too much cold, so the shower feels lukewarm no matter how you set the tap.
• Wild temperature swings: hot, then cold, then hot again can also point to a tempering valve on its way out.

Scald safety comes first

A scald is not a minor thing. At 50°C it takes several minutes to cause a full-thickness burn, but at 60°C it takes only a few seconds, and hotter water is faster again. If your water has gone scalding, treat it seriously, especially around young children, elderly family, or anyone who can't move away from the tap quickly. Turning the tank thermostat down is not the fix, because the tank needs to stay hot enough to kill bacteria. The correct fix is a working tempering valve doing the blending, and that is licensed plumbing work, not a DIY adjustment.

How to tell it's the valve and not the system

There's a simple pattern that points at the tempering valve. If the bathroom taps have gone scalding but the kitchen and laundry feel about the same as always, that's a strong hint, because the kitchen and laundry aren't fed through the tempering valve, only the bathroom is. The other tell is timing: a tempering valve failure comes on fairly suddenly, often over a few days, rather than the slow decline you'd get from a dying element or a tank full of sediment. If the whole house runs lukewarm instead, the issue is more likely the system itself. Either way, Terry checks the temperature at the tap with a thermometer to be sure before quoting, so you're not paying to replace a part that's fine.

Repair or replace?

Tempering valves are almost always replaced rather than repaired. They're a sealed, relatively inexpensive part, and swapping the whole valve is quicker and more reliable than trying to rebuild one. Fitting a replacement is a routine job for a licensed plumber and usually falls in the rough $150 to $450 range, depending on access and the valve. While he's there, Terry checks the temperature at the tap with a thermometer to confirm it's landing at 50°C as it should. If your place is 5 to 10 years old and the valve has never been looked at, it's worth a check before it fails.

If your hot water has turned scalding or gone stone cold, don't risk a burn. Call Terry on 0415 274 542 for a fixed-price fix. Licensed plumber and gas fitter, servicing Berala and Western Sydney since 1986.