What's Best for a Pressure Washer
PSI or GPM?


When evaluating a pressure washer for purchase the majority of shoppers only consider the psi component of the pressure pump. This is probably because psi is a simple concept to understand, psi stands for "pounds per square inch" pressure. Even a small machine of 1500 psi sounds impressive when viewed only by this parameter.

Why then does the contract cleaning pro give equal to more weight to the GPM rating of a pressure washer pump? GPM is "gallons per minute" and the reason pros prefer GPM is simply because time is money.

These two ratings represent two very different aspects of a pumps' performance.


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The only purpose of the PSI rating of a pump is to determine the pressure applied when breaking the bond between the grime to be cleaned and the surface (substrate) it is adhering to. Naturally it requires more psi to break the bond between paint and concrete walls than to break the bond between oil and concrete driveways. These are the basics to understanding PSI.

Generally these are some PSI ranges for various tasks.

1000-1900 PSI - cars, boats, gutters, bricks, fence, deck, etc ...

2000-2900 PSI – Houses, airplanes, bricks, truck fleets, pools, patios, sidewalks, etc.

3000-5000 PSI - Stripping paint, Industrial, heavy greese stains, stubborn concrete spots, etc.

You are wasting your money if you purchase a high PSI machine and all you are planning to do is wooden decks, or the type of grime where the bond is easily broken.

The GPM or gallons per minute is where the true efficiency of a pressure washer comes into play. A 4 gallon per minute pump will wash the dirt away twice as fast as a machine at 2 gallons per minute.The pro's understand that as long as the psi is sufficient for the type of work they are performing the GPM will determine the total job time required. And to the pro time really is money.

So how do you decide which machine is best for you?

A formula was devised that eases the comparison task and provides an excellent proxy for a machines' cleaning efficiency. It is referred to as e.c.u.which stands for "effective cleaning units". This is a super simple calculation , all you have to do is multiply the psi X gpm.

Where 3000 psi X 3.0 gpm yields 9,000 e.c.u. and 1500 psi X 2 gpm yields 3,000 e.c.u. Notice the first is 3 times as effective as the latter. So now you can understand why a high psi unit with a low gpm rating is nothing more than an oversized waterpic on steroids. .

What is generally understood in the painting and equipment rental industries is that it takes about 9,000 cleaning units or more to effectively strip paint when prepping a house for painting.

So if house painting is your goal and paint needs to be stripped don't go out and buy a 1500 psi washer for $350.00 at your local box store. It just won't get the job done and probably won't last too long either. Consider keeping your GPM requirement to 4GPM -4.5GPM or less as many municipal water systems may not supply the flow to support more.

You can easily test the flow by filling a 5 gallon bucket from a water hose and stopping at 60 seconds. It is best if your water supply exceeds your pumps' requirements by at least one gallon or more. If your water supply is on the closer end make extremely sure there are no restrictions or excessive bends in your water supply hose while the pressure washer is operating. One of the most severe types of pump damage is cavitation which is most commonly caused when a pump is starved for water.

These are the primary reasons why so many contract cleaners carry additional water supply tanks and employ the use of bond-breaking chemicals in an effort to achieve more cleaning efficiency The wide variety in pricing of different machines will become much more clear when the e.c.u. filter is applied to your purchase comparison of pressure washer models.

 

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