Can you test a closed refrigerant lineset/equipment with pressurized air instead of nitrogen? I am guessing not but what would happen if you did?

  • 6
    Nitrogen is known as a neutral/non reacting gas and dry. Pressurized air can have all sorts of junk(moisture, nasty fumes, smoke) in it. I am assuming the air comes from an air compressor you have plugged in and not a tank of air you brought.
    – crip659
    Jun 5 at 17:11
  • @crip659 air compressor, correct
    – amphibient
    Jun 5 at 17:25
  • Man, you've been working on this for like two weeks.... Can you get another year out of the system or is it that bad? SOP is fill it (even knowing there's a leak), try to find the leak, know that no one is going to come out and solder your coil, if that's where it is - a bad braze in the lineset? no problem. Anywhere else... you're buying a new AC next year (which is everything except the air handler, so... might as well not have a 20y AH - that's like $1k on a $20k ticket). Or you can repeat the process every year, which moves this from gotta do what you gotta do, to IDGAS.
    – Mazura
    Jun 8 at 2:07

3 Answers 3


You can fill the lines with anything that is compatible with all the materials used for the lines and equipment, including oil, refrigerant, nitrogen, air, water, and probably other materials. However please don't. We use nitrogen for a few reasons:

  1. It's easy to get at really high pressures. Testing at 400 psi is easy to do with a nitrogen tank, and impossible with most air compressors.
  2. It's a consistent gas, so the properties and behavior are well known. This includes the fact that the molecule is much smaller than refrigerants, so a system which holds nitrogen well should also hold refrigerant well.
  3. It's available dry, meaning you can use it to drive moisture out of the system.
  4. It won't contain dangerous flammable materials such as methane, which could cause serious problems.
  5. As a bonus, it's suitable for backpurging joints as you braze them, so you don't need a separate material on hand to make quality joints in refrigerant piping.

There's probably more, that's just what comes to mind for me. To answer your specific question 'what would happen if you did [use compressed air]?' You would end up with higher levels of moisture in the system which is a common cause of HVAC compressor failures, you would potentially end up with dirt and debris in the system from the compressor, and you could end up with incompatible oil in the system from the compressor.

  • Good point, all the nitrogen will evacuate. Things in compressed air like dust or oil will not. Jun 5 at 18:16
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    The installation instructions for LG mini splits actually do a great job of laying out the goals for using nitrogen as a purge.
    – KMJ
    Jun 5 at 18:24
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    An additional point: 400psi is about 27 atmospheres. Air at that pressure will have the partial pressure of oxygen >5 atmospheres. If there is any flammable lubricant in the system, it will probably burst into flames. Jun 6 at 9:14
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    That's one of the many reasons readily available air compressors rarely go over 200 psi. There's just so much more you have to think about at higher pressures. Honestly if I think about it too hard, it still freaks me out a bit how high the pressures are in some hydraulic systems.
    – KMJ
    Jun 6 at 16:27

Pressurized air contains moisture levels not found in Nitrogen. We all know moisture causes rust so not a good idea to use air. Plus Nitrogen can detect much smaller leaks than air so you might not find the existing leaks with pressurized air.

  • Nitrogen makes up about 75% of the air. When checking for leaks, any leak that nitrogen would reveal would also be revealed by pressurized air.
    – Mark
    Jun 6 at 3:09
  • @Mark but what's the other 22%? (Nitrogen is *78*% of the air.) Reactive stuff (oxygen and moisture) and contaminants from the compressor.
    – RonJohn
    Jun 6 at 6:03
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    @RonJohn, never said it wouldn't mess up the system, just that pure nitrogen is only marginally better than compressed air at detecting leaks.
    – Mark
    Jun 6 at 6:32
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    "Plus Nitrogen can detect much smaller leaks than air because of Nitrogen's smaller size of molecules" - err, no. Almost all the molecules in air apart from nitrogen are oxygen - and oxygen molecules are actually smaller than nitrogen (.152nm rather than .155nm). Jun 6 at 9:19

You are asking about the supply of "gas", whether Nitrogen (N2) can be replaced by something else.

With "Air" you may mean

  • dry air as the dry mixture of components we find in the atmosphere: 78% N2, 20% oxygen (O2), 2% all the other stuff (noble gases, 400ppm = 0.04% of CO2, etcetc);
  • air as the mixture of dry air and "humidity" aka water: H2O ;

or in the worst case, maybe you are thinking of replacing N2 with pure Oxygen (O2).

As you may see, air is N2 plus something else. And this something else is largely O2, which is very reactive, leading to oxidation (ox-ygen ...) or O2 plus a very strong solvent that may condensate (H2O), blocking microscopic cracks that therefore will go unnoticed or getting stuck in some meniscus/curves, promoting all kind of future reactions/blockage inside your piping system.

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