How to flare a mini split line set
Cut, ream, flare, check, and torque. The connection that causes more callbacks than anything else on a ductless install.
- Time on the job
- 20 to 40 minutes
- Skill
- Moderate
- Written by
- [Tech Name], HVAC tech
- Updated
A flare joint seals metal to metal. The cone you form on the end of the tube gets pressed against the fitting by the nut. There's no gasket. That's why every step here matters. A burr, a scratch, or the wrong torque is enough to make it leak.
1Cut the tube square
Use a tubing cutter and turn it slow, tightening a little each turn. Pushing hard crushes the tube out of round. Point the open end down while you cut so chips fall out instead of in.
2Ream the inside edge
The cutter leaves a lip on the inside of the tube. Take it off with a reamer, tube end pointing down. Remove just the lip. Take off too much and the flare comes out thin.
3Slide the nut on first
Put the flare nut on the tube, threads facing the end, before you flare. Everyone forgets this once.
4Make the flare
Clamp the tube in the flaring block at the height the tool calls for. A flare gauge takes the guessing out of this. Turn the cone down until the clutch slips or it stops, then back it off.
Tech note. Eccentric flaring tools that burnish the cone as they turn make a much better flare than the old hammer-style kits. Worth buying if you don't have one.
5Check it before you connect it
A good flare is even all the way around, smooth and shiny, with no cracks or splits at the edge. It should fill the nut's inner taper without sticking out past it. If anything looks off, cut it and make it again. Copper is cheap and callbacks aren't.
6Connect and torque
Line the flare up with the fitting and thread the nut on by hand until it's snug. If it binds, stop and realign. Hold the fitting body with a backup wrench, then tighten the nut with the torque wrench to the manual's number.
Stop. Don't use an adjustable wrench to tighten by feel. Overtightening is the most common flare failure I see on DIY jobs, and it rarely leaks on day one.
7Test it
Every flare gets checked during the nitrogen pressure test with soap solution. The steps are in the full install guide.
Questions people ask me
Should I put oil on a mini split flare?
A drop of refrigerant oil on the back of the flare and the nut threads helps the nut seat without galling. Keep oil off the sealing face if your manual says so, and never use thread tape on a flare.
Can I reuse a flare nut?
I don't. Use the nuts that came with the unit, and cut off and remake a flare that didn't seal the first time.
How tight should a mini split flare nut be?
Tight to the torque in your manual, and no tighter. Too loose leaks. Too tight cracks the flare, and that leak often shows up weeks later.