Tempering Valves And Hot Water Scalding

Hot water in a building has to satisfy two requirements that pull against each other.

It has to be stored hot enough to prevent bacterial growth in the cylinder, and it has to arrive at a bathroom outlet cool enough that it cannot scald somebody. The device that resolves that is a tempering or thermostatic mixing valve, and it is a maintenance item rather than a fitting.

Why both limits exist

Stored hot because Legionella and other bacteria multiply in warm water and are killed at higher temperatures. Which is why a storage cylinder is kept above the growth range rather than at the temperature you would want out of a tap.

Delivered cooler because water at storage temperature causes a serious burn in seconds, and far faster than that for a child or an elderly person whose skin is thinner and whose reaction is slower. The requirement for bathroom outlets reflects that.

So the system runs hot and the valve blends it down on the way to the sanitary fixtures, while the kitchen and the laundry are generally served at the hotter temperature because those uses need it.

What the valve does and how it fails

The valve mixes stored hot water with cold to a set outlet temperature, and it holds that temperature as the pressures and the incoming temperatures change.

It fails in three ways.

It drifts, delivering progressively hotter or cooler water than it is set to. The hotter drift is the dangerous one and nobody notices until a shower is suddenly too hot.

It seizes, usually from scale or from sitting in one position for years, and the outlet temperature stops responding at all.

It fails toward hot, which is the failure mode that matters, and it is why these valves are a scheduled service item rather than a fit and forget one.

The cartridge is the serviceable part, and the manufacturers specify a service interval. Which almost no residential property observes, and which is routinely overlooked in strata buildings where the valve sits in a plant room nobody opens.

The symptoms worth acting on

Shower temperature that wanders while you are in it, or changes when another tap is used.

Water that is suddenly much hotter than it used to be, which is the one to treat as urgent.

Water that will not get hot enough at a bathroom outlet while the kitchen tap is fine, which points at the valve rather than at the hot water unit.

A long wait and then a temperature swing, which can be the valve or can be a dead leg in the pipework, and the second of those is the problem in [hot water that takes forever to arrive](/blog/hot-water-that-takes-forever-to-arrive).

No hot water at all, which is more likely the unit than the valve, and the repair or replace question is in [hot water, when to repair and when to replace](/blog/hot-water-when-to-repair-and-when-to-replace).

Who it protects most

The requirement exists because of who gets burned.

Children, because their skin is thinner and they cannot get out of the way. Older residents, for the same reasons plus slower reactions. Anybody with reduced sensation or reduced mobility, who may not feel the water is too hot or cannot move quickly once they do.

Which means in a property housing any of those, the valve is a safety device with a specific purpose rather than a compliance box. In supported or accessible housing it sits alongside the rest of the bathroom safety thinking in [grab rails and a safer bathroom](/blog/grab-rails-and-a-safer-bathroom).

What proper service involves

Measure the delivered temperature at the outlets, with a thermometer, rather than assuming the setting is being achieved.

Service or replace the cartridge at the manufacturer's interval.

Check the strainers, because debris from the system is what causes most of the seizing, and a valve that is replaced without cleaning the strainers fails again.

Check the isolation valves either side of it, because a valve that cannot be isolated cannot be serviced without shutting down a riser. On a strata building that distinction is the whole of [isolation valves and why a building needs them](/blog/isolation-valves-and-why-a-building-needs-them).

Record the date and the measured temperatures, so the next service has a baseline and so the property has evidence the obligation was met. That record belongs in the schedule described in [the annual maintenance plan and what goes in it](/blog/the-annual-maintenance-plan-and-what-goes-in-it).

Where it sits in a building

In a house, usually close to the hot water unit, serving the bathrooms.

In an apartment, either a unit valve within the apartment or a central valve serving the building, and which one you have decides whose responsibility it is. A valve inside the lot is generally the lot owner's; a valve in the plant room serving the building is generally common property. The line is the one in [common property or lot owner, who pays](/blog/common-property-or-lot-owner-who-pays).

In an older building, possibly not present at all, because the requirement came in at a point in time and a system installed before it may never have had one. Which is worth checking on any pre-existing installation rather than assuming, particularly where the property now houses children or older residents.

The related failures

A tempering valve problem is often part of a larger hot water picture: a unit at the end of its life, a system with long dead legs, mixer cartridges worn out at the outlets, or pressure problems through the building.

Which is why the sensible move on a property with hot water complaints is to look at the whole system in one attendance rather than replacing the thing that was closest to hand. The outlet side of it is in [dripping taps and mixer cartridges](/blog/dripping-taps-and-mixer-cartridges), and the pressure side in [water pressure problems in apartments](/blog/water-pressure-problems-in-apartments). One team looking at all of it finds the actual cause instead of the nearest one.

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