Connecting Two Air Compressors

There are sometimes good reasons for connecting two air compressors.

A single air compressor may not offer enough flow to supply a higher demand air tool.

Another, single air compressor, can have the same problem with that same air tool. But if you connect two air compressors into one line to the air tool, you may have enough air flow from the two air compressors to use that higher demand air tool.

How Compressors Run

A quick review of how compressors work (also covered in detail on a page on this site) is that they have an air tank into which air is pumped. When the tank pressure reaches the cut out setting of the pressure switch, the pump stops, and nothing else happens until you begin to use the compressed air that’s stored in the compressor tank.

As you use air, the tank air pressure drops. When the tank pressure drops to the pressure switch cut in level, the pump will start, and add more air to the tank until the tank reaches the cut out pressure setting, and the pump stops.

Fix My Compressor - sometimes two air compressors are better than one

Sometimes two air compressors can provide enough air for a high demand air tool.

The Compressor Pump May Not Stop

However, if your air tool has a greater demand for compressed air than the compressor can deliver you will use air in the tool at a greater rate than the compressor can supply it, and even though the compressor is running, since it cannot keep up with the demand of the air tool, the tank pressure will keep dropping until there is not enough compressed air to run the air tool.

To use a higher demand air tool you will need a bigger compressor. Or, connecting two air compressors to the same air line to the tool, if you have a couple of small air compressors around, might work.

Connecting Two Air Compressors

If you have two air compressors, select which will be the primary air compressor and plumb a tee into the discharge coupler on that one.

One leg of that tee is the supply from the primary air compressor, one leg of this tee is the supply from the secondary air compressor, and the last leg is where you install the air line to your air tools.

If we assume that both air compressor tanks are full of compressed air, when you start using your air tool, compressed air will be drawn from both tanks… at least, in theory.

The reality is, compressed air always flows through the easiest path from high pressure to low. As air is drawn from the two compressor tanks to the air tool, compressed air from one tank will flow into the other tank and vice-versa, rather than all of it flowing down the air line to the tool.

By installing a check valve in the line between the secondary air compressor and the tee of the primary compressor, this will prevent air from the main tank from flowing back into the secondary, improving the efficiency of the air flow.

Once the pressure in the hose to the air tool drops due to air use, air will be drawn from the secondary tank.

However, once again, air from the secondary tank may flow back into the tank of the primary air compressor rather than down the line to the air tool.

Solution? Another non-passing valve before the tee in the discharge coupler of the primary compressor, allowing air out of the tank, but not back into it from the other compressor.

Fix My Compressor - compressed air check valveThe image above is of a simple, in-line, check valve for compressed air. It’s also known as a non-passing valve. If you Google air check valve you will find many sources with a variety of attachment options.

With the male thread on either end of the check valve in the image, installation is not complex. Note the arrow on the body. That shows the direction  that the compressed air can flow through this valve. Take a care not to install it backwards!

Comments

  1. Ari Goldman says:

    I had struggled with the same problem – especially when I decided to connect three compressors together, while putting some check valves and adjusting pressure works ok I used this thing to network all the air compressors: http://compressorcontroller.com

  2. Any possibly way that you could add links for a check valve and a tee and what else could possibly be needed for this process?

    Thanks

    • Difficult to do, Zach, as air lines on different compressor makes and models are different sizes. If you Google compressed air check valve and air line tee, you’ll find lots of sources. You then need to pick the type and size that suits your compressor setup.

  3. Hello, I am running two compressors in tandem and everything works fine except the pressure regulator on one sometimes leaks air and hisses. This only happens when running the compressors together, but is fine when run by itself. Is that one regulator bad or something else???

    • If the regulator setting is below the air pressure in the line downstream from the regulator, air pressure will bleed from the regulator to allow the downstream air pressure to drop to the regulator setting. The solution is to have a check valve in the line downstream of the regulator so that air pressure in the line downstream that may be higher – when the other compressor regulator setting is higher for example – cannot backflow to be vented.

  4. Hello, I have two compressors I’m hooking together and I have a check valve for the secondary but not for my primary. I am painting a vehicle and need the extra capacity, can I get by without buying a check valve for my primary compressor? The primary holds around 150psi, and my secondary holds about 120psi max, I want to keep my main compressor from running all the time and burning up.

    Thanks.

    • If you have only one check valve, and the secondary compressor comes on but the primary is off, the air from the secondary will leak out of the primary instead of going to the tool.

  5. I’ve tried that process but one of the air compressor did not start automatically in cut-in. Even though they have the same cut in and cut out setting..

    • Typically folks will have a primary compressor and a back up, to kick in when the flow requirements exceed the CFM capacity of the first compressor. With both pressure switches set at the same level, you would think that both would kick on at the same time. We suspect that the pressure switches have a range of accuracy, and even though both are set – or you think they are set – at the same pressure, clearly one reacts faster to the drop in tank pressure and fires up before the other can. And, of course, air pressure immediately starts to rise, and the second pressure switch never reaches the cut in.

      That may not be a bad thing though as it defaults to the typical set up for two compressors. Depending on your air use, the air pressure may drop far enough to have the second compressor kick on, assuming its switch works.

      If that comes to pass, and you use the compressor(s) daily, maybe every couple of months change the settings on what is the primary compressor to make it the secondary one, so that you get even wear on both.

  6. what if you haVe one tank and 2/ compressor. model tc.20 how wood you do that????

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