GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Audio Play Icon Headphones Icon Plus Icon Minus Icon Check Icon Print Icon Picture icon Single Arrow Icon Double Arrow Icon Hamburger Icon TV Icon Close Icon Sorted Hamburger/Search Icon

Community and Q&A

Mini-split final install Qs

Scott Mcllarky | Posted in Mechanicals on

Hello All,

The info I’ve found in other threads regarding DIY mini-split installations have been invaluable, and I am very appreciative.

I am now at the stage in my own, 3 zone, install that it’s time to pressure test, evacuate, and release the 410a. I purchased pro equipment to do the job and I just have a few lingering questions I’d like answers to, along with any advice, before proceeding.

First, the manufacture and seller of my unit, Thermocore, does not recommend pressure testing the system with nitrogen. They say pulling and holding a deep vacuum is check enough. It just seems to me that applying pressure is a better way to check those flare connections. Opinions/advice?

Next, if I do test with nitrogen and there are no leaks, I assume that I evacuate the nitrogen with the vacuum pump in the same operation as pulling the deep vacuum ahead of releasing the 410a, correct? Does pulling the vacuum assure there is no nitrogen left in the system to contaminate the refrigerant?

I will begin with fresh oil in the pump. Will I need to change the oil between each zone vacuum operation?

Next, I am using a core removal tool on my hose setup. After pulling the deep vacuum on a zone, do I reinsert the valve before I release the refrigerant, or after?

And lastly, am I correct that I proceed one zone at a time? Meaning, beginning with zone A, pressure test, evacuate, release refrigerant, power test, then on to zone B, according, then zone C?

Thank you, Scott

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Hobbit _ | | #1

    I can't necessarily comment on the order-of-operations aspects other
    than thinking that releasing the refer might want to be the very
    *last* thing you do. I'd venture a guess that pump oil changes
    might depend on how crudded-up you suspect it's getting depending
    on size of the components you're evacuating and what's in them.
    And I applaud your decision to dive into this yourself -- did you
    have any trouble obtaining the gear and the supplies if you don't
    already do this stuff for a living??

    However, I can't imagine why the company would disrecommend a pressure
    test. The gear is going to run at typical R410a pressures, right?
    So why not make sure it can hold that and then some? My install
    passed vacuum tests but when it turned around into heat mode and the
    vapor line became the high side, it turned out to have a leak in
    the larger flare. Vacuum tests can cause small faults to close up
    and not leak, whereas pressure forces them to open up and reveal
    themselves. Are they worried about high pressure nitrogen getting
    into places it shouldn't and then not able to be vacuumed back out?
    I'd be very interested in their answer if you go ahead and call
    them on this.

    If the unit components have an "open all valves" test mode, that
    might be good to have in effect when doing all these tests and pulling
    the vac, so closed expansion valves aren't slowing things down. Not
    sure about per-zone but somehow it feels like doing the whole system
    in one shot might be the most reliable?


  2. Scott Mcllarky | | #2

    Thank you so much for taking the time to respond. That was my thinking, exactly, about a vacuum possibly "closing up" an opening that pressure would certainly open. I don't see a way to do all three zones as once because they aren't opened to each other until after the refrigerant is released. As for tools, I'm spending some money on those, but plan to sell them afterwards to recoup some of the money. It will still be less expensive and I'll know it was done right. split-air systems are practically unheard of in my area, and could not find an HVAC person around with any experience with them.

    One more Q. re: pump oil, can you visually see that it needs changing in the glass? If so, at what point do you know it needs to be changed? Thank you, again.

  3. Stephen E | | #3

    DIY a mini split invalidates the manufactures warrantee. If you want to DIY use a pthp system. Mini splits call in the experts if there is any in your area that will touch them. Mini splits are excellent systems if done properly.

  4. Igor Obraztsov | | #4

    Nitrogen will not contaminate anything. The reason for using dry nitrogen is specifically to minimize the contaminants: oxygen and water. If the system can hold 400+ psi on the high side, 150psi or so of nitrogen should be nothing to it. Incidentally, I recommend using Leak Lock (blue stuff) to seal your flares. Works perfectly. I would do the pressure test. My AC's are LG, and LG recommends a 150psi nitrogen test.

    For pump oil, just follow the manual directions. Generally, if your linesets arrived with sealed ends, they should be real clean inside, same with your unit coils. A single batch of oil should stay clean pretty long, likely long enough for all three zones.

    My usual procedure is pressure test, then pump down to 800-1000 microns or so, then fill it up with a few psi of dry nitrogen, then do the deep pump-down to under 500 microns. The extra step mixes the remaining moisture in the lines with nitro, and what you have left after final pumpdown will be 99.999% nitro, which is what you want.

    About that core remover, here is how I prefer to do it: While pump is still running, open the ball valve, quickly push in the plunger, screw in the core, retract the plunger and close the ball valve. Then, shut off the manifold gauge, and shut off the pump. Now, open the gas and liquid valves on the unit, releasing freon into the lines. Now you can slowly open the ball valve of the core remover, detach it, and cap the service port. This sequence assures that you do not expose the service port schrader to atmospheric pressure while the system is under vacuum. Otherwise air outside will push the valve stem and enter the system. Which answers your last question: you finish all work on one zone before going to the next.

  5. Scott Mcllarky | | #5

    Many, many thanks, Igor. I REALLY appreciate your time and sharing your expertise. One Q. After pressure testing, assuming no leaks, do I just release the nitrogen for the pressure test step to the atmosphere by disconnecting the hose? The pull first vacuum per you suggestion?

  6. Igor Obraztsov | | #6

    Well, atmosphere is more than 75% nitrogen already, so a bit more will not hurt, just bleed it out slowly. You generally want to bleed it to atmospheric or close to that (1-2 psig) before connecting a vacuum pump. Most electronic vacuum gauges don't mind a bit of extra pressure, but vacuum pumps don't like it.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |