Do You Need Flux To Solder Copper?

Copper is arguably the best looking material you can have in your plumbing systems and is a durable way to supply water through out your home.

Copper has not only been used for water supply, but also for drainage pipe as well. As far as installing copper into ones home however, where it shines in aesthetics, it lacks in speed and ease. Soldering pipe takes time, and skill in order to effectively maneuver a good joint.

But do you need flux to solder copper? Yes, flux is a vital proponent in joining pipe and fitting via the process of soldering. Flux is acidic in nature and not only acts to clean the copper pipe, but attracts the flame towards it, which brings the melted alloy with it, creating a water tight seal.

Ensuring a proper connection when soldering is of utmost importance to prevent leaks from occurring and creating a potential for water damage.

Stick around to learn more about why flux is required when soldering, and why attempting to solder without flux is a bad idea!

What Is Flux?

Flux for soldering copper is either a rosin or ammonium chloride with rosin being low to non-corrosive and ammonium chloride being far more corrosive, but both acidic in nature.

Flux rosin is actually also used in electronics for soldering metal parts together, where ammonium chloride would eventually eat away and corrode the metal over time.

This paste is what plumbers use during the installation of copper pipes into homes and businesses. Copper pipe is used to supply water throughout the buildings such as potable (drinking water) as well as non-potable water.

Soldering is the process of creating a water tight seal to ensure there are no leaks through out the water supply.

Flux is a paste you can pick up at just about any hardware store and requires a brush for use.

Why Is Flux Important For Soldering?

Flux is used to remove the oxidation from metals (such as copper) when heating process is applied to it. Oxidization is a form of corrosion and will force electrons to shift to oxygen molecules from the metal molecules.

By applying flux to the copper, you are actually preventing this process from happening as well as taking away any impurities that happen to be on the pipe at the time.

What Happens When You Solder Without Flux?

Attempting to solder without a flux paste added to the pipe will spell disaster for you.

Heating up the copper pipe and melting the alloy (solder) will work no problem, but won’t do much of anything.

When soldering a pipe, your aim is to fill in between the fitting and pipe as much as possible with solder so as to create a perfect water tight seal.

Without flux applied, the flame will not be able to attract the solder to get sucked into the fitting to make the seal.

Instead there will be beads of metal that will sloppily stick to the pipe. Because there is no flux, you will also have to manually spin the copper pipe and apply solder 360 degrees around the pipe.

You may even find that the solder does not stick to the pipe at all.

The end result: What you will end up with is a very very weak joint that should be able to get pulled apart with your bare hands.

If for some reason the connection can’t be pulled apart by your hands, installing it into a plumbing system and having water flow through it will give you major leaks for that incorrectly soldered joint.

Can You Over Heat The Pipe When Soldering It?

Generally, when soldering pipe you will require quite a bit of heat, however it is also possible to overdo it on the heat and mess the joint up completely.

When we’re talking about heat, we’re not talking about using a cigarette lighter to apply heat to pipe to heat it up, but rather a full on torch.

Solder gets applied to pipe and fitting, copper pipe gets heated up fully around the pipe and fitting to ensure it’s hot enough.

When it’s time to apply the solder, you simply have to press it up against the copper until it just melts like butter and into the fitting. As soon as you see the solder melt and get sucked into the fitting, you should remove the heat source from the pipe.

What happens when you keep the heat source applied while the solder is hot enough to melt is that it will actually melt away the flux and give you an uneven solder. Uneven soldering will give you leaks and a comprised connection.

How Do You Use Flux To Solder?

Flux is a very easy thing to use and rather inexpensive. When using it for soldering, all you need to do is apply the flux around the circumference at the end of the pipe you wish to solder and the fitting.

You only need to apply as much paste on the pipe as the fitting will allow the pipe to fit into it.

Before you apply the paste:

Before you get into painting on the paste all over the pipe and fittings like your Picasso, make sure you sand down the portion of the pipe that will fit into the fitting as best as you humanly possibly can.

Sanding down the pipe will remove most of the dirt and impurities that naturally occur on the pipe. You’ll even want to sand a bit further past where the fitting can fit onto to ensure you can make the best connection.

You will essentially be looking for the pipe to look brand new and shiny. You don’t want to see any marks left on the pipe and then you can add the paste.

It’s good practice also to use a wire brush or the sand cloth to sand the inside of the fitting as well.

Once everything is sanded down, pasted up and it’s time to solder, you’ll want to fit the fitting fully onto the pipe and begin applying heat around the pipe, with solder readily available in your other hand.

*Note – Make sure to work in a safe area, away from flammable materials to ensure your own safety. If ever unsure about anything, consult your local plumber to assist you.

On smaller sized pipe such as 1/2″ diameter and 3/4″ diameter, it won’t take very long at all before you are ready to apply the solder.

Position your flame so it is at the bottom end of the fitting and facing up toward the pipe and fitting. When you apply your solder to the end of the fitting where it meets the pipe, the flame will attract the solder inwards to create the desired seal.

As soon as your solder begins to melt, remove your source of heat and dab the solder around the pipe a few times to ensure it fully adheres.

Solder hardens really quickly, and can be dipped into water after 6 seconds. Dipping the fitting and pipe into water will rapidly cool it off enough for you to (cautiously) be able to touch again.

Grab a rag and wipe around the pipe and fitting where the solder and flux were to remove all of the grease. Removing this grease will clean the pipe and help to prevent it from dirtying up over time, keeping it aesthetic.

Can You Re-Solder A Joint?

If for whatever reason you feel the need to re-solder a joint, it will be possible to do so.

You may end up with a joint that didn’t fully adhere the solder around, and that’s ok, it happens as the littlest bit of dirt can mess the connection up. You will have to redo it in this case.

The best course of action however will to not just try and heat the pipe/fitting up again and apply solder, but rather heat the pipe/fitting up, wiggle the fitting off with a pair of channel lock pliers.

Once the fitting and pipe are separate, apply more heat to where the solder is left over so that it melts. As it melts, wipe it away with a rag until you can get most of it off.

*Note – This is actually not quite as easy as it sounds and may take you a while to be able to accomplish. Also, be cautious here as there is potential for you to burn your self if you touch either the copper or the flame as you’re trying to wipe away the solder.

Once the solder is fully removed, you should be left with a silver end of your pipe. Simply rinse and repeat the steps. Sand down the fitting and pipe again, apply flux, apply heat, apply solder and voila.

If possible, you could even just cut that piece of the copper pipe off and get a new fitting if you don’t feel up to melting away the solder.


Tyler Takacs

My name is Tyler, I live in Ontario Canada and enjoy learning about common plumbing issues in the household. I have spent just over three years in the trades as a plumbers apprentice, but am now onto a less physical job. I still enjoy studying and learning about my own house's plumbing as well as finding ways to help others with their issues.

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