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Water Heater Mixing Valve (Add Up to 50% Capacity)


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A water heater mixing valve, also known as a thermostatic mixing valve, is an easy and relatively inexpensive way to increase the capacity of your water heater. In other words, how to get more hot water out of your water heater.

In addition, they'll also ensure your hot water is at a safe temperature to prevent scalding and disease. This article will show you how to install a water heater mixing valve and provide you with other helpful information. 

Water Heater Mixing Valve on a Gas Water Heater

What is a Water Heater Mixing Valve?

A mixing valve is simply a mechanical device that delivers tempered water by mixing hot and cold water together.

When the hot water enters the mixing valve it blends with cold water which lowers the temperature of the hot water.

Mixing valves can be installed under sinks or other hot water outlets. however, many homeowners prefer to install a water heater mixing valve which will temper all the hot water in your home.

How it Works

When a water heater mixing valve is installed, the tank temperature is increased to 140°F.

As hot water is drawn from the tank, it'll enter the mixing valve at 140°F and mix with cold water. Once mixed, the water will enter your home's plumbing system at 120°F.

By mixing the hot and cold water, your tank will have more hot water available, thus increasing the capacity of your water heater.

Below is a quick video showing how a mixing valve works.

Watch the Video

How Much Capacity Does a Mixing Valve Add to a Water Heater?

Adding a water heater mixing valve will give you at least 25% more hot water. Some professionals estimate that a mixing valve could even increase your capacity by up to 50%.

The increased capacity will allow you to purchase a smaller water heater, yet still be able to meet your household's hot water needs.

Water Heater Mixing Valve on an Electric Water Heater

Purpose of a Water Heater Mixing Valve

Water heater mixing valves serve two primary purposes.

  1. Increase the hot water capacity of the tank
  2. Safety 

Increased Capacity

In 2015 the National Appliance Energy Conservation Act (NAECA) required manufacturers to increase the Energy Efficiency of their water heaters.

Water heaters manufactured after 2015 will typically be up to 2" wider in order to accommodate the added insulation required.

This means if you need to replace a water heater manufactured prior to 2015 you may find it difficult to install the same tank capacity into a small space.

In other words, if you're replacing a 50-gallon tank, you may need to choose a smaller tank or make structural changes to the space where your water heater is located.

Most homeowners don't want to knock out walls in order to install a water heater, so they opt for purchasing the smaller tank size. However, the trade off is a reduction of deliverable hot water.

Your new water heater may not be able to keep up with your household hot water demand.

That's where the water heater mixing valve comes in, and to be fair, you could also install a water heater booster that will also increase your tank capacity. But they tend to be more difficult to install and should be done by a professional.

> > Read More About Water Heater Boosters HERE < <

More Deliverable Hot Water

Once the mixing valve is installed, the water heater temperature is turned up to 140°F, which is typically 20°F higher than recommended due to scalding.

When water is drawn from the tank, its mixed with cold water and delivered to the faucet at 120°F.

By mixing the hot water with cold, your water heater will have more deliverable hot water.



When talking about safety, there are really two issues to consider: Serious burns and Legionnaires disease.

Serious Burns

Most hot water heater manufacturers automatically set their units to 120°F. This is considered a safe setting for both disease and burns as it would take more than 5 minutes for a serious burn to occur.

However, increase the temperature by only 10°, and a serious burn can happen in about 30 seconds. 

Hot water at 140°F, will take less than 5 seconds! And children and the elderly are especially prone to scalding.

Installing a mixing valve will allow you to lower the water temperature within the tank to prevent scalding, without the fear of Legionella bacteria.

Legionnaires Disease

Water heaters should always be set to at least 120°F, even if you are worried about the risk of scalding or burns.

When water is held below 120°F there's a risk of legionella bacteria growth.

Legionella bacteria can cause Legionnaires disease which is a serious pneumonia spread through water droplets when taking a shower, or other exposure to hot water mist.

Using a hot water heater mixing valve will allow you to keep the hot water within the tank at a safe temperature, yet lower the output at the faucet to a temperature that will not cause burns.

Plumbers Tools

Installing a Water Heater Mixing Valve

One of the advantages of using a water heater mixing valve vs. a booster tank is that they are easy to install and don't require electricity to operate. They're also typically less expensive.

Mixing valves for your water heater should be available at all hardware stores as well as online retailers such as Amazon.

When buying one, you'll want to verify that it'll work with your pipe type and size. Although this shouldn't be a problem, it's always best to take the time upfront before purchasing.

Cash Acme 3/4 Inch Tank Booster Water Heater Thermostatic Mixing Valve with Temperature Gauge and Braided Hose, Domestic and Commercial Application, PEX Pipe, PE-RT, CPVC, Copper, 24644

Water Heater Mixing Valve

The Cash Acme Water Heater Mixing Valve works with 3/4" pipe connections.

It's important to note, that although installing a mixing valve on your water heater is a relatively easy task, many homeowners may not be comfortable with the process.

Hiring a professional plumber is never a bad decision, especially if you're unfamiliar with working with plumbing.

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How to Install a Water Heater Mixing Valve

Step One - Turn off the water heater and shut off the water supply.

Step Two - Drain some water from the tank to relieve the pressure.

Step Three - Disconnect the hot and cold water feeds from the dielectric nipples on top of the water heater.

Step Four - Attach the hot water mixing valve to the hot water dielectric nipple, and the cold water valve to the cold water dielectric nipple. Be sure they are facing in the same direction.

Step Five - Attach the cold water transfer tube to the hot water and cold water valve. This will connect the two valves together. 

Step Six - Set your water heater's temperature to 140°F.

Step Seven - Set the mixer temperature to 120°F using the adjuster screw on the mixer valve. You may want to take a temperature measurement, learn how to do that HERE.

Watch the Video

What Happens When a Mixing Valve Fails?

Unfortunately, nothing lasts forever and even mixing valves need to be replaced.

If you suspect your mixing valve is not working properly there are three tell-tale signs you may notice:

  1.  Drips or leakage around the mixing valve.
  2. Hot water from the tap is either too hot or too cold.
  3. Flow reduction in your hot water from the tap.

You can either replace the mixing valve yourself, or hire a professional plumber to replace it for you.

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