How To Join Steel and Copper Pipes

Sometimes, you have to combine steel and copper pipes, especially when you’re repairing older plumbing systems. Joining dissimilar metals is a challenge, though, because they can get corroded over time. In our guide, we’ll explain why this corrosion occurs and how to join steel and copper pipes the right way so that you prevent leaks and water damage. We’ve also included a video in which This Old House’s plumbing and heating expert, Richard Trethewey, walks a homeowner through the process of connecting dissimilar metals using a dielectric union.

Understanding the Challenge of Joining Steel and Copper

Several issues can arise when you connect steel and copper pipes. The primary concern is a process called electrolysis, which happens when two different metals come into contact with water or moisture.

What Is Electrolysis?

Electrolysis is an electrochemical reaction that can cause rapid corrosion. This can happen at the spot where dissimilar metals meet. Plumbers run into this issue often when steel and copper pipes join because water helps electrons transfer between the metals, acting like an electrolyte. When this happens, it can cause pipes to deteriorate and leak. If it goes unchecked long enough, it can result in water damage to your home and mold growth.

The Importance of Proper Joining Techniques

You can avoid electrolysis and make a safe, long-lasting connection between steel and copper pipes by using the right technique and materials. A dielectric union is an effective tool because it provides a barrier between the metals while keeping a watertight seal.

Tools and Materials You’ll Need to Join Steel and Copper Pipes

If you have do-it-yourself (DIY) plumbing experience, you can take on the project with the right tools and materials. Here are the items you’ll need to get started:

• Dielectric union
• Emery cloth or sandpaper
• Flux
• Garden hose (for draining water)
• Pipe wrenches
• Pliers
• Propane torch
• Solder
• Teflon tape
• Tubing cutter

Step-by-Step Guide to Joining Steel and Copper Pipes

In the video, Trethewey walks you through this process if you need a visual guide.

Here are the steps to use a dielectric union to join copper and steel pipes:

1. Turn off the water supply and power to the water heater.
2. Drain the water from the system using a garden hose.
3. Cut the copper pipe just above the corroded connection using a tubing cutter.
4. Remove the old union from the steel pipe using two pipe wrenches.
5. Clean the end of the steel pipe and wrap the threads with Teflon tape.
6. Install the lower half of the dielectric union onto the steel pipe, tightening it first with pliers and then with a pipe wrench.
7. Clean the end of the copper pipe with emery cloth or sandpaper.
8. Apply flux to the cleaned end of the copper pipe and the inside of a copper coupling.
9. Solder the coupling and a small length of copper pipe to the existing copper water pipe.
10. Remove the rubber gasket from the dielectric union to protect it from heat damage.
11. Apply flux to the top half of the union and the new copper pipe section.
12. Solder the top half of the union to the copper pipe.
13. Allow the soldered connections to cool, then replace the rubber gasket in the dielectric union.
14. Use a wrench to tighten the two halves of the union together.
15. Turn the water and power back on, then check for leaks.

Understanding Dielectric Unions Between Steel and Copper

Dielectric unions are fittings or adapters that prevent direct contact between two metals. Below is an overview of how they work and their benefits.

How Dielectric Unions Work

A dielectric union has two main parts:

• Brass or plastic side that connects to the copper pipe.
• Steel side that connects to the steel pipe.

A nonconductive gasket made of rubber or plastic sits between those two sides described above. The gasket is a physical barrier that disrupts the electrochemical reaction we mentioned earlier. Since the barrier prevents the metal from making an electrical exchange, they won’t get corroded. 

Benefits of Using Dielectric Unions

There are several advantages to using these fittings when you want to join two metals together. A dielectric union does the following:

• Allows for easier future repairs or replacements.
• Complies with most plumbing codes.
• Extends the lifespan of the plumbing system.
• Prevents galvanic corrosion.
• Reduces the risk of leaks and pipe failures.

Common Pipe Joining Mistakes To Avoid

When you’re ready to get started, keep in mind the following common pitfalls and why they occur:   

• Forgetting to turn off the water supply: If you forget to shut off the water and drain the system before beginning plumbing work, it could lead to water damage in your home.
• Neglecting proper preparation: You have to clean and prepare the pipes before starting the project, or it could result in poor solder joints and leaks.
• Overtightening connections: If you make the connection too tight, it could damage the gasket or threads and compromise the seal.
• Using the wrong type of solder: Make sure you use lead-free solder that’s approved for potable water systems.

Maintenance and Inspection for Steel and Copper Pipes

After joining steel and copper pipes, remember to maintain and inspect your connections so that you extend the life of your plumbing and prevent minor issues from snowballing into major problems. Here are a few tips:

• Have a plumber check the steel-to-copper pipe connections during your yearly plumbing inspections.

• If you find any discoloration, mineral buildup, or moisture around the connection, investigate the cause and fix it immediately.
• Look for signs of corrosion or leaks around the dielectric union.

Common Pipe Joining Errors and Their Solutions

Even if you follow the instructions carefully, you could still encounter some problems. We’ve provided a few common ones and their solutions below.

Issue: Soldering creates a weak joint.
Solution: Check that the surfaces are clean and free from oxidation, and then use the appropriate flux and lead-free solder. Apply an even heat.

Issue: Leaks after reassembling the union.
Solution: Make sure that the gasket is in the correct position and the union isn’t too tight or loose.

Issue: Difficulty in disassembling old unions.
Solution: Use penetrating oil to loosen rusted threads and give it time to work before trying to undo the nuts.

When To Call a Professional to Join Steel and Copper Pipes

Many DIYers can join steel and copper pipes successfully, but there are times when you’ll want to call a professional plumber. Professionals have the tools and expertise to handle complex plumbing tasks. Consider hiring someone if you run into the issues below:

• Local building codes require licensed plumbers for certain tasks.
• The pipes are in a space that’s hard to access.
• You’re dealing with large-diameter pipes or high-pressure systems.
• You encounter unexpected complications during the project.
• You’re unsure about any step in the process.

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