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Like a lot of people, I often forget to drain my air compressor.  It’s not a big enough problem that I’m gonna shell out for a high-tech, high-dollar automatic compressor drain, but I’m willing to give this low-buck Harbor Freight version a look.

First you have to cut into the high-pressure line to install the auto drain.  Once it’s installed, whenever the compressor cycles on or off and the valve notices a difference in pressure, the auto drain releases a small burst of air from the compressor.

This automatic compressor drain sells for about $10 at Harbor Freight.  Other manufacturers make electric-powered versions that run from $80 to several hundred dollars — you can find those by following the street pricing link below.

Automatic Compressor Drain [Harbor Freight]
Street Pricing [Google]


17 Responses to Automatic Compressor Drain

  1. tmib_seattle says:

    I have one of these on my compressor tank. It works as described, a small blast of air out the drain valve every time the compressor turns on or off.

    Mine lasted a couple years before it started leaking. When it did, I just bought another kit and replaced the valve portion.

    The only real caveat to it is that the drain valve sticks out the bottom of the tank by a couple of inches. It may be needed to raise the tank up a little bit to get enough ground clearance for the valve.

  2. Patrick says:

    Maybe I’m missing something, what is the benefit of one of these things?

  3. SirFatty says:


    So you don’t get a puddle of water in the bottom of your compressor tank, rusting from the inside.

  4. BC says:

    I wonder if this could somehow be fitted to an air line. In our shop (60′ x 120′) we have 3/4″ black pipe air lines with strategically placed pipe nipples to hold condensation. Right now, in order to drain them, we have to depressurize the whole system and remove rusty caps from the ends of the nipples.

  5. PeterP says:


    Could you just install ball valves in place of the caps?

  6. Joe says:

    Would a “small burst” be enough to purge the water from the tank? When I’ve used my compressor a lot over a whole day, I’ll get a pretty big puddle when I drain it.

  7. I drain my pancake compressor every couple of months have it on my calendar, although I’ve never gotten more than an oz or two after that amount of time.

    I do it thought because I’ve seen problems first hand.

    Where I used to work we had a humidity chamber on the production line for testing our hygrometers. It had a compressor with a little five gallon tank that was under 150psi. The humidity generator ran at least 4 hours a day and the compressor ran pretty much continuously.

    On day I get a call from production saying the humidity generator wasn’t working right. The first thing I check was the drain valve on the tank, which was supposed to be checked every day. Well evidently it wasn’t because water started streaming out of it. So I stopped draining it and got a bucket. I drained about 3 gallons of water from the tank…no kidding.


    I’ll have to look into one of these when I get a proper compressor.

  8. Joe, it operates every time the compressor cycles, so unless you’ve got an undersized compressor that runs continuously, this will drain many times throughout the day.

    The pressure switch that controls the compressor’s cycling also has an “unloader” that depressurizes the cylinder so the motor can start next time it’s needed. It’s the hiss you hear whenever the motor stops. This hooks into that line, and it’s that pressure drop that triggers the mechanism.

  9. Al Soup says:


    I have been researching a dedicated air compressor system for my shop (permanently mounted compressor and plumbed airlines) and found a good review of the Harbor Freight auto release system here: http://www.paragoncode.com/shop/compressor/

    The authors caveat has to do with the plastic line – he recommends upgrading it to copper.

    BTW: It looks like you can “vent” the outflow from the valve if you live in a higher humidity climate.


  10. meetoo says:

    I have one and it’s worked fairly well for over a year. It did need to have the petcock cleaned out once, since the old rust was eventually all excreted. I bought two on sale because they were really cheap. One is a backup or may get installed on new compressor. Honestly, I can’t buy the brass parts for less than what they sell an entire kit for.

    Anyway, a big plus of this system is that it depressurizes the compressor piston on the “blow-off”. So you can kick in the compressor whenever you like (back pressure relief). Some compressors won’t start (starter cap, long power cord, head pressure, etc.) unless they get below ~80 psi. I have tools that won’t work well unless they run at 90 to 100 psi. So it now keeps me from bleeding off 10 to 15 psi down to the restart threshold.

  11. meetoo says:

    One more thing. Yes it required me to raise my compressor. But it was a project worth doing since I upgraded to some of those HF knobby tires to replace the hard-rubber wheels and beefed up the axle. Makes it a breeze to move now.

    I use an aluminum pie pan underneath to catch any of the water. At first it spewed plenty of water and rust. But a good time afterward you can can hardly notice any water. It’s doing it’s job. And it reminds me to eat some pie and put a new pan underneath. 🙂

  12. Nate says:


    So here’s the deal with the Harbor Freight Automatic Compressor Drain Kit as I see it:

    In diagnosing a problem with mine (stayed open after compressor shutdown, causing the tank to drain, due to a unseated O-ring), I found that so little air “PFFTs” through that any water had better be accumulated right there inside the valve, ready to squirt through, or else just air is going to come out.

    Due to the stock setup with the ‘T’ and the petcock, the water will accumulate at the petcock first then eventually fill to a level where it reaches the automatic valve.

    A problem that I had was the ‘T’ that connects to the unloader hose was becoming very hot while the compressor was running, and the only conclusion I could reach was that air was flowing through the unloader into the valve, somehow bypassing the diaphragm that’s supposed to prevent this. Eventually the hose supplied with the Harbor Freight kit melted and leaked just past this ‘T’. What a joke – there should be no airflow in this line until the unloader kicks in, just pressure. Oh well.

    I found it more productive to install a copper tube extending out to an easily accessible ball valve. The copper pipe has the capacity to store enough water so that it’s not sitting in the bottom of the tank if I forget to drain it. It won’t clog and it should be a whole lot more reliable than this Harbor Freight brass paperweight.

    Then again, reviews seem to be mixed and a bunch of users are happy with theirs. For $10, it’s not too expensive to pick one up and see for yourself.

  13. Tim says:

    I think this HF kit would work fine if your compressor never built up over 100-120 PSI. The manual states that the kit must not be used on anything over 100 PSI. I thought that was simply a restriction due to the plastic tubing so I ran all copper lines to it and figured I would be OK. (The copper refrigerant line and fittings cost me more than the HF kit.) My compressor builds to a solid 155 PSI before cutting off. As soon as it got to about 120 PSI, the pressure blew past the O-ring inside the drain valve and the sucker started leaking (big-time). So I drained off all the pressure in the tank, crawled under it and took it all back apart and opened up the valve and fixed the O-ring. It was easy to see where the air blew it out of its seat and deformed it. Put it back together and tried again. Same deal. Fine up to about 120 PSI. Then PSSSSSHHHHHH…

    So, $10 (plus the cost of a bunch of copper tubing and fittings later), I put the compressor back to the way it was originally from the factory and tossed the HF kit in the dumpster. The idea’s cool, but it’s a piece of junk. Almost any decent air compressor is going to build a tank pressure more than this thing is rated for.

    Don’t waste your money and time.

    • Pierre says:


      I have an air compressor which also goes to 150 psi. I was looking at this and was wondering which side of the relief valve is susceptible to blowing out with pressure? In other words, could I simply put an inline regulator on the plastic tubing line and limit it to say 80 psi, or would I still have problems because the internal tank pressure is still being applied to the relief valve in the purging pathway?


  14. ryan says:

    After i purchased this kit i read in the manual that “it is not for use on compressors which use copper or aluminium unloader lines.” I think i will take it back so i dont ruin my $600 compressor investment over a $10 drain…

  15. Joe says:

    I am on my 2nd kit. Only problem with the drain kit is that the O-rings either break or shred. I cannot find the right size O-rings to replace them and that includes most hardware stores, O-ring kits etc. The thickness is not standard and replacements do not work well. Anybody have any ideas?

  16. Tyler says:

    Buy a new one! You’re going to spend more than 10 bucks in time and gas just to get the parts for repair!

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