by Chris Woodford. Last updated: February 7, 2019.
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: Left: 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.
Why pressure jets get things cleaner
Photo: A small Kärcher pressure washer suitable for home use. This one manages a pressure of about 1400psi, which is typical for an electrically powered washer, and uses about 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 different things.
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.
These, then, are the main parts you'll find inside a pressure washer:
- Water inlet: A hose that connects the pressure washer to the main 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 pump.
- Water pump: This is the heart of a pressure washer. It's a bit 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 1–2 gallons (4–8 liters) 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, you can 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).