By Patrick Gordon of Calafrica (

Patrick continues his series on valves and how they operate.

Q – How does a Thermostatic Mixing Valve work?

Most domestic hot water systems support temperatures of more than 60°C. This temperature is maintained to make the system safe from bacteria like Legionella. This temperature however exposes the user to the risk of scalding. Under normal conditions this is not a problem as the average person’s reaction speed is such that they can remove themselves from the exposure before suffering serious injury. However young children, old people and disabled people have slower reactions and are thus at risk. The set temperature of the system cannot be reduced without the risk of bacterial infection. The only way to make the system safe is to regulate the terminal temperatures by using a thermostatic mixer with an anti-scald feature.

Image credit Patrick Gordon

Image credit Patrick Gordon

But how does it do this:

Image credit Patrick Gordon

Image credit Patrick Gordon

The regulating element in the thermostatic mixing valve consists of a temperature sensor that is fully immersed in the mixed water outlet pipe which, by expanding and contracting, continuously ensures the correct proportioning of hot and cold water at the outlet. The water proportions are regulated by a piston that slides inside a special cylinder that is placed between the hot and cold-water passage channels.

Even if the pressure drops due to hot or cold water drawn-off by other users or temperature variations at the inlet, the mixing valve automatically adjusts the water flow rate until the set temperature is achieved. A mixing valve with an Anti-scald feature will totally prevent hot-water flow if the cold-water flow should be interrupted. This may inconvenience the user, but it will safeguard them against serious injury.

After you have adjusted the valve to the desired temperature, you can lock the setting by using the control knob. To achieve this, unscrew the retaining screw at the top of the control knob. Remove the knob and then replace it so that the spline on the inside of the handle matches with the spline on the adjustment mechanism.

Image credit Patrick Gordon

To protect our valuable friends and family, get a qualified plumber to fit a Thermostatic Mixing Valve to your system.

Patrick Gordon

Image supplied

For any further information regarding this topic please feel free to contact Patrick Gordon on: +27 (0)83 303 1437.

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