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Mitsu Mr. Slim line set length question

For a very short line set run, do I subtract some charge out or leave it at factory fill. The manual one sites adding for longer run. This is a single head H2i type unit, and the line set is going to work out to be under 10ft, something like 6.



  • Jack
    Jack Member Posts: 1,046
    Do not adjust tHe charge

    For the shorter line set. The unit is built to function perfectly up the stated limit with the pre-charge. One thing to look for however. Does Mitsu list a minimum line set length and if so have you met that requirement?
  • russiand
    russiand Member Posts: 73
    I only saw a max

    That's just the thing I didn't see a minimum...
  • meplumber
    meplumber Member Posts: 678
    Minimum lineset length.

    There is a minimum lineset length of 15' on those units.  
  • MTC
    MTC Member Posts: 192
    If you can handle the aesthetics of it...

    just put a small loop of lineset at the compressor and use the 15' min set. Make sure to follow minimum bend radius guidelines for the loop. You can usually hide the loop pretty easily.
  • Harvey Ramer
    Harvey Ramer Member Posts: 2,225
    Oil Trap

    If you put in a loop to increase the length, make sure you keep it horizontal so you don't create an oil trap.

  • MTC
    MTC Member Posts: 192
    Is this really a concern?

    I'm no pro, but pretty sure I've read that in a longer vertical lineset you're required to put in a vertical loop every so many feet. In a short one would it really create a problem?

    I'm not an A/C pro - trying to learn, not question you :) I know I've seen the vertical loop done many times to take up the extra line, right or wrong...
  • Harvey Ramer
    Harvey Ramer Member Posts: 2,225
    I've been

    taught this way and furthered my opinion this direction.

    If a HP condenser is at a higher elevation than the evaporator, then an inverted trap may need to be installed on the vapor line. This is to keep oil at the compressor. Other than that piping should be as straight as possible with the smallest number of bends possible. It becomes necessary to install expansion loops during long straight runs. These should also be put in horizontally or at a slight upwards gradient.  

    I see people using hard copper 90's in refrigerant tubing all the time. In the same breath I would like to say, I have dumped out evaporator coils full of compressor oil.

    Consult manufacturer guidelines. They are pretty specific.

  • MTC
    MTC Member Posts: 192
    I'll submit to the pros...

    Thanks for the info, good to know.

    Still, you could use a loop horizontally to solve the problem, if that works. Maybe around the base of the compressor or something... it seems to be a common way to get around that problem one way or another. Many are using fixed length lines with connectors instead of flaring or brazing, so there has to be a good way to deal with the extra length inherent to this situation.
  • Techman
    Techman Member Posts: 2,144
    edited May 2014

    15' lineset is the minimum for two reasons, 1- to absorb any noise/vibration from the comp/outdoor unit 2- to give the TXV a little bit of room to do its throttling/hunting. Having "oil traps" in the vertical suction line every so many feet is  to help insure oil return back to the compressor on "regular "systems. On inverter systems the "oil trap" is not needed at the same height of separation or not needed at all, DEPENDING! What kind of a "connector" is used in the lineset if not a flare/solder/braze fitting is not used?

    Using elbows in ACR piping is normal. Using LONG RADIUS ells is normal. Using SHORT RADIUS ells is normal for Hoopels. Using PLUMBERS RADIUS ells is normal for Knuckleheaded Hoopels. As Harvey mentioned ,the oil leaves the comp, & the oil accumulates in the evap, the comp runs out of oil and the comp goes WHACK. There are several reasons why the oil stays in the evap besides the problems caused by the Hoopels or the Knuckleheaded Hoopels. And the horizontal loop of tubing to fulfill the min lineset length, has to be looped from top to bottom to help prevent oil traps. TOP TO BOTTOM! A good way to deal with the length of lineset is to put the outdoor unit 20-25 away from the indoor unit.
  • Harvey Ramer
    Harvey Ramer Member Posts: 2,225

    I was referring to PLUMBERS hard copper 90 ells. I see a lot of them installed on residential units in the area.

    I saw an 18 seer Luxaire HP get killed in less then a year. When I arrived at the job the compressor was toast. The home owners showed me the service record. The condenser coil, 4-way valve, compressor and evaporator coil had all been replaced previously. Within a year of the install!

    The unit never had a chance. The condenser was on the same level as the evap. Walk in basement. Lineset left the condenser with a couple hard 90 ells, entered the wall and went up through the wall cavity turned horizontal with another ell and then ran across the ceiling to the equipment room. There he used a couple more ells to run the piping down and line it up with the evap. Of course he had a small leak somewhere in this catastrophe. Then fall came and the unit was functioning in HOT HP Mode. As soon as enough Freon leaked out, WHACK there went the first compressor. Installer had already fled the state with multiple lawsuits pending. The next company to work on it started throwing parts at it. Replaced all of the above mentioned parts to no avail.

    What a mess!