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10 Reasons Why a Mini-Split Ductless Flare May Leak Refrigerant!

Updated: Jun 26, 2023

On a mini-split ductless system, two copper tubes are needed to transport the refrigerant. These copper tubes are referred to as the line set and they connect the indoor head unit to the outdoor heat pump or condensing unit. Two flare connections are needed at the indoor head unit and two flare connections are needed at the outdoor mini-split unit.

Things to consider:

· A standard system has four flare connections while a multi-zone system has more flare connections for the additional indoor head units.

· The pressure of the refrigerant in the system will be anywhere from approximately 100-400 PSI depending on whether the unit is in heating or air conditioning mode.

Therefore, it is essential to have all the flare connections correctly sealed so that refrigerant does not leak out of the system. Listed below are the ten most common reasons mini-split flares may leak.

Problem #1: The Flare Size is too Small

· At the end of the copper tube, the surface on the flare face must be big enough to take up the full amount of space inside the flare nut. The reason for this is so that the tube's flare face entirely covers over the seat of the flare adapter. This will give you the best chance at sealing this connection point.

· If the tube flare is not very wide, it will only cover half of the connection point and will barely seal the joint.

· The flare may accidentally be made too small if the flare block is not tight enough. If the block is not tight enough, the copper tube will slide while you are trying to make the flare. Use the rod that comes with the flare block to tighten the block as hand tightening may not be sufficient.

Problem #2: Over-Tightening the Flare Connection

· This could be caused by using adjustable wrenches that are too large. If the wrenches are too large, you will not feel when the joint is tight and you will easily over-tighten the joint.

· Over-tightening can cause the threads of the flare nut and adapter to become stripped/dethreaded.

· This causes the flare face inside the nut to be crushed or cracked.

· Over-tightening may cause the tubing inside the flare nut to spin, which scratches the flare seat or flare face so that it no longer has a good seal.

· Always use a torque wrench when connecting mini-split lines. The mini-split manufacturer’s installation literature should list the correct amount of foot-pounds that the connections need to be tightened to. Each manufacturer's required foot-pounds may be different.

Eight Head Mini-Split Torque Wrench:

Problem #3: The Cone is Scarring up the Flares

· This may be due to the fact that refrigerant oil was not added to the cone of the tool before flaring the copper.

· It could be because the cone itself is bad or rough.

· An eccentric flaring tool would likely be the best tool to avoid scarring the flare face. This is because the cone of this tool has the least amount of surface contact while forming the flare out of any tool. Eccentric flaring tool:

Problem #4: No Refrigerant Oil on the Flare Face

· A small amount of refrigerant oil or Nylog Blue (which is compatible with all refrigerants) should be applied to the flare face before tightening the connection.

· Make sure that oil or Nylog does not get onto the threads of the flare as this will act like a lubricant. This may cause you to accidentally over-tighten the flare or to strip the threads on the nut or the flare adapter.

· You don’t want to get the Nylog Blue inside the tubing as it may clog strainers if it is pushed into the system by the refrigerant.

· Carry a small container of POE oil, PVE oil, mineral oil, or other comparable oils with you if you choose not to use Nylog. Small Nylog Blue:

Problem #5: An Incorrect Flare Nut is Being Used

· When working on a mini-split system, be sure to only use flares nuts that have been specifically manufactured for mini-split ductless units.

· A flare nut could crack over time if it is not designed to handle the pressure of R-410A or other refrigerants.

· A flare nut could crack if it is not designed to be exposed to the outdoor environment.

Problem #6: A Scratch on the Flare Face or Flare Adapter

· If there is a scratch on either the flare face or the flare seat of the adapter, it may be enough to allow refrigerant to leak out of the system.

Problem #7: Preparation of the Copper Tubing

· If the copper tubing is squished while it is being cut, it usually cannot be used no matter how much you deburr the inside of the tube or re-round the outside of the tube.

· Using a stick reamer or a unibit to ream the hole in the end of the copper tubing may not be a good idea. It could result in the inside of the tube being scarred, which you don’t want to happen. This could cause your flare face to be scarred before you even make the flare!

· Deburr the hole at the end of the copper tubing well before starting to flare.

Problem #8: Using an Improper Flaring Tool

· The correct flaring angle to use for HVAC and plumbing is one with a 45 degree flare. There are flaring tools out there that have different angles such as 37 degrees, which you don't want for flaring refrigerant tubing.

· A standard flaring tool, a process flaring tool, or an eccentric flaring tool will create the correct 45 degree angle needed for a refrigerant tube flare.

· A spin flare will create a smaller angle flare but while the copper tube is still hot, it can be tightened to the flare adapter which will stretch the flare face to 45 degrees.

Here is a video I made using spin flaring and spin swaging bits:

Problem #9: Improperly Torqued Flares

· Read the manufacturer’s literature to learn the recommended foot-pound torque. Some manufacturers data will only be in metric. Make sure to have a torque wrench with heads that include standard and metric. Eight head torque wrench:

· Remember that the torque value may vary from manufacturer to manufacturer and it will vary depending on the size of the copper tube, flare adapter, and flare nut size.

· An adjustable head torque wrench can be used when working on systems with many flare nut sizes. This allows you to speedily adjust your torque wrench. Adjustable head torque wrench:

Problem #10: The Pressure Test

· If you pressure test the system at a low PSI, a leak may not show up until you run the system in heating mode.

· When using a compound manifold gauge set, you may have to let the pressure test sit on there longer because the incremental changes in a compound gauge are very small.

· A leak shows up quicker when using a digital manifold gauge compared to a compound manifold gauge set because the digital display indicates changes in tenths of a PSI.

· You want to pressure test the system below the max design pressure of the unit. Follow the manufacturer’s installation literature for pressure testing so that you do not over-pressurize a system.

· If a leak is detected, use a non-corrosive bubble leak detector on the exposed joints in order to find the leak or use an ultrasonic leak detector.

Bubble leak detector spray bottle:

8-ounce bubble leak detector with dabber:

Ultrasonic leak detector:

· Do a pressure test before doing the vacuum procedure, otherwise if there is a leak, you will pull in the humid air surrounding the outside of the leak spot into the tubing.

· When performing a vacuum, pull the vacuum down below 500 microns and perform a standing vacuum test. Try to target at least 200-300 microns while the vacuum pump is running, before performing the standing vacuum test.

Here is a video on vacuuming a mini-split ductless unit and breaking the vacuum with refrigerant from the bottle:

Here is a video showing the 10 reasons why mini-split flares may leak:

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Published: 5/20/2020 Author: Craig Migliaccio

About the Author: Craig is the owner of AC Service Tech LLC and the Author of the book “Refrigerant Charging and Service Procedures for Air Conditioning”. Craig is a licensed Teacher of HVACR, Sheet Metal, and Building Maintenance in the State of New Jersey of the USA. He is also an HVACR Contracting Business owner of 15 years and holds an NJ HVACR Master License. Craig creates educational HVACR articles and videos which are posted at & &

1 comment

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Thanks for shedding light on this common issue. Understanding why mini-split ductless flares may leak refrigerant is crucial for maintaining system efficiency and preventing costly repairs. Contact HVAC Contractor Davie for an instant solution at a genuine cost.

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