Why Can’t You Connect Pex Directly To Water Heater?

When it comes to plumbing and water heating systems, PEX piping has become increasingly popular due to its flexibility, durability, and ease of installation. However, there’s a common question that arises in the context of water heaters: “Can you connect PEX directly to a water heater?” The short answer is no, and this article aims to explain why.

In the world of plumbing and water heating systems, selecting the right materials and connections is paramount to ensure both efficiency and longevity. When it comes to the versatile material known as PEX (cross-linked polyethylene), questions often arise about its compatibility with water heaters. 

This article explores the crucial considerations regarding the use of PEX in direct connection to water heaters, shedding light on why it’s generally not recommended and the alternative solutions that can preserve the integrity of both your plumbing system and your water heater.

Understanding PEX Piping

What is PEX?

PEX, short for cross-linked polyethylene, is a flexible and durable plastic piping material used in various plumbing and heating applications. PEX tubing is known for its versatility and is commonly used for transporting potable water, as well as for radiant floor heating systems. It has gained popularity in the plumbing industry due to several key advantages:

  • Flexibility: PEX is highly flexible, which makes it easy to install in tight spaces or around obstacles without the need for as many fittings or joints as traditional rigid pipes.
  • Durability: PEX is resistant to corrosion, scale buildup, and chemical reactions, making it a long-lasting and low-maintenance choice for plumbing systems.
  • Freeze Resistance: PEX has some resistance to freezing, allowing it to expand and contract without breaking, reducing the risk of pipe bursts in cold climates.
  • Cost-Effective: PEX is often more cost-effective than traditional copper or steel piping due to its ease of installation and lower material cost.
  • Reduced Noise: PEX can dampen water flow noise, offering a quieter plumbing system.

PEX comes in different colors, such as red and blue, to indicate hot and cold water lines, making installation and maintenance more straightforward

PEX Piping and Water Heaters

Water heaters, whether tankless or traditional, require a specific approach when it comes to connecting piping. While PEX is an excellent choice for many plumbing applications, it’s not suitable for direct connection to water heaters. The reason behind this is the potential for a phenomenon known as galvanic corrosion.

Reasons Why PEX Isn’t a Suitable Pick for Water Heaters

Here are four reasons why PEX isn’t a suitable choice for connecting directly to water heaters:

  • Galvanic Corrosion Risk: PEX is non-conductive, and connecting it directly to a water heater’s metal components can create the potential for galvanic corrosion, which can lead to premature water heater damage.
  • Leakage and Water Damage: Without the use of dielectric unions, the risk of leaks and water damage increases when PEX connects directly to the water heater. This can result in costly repairs and potential water-related issues.
  • Water Quality Concerns: Direct PEX connection can compromise water quality as it may allow rust or corrosion particles from the water heater to enter the water supply, affecting its purity and safety.
  • Non-Compliance with Codes: Connecting PEX directly to water heaters often violates plumbing codes and regulations, which are established to ensure the safety and functionality of plumbing systems. It’s essential to follow these codes for a compliant and secure installation.

The Importance of a Dielectric Union

To prevent galvanic corrosion, a dielectric union is used when connecting dissimilar metals or materials in a plumbing system. A water heater typically contains metal components, such as the heating element or anode rod, and connecting PEX directly to these metal parts can lead to corrosion and premature water heater failure.

How Dielectric Unions Work

Dielectric unions are made up of two different metal components, typically brass or steel, that are separated by a non-conductive material, such as rubber or plastic. This separation prevents a direct electrical connection between the dissimilar metals, reducing the risk of galvanic corrosion.

Benefits of Using Dielectric Unions

Incorporating dielectric unions during the connection of PEX to a water heater presents several advantages, which include:

  • Prolonging Water Heater Longevity: Dielectric unions contribute to extending the operational lifespan of the water heater by mitigating the detrimental effects of galvanic corrosion and reducing the risk of premature deterioration.
  • Leak Prevention: These unions act as a protective barrier, effectively decreasing the likelihood of leaks, and thereby safeguarding your plumbing system against potential water-related damages.
  • Preserving Water Quality: By impeding the formation of rust or corrosion, dielectric unions help maintain the quality of the water supply. This ensures that the water remains pure and safe for consumption.
  • Regulatory Compliance: The utilization of dielectric unions is imperative for aligning with plumbing codes and regulations. These are in place to ensure the safety and adherence of your plumbing system.

Installation of Dielectric Unions

The installation of dielectric unions is a relatively simple process. It involves threading the union onto the water heater’s inlet and outlet ports and then connecting the PEX piping to the other end of the union. It’s crucial to follow manufacturer guidelines and local plumbing codes during installation.

Compatibility with Different Water Heaters

Dielectric unions can be used with various types of water heaters, including tankless and traditional tank-style heaters. They are a versatile solution to address galvanic corrosion concerns.

Common Issues When Not Using Dielectric Unions

If PEX is connected directly to a water heater without dielectric unions, several issues can arise, including:

  • Corrosion of the water heater’s components.
  • Reduced energy efficiency.
  • Water contamination due to rust or metal particles.
  • Increased risk of leaks and water damage.

Maintenance and Care Tips

Proper maintenance and care are essential to ensure the longevity and optimal performance of your water heater and the overall plumbing system. Here are some valuable maintenance and care tips to keep in mind:

Regular Inspection of Dielectric Unions

Dielectric unions play a critical role in preventing galvanic corrosion, but like all components, they require periodic inspection. Check these unions for any signs of wear or corrosion. This visual assessment helps you identify any issues early, allowing for timely repairs or replacements. 

By addressing problems promptly, you can maintain the integrity of the union and prevent potential damage to the water heater.

Scheduled Water Heater Flushing

Regular flushing of the water heater is a crucial maintenance task. Over time, sediments and mineral deposits can accumulate at the bottom of the tank, affecting heating efficiency. Following the manufacturer’s recommendations, flushing the tank at specified intervals helps remove these deposits and keeps your water heater operating at its best. 

This not only extends the heater’s lifespan but also ensures you have access to hot water without energy waste.

Monitoring Water Quality and Taxt

Pay attention to the quality and taste of the water coming from your faucets. If you notice any unusual changes, such as a metallic taste or discolored water, it could be an early indicator of issues with the water heater or the plumbing system. 

These changes might be related to corrosion or sediment buildup. In such cases, it’s advisable to consult a professional plumber to assess and address the problem promptly. Timely action can help maintain the quality and safety of your water supply.

Safety Considerations

When working with water heaters and plumbing systems, safety should be a top priority. Always follow manufacturer instructions and consult a licensed plumber for any installations or repairs. Safety precautions are essential to avoid accidents and ensure the proper functioning of the system.

Reasons Why PEX Shouldn’t Connect Directly to Water Heaters
ChallengesPotential Consequences
Galvanic Corrosion RiskPremature water heater damage
Leakage and Water DamageCostly repairs and water-related issues
Water Quality ConcernsCompromised water purity and safety
Non-Compliance with CodesViolation of plumbing regulations

This table provides a concise summary of the main reasons why it’s not advisable to connect PEX directly to water heaters and the potential consequences of doing so.


Why is PEX not allowed?

Galvanic corrosion and associated damage may restrict the direct connection of PEX to water heaters.

Can you use PEX for the water heater discharge pipe?

Its temperature limitations generally make PEX not recommendable for use as a water heater discharge pipe.

Can you connect PEX to the boiler?

You can connect PEX to a boiler for certain applications, but it depends on the specific boiler type, its temperature requirements, and local regulations.

How much heat can PEX pipe withstand?

PEX can typically withstand temperatures up to 200 degrees Fahrenheit (93 degrees Celsius), making it suitable for hot water supply but not for extremely high-temperature applications.


In conclusion, while PEX piping is a fantastic choice for various plumbing applications, connecting it directly to a water heater can lead to galvanic corrosion and a host of related problems. To prevent these issues and ensure the longevity of your water heater. For a plumbing system, it’s crucial to use dielectric unions. These simple components act as a barrier, preventing the corrosion that can occur when dissimilar metals meet. By taking this precaution, you can enjoy a reliable and efficient water heating system for years to come.

In residential and commercial plumbing projects, people commonly use it for water supply lines, radiant heating systems, and even some applications in the HVAC industry.

Leave a Comment