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 washing bricks. Note the extremely thin metal
pipe that directs the high-pressure jet downward. Photo by Senior Airman Ciara Wymbs courtesy of
US Air Force.
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.
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!
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
These, then, are the main parts you'll find inside a pressure
Water inlet: A hose that connects the pressure washer to
water supply. There's usually a filter in the inlet to stop dirt and
debris entering the washer and clogging up the works. Little bits of
grit are the last thing you want inside your washer—especially since
they could come blasting out of the other end at high speed!
Electric motor or gas engine: Most smaller, pressure
washers (such as the very popular ones made by Kärcher)
run off the domestic electricity supply, but bigger models are
powered by compact gasoline engines.
The engines are similar to the ones you'd find in lawnmowers
(typically power rated at around 3–4kW or 3.5–5.5HP).
Gas engine models are great if you're working outside in places where an electricity supply is hard
to find (or where a long trailing cable would be dangerous or
inconvenient). The motor or engine is designed to power the water
Photo: Although domestic pressure washers are typically powered by electricity, bigger ones are often driven by small gasoline engines and produce
considerably more powerful water jets. Here's the engine inside a typical gas-powered washer used by the US Marine Corps being refilled ready for cleaning helicopters. This one's made by Jenny and rated
at a pressure of 3400 psi (roughly 230 times normal atmospheric pressure), which is about three times more than an
average home Kärcher. It's just about the most powerful washer you'll come across! Photo by LCpl. Jesse D. Leger courtesy of US Marine Corps.
Water pump: This is the heart of a pressure washer. It's a
like a hand-operated ground-water pump—only it's driven at high
speed by the electric motor (or gas engine) instead of your hand.
When the engine pulls the pump one way, it sucks water in from the
faucet; when it pushes the pump the other way, the water squirts out
in a high-pressure jet. Pumps are designed to handle a water flow of
around 4–8 liters (1–2 gallons) per minute.
High-pressure hose: This is the tube that runs out from
the washer to whatever cleaning attachment you've decided to use. An ordinary
bit of tubing wouldn't be able to survive the high-pressure of the
water flowing through it. High-pressure hose is reinforced with wire
mesh and has two or more layers of high-density plastic. It's
important to use hose that has a higher pressure rating than the pump
in your pressure washer but, if your washer came with your own hose,
there shouldn't be anything to worry about. Typically, the safety
margin on pressure-washer hoses is about 300 percent, so if your
washer is rated at 2000 psi, your hose should be able to withstand
pressures of at least 6000 psi.
Cleaning attachment: Depending on what you're cleaning,
switch from a simple trigger gun (essentially just a valve that lets
water through only when you squeeze the handle) to a spinning wand
spray or a rotating brush to scrub your drive. Powered attachments
are driven by the force of the water flowing through them.
Photo: The trigger gun from a Kärcher pressure washer. The reinforced, high-pressure hose runs up inside the plastic casing, through a valve, and out of the open end on the right.
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
Here's a quick summary of the basic principle:
Detergent flows in from a bottle or container through one hose.
Cold water flows in from a faucet (tap) through another hose and is filtered on the way in.
An electric motor or diesel engine powers the washer.
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).
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?
Artwork: Kärcher pressure washer patent courtesy of US Patent and Trademark Office (with coloring and annotation added for clarity).
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:
Main outer plastic housing (yellow).
Electric motor (red).
Insulating plastic foil (blue): This ensures no water penetrates inside the motor.
Central shaft of the motor spins around at high speed, powering the water pump.
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.
Pump piston (orange): This is the chamber where water is pumped to high pressure.
Water pipe (blue): This is where water is sucked in and pumped out.
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
Pressure washers can get things really clean, but they have a few problems too:
They use a lot of water (typically 4–8 liters or 1–2 gallons per minute). That water needs to move away quickly, so you'll need good drainage where you're working if you don't want to cause a flood.
They're noisy (sound levels are usually about 75–90dBA).
If that's likely to be a problem for you, seek out quieter models by comparing the actual dBA
values quoted in technical specs. Kärcher, for example, has produced models described as "Silent," which claim to be 50 percent quieter, although the spec for the (now-discontinued) K4 Silent still quotes a
maximum noise level of 81dBA (hardly silent).
They can make everything around them (including you!) very dirty by blasting filth in all directions.
Think carefully about the order or direction in which you'll be working to minimize how much cleaning up
you'll need to do later. Choose your attachments wisely to avoid making too much mess, and be sure to wear waterproof, protective overalls and shoes.
Although power washers are insulated and very safe to use, there's always a risk of using electricity near water, so be sure to work with an RCD unless you're using a gasoline-powered machine.
Pressure washers are blunt instruments, not always suitable for cleaning delicate fixtures and fittings.
Used the wrong way, they can damage wooden decking and penetrate the weakening seals of old windows, making them more prone to leaks or (in the case of double-glazing) "fogging up." If you're unsure, try your washer on an inconspicuous area first—and use with a low-power setting (or a wide nozzle) before upping the power (or using a narrower nozzle) once you feel it's safe to do so.
US Patent #6,092,998: Pump for a pressure washer by Shane Dexter et al, Mat Industries LLC, granted July 25, 2000. There's some discussion here about the technical difficulties of designing vertically oriented pumps for typical domestic washers, which are used in an upright position.
Outdoor Cleaning with Pressure Washers: A Step-by-Step Project Guide by Thomas G. Lemmer and Pat Simpson. Quarto Publishing, 2004. I've flicked through this and it seems to be a very comprehensive, hands-on guide with lots of photos and illustrations and plenty of practical text. It starts off explaining the parts of a pressure washer and how it works, and then explains how to use it in various specific cleaning jobs (including cleaning gutters, bricks and mortar, and patios and drives).
How to pressure wash your home siding by Mike Kraft, Lowe's. A simple, 5-minute run-through of what you can do with pressure washers and some pitfalls to avoid. The tips about how to prepare for pressure washing are also very helpful.
↑ 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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