The pump is the heart of a pressure wash system.  Basically the pump pushes water using pistons or plungers, inside a cylinder.  When the water is pushed out of the cylinder a small valve closes, not allowing the water to come back into the cylinder.  Instead, new water is brought into the cylinder, past a similar valve which opens when the piston pushes the pressurized water out.  This process is repeated many times per minute, creating flow. 

Since water can not be compressed and pumps are positive displacement the water needs a place to go.  The pump creates the flow. The size of the orifice (nozzle) the flow is being pushed through creates the pressure.  

How much flow is a result of displacement and pump speed.  How much pressure depends on orifice size.  This depends on enough horsepower to push the flow through the orifice, without slowing the pump down.  

There are two types of pumps used in the pressure wash industry:


The rotary pump works by turning an angled plate which has pistons held against it by springs.  As the plate turns it moves the pistons, which in turn push water through a valve.  The pistons are held tight to the plate by springs.  Think of it as a cam shaft in an engine, which open valves as it turns.



Piston and plunger pumps work in the same way to produce flow.  The sealing system is the major difference. In a piston pump the sealing system (rings, packing etc) is attached to the piston and moves with it during its stroke. The sealing system for the plunger pump is stationary and the plunger moves through it during its stroke.

The main difference to a rotary pump is how the pistons are moved.  While the rotary pump turns an angled plate to move the pistons, the piston/plunger pump pistons are turned by a rotating shaft with connecting rods attached to pistons.  There are no springs to pull the pistons back since the shaft does both jobs.  Think of it as the crankshaft in an engine, which moves pistons up and down as it turns.

Regardless of the pump type, all pumps have two separate sections.  The water side and the oil side.  

The oil side lubricates the metal parts as the pump turns.  There is not much to maintain here but from time to time you will want to check the oil level in the pump.  Occasionally the oil will need to be changed.

The water side is flooded with water to be pumped, which also helps the cool the pump.  The valves and seals in the water side require periodic replacement to maintain pump performance.

Pump speed is an important factor when deciding on your pressure washer.  Because rotary pumps are made for the lower-end big box seller, using gas engines, virtually all rotary pumps run at 3400 RPM.

Plunger and piston pumps are, for the most part, made for the commercial and industrial markets.  Pump speeds can vary from a few hundred RPM to 3400 RPM.  Slower turning pumps will last longer.  However, they are larger, and more expensive.  Slower turning pumps can be driven by any speed motor through a belt system or gear box

A positive displacement pump needs a place to push the flow it is producing.  If pressurized water flow from a pump was never turned off the pump would not need a method of controlling this flow  But, this is very rare circumstance. 

All pressure washers need a way to deal with the water flow, considering the flow is turned on and off many times by the operator opening and closing the trigger gun.   The unloader valve solves the problem by diverting water through a bypass when the flow is stopped.