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I'm about to install a mini-split system in my shop, and several online sources have recommended strongly against coiling extra line set length, because it creates a low spot that traps condensed refrigerant oil.

But in practice I've seen many installations that have low spots in the lines. How bad is this really?

In my case, I have about 5' of line outside, but I'd like to mount the unit as high on the wall as possible. I can't get much higher than where the line set exits the building, and I can run it horizontally from there, but that makes things a little less aesthetic. I'd like to know what my options are.

I don't want to cut the line if I can avoid it, because I don't want to risk flaring the ends (it would be my first time).

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  • Only professionals can handle refrigerant these days. No more going "Pssssssss" to your car's A/C system when recharging it. That's why people have to wear sunblock now. Now, all freon work must be done with pros with freon recovery apparatus, the freon is then purified and reused; or annihilated. (more the former for the precious R12/R22). It's working; the ozone layer is healing.
    – Harper - Reinstate Monica
    Feb 5, 2021 at 2:25
  • There's no release of refrigerant to the atmosphere with these systems (unless there's a mistake, but it's not hard to check for leaks before).
    – Rick
    Feb 5, 2021 at 2:47
  • Actually some auto parts sell r134 recharge kits 1 or 2 lb , it’s quite expensive but no license required. I thought it was not true “r134” but it was. I think it was 30$ per lb or close to 5x to what I pay per lb. it had oil and possibly die, I asked the guy at the counter how they could sell it, he had no clue but said it was available on line also. It’s possible since 134 is used as a duster the mfg took the epa to court because the law says when used for hvac equipment a license is required. But I was surprised to see it for sale with no license required.
    – Ed Beal
    Feb 5, 2021 at 17:56

5 Answers 5

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one of the diy systems with precharged line sets there instructions specifically state to coil the excess at the system.

Will a low spot be a real problem? I have not seen one although in theory it sounds like it should. Remember the oil is actually carried with the gas/ liquid to an extent so it is being pushed through the system I doubt you will have problems.

The precharged line sets I have seen are 25’ long and when I install a mini split on a single story I usually use less than 12’ so that would leave 13’ to coil up at the compressor.

Get a flaring tool expensive pro ones are under 30$ Cheap ones under 10$. Cut and re flair. Honestly even if I purchase line sets I almost always cut factory flairs off because they are usually sloppy and uneven. On the systems I have pumped down for others the leaks have always been at the factory flairs.

I reflair and wet the surface with nylog (a sealant made for this purpose) torque them down and pump down again with no issues. The extra time costs so don’t be afraid to flair it is quite easy.

As others have mentioned charging a system requires special licenses and training and equipment in most of the world. When the system is pumped down leaks are usually very obvious but the leak location can be harder to find quality flairs are not hard to make just leave an extra foot at the compressor, practice on the pice you cut off then do the real one. (Don’t forget to put the nut on first I have forgotten this when having a beer and BSing with a friend while doing his system, you figure it out real fast lol).

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  • If a system is sold, pre-charged with the assumption you will use 25' of line, does it affect the amount of refrigerant needed if you cut the lines in half?
    – JPhi1618
    Feb 5, 2021 at 16:26
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    @jphi1618 no the systems there pre charged for 25’ lines are fine unless the line set is extremely short like 5’ , the mfg instructions usually state what the max and min line length is with the recommended line set size.
    – Ed Beal
    Feb 5, 2021 at 17:43
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Some things you should hire a pro for - which is a generally frowned upon response around here, but in this case it's illegal (In the USA, anyway) to install most mini-split systems without the approriate refrigeration / EPA license for the refrigerant used. Along with that comes the appropriate tooling and experience, normally. Then your system does not leak and release refrigerant (bad for the planet, and also bad for your system.)

In addition, excess line length makes the system inefficient .vs. just enough, even if you arrange the excess so it does not trap refrigerant.

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    I'm guessing in this case the OP is installing one of the MrCool ones which are legal to DIY-install. The way they do that is by including linesets that are pre-charged with refrigerant, so you don't have to actually deal with adding any, but of course the downside is that you can't trim the linesets to length so you always have extra. If that's the case, it's a valid question.
    – Nate S.
    Feb 5, 2021 at 1:28
  • As far as I know it's perfectly legal to install a pre-charged mini-split. Most of them have the condensor charged, and you have to draw vacuum on the line sets before opening them up. There are plenty of YouTube videos showing how to do this, even by professional installers. Also, the point is not only to condition my air, but to learn.
    – Rick
    Feb 5, 2021 at 2:01
  • @rick you can install a system that the lines are precharged and sealed. It is illegal to charge the lineset in the case of a “flared” type or soldered connection without a license, epa fines up to 20k for violations. The level one license is easy and allows you to do up to 5lb systems. if you get inspected you better have an approved recovery system, proper gauge set, vacuum pump and gauges. A friend of mine that had a 609 license got tagged for doing a mini split, he had the equipment and appealed the fine I think it was 1k, he took the level 1 corse and got the 608 level 1. He won No fine.
    – Ed Beal
    Feb 5, 2021 at 18:08
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    Data point, not answer: I have seen lines go well above when routed through attics, I've seen them go well below when routed through basements. In my case the manifold in the basement feeds (among other places) up to the attic before coming back down into the second-floor rooms. I don't have a comparison for a similar house, but this was a professional install and is working almost as well as I could wish it to. (My gripes have to do with airflow through the house and the controls, not the heat pump itself.)
    – keshlam
    Mar 16, 2023 at 23:19
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Coil the excess horizontally instead of vertically. This is specified by at least one manufacturer (MRCOOL).

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Think about this from a physics point of view.

A siphon does not care about anything but the relative height difference between the inlet and outlet.

Imagine a scenario where a heat pump line has to go up 20 feet to the air handler. If there was such a thing as an "oil trap" you don't need a coil to create the trap, the oil would get trapped at the bottom most point of the line, which would be at the heat pump's output location. Obviously this is not an issue.

This entire concept of an "oil trap" is nothing but a fake thing that's been spreading among HVAC technicians for decades as practically none of them have any real science knowledge. The extra line can be coiled any which way you want and it will absolutely not matter so long as you make sure the line is not kinked or pinched and that it's fully insulated.

There is some additional friction that comes from having too much extra line but keep in mind the compressor is designed to raise the pressure from the cold side to the hot side by over 200psi. Pushing against 20ft of gravity is barely 5psi for a 3/4 line, so the pump won't even notice the difference.

Lastly, the high pressure side emits liquid r410a at a density similar to water. There isn't going to be any layer separation in that line. It's not a mix of liquid and gas.

The low pressure side is mostly gaseous when it enters the pump which at that low density means the flow is quite fast, even if there is some liquid it's going to be pulled through the line fully. The only time you'd maybe get some fluid to pool up at a low point is when the heat pump is turned off, but it wouldn't make any difference after the pump starts again.

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  • Further, R410a is highly miscible with the lubricant in all phases (gas/liquid) so it seems unlikely that a trap could have any effect even if you wanted it to. Maybe with older refrigerants it was necessary, but the rule about traps has not kept pace.
    – Jim W
    Dec 5, 2023 at 3:19
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I have heard the same thing. Looping traps oil, etc. My first mini-split went four years before needing a touch-up recharge and the extra coil of copper tubing was all over the place with bends and loops. The purpose of the looping and one coil was to use up the extra 10 feet not needed. I was told not to cut to a different length because the amount of freon in the line was exact and if I cut this precharged line? Well, game over. The extra loop and coiling did not seem to make any difference. Also, a small side note, or, observation -- the old freon is banned -- gone -- and all new systems use the new R134, the good stuff that will not harm the Ozone, right? Why is everyone so adamant about having to use tech to recharge a unit? If we are not using the old stuff and only the new, good stuff then what is the problem?

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    Your modern HFC refrigerants (R125, R32, R134a, & R410a aka a blend of the first two) all are fine for the ozone layer, but sadly are really wickedly effective at trapping solar radiation (>1000x as effective at that as CO2 is)
    – ThreePhaseEel
    Jun 9, 2022 at 2:55
  • If you have a new question, please ask it by clicking the Ask Question button. Include a link to this question if it helps provide context. - From Review
    – FreeMan
    Jun 9, 2022 at 10:40

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