Thermostatic Mixing Valve - Solution to Scald Risk

Anybody that is familiar with the temperature method of Legionella control, knows that while cold temps might hold the bacteria in stasis, it’s high temperatures that will actually remove it. Now that may seem all well and good, turn the heat up and no more Legionella issues, while that may be true, it does come with quite a nasty side effect.

Let’s quickly cover the basics of temperature control, just to better understand how scald risk comes into play and how an effective control programme will enforce a strict limit for water temperatures within a property. Cold water is stored and distributed no higher than 20°C and hot water is stored at 60°C and distributed at either 50°C or 55°C, depending on the building’s purpose. Primarily whether it’s healthcare or non-healthcare.

Now although tolerances may vary, it is generally accepted that anything above 49°C poses a scald risk, with the HSE suggesting that for vulnerable people (the young and elderly alike), 44°C poses a scald risk. So it’s easy to see that proper bacterial control, can have some potentially problematic effects.

Looking particularly at the two most at-risk groups, over three-quarters of severe scalds are suffered by children under five and almost three-quarters of fatalities are people aged 65 and over.

Children and the elderly are more susceptible to bath water scalds because their skin is thinner and less tolerant to high water temperatures when compared to other age ranges.

This means scalds are sustained much quicker, at lower temperatures and often to a worse degree. Additionally, those with a reduced awareness or perception of risk and the inability to react to hazardous situations also have a greater risk of scalding.

The Solution

As we previously mentioned in our Methods of Control article, a solution does exist and it’s a simple, yet effective device known as a thermostatic mixing valve or TMV for short. It’s a common anti-scald measure used in a number of premises to counteract high temperatures and help ensure protection.

They come in a variety of makes and models to suit anything from basins and showers to baths and bidets. Every thermostatic mixing valve will be classified as either grade 2 or grade 3, but what’s the difference between them?

Well truth be told, there’s no physical difference, both grades provide the same protection and have the same features, but the grade 3 valves are independently tested external to the manufacturer. This testing is usually required for use in healthcare settings, particularly when it comes to the NHS.

How It Works

So now we know what a thermostatic mixing valve is, let’s take a look at how they work.

At its simplest, the valve takes in both hot and cold water, mixes the two together and provides water at a predetermined temperature of your choosing, which should comply with guidance, but can be altered providing there are extenuating circumstances and relevant precautions are taken.

To go a little deeper, commonly within most valves are a few components:

  • Thermostatic Element – The core of the device which actively expands and contracts to changes in the water temperature and pressure.
  • Piston – Typically connected to the thermostatic element and moves back and forth over the hot and cold ports of the valve, to change the proportion of water entering the valve.
  • Return Spring – When the thermostatic element expands it moves the piston down and compresses the return spring. When the thermostatic element is cooled, it contracts and the spring pushes the piston back.
  • Temperature Control – This component can be adjusted to maneuver the piston and control the temperature by allowing more or less water to enter.

As the thermostatic element expands and contracts, it moves the slide valve to increase or decrease the amount of water that is entering the central chamber and should the supply of cold water gradually drop, this would reduce the amount of hot water to ensure a perfect blend is always issued.

In the event that the cold water is completely shut off, the element will completely expand and prevent any hot water from leaving the outlet.

Digital Thermostatic Mixing Valve

Although, less common in the UK there are also digital thermostatic mixing valves that operate on a similar principle, but the temperature setting can be more easily set and monitored.

They may also have some other features such as an automated flushing routine or temperature records, but these will vary from model to model.

A Double-edged Sword

At this point, it’s worth mentioning that although these valves are an almost perfect solution to counteract the elevated temperatures required for proper Legionella control, they can lead to situations that actually allow the bacteria to thrive.

Just a little prior knowledge before we get too far into this, one of the main factors that allow for bacterial growth in water systems is stagnation. In summary, it provides the opportunity for the bacteria to group together and start to form biofilm.

With that in mind, imagine a tap either at a basin or bath and it is fed by a thermostatic mixing valve which is correctly serviced, maintained and set to the appropriate temperature.

Given that the set temperature is perfect for use, no need to increase or decrease it, it’s entirely possible that the cold tap is going unused and thus the water within the pipework that feeds the cold outlet is sitting there and stagnating.

Luckily this kind of scenario seems to somewhat of a rarity, usually both the hot and cold sides of an outlet are used or flushed regularly and the valve will be fitted as close as realistically possible to reduce the amount of water that might be left to stagnate.

A Lifetime of Service

Once these measures are in place, you might think that’s the end of it and it is for the most part. However, as with many mechanical devices, they are prone to failure and require regular servicing to ensure the protection you’re expecting is still being upheld.

Now to ensure they operate as intended, they need to be serviced at least annually and by a qualified professional.

When these valves are serviced, 3 factors are assessed to ascertain the thermostatic mixing valve’s functionality. The factors are:

  • Temperature Adjustment – The core feature of TMV’s is to control the temperature of the water it supplies, so if a valve’s temperature can’t be changed, then it’s basically not doing its job.
  • Crossover – This might not be as obvious, but the device must make sure that a change in pressure on either side of the valve doesn’t result in hot water entering the cold pipework and vice versa. The importance of this check is to avoid temperatures being elevated or lowered to a range where bacterial growth is supported.
  • Failsafe – Hopefully something that will never have to be used, but still important to know it works. The function of the failsafe feature only comes into play when the cold water supply is reduced or stops and its primary purpose is to stop the hot water and prevent 50°C+ water reaching the outlet and ultimately the user.

While the checks might appear to be fairly straightforward, we would highly recommend that nobody take it upon themselves to carry out these checks, unless suitably trained.

We’ve covered a lot of information in this article and hopefully, you’ll leave with a better understanding of these measures and the service they provide.

If you’d like to know a bit more, please check out our thermostatic mixing valve page or get in touch.