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HOW-TO GUIDEHow a Pressure Washer Actually Works
A pressure washer takes ordinary tap water at roughly 2 to 3 bar and uses an electric or petrol motor to drive a pump that squeezes that same water up to 100 bar or more. Nothing is added and no water is created, the machine simply forces a modest flow through a very small nozzle opening so it leaves at high speed. That combination of pressure and speed is what lifts algae, moss and grime off a surface instead of just wetting it.
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The nozzle is the part that turns pump pressure into cleaning force, so it has more effect on results than almost anything else on the machine.
See Our Top Pick →The water path from tap to nozzle
Follow the water and the whole machine makes sense. It travels through five stages, and every common fault sits somewhere along that route.
- Inlet and filter. Water arrives from a hose or a tank and passes a small mesh filter that catches grit before it reaches the pump.
- Pump. Pistons or plungers driven by the motor draw water in on one stroke and force it out on the next, through one-way valves that stop it going backwards.
- Unloader valve. When you release the trigger the pump keeps turning, so this valve diverts water back around the pump instead of letting pressure spike.
- Hose. A reinforced high pressure hose carries the water to the gun without ballooning.
- Lance and nozzle. The gun trigger opens the flow and the nozzle orifice restricts it, converting pressure into a fast moving jet or fan.
Why a small hole makes such a difference
The pump does not create speed on its own. It creates pressure, and the nozzle converts that pressure into velocity as the water escapes through an opening often less than 1.5 mm across. Fit a nozzle with a larger orifice and the same pump will show a lower pressure reading on the gauge, because water can leave faster than the pump can pressurise it. Fit one that is too small and pressure climbs, the unloader valve starts working harder, and the machine may cycle on and off.
This is also why a blocked nozzle causes odd behaviour rather than simply reducing the spray. Partial blockages narrow the effective opening, pressure rises, and many machines respond by pulsing or stalling.
Quick Tip
Always turn the tap on fully and squeeze the trigger for a few seconds before starting the motor. Pumps are designed to move water, not air, and running one dry even briefly wears the seals faster than hours of normal use.
Bar and litres per minute both matter
Pressure gets all the attention on the box, but cleaning is really the product of two numbers. Pressure, quoted in bar or PSI, is what breaks the bond between dirt and the surface. Flow rate, quoted in litres per minute, is what carries the loosened dirt away and covers ground.
A machine with high pressure and low flow will strip a stubborn patch but take a long time to work across a whole driveway. A machine with modest pressure and generous flow rinses large areas quickly but may not shift ingrained moss. Domestic units typically sit somewhere between 100 and 150 bar with 5 to 8 litres per minute, which suits patios, fencing, cars and garden furniture.
Worth knowing: the bar figure printed on the box is usually the maximum the pump can reach against a closed valve, not what you get while cleaning. Working pressure is commonly 20 to 30 percent lower.
Electric against petrol
Electric machines use a mains motor driving the pump directly or through a gearbox. They are quieter, lighter, need no fuel and start instantly, but they are limited by what a domestic socket can supply, so most top out around 150 bar.
Petrol machines use an engine instead, which removes the electrical ceiling. They reach higher pressures and flow rates and go anywhere without a cable, at the cost of noise, weight, servicing and exhaust fumes that rule out indoor or enclosed use. For most household jobs an electric unit is the sensible default, and petrol earns its place on large areas, farm work or sites without power.
How detergent gets into the water
Pressure washers do not pump soap through the high pressure system, since detergent would not survive that treatment and neither would the seals. Instead they use a low pressure route. Some have a tank or bottle feeding a venturi, where fast moving water creates suction that draws detergent in. Others use a separate foam lance that mixes soap at the nozzle end.
Either way, detergent only flows at low pressure, which is why fitting the wide fan or dedicated soap nozzle is essential. With a narrow high pressure nozzle in place, the pressure never drops enough to pull detergent through, and it looks like the machine is faulty when it is working exactly as designed.
What the motor is doing between triggers
Most domestic machines use total stop systems. Release the trigger and a pressure switch cuts the motor within a second or two rather than leaving it spinning. This saves electricity, reduces noise and keeps the pump cool.
The trade off is that repeated brief bursts mean repeated starts, and each start puts load on the motor and switch. If you are working across a large surface, keep the trigger held for longer sweeps rather than tapping it dozens of times a minute. Machines without total stop rely entirely on the unloader valve to recirculate water, and that recirculating water heats up, so leaving one idling for more than a couple of minutes is worth avoiding.
Does a pressure washer use a lot of water?
Less than most people expect. At 6 litres per minute a pressure washer uses roughly half of what an open garden hose delivers, and because it cleans faster the total for a job is usually lower. A twenty minute patio clean comes to about 120 litres.
Can a pressure washer heat its own water?
Standard domestic machines cannot. They deliver water at whatever temperature comes out of the tap. Hot water pressure washers exist with a built in burner, but they are commercial equipment and cost several times more.
Why does my pressure washer feel weaker than the bar rating suggests?
Usually because the quoted figure is maximum rather than working pressure, and because a worn nozzle has widened. Nozzle orifices erode over time, and a nozzle that has grown by even a fraction of a millimetre drops working pressure noticeably.