Ductless Line Set Minimum Length

jb9

Hello,

I know this topic has been beat pretty hard... but I need to install a very small (9k) ductless system in my workshop and I'm hoping to get away with around 7' to 10' of lineset. I need to put the outdoor unit on a gable end (snow) and I think if I run the lineset into the crawlspace and send it to the floor mounted unit, I should be able to hit 7'-10'. The space is small (12'x16') so I don't have a lot of room.

I am hoping to use every inch...

I am attaching a rendering with an annotation showing the lineset and floor unit.

Does this look like a reasonable solution?

Thanks in advance.

ratio

Does the installation instruction list a minimum length?

Maybe take it up to the attic & back down to (looks like) the low wall unit. You might be able to gain a little length that way. If you're going to run it in a crawl space, you could leave a coil or two down there.

jb9

Thanks ratio. Does anyone have a link to what a properly set "coil" would look like? I keep reading about how an oil trap is a death knell for the compressor.

ratio

Just lay the coiled lineset down on it's side, unrolling what you need for each end. Remember, that same unit would work just fine of the lineset went up to the eaves & then over and down. Maybe lay a piece of foam insulation board under it; direct contact with earth probably wouldn't do it any good.

Does the mfgr list a minimum length for that unit? It might be worth a call to tech support.

pecmsg

Fujitsu's have a 16' minimum length.

I mount the line set to the outside wall horizontal 2 or 3 times to take up the slack and allow oil return.

jb9

I kinda like the horizontal coil idea. I may also run alongside a floor joist and then up through the floor to the floor mounted unit.

Here's an interesting thread on an inspection forum showing what appears to be confusion over what an oil trap actually is, right?

The photo in the post does appear to show the idea you are suggesting ratio, right?

https://www.nachi.org/forum/f20/refrigerant-line-coil-79242/

The goal is to PREVENT the oil from getting caught in a trap and returning to the compressor, right?

I just want to make sure I understand the oil trap problem and how it relates to the compressor.

pecmsg

You want to avoid oil pooling. During the off cycle it should be able to flow down hill.
If you make several loops then you have several oil traps.

ratio

We want to prevent trapping a significant amount of oil. It is only used in the compressor, we allow small amounts to travel through the system merely as a convenience, since it would add costs to keep it in the compressor. If a significant amount is out in the system, the compressor may be starved for oil; and it may return as a (non-compressable) slug with mechanical implications for the compressor.

That said, the system is designed to allow some oil to travel through the system, and with the coils horizontal there really isn't much difference between that and a horizontal run. IDK which system you're installing, but (some) Mitsubishi mini splits are pre-charged for up to 70' of lineset—no additional oil or refrigerant is needed. (I don't recall if they had a minimum length listed.)want to trap oil, albeit as small an amount as possible: large systems with double risers use an oil trap to close off one of the risers to allow adequate suction velocity during low load operation. But I don't think we have that application here.

Big Ed_4

The mini split manufactures insist with too short of a line set , the returning oil comes back into the compressor too fast ... With a 10' coil laying flat I am thinking portion of the compressor oil is sitting in the return pipe ... On a normal split system we use traps to force up oil to a higher sitting compressor ... Where the mini splits do not like traps ..... Some what confusing to what I understand .... but one is best to instal how it is written, just incase the lights on the indoor unit start to flicker ...

Terry O

LG LS090HSV4 minimum line set length is 6.6ft

Jack

When the first inverters came out we were told that the line set had to be about 12 ft for the smaller units and 18k and above 16'. Originally we were told it was a noise transmission issue. You could turn the evap into a speaker. The following year in discussion with one of the engineers I asked for more detail on the noise. He offered that it was the compressor beating itself up. I think you could lay off the line set in a serpentine run in the crawl space and gain quite a lot. Serpentine is the right way on the vertical wall behind the condensing unit as well. As noted do not coil it as you will trap oil.

GBart

This blows my mind every time this type of question comes up

seriously.........follow the installation manual period, there is no opinion about it

Harvey Ramer

I saw an LG unit with about 3' of lineset. No noise but the unit kept hunting.

ChrisJ

What is the reasoning for the minimum lineset length on these units?

Can you recover some charge and use a shorter length?

ratio

Shouldn't need to recover any refrigerant. With a true mini split (expansion device at the outdoor unit), all the lineset is basically suction line.

I don't understand all the worry about minimum lengths. I can't recall ever doing an install where I couldn't stick an extra 10' of lineset somewhere. OTOH, the lineset on my A/C is about 6' long. I had to recover some of the factory charge to happy it up.

ChrisJ

> @ratio said:
> Shouldn't need to recover any refrigerant. With a true mini split (expansion device at the outdoor unit), all the lineset is basically suction line.I don't understand all the worry about minimum lengths. I can't recall ever doing an install where I couldn't stick an extra 10' of lineset somewhere. OTOH, the lineset on my A/C is about 6' long. I had to recover some of the factory charge to happy it up.

I mean, other than the fact the line between the txv and evap should be carrying liquid and not vapor.......

ratio

"Mixed phase". After the expansion device, it's a low pressure liquid that really wants to boil, hence the insulation on both lines. But some units called mini splits have an expansion device in the lead & a real liquid line that doesn't need insulation. Carrier has a 4 ton "mini split" that wires & pipes like a regular split AC save that the TXV is in a box that mounds under the indoor unit. & pipes in to the head with a 1-1/8" flare. I had to buy a new tool to make that flare.

ChrisJ

> @ratio said:
> "Mixed phase". After the expansion device, it's a low pressure liquid that really wants to boil, hence the insulation on both lines. But some units called mini splits have an expansion device in the lead & a real liquid line that doesn't need insulation. Carrier has a 4 ton "mini split" that wires & pipes like a regular split AC save that the TXV is in a box that mounds under the indoor unit. & pipes in to the head with a 1-1/8" flare. I had to buy a new tool to make that flare.

I suppose my point was that line is primarily liquid and therefore holds a lot of refrigerant vs a suction line