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I just got a new air compressor for occasional finish nailing and tire inflation. I understand that I need to open the bottom drain to let out any condensation (i.e., this question) after each use.

Should I also store the compressor with this valve open? Is it a bad idea to do so? If it makes a difference: I only plan to use it once or twice a month and live in a very dry climate.

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Anecdotally, my experience (father and grandfather's practices, handed down) has always been to drain the tank completely after each use, then close the drainage valve for storage. This prevents environmental contaminants (moisture, dust, insects) from getting into the tank. But now I'm curious if there's a more formal answer to this. – Scivitri Jan 26 '12 at 19:40
I always let it drain after each use, and then close it back up again. I think I'm doing it right. – Doresoom Jan 26 '12 at 21:50
I've noticed my shutoff valve doesn't work as well, and I used to always leave it open during storage. Perhaps a seal dried out or else it's just a cheap valve. With a tank designed to handle over 100psi, the natural atmospheric pressure changes shouldn't effect it in the slightest. – BMitch Jan 27 '12 at 15:30
up vote 8 down vote accepted

Leave the valve closed during storage to prevent moisture, dust, and insects from entering the tank (as @Scivitri wisely mentions in his comment).

Even though any additional moisture could be drained form the tank later, it's more difficult to get the dust and insects out. And they're what's likely to clog up any attachments. They'd also potentially clog the drain valve itself and essentially cause a slow leak until they were removed.

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It sounds logical, but also conflicting with the owner's manuals cited below. Perhaps if the debris/bugs is a concern it would be best to leave the valve barely open, or to cover it tightly with something like a zip-tied cloth to act as a permiable filter/barrier – STW Feb 10 '15 at 0:06

Should be covered in the owner's manual if you can find it. The manual for mine says to leave the valve open in storage. Presumably to let the internal/external environment stay equalized to minimize condensation.

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Directly from owner's manual (Central Pneumatic 6 gallon model 67696)

Drain tank of moisture after each day’s use. If unit will not be used for a while, it is best to leave drain valve open until such time as it is to be used. This will allow moisture to completely drain out and help prevent corrosion on the inside of tank.

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I did scuba diving for years. The general rule was to always leave a minimum amount of air pressure in the vessel, when not being used, so that moisture, or anything else, could not get into the tank. This sounds like good advice for any pressurized vessel as moisture is the #1 cause of corrosion/damage.

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Ben, our SCUBA fills are very dry air to prevent dangerous corrosion and contamination. Thus, we store with a bit of positive pressure. Air compressors do not condition (dry, filter, etc.) their air first and thus there's plenty of moisture to drain from the tank. My manual also states to leave the drain open during prolonged storage. It's a Central Pneumatic brand from Harbor Freight. – user32599 Feb 9 '15 at 23:50

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