I was considering buying a cheap mini-split AC unit to cool a small room (10ftx10ft) over the summer now that we spend more time working from home. There are several units on the market. However I'd like to mount the two parts of the system pretty much back to back, one on the outside wall, the other on the corresponding inside wall, about 8-10 inch apart (I just have to check how thick is the stucco etc). However I read there is a 10ft minimum distance between the two units, and this would complicate the job a whole lot. What is the reason for a minimum distance? Too much final pressure of the pre-charged gas comes to mind perhaps?

  • What comes to mind is the access required to make the connections. – Jim Stewart May 29 at 21:31

The manual for the Pioneer mini split system I'm installing this weekend says the unit is pre-charged for 16 foot lines. For distances less than 25 feet the manual says no change to the refrigerant charge is needed. Nowhere that I can find does it say the pipes have a minimum length.

Maybe see if the installation manuals are available on the Internet for the unit you intend to purchase. They should have similar guidelines.

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  • rtfm is what I'd have to do if you gave me a model number. But you need the installer's, not the user's. – Mazura May 30 at 3:47
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    Good idea, the installation manual states: "The length of refrigerant piping will affect the performance and energy efficiency of the unit. Nominal efficiency is tested on units with a pipe length of 5 meters (16.5ft), in North America, the standard pipelength is 7.5m (25’). A minimum pipe run of 3 meters is required to minimise vibration & excessive noise." The language in this manual is open to interpretation. It does say there is a minimum but only for "excessive" noise and vibration... – Alessio Sangalli May 30 at 4:17
  • "no change to the refrigerant charge is needed" for it to NOT freeze (that happens when it's too low on gas). - "Nominal efficiency is tested on units with a pipe length of 5 meters" presumably that's the correct charge for 5m. Efficiency drops like a rock outside of that pressure. 'In NA the standard SOLD is 25 feet'. I've never done a mini, but I've done more splits than I can remember, and only one of them was ever even close to operating psi with the charge that came in the condenser. Sure it would've worked, but that's not why they called me. – Mazura May 30 at 10:31
  • There's (more to it than I know and this is half wrong, because : 'beer can cold; you're too old'), but the closer you can get it to freezing the colder the air, obviously, but you have to leave room for not only installer error, but marginal weather which changes the superheat if it has a TXV, or the subcooling if it doesn't. Otherwise it will freeze up when it's 70 outside and you want 69. Again, you need the manual... and a bunch of stuff. – Mazura May 30 at 10:43

Well no there is not a minimum in any case I have seen. Now let’s consider what is practical the inside unit is normally required to be above the outside unit , the elevation is not specified, those that need more heating the inside unit will be close to the floor, others with more cooling or a mix are usually high with an inside unit that has a sweeping motion. But is that required? NO what is required is the drain for the condensate is below the inside unit. Many commercial compressors are above the evaporator the same can be true for a split system , the systems are shipped with a charge that will work within the specified lengths of line sets.

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  • I should add that very cheap DIY split systems are out there , they have special seals and a preset long length of 25’ I think but pro installed can be much shorter as I mentioned above. – Ed Beal May 30 at 1:19
  • Good catch to think about the condensate line. I was considering an installation that takes the lines up above in the attic and then go down again below. Unless there is an internal pump for condensate it sounds like I won't be able to do that... – Alessio Sangalli May 30 at 4:21
  • "It'll Work..." – Mazura May 30 at 10:49

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