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How a Pressure Washer Works

View All Karcher Pressure Washer Spares

The principle of an electric pressure washer is simple – water is forced at high speed through a fine nozzle, which produces a strong jet to dislodge dirt. Most models also allow you to add detergent to the water, which increases the cleaning power of the jet.

  1. The cold-water feed hose is attached to the domestic water supply and the unit is turned on.
  2. The electric motor drives the pump which pressurises the water. When the water reaches operational pressure (usually around 110-120bar), a pressure switch inside the pump is activated, turning off the electric motor. The water remains under pressure in the pump.
  3. When the trigger is pressed it opens a valve, causing the water to flow along the pressure hose into the spray lance. The lance is fitted with an adjustable nozzle, which widens or narrows the hole from which the water exits. This reduces or increases the pressure of the water jet.
  4. When the trigger is pressed, the motor is turned back on. The pump then continues to pressurise the water entering from the mains until the trigger is released.

Alternative Design

In some pressure washers the electric motor runs constantly, re-circulating the water in the pump even when the trigger is closed. A thermal switch prevents this type of washer from overheating. If the temperature exceeds safe levels, the switch turns the machine off.


Pressure washers are mainly used for the following domestic cleaning tasks:

  • Cars, motorbikes, caravans, boats and bicycles
  • Patios, fences, garden walls, pathways, driveways and house facades – they are particularly useful for cleaning blocked driveways
  • Removal of algae and moss from roofs, brickwork and guttering
  • Garden tools and machinery, furniture and barbecues
  • Hygienic cleaning of animal pens and their surroundings

With the addition of accessories such as the Karcher Drain Cleaning Kit, or the T-Racer T-300, they can also be used for:

  • Clearing blocked drains and pipes
  • Removal of old paint and rust from garden furniture and gates

All pressure washer spares featured in this article are available at

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