TL;DR

Make a proper flare and you will not have to worry about the connection. Cut a fresh end of copper tubing or copper pipe, deburr it tipped down so shavings do not get inside, and slide the flare nut on first. Seat the copper against the stop in the flare block, add a drop of vacuum pump oil, and make the flare. Then check it with a flare gauge: it should go in and stop at the first hole. All the way through means too small; will not go in means too big. Protect the face with painter’s tape.

How Do You Flare Copper Tubing for HVAC Systems?

To flare copper tubing for an HVAC system, cut a fresh end and deburr it, slide the flare nut on first, seat the copper against the stop in the flare block with a drop of vacuum pump oil, make the flare, and check it with a flare gauge before you connect anything. The same sequence works on every line size. On a larger line such as a 5/8, avoid bending the copper around more than you have to, because it work-hardens.

How to Read a Flare Gauge When Flaring Copper Tubing

A flare gauge tells you whether the flare is the right size. It is important to use it so you know the tool is doing what it is supposed to do.

How Do You Make a Proper Flare, Step by Step?

Making a proper flare is a short sequence.

Step 1: Remove the cap and cut a fresh end

Remove the cap from the line first, and do not let it or any debris end up inside the refrigerant line. Then cut a fresh piece off the end, because the tubing can be damaged where it was originally cut, and you want a good smooth start. Use the right size tubing cutter: on smaller lines, like a quarter-inch line, a smaller tubing cutter leaves a smaller burr than a full-size cutter. Do not over-torque the cutter; do it little by little.

Step 2: Slide the flare nut on first

Put the flare nut on the tubing before you make the flare, so you do not forget to put it on.

Step 3: Deburr with the tubing tipped down

Deburr the freshly cut end, and tip the tubing down while you do it so you are not getting any shavings inside the copper.

Step 4: Seat the copper against the stop and lubricate

Set the tubing in the flare block with the stop in place, and make sure the copper is up against the stop. Add a little lubrication, just a drop of vacuum pump oil, especially on a new tool. Then make the flare.

Step 5: Check the flare with a flare gauge

A flare can look nice and still be the wrong size, so check it with a flare gauge. A proper flare goes in and stops at the first hole, where a small metal ring marks the stop. If it pushes all the way through, the flare is too small. If it will not go into the hole at all, the flare is too big.

Step 6: Protect the flare face with painter’s tape

Put a quick piece of painter’s tape over the finished flare to protect the face. Use painter’s tape specifically, because you do not want adhesive glue stuck onto the sealing face of the flare.

How Do You Adjust a Flaring Tool That Is Making the Wrong Size Flare?

If the gauge shows the flare is too big or too small, the flaring tool needs adjusting. Flare tools include instructions, and the adjustment is usually to loosen or tighten a hex nut to change the flare size. Make a test flare, check it against the gauge, and adjust until the tool is flaring to the right specification. A tool can be out of calibration even when it is brand new, so verify with the gauge rather than trusting the tool blindly.

Do You Need Thread Sealant or Loctite on a Flare Connection?

No. Red Loctite does nothing for a flare. Make a proper flare and verify it with a gauge, and you will not have to worry about a sealant or Loctite.

What Flaring Mistakes Cause Callbacks?

These are the mistakes you’ll want to avoid when flaring copper tubing for an HVAC system:

• Leaving the cap or debris inside the refrigerant line.
• Flaring off a damaged or rough end instead of cutting a fresh one.
• Letting metal shavings fall inside the copper while deburring.
• Forgetting to slide the flare nut on before flaring.
• Not seating the copper against the stop in the flare block.
• Skipping the flare gauge, so a too-big or too-small flare goes unnoticed.
• Over-bending and work-hardening larger lines, such as a 5/8 line.
• Relying on red Loctite instead of a proper flare.

For Additional Insights on Making Proper Flares

Watch this field demonstration of making proper flare connections and checking them with a flare gauge:

Frequently Asked Questions

How do you know if a flare is the right size?

Check it with a flare gauge. A proper flare goes in and stops at the first hole. If it pushes all the way through it is too small; if it will not go in at all it is too big.

Should you put the flare nut on before or after flaring?

Before. Slide the flare nut onto the tubing first, so you do not forget to put it on.

What lubricant do you use when making a flare?

A drop of vacuum pump oil, especially with a new tool. Just a little is enough.

Why cut a fresh end before flaring?

The tubing can be damaged where it was originally cut, so cutting a fresh piece gives a smooth, clean start for the flare. Use the right size tubing cutter to keep the burr small, and do not over-torque it.

Do you need thread sealant on a flare?

No. Red Loctite does nothing for a flare, because that is not how a flare works.

Why use painter’s tape on the flare?

To protect the sealing face after you make the flare, without leaving adhesive glue on the face the way other tapes would.

Why does my flaring tool make the wrong size flare?

The tool may be out of calibration, even when new. Adjust the flare size with the hex nut per the tool’s instructions, and verify each test flare with a flare gauge until it is correct.