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on Pressure Washers
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The Pressure of your Water Supply

OK, please don’t shoot me because of this one. You’ve finally decided on the brand and model of pressure washer that’s right for you. It has the right amount of power and water flow you need. Plus, you’ve managed to get all those features that make it easier to use and maintain. You’ve also figured out what you are going to use it for, and decided on the appropriate nozzle and attachments. So the next thing you do is go online and order that pressure washer, right?

Wrong! Even though you’ve found the right unit, you can’t buy it without checking your water supply. The GPM number on the pressure washer has to match the capacity, or the pressure of the water supply.
Even if the GPM water flow of your water supply is just a little bit lower than the GPM specs of the pressure washer, you can wind up ruining the unit.

Let me give you an example. You buy a 2.8 GPM unit. This means the water supply the pressure washer is hooked up to has to be able to deliver 2.8 GPM as well (preferably more), because the slightest difference in favor of the pressure washer is enough to damage its pump irreversibly. To find out how to prevent this from happening, keep reading, but first let me explain how this occurs.


low pressure causing cavitiesThe main problem you face if the pressure of your water supply is not sufficient is cavitation (the name comes from air cavities that form inside the water supply, due to insufficient pressure). There are several signs that appear simultaneously. The first thing is a loss in capacity, and usually there is a significant drop in efficiency as well.

As the water inside the water supply becomes saturated with a cavity (which is just a fancy word for bubbles) the pressure washer will become noisier and it will vibrate a lot more. This happens because cavities leave the area of low pressure and pass into the area of higher pressure. More often than not, this causes explosions that are powerful enough to blow the pump into pieces, including the metal parts.

Why It Happens

There are different reasons why cavities form, but since the end result is the same all of them are categorized as cavitation. The first reason is vaporization which happens when the water temperature is too high. All pumps are designed to be used with fresh water, so the temperature should never exceed 68F.

Another common cause is air ingestion. Even though air ingestion has a different cause, pretty much the same thing happens as in the case of vaporization. You can tell that air ingestion is taking place by the loss of capacity. Both lead to pump damage although the consequences of air ingestion are somewhat less severe.

Another cause of cavitation is turbulence, but instead of boring you with the details any further (you probably get the idea by now), let’s find out what you can do to check if you have sufficient pressure in your water supply.

How to Check the Pressure of the Water Supply

the bucket testOK, this is very simple. All you need is a 2-gallon bucket if you want to have an electric pressure washer or a 5-gallon bucket if you are planning on using a gas-powered pressure washer, a stopwatch and to be sure a 25-foot, 3/4″ diameter hose (doing the test without a hose is usually good enough tough because in most situations you will see a big enough surplus anyway).

Now open the tap all the way and fill the bucket with water. In the case of a 2-gallon bucket, if it’s full in less than 60 seconds you have sufficient water pressure. In the case of a 5-gallon bucket, if it’s at least 3/4 full in 60 seconds your water pressure is sufficient even for the more powerful gas powered washers.

To be more accurate you can also count how long it takes to fill a 1 gallon bucket and then divide 60 by the number of seconds you counted for 1 gallon. So if, for example, it took you 15 seconds to fill the bucket your water flow is 60 / 15 = 4 GPM.


There you go, a simple way of determining if your water supply is able to provide enough pressure in order for the pressure washer to function properly. The only thing I will say is it’s really important you check this first.

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