Should I Install a Mixing Valve in my Water Heating System? 
Yes! & Here's Why

A thermostatic mixing valve can save someone in your family from being scalded seriously, so in many ways, it's an easy and sensible decision to make. It could save someone's life, but it will also save you money.


So, what is a mixing valve?

A mixing valve allows you to control the water temperature at the point of delivery, such as sinks, showers, bathtubs, or wherever you use hot water. The cold and hot water supplies feed the valve, and the water coming out of the unit can be preset, so there's no risk of scalding. They can also be individually controlled by rotating dial on the unit, which allows the user to regulate the temperature up and down as they use the water.

The thermostatic mixing valve is a relatively modern invention from 1936 by Sergius Vernet (USA 1899 – 1968). He gained a patent in 1938 for a wax thermostatic valve, which works based on the thermal expansion of a wax pellet within the valve.

The wax pellet rapidly expands and contracts when heated and cooled, respectively, thereby controlling the temperature and flow through the valve. Its first use in the heating and plumbing industries dates back to 1970. It's the same principle used today in modern thermostatic valves.

Today we have a vast range of thermostatic valves available for every type of project or requirement. The most common type is used in water heating to control the flow and temperature of hot water in our home. It doesn't matter what the incoming cold water temperature is; it will lock in the temperature you set and keep it there.

A thermostatic valve will prevent burns due to unexpected changes in water temperature. They usually have a safety system built in that will shut the water off immediately in the event of an error or loss of cold water, leading to scalding.

In the past, many people didn't install a thermostatic mixing valve on their hot water systems. The initial setting on the thermostat on the heater would remain once set up and forgotten. The fact is that many homes actively ignore their water heater and afford it little attention, so after it's installed and set up, it's forgotten.

Until it goes wrong, that is.

Why do I need a thermostatic mixing valve installed?

Water heaters are usually delivered preset at a safe setting of around 120 F. However, it could still be dangerous for a child's thin skin, and the same applies to older adults and those who are frail. As we age, our skin loses thickness, so it takes less heat to burn us seriously.

Many of us will have experienced the shock of turning on a shower, and after setting the water temperature just right, we get blasted by a stream of very hot or freezing cold water. This can happen where there is no thermostatic control of the water system, often in hotels in countries where the regulations are lax, don't exist, or are ignored.

Being drenched with ice-cold water won't hurt you, perhaps only waken you up a bit. However, if the water is scalding hot and you don't move quickly enough, you could receive severe burns, which is why children and the frail are particularly vulnerable.

Safeguarding your family is your primary concern. Fitting a suitable thermostatic valve at your water heater is a simple job that should be a top priority for you.

Of course, you could turn the water heater temperature down to a lower setting. That would undoubtedly avoid scalding issues; however, it introduces a further possible complication – bacteria.

Yes, bugs like Legionella, which causes Legionnaires Disease, a severe type of pneumonia that can kill. It thrives in water between 95 F and 110 F. If Legionella grows and reproduces in a home hot-water system, water containing it can spread in droplets of water in the air, tiny enough for people to breathe in, like in a shower, for example.

Fitting a thermostatic mixing valve in the hot water line from your water heater will allow you to keep the water heater temperature up to 120 F or slightly above. By doing that, it will kill Legionella. You can then set the water temperature lower in the thermostatic valve to ensure everyone is safe from scalding.

 Should I install it myself?

Installing a mixing valve is a simple job for a competent DIYer.

But if you are unsure, ask your local professional plumber to fit it for you; it should only take a few minutes to do. If you are going to do it yourself, purchase a suitable three-way thermostatic valve such as this one from Caleffi. It's connected to the hot and cold water supplies; then, it feeds your home with water at the temperature you choose. This will save you money and allow you to raise the heater temperature to ensure all bacteria are eliminated.

If you decide to do it yourself, follow the step-by-step instructions below.

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What does a thermostatic mixing valve look like?

The valve has a cold waterside and a hot waterside. The valve balances the temperature between the hot and cold supplies and compensates for any pressure differences (within its range). The hot and cold sides are marked clearly.

You will have to adjust the run of the pipes to accommodate the length of the valve or use suitably rated flexible connections.

These valves are connected to your water heater (point of source); the water is mixed and leaves the valve at the temperature you select and lock into the unit.

The valve has a plastic cap that is levered off easily, and the setting adjusted with a screwdriver, then locked for safety. The mixed water will always come out at that temperature unless there is a water supply failure, then it will shut down for safety.

You can also fit thermostatic valves at sinks, showers, and tubs for added safety; this is called a point of delivery mixing.

Step-by-step instructions for installing a thermostatic mixing valve (video attached)

Mixing valve installation

Note: Your pipework layout will probably be different from what's shown here; however, if you identify each pipe and what it does and which direction it flows in, then follow this guide through; it should be a simple task.

  • Switch off all power (and gas) to the water heater.
  • Close the hot and cold valves and run off some cold and hot water from nearby taps to drain the pipes while you work.
  • Allow the water heater to cool for a couple of hours before you start working on the job to avoid scalding risks.
  • Make sure you select the correct size of the thermostatic valve to suit your pipe diameter. Mixing valves come in a range of sizes to suit your installation.
  • Select the type of connection you prefer to use; the valves are available in solder, threaded, PEX, or CPVC. Copper fittings are probably the most common with compression or soldered connections.
  • The thermostatic valve has a cold water inlet, a hot water inlet, and an outlet with water at the required temperature, which you can set and lock on the valve.
  • You are going to need to divert the hot water feed (1) and the cold feed (2) to the water heater through the thermostatic valve (4). Make sure you connect each to the correct side of the valve.
  • Install shut-off valves (5) in both the hot water feed and the cold water feed to give you additional control. Close each valve off.
  • Plan where to cut into each pipe and work out the new route for the piping through the shut-off valves to the thermostatic valve. Cut the copper pipe to length with a pipe cutter tool and ensure the ends of each tube are smooth and burr-free.
  • It's also vital to check the output temperature setting on the new valve out of the box. It may be factory preset to 110 F, but always check and reset it to the temperature you want before installing it.
  • Make your connections, and if you are using threaded joints, wrap the threads with the plumber's Teflon tape. (Do not use it with the flex lines, flared fittings, or gas lines).
  • Install the temperature gauge (6), if you are using one, in the feed line to the house supply (3). It's a useful thing to have, and you can see at a glance what the water temperature setting is and if it changes.
  • The water temperature control is under the plastic cap on the valve. It flicks off, and you can set the temperature using a screwdriver. After setting, it's locked in place to avoid accidental changes, which could cause scalding.
  • If you're not confident with copper work, then flexible braided connectors may be ideal for you. They come in varying diameters and lengths to suit all applications and are designed for hot water applications. They are simple and easy to connect and require no specialist tools or skills to install.
  • Ensure all your connections are correct and tight, turn on the hot and cold water supplies through the valve and check for any leaks.
  • Turn on the mains water supply to the water heater again and power it up. (If it's a gas heater, then turn on the gas and run it up).
  • There will be air in the system, which will clear once the tank is full again. Test the water temperature coming from the valve, and it should be as you set it; however, if you need to change it up or down, remove the plastic cap and adjust the gauge with a screwdriver and lock it up again.

Remember, your plumbing layout will be different from what's shown here, but the steps are the same. Make sure you identify the pipe first, then tackle the job carefully and methodically; you will do it!

Other uses for thermostatic mixing valves

For faster response, a thermostatic mixing valve can be fitted closer to the point of delivery. For example, in a child's or frail adult's bathroom. These valves respond faster and cannot be set above 120 F to prevent scalding. They also cut off instantly should the cold water fail.

Another area where a thermostatic valve is essential is in a shower room or bathtub, where it can be unsafe to have water above 120 F. A thermostatic valve will ensure water at a controlled temperature is delivered without any risks.

Thermostatic valves are also used in radiant heating systems with baseboard radiators or wall-mounted panels to give you accurate temperature control throughout your home. They can be adjusted to suit the room's specific requirements and, as necessary to provide you with complete zonal control.

In hydronic radiant systems, thermostatic valves allow the temperature to be higher. This system can sometimes require up to 130 F. Again; if the hydronic system is zoned and using underfloor tubing, it can be accurately controlled using thermostatic valves for each zone.


Thermostatic mixing valves on your water heater are essential to prevent scalding and control water temperature at the delivery point. It allows you to increase the heater temperature, killing off harmful bacteria.

It may seem counterintuitive to set your heater at a higher level if you want to save money, but remember, using a thermostatic valve, you will use less hot water, so there is a possible saving there.

Ensure you and your family are safe from scalding and save yourself money by using your expensive hot water wisely.

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