How to drain an air compressor tank

Do it at the end of every session, while the machine is still warm, because warm air holds more of its moisture in suspension and a warm tank gives it up more readily than a cold one.

Switch off and unplug first. A pressure switch can restart the motor unattended, and you do not want the pump running while the drain is open. Then wind the regulator to zero and bleed the hose - pull a blow gun trigger or disconnect at the coupler - so there is no pressure downstream.

Position something to catch what comes out. It will be water mixed with rust-colored sediment and, on an oil-lubricated machine, a little oil carryover. It will stain concrete. A shallow pan or a rag is enough on a pancake; on a 20 or 30-gallon machine put a bucket under it, because there can be a surprising amount.

Open the drain gradually, standing to one side. On a petcock that is a quarter-turn; on a ball valve a half-turn. Leave it fully open until the hiss stops completely and no more liquid appears, then tilt or rock a portable machine so the far end of the tank drains too - the drain sits at one low point, and a tank on uneven ground holds water at the other end.

Close it firmly but do not force it. A drain cock over-tightened eventually will not seal at all, and a drain left slightly open is one of the most common overnight air leaks there is.

Two faults account for nearly every complaint about draining, and both are fixable.

A seized drain cock, first. These are small brass taps living in standing water and they corrode shut. Depressurise the tank completely, then work the tap gently with penetrating oil and a correctly-sized wrench rather than pliers - rounding the flats off turns a cheap job into a hard one. If it will not move or will not seal afterwards, replace it. While you are there, consider fitting a ball valve instead of a petcock: a ball valve is easier to open fully, easier to confirm fully closed, and far less likely to clog with sediment.

An unreachable drain, second, which is mostly a vertical-tank and a stationary-install problem. If the drain sits an inch above the floor behind a wheel, you will not use it, and a drain you will not use is the same as no drain. Fix it with geometry: fit a short length of appropriately rated pipe and an elbow to bring the drain out to a usable position, or raise the machine on blocks or an anti-vibration pad. Do not fit a flexible plastic tube and leave it curled on the floor, because the low point in the tube becomes the new place the water sits.

And if you simply will not remember: an automatic tank drain is a real product category that solves this by timing or by float. It is the right answer for anyone who knows their own habits, and it is much cheaper than a tank.

Two different problems get confused here. Draining the tank protects the tank. Filtering the line protects your tools and your paint. You need both, and one does not substitute for the other.

Water that leaves the tank with the air goes down the hose and into whatever is on the end of it. In an impact wrench it washes the oil out of the motor. Through a blow gun it marks bare metal. Through a spray gun it is a finish defect you will be sanding out. That is why a filter belongs downstream of the tank rather than at the tool.

Match the filtration to the job. For general garage work, a particulate filter and regulator like the PneumaticPlus SAW300 - published 10 micron, 53 SCFM, 3/8-inch NPT, with a 0 to 150 PSI gauge - covers it. For spraying, step up to a multi-stage unit: the SAU430 publishes a 10-micron particulate stage, a 0.1-micron mist separator and a regulator on 1/2-inch NPT. For paint work where the standard is higher again, the Motor Guard M-60 publishes 0.01 micron filtration at up to 100 CFM at 80 PSI.

Then plumb it properly. Take drops off the TOP of a horizontal main rather than the bottom, so condensate keeps running along the main to a drain leg at the end instead of falling into the drop. That single detail does more for air quality than an extra filter stage.

Cold changes the stakes. Water left in a tank or a drain cock before a hard freeze becomes ice, and ice in a drain cock means you cannot drain the tank at all until it thaws - which is exactly when condensate accumulates fastest, because the machine is being used in short bursts in a cold garage.

So before the first freeze: drain the tank completely, drain the filter bowl, and leave the drain cock open if the machine will sit unused through winter. An empty tank with an open drain cannot freeze shut and cannot rust under standing water.

Coming back after a layup, drain before the first start rather than after. A machine that has sat for three months has condensate in it and possibly a skin of surface rust on the tank floor; getting the water out before you pressurise it is a better order of operations. Then check the oil on an oil-lubricated pump, because oil degrades sitting still as well as running.

And if the machine lives in an unheated garage year-round, read the cold-weather page before the first cold session. Oil-lubricated pumps, tank condensate and hose flexibility all behave differently below about 40 degrees F, and the answers are not the same as the summer ones.