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A man pressure washing a train with a high-pressure hose

Pressure washers

You've tried water. You've tried soap. You've tried scrubbing and scouring. You've tried nasty chemicals that don't do what they say. So what do you do when it won't come clean? Roll out the pressure washer! Many people now routinely use these super-powerful hoses (also known as "power washers") to blast things clean with water jets pressurized at about 100–200 times the pressure of the air around us (that's 1500–3000 pounds per square inch or psi). They're brilliant on patios, drives, lawn furniture, barbecue grills, and all kinds of other outside grime. Let's take a closer look at how they work!

Photo: Pressure (power) washing a train. Note the extremely thin metal pipe that directs the high-pressure jet downward. Photo by Jordan Colvin courtesy of US Air Force.

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  1. Why pressure jets get things cleaner
  2. Parts of a pressure water
  3. How a pressure washer works
  4. Drawbacks of pressure washers
  5. Find out more

Why pressure jets get things cleaner

There's a good scientific reason why water gets things so clean: its molecules have a slight electrical polarity (one end is positively charged and the other is negatively charged), so they tend to stick to things all by themselves. Detergents (soap chemicals) help water to do its job even better by breaking down gunge and grease and making it easier for water to flush away. But some kinds of ground-on dirt just won't budge, no matter how hard you try.

That's when a pressure washer comes in really handy. It uses a narrow, high-pressure jet of hot or cold water to blast dirt free. Because the water is traveling fast, it hits the dirty surface with high kinetic energy, knocking dirt and dust away like a constant rain of tiny hammer blows. It's only water, though, so it doesn't damage most hard surfaces. Having said that, it's a good idea to test a pressure washer on an inconspicuous area before you start work to make sure it doesn't harm the surface you're cleaning. Always read the instructions before you use a pressure washer!

Kärcher pressure washer

Photo: A small Kärcher pressure washer suitable for home use. This one manages a pressure of about 1400psi (about 95 times atmospheric pressure), which is typical for an electrically powered washer, and uses about 5 liters (1.4 gallons) of water per minute. As you can see, it's pretty dirty: bear in mind that pressure washers blast dirt everywhere!

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Parts of a pressure water

A pressure washer is less sophisticated than it sounds. It's really just a water pump powered by an electric motor. The washer takes in ordinary water from a faucet (that's a tap to you folks in the UK), the pump accelerates the water to high pressure, and then squirts it from a hose at speed through a trigger gun. You can fit various other attachments to the end of the hose for cleaning different things.

These, then, are the main parts you'll find inside a pressure washer:

Kärcher pressure washer gun and circular spinning attachment for cleaning patios and drives

Photo: Pressure washers come with a basic trigger gun to which you can add all kinds of different attachments. Left: The trigger gun from a Kärcher pressure washer. The reinforced, high-pressure hose runs up inside the plastic casing, through a valve that you operate with the trigger. Right: The circular spinning brush attachment you use for cleaning patios and drives. In use, it points down toward the paved surface. As water enters, the black bar spins around, washing your drive clean and trapping the dirt inside the yellow plastic cover. The black circle around the edge is a brush.

Some pressure washers have additional features. Water and electricity are not a good mix, so many power washers have ground-fault circuit breakers, also known as residual current devices (RCDs), built into the power supply to protect you in case of an electrical fault. Most washers work in the same way and do exactly the same kind of thing, but the more expensive ones tend to operate at higher water pressures (and have better cleaning power).

How a pressure washer works

Diagram showing the basic parts of a high-pressure power washer and what they do

Here's a quick summary of the basic principle:

  1. Detergent flows in from a bottle or container through one hose.
  2. Cold water flows in from a faucet (tap) through another hose and is filtered on the way in.
  3. An electric motor or diesel engine powers the washer.
  4. Powered by the engine or motor, a water pump (impeller) draws in the detergent and water and mixes them together. Most washers also heat the water to a temperature of 50–70°C (125–155°F).
  5. The pump squirts out the hot, soapy water through the reinforced, high-pressure exit hose (and whatever attachment is fixed onto it). The narrow nozzle on these attachments helps to increase the pressure of the water jet even more. The high-pressure of the jet not only cleans more effectively but means you're wasting around 80 percent less water than if you used an ordinary low-pressure hosepipe (which is more economical if your water is metered).

What's a real washer like inside?

That's a hugely simplified version; in reality, a pressure washer is quite a bit more complex inside. There are several pumps, for a start, and for safety reasons quite a lot of attention is paid to keeping the wet parts of the machine completely insulated from all the electrical parts. I can't take a washer apart for you, but I can do the next best thing: show you a technical diagram of the inside of a typical Kärcher pressure washer from one of the US patents the company has filed. I've colored it and greatly simplified the numbering so it's easier to follow:

Technical patent drawing of the parts inside a typical Kärcher pressure washer

Artwork: Kärcher pressure washer patent courtesy of US Patent and Trademark Office (with coloring and annotation added for clarity).

  1. Main outer plastic housing (yellow).
  2. Electric motor (red).
  3. Insulating plastic foil (blue): This ensures no water penetrates inside the motor.
  4. Central shaft of the motor spins around at high speed, powering the water pump.
  5. Reciprocating water pump (gray): The motor turns around (rotates), but the pump moves back and forth (reciprocates) to convert the water to a high-pressure jet. There are several pump units inside a pressure washer and (for simplicity) only one is shown here.
  6. Pump piston (orange): This is the chamber where water is pumped to high pressure.
  7. Water pipe (blue): This is where water is sucked in and pumped out.

You can find much more detail about all the parts and how they work by looking at US Patent #5,886,436: High-pressure cleaning apparatus (via Google Patents) by Josef Schneider et al, Alfred Kärcher GmbH & Co., granted March 23, 1999.

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Drawbacks of pressure washers

Cleaning memorial headstones with a high-pressure power washer

Photo: Pressure washing memorial headstones before (left) and after (right). Unfortunately, the dirt has to go somewhere, so think about where you're spraying and plan the direction in which you're working. Photo by Charles D. Gaddis IV courtesy of US Navy.

Pressure washers can get things really clean, but they have a few problems too:

Pressure washing an army truck

Photo: Pressure washing a fire truck. Photo by Cameron Thurman courtesy of US Army and DVIDS.

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Find out more

On this website



Like a bit more technical detail? Try these!




  1.    See for example the specification for High pressure washer K4 Silent Home [Archived via the Wayback Machine].
  2.    In the Kärcher patent described up above (US Patent #5,886,436: High-pressure cleaning apparatus), there's a good description of the various different bits of electrical insulation that keep users safe, including a non-conducting plastic connection between the electric motor and the pump. Suitable plastics mentioned in the patent include "polyamides, polyesters, polyurethanes or various thermosetting plastics."

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Text copyright © Chris Woodford 2008, 2023. All rights reserved. Full copyright notice and terms of use.

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Woodford, Chris. (2008/2023) Pressure washers. Retrieved from [Accessed (Insert date here)]


@misc{woodford_pressure_washers, author = "Woodford, Chris", title = "Pressure washers", publisher = "Explain that Stuff", year = "2008", url = "", urldate = "2023-06-17" }

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