I've just got my first air compressor, it's a Nuair Belt Drive 3HP 150Litre 10Bar machine.

The instructions say to drain water from the tank after use. There is a tap on the bottom to do this.

Is it safe to open the tap when the tank is full or do I need to reduce the pressure in the tank before opening the valve?

Also should the tank be depressurized after use?

  • It needs some pressure to move the water out, unless you are opening something else to admit air....
    – keshlam
    Commented Mar 29 at 1:34
  • 1
    Question should be how to drain an air compressor. Which is, while pressurized, open the valve only enough that it starts to pee. Let it start to spit for a little bit and then you're done. If you can't get it to pee then it was unnecessary, or you're opening it too far.
    – Mazura
    Commented Mar 29 at 21:08

2 Answers 2


It is safe to open the drain while the tank is fully pressurized. The compressed air helps force the water out of the tank.

You do not have to de-pressurize the tank after using the compressor.

Edit: not de-pressurizing the tank is something I and co-workers have always practiced. We never de-pressurized. However, two compressors' owner's manuals I looked at say to release the tank air down to 20psi pressure then drain the water. So the water drain process when done by the book does effectively release all the air. On the other hand, my boss's auto-drain compressor did not require full depressurization to auto-drain....

  • 3
    It might be a waste of compressed air, but what else are you going to do with it? Most compressors I’ve lived with will seep to a low level over a week or two… Commented Mar 27 at 18:04
  • 2
    Having a pressurized tank means that less time is involved until the water is drained at which point the valve can be closed, retaining the remaining pressure, until it leaks of its own accord. Note that the water forced out under pressure will spray rather than dribble or pour under control. My self-draining tank sprays to the back wall, but I have a Franzinator installed which means zero water in the tank anyway.
    – fred_dot_u
    Commented Mar 27 at 18:14
  • @fred_dot_u I thought you were joking until I saw: route60garage.blogspot.com/2009/04/wtf-is-franzinator.html Commented Mar 28 at 2:25
  • @triplefault thanks for your answer. I though this may be the case but, having no prior experience of compressors, I wanted to make sure Commented Mar 28 at 7:30
  • @CraigEstey, I'm in a humid part of the country and built a three-stage unit. Elsewhere on the 'net, one can find a statement from "the Franz" in which he suggests going beyond two stages is foolish, but I consistently remove water from the third stage. The self-draining "sprayer" never shoots water and the in-line dessicant beads remain unchanged as well.
    – fred_dot_u
    Commented Mar 28 at 21:20

Also should the tank be depressurized after use?

5 people are voting here for You do not have to de-pressurize the tank after using the compressor.

Should I release the pressure out of an air compressor when it is not in use?

^^^ However, from there 10 people voted for drain the tank after each use or daily and also for storage, and other 5 for a separate answer of Yes, definitely drain it after each use., and another 2 for a separate answer of I always drain my compressor when finished. So 17 total for drain it, versus only 5 here for don't have to. So following the science if more people are draining it after each use versus not draining it, then the correct course of action seems to be drain it after each use.

you could also inquire from the manufacturer if they offer an air compressor training class.

  • FWIW crowd sampling doesn't necessarily reveal what's the right thing to do. The crowd may be wrong. Also, though the linked question does ask whether releasing all air from the tank is necessary, the answers dwell far more on draining the water out of the tank. It would be good to drain the water from the tank regularly. That said, it turns out my compressor's instruction manual outlines a procedure for draining the water that would also end up releasing all the air. It doesn't look like releasing the air is necessary, but the manual has it done to get the water out.... Commented Mar 28 at 21:54

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