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About Pressure Washers

Learn About Pressure Washers

How does a pressure washer make things clean?

Understanding the cleaning process is essential to the effective use of a pressure washer. A pressure washer cleans by creating a high pressure stream of water that hits the surface with a large amount of kinetic energy, thus removing the dirt mechanically, while providing a constant flow of water to wash the dirt away. Often dirt is mixed with grease or oil, or other substances, which chemically bond the dirt to the surface you are trying to clean. Sometimes even the force of high pressure water needed to break these chemical bonds is insufficient to get the surface clean without requiring excessive pressure, that is either impracticable or would damage the surface. Adding a detergent, or other cleaning chemical, or using hot water, while agitating or brushing the surface, significantly enhances the cleaning power of the pressure washer.

Detergents and soaps are used for cleaning because pure water can't remove oily, organic soiling. Soap cleans by acting as an emulsifier. Basically, soap allows oil and water to mix so that oily grime can be removed during rinsing.

Detergents cannot accomplish much until some mechanical energy or agitation is added into the equation. This mechanical energy is provided by the pressure washer. As the detergent emulsifies the grime, breaking the chemical bond, the blasting effect of the pressurized water strips the dirt from the surface, while the flowing water rinses the detergent and soil away. Warm or hot water, or steam, melts fats and oils so that it is easier for the soap or detergent to dissolve the soil and pull it away into the rinse water.

What is a pressure washer?

A pressure washer is a mechanical device that uses a pump to generate a high-pressure stream of water to clean surfaces. The pump is typically powered by a gasoline engine, an electric motor, or a diesel engine. The pump is a positive displacement pump that forces a specific amount of water flow from the outlet.  By restricting the flow with the nozzle, much like you do by placing your thumb over the end of a garden hose, the pressure washer produces pressure. Without the nozzle the impact of the water from the end of the pressure washer hose is similar to that from a garden hose. The narrow diameter of the nozzle increases the water's velocity and, consequently, its impact and cleaning power. The smaller the nozzle orifice, the more pressure is potentially produced. However to produce greater pressure, more horsepower is required to force the water through the smaller hole. Each pressure washer calls for a specific size nozzle to match the flow of the pump and the horsepower of the motor. Putting too small a nozzle on your pressure washer strains the components and may cause excessive wear to the machine; too large a nozzle sacrifices cleaning power.

All pressure washers produce pressure and flow. Both are important to the cleaning process. Most pressure washers also provide a method for dispensing soap or detergents, either under low or high pressure. The proper use of soap is often key to the cleaning process. Hot water pressure washer are equipped a burner system to continuously heat the stream of water, further aiding the aiding the cleaning process. Most hot water pressure washer can generate wet steam of approximately 240 degrees F, by turning down the pressure to around 150 PSI. There also true steam cleaners that generate a dry steam of approximately 320 degrees F at 150 PSI. Understanding the basic concepts of pressure flow and cleaning units will help you pick the right pressure washer for the cleaning job at hand.

Pressure and flow

Pressure washers use the impact of the water to break the bond of the dirt to the surface you are trying to clean; the higher the pressure, the greater the impact. The type of nozzle you use makes a huge difference in how much impact your pressure washer creates. A zero degree tip gives you the full impact of your pressure washer but is almost useless for cleaning because it hits such a small area. A 15 degree nozzle increases the area you can clean at one time, but reduces impact by 70%. A 40 degree increases the surface area cleaned further, but has 88% less impact than a zero degree nozzle. This is why a turbo nozzle is so effective; for it is a 0 degree nozzle rotating thousands of times a minute creating a 25 degree cone of full impact.

The amount of water, or flow, a pressure washer puts out makes a big difference in cleaning. The flow is what carries the impact to the surface. The more flow, the more impact; there is a direct relationship. A 13 HP pressure washer putting out 4000 PSI at 3.5 gallons a minutes flow has one third less impact than a 20 HP unit putting out 3,500 PSI at 5.0 gallons per minute. Moreover, flow is what is needed for effective rinsing. This is especially true when cleaning large open surfaces, like driveways; or quickly rinsing vehicles in truck washing operation.

Cleaning units

A pressure washer pump creates a positive water flow, and by restricting that flow at the nozzle creates pressure. The more horse power driving the pump allows more flow to be pushed through the pump and the more pressure to be created at the nozzle. For a given amount of horsepower you can have only so much pressure and flow; there is a tradeoff. Most people focus on pressure (PSI), but you really need to consider both pressure and flow. One way of comparing pressure washers is to use the measure of “cleaning units”. To calculate cleaning units, multiply the pressure times the flow of the pressure washer. All other things being equal (i.e. chemical use and heat) a unit with higher cleaning units will clean better than a unit with lower cleaning units.

Cleaning unit application guidelines

Home use, light duty: 2,000 to 3,750 cleaning units.

Medium duty: 3,800 to 7,500 cleaning units

Heavy duty: 7,600 to 15,000 cleaning units

The measure of cleaning units is a good way to compare the power of one pressure washer relative to another, with the following proviso. When two pressure washers have different pressure and flow specifications, but the cleaning units are equal, remember the unit with the higher pressure unit will make a rotary nozzle work better, while the higher flow unit will rinse better and make flat tip nozzles work more effectively. With that in mind, the following guide can be used on how much pressure and flow is needed to perform certain tasks. Some of these tasks may require a rotary Dirt Killer nozzle as well if you are using the minimum PSI. The minimum specifications are what are required to get the job done. Having more pressure and flow will make the job go faster. The column marked preferred CU shows how much cleaning power is required to get the job done more quickly. (CU means Cleaning Units: cleaning units = Pounds per Square Inch of pressure (PSI) times Gallons Per Minute of flow (GPM).


Task Min PSI Min GPM Min CU Preferred CU

Wash a car 1000 1.5 3,000 5,000

Deck 1500 1.5 2,250 7,000

House 1000 1.5 1,500 7,000

Boat         2000 1.5 3,000 7,000

Barnacle removal 3000 2.5 7,500 12,000

Surface clean 1500 3.5 5,250 15,000

Brick 2000 2.5 5,000 8,000

Heavy Equipment 1500 2.5 3,750 8,000

Drives and sidewalks 1,500 1.5 2,250 12,000

Paint prep 2,000 1.5 3,000 7,500

Fleet wash 1,500 3.0 4,500 8,000

Graffiti removal 3,000 3.0 9,000 12,000

Parking lots 2,500 3.5 8,750 14,000

Nets/crab/lobster pots 2,500 3.0 7,500 12,000