Choosing an Air Compressor Q&A

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Dear NH,

I was wondering if you could give me the minimum (and suggested) pressure, horsepower, and other relevant specs for selecting an air compressor for spraying ceiling texture. I will probably spray 1-2 homes per year. I'd like enough power to possibly run a coil nailer or brad nailer occasionally.

I would not want to over-work the unit by buying something too small for my needs. Any clue where to find out ratings on different makes and models?

Dear MH,

Though I don't have specific product information available here and, therefore, can't recommend brands, I can give you some general information that will hopefully help in your quest!

First, I have to ask... do you really... I mean really... want to buy a compressor? If you are only going to spray once or twice a year, you might be pleasantly surprised to find that renting a unit and figuring the cost into your quote might be a better choice. Then, you won't have to put out any money up front, nor bother with compressor maintenance and repair. Many smaller painters do this with paint spray equipment and power washers, too! Carpenters and handymen who do only occasional power nailing or stapling have also discovered how economical renting can be.

Air compressors are rated by horsepower, pressure and CFM (cubic feet per minute of output at a given tank pressure). If you really feel that you need to own a compressor, the only sensible way to purchase one is to first decide which tools you are going to use with it! Each type of air-powered tool has requirements for air pressure and CFM. The general rule is that the compressor should exceed the CFM of the largest tool by 1.5 times. If you plan to run more than one tool at a time off the same compressor, add the CFM's of the tools together before multiplying by 1.5. Proper compressor sizing means longer life, since it will never be overstrained.

Certain types of tools, such as nailguns or impact tools, use short bursts of pressurized air and do not need a high powered, high volume compressor to function properly. Other tools such as grinders, sanders, sandblasters and drills need a continuous flow of air and thus need larger, more powerful compressors to work properly.

Another important factor to understand is what is known as the "duty cycle". The duty cycle is how much time the compressor can safely run within a given period of time, expressed as a ratio. For example, a common duty cycle for compressors is 50/50, meaning that the compressor motor can run about half the time it is supplying air to your tools... 50 minutes on and 50 minutes off. During the "on" time, the motor is pressurizing the tank. During the "off" time, your tools are running on the air stored under pressure in the tank. If your tools are draining the pressure off too fast, the compressor engine must run at more than a 50/50 rate which can lead to overheating and significantly shorten compressor life.

This also applies to gasoline powered compressors. Yes, the motor runs all the time, but it is not always under a load. The duty cycle applies to the amount of time the motor is under a load vs. time not under a load. Of course, the most sturdy compressors have a 100% duty cycle, meaning that they can output air continuously. These, of course, are also the most expensive commercial units.

What about horsepower ratings? You can pretty much ignore horsepower when shopping compressors, especially when comparing different manufacturers. First, horsepower figures are not reliable from manufacturer to manufacturer, especially on electric-powered units because there are different ways to measure it. Secondly, the CFM and pressure ratings of an air compressor are a true measure of the overall power and efficiency of the compressor. For example, a more efficient air pump can produce greater CFM with less horsepower!

So in summary:
1) Choose the air tools you are going to buy.
2) Determine the "highest" pressure needed by them and the "total" CFM needed… add the requirements of all tools you may run simultaneously.
3) Multiply your needed CFM by 1.5 to allow for a little "cushion" so that your compressor is not overworked. Add a little more to the CFM number if you anticipate future air tool purchases.
4) Go shopping! Now that you know the basics, you will be able to focus in on a few compressor models making your comparison shopping less confusing.