However, when you shut that flow down horsepower takes over.  When you have a positive displacement pump , the pressure builds up the more you restrict the flow.  When you continue to restrict the flow the pressure continues to build until either the engine runs out of horsepower and stalls, something blows or the unloader valve kick in.

The size of a nozzle is critical to your machine operating at it's capacity.  It's all in the hole size.  Too big a hole lets out too much water and reduces pressure.  Too small a hole and the pressure rises to the point the unloader valve starts to dump water through the bypass, reducing flow. If you are working on a sensitive area and want to reduce pressure the best way is to install an oversize nozzle.  This reduces pressure while maintaining flow.  To choose the proper nozzle look at our nozzle sizing chart or use this formula to work out the size....... 

Always round up if your calculation falls in between sizes.

It is important to keep your nozzle clean.  A small speck of debris can reduce your flow and increase the machine  work load.  Many contractors carry a set of welding tip cleaners to make cleaning easy.  

There are many different types of nozzles out there, both for the professional pressure wash contractor and the homeowner:

MEG AND WEG
MEG Nozzles (female pipe thread) and WEG Nozzles (male pipe thread) come in a myriad of shapes, spray patterns, styles and sizes.  The most common you are likely to find on a pressure washer wand is 1/4 inch MEG.  Because MEG and WEG nozzles need to be screwed onto the end of the wand they are generally limited to one application.  For instance, if you are cleaning the same thing all the time in a wash bay fixed system then chances are you only need one size nozzle...use a MEG.  If you are a homeowner or contractor, cleaning many different types of surfaces then you need lower pressure for wood decks and higher pressure for concrete sidewalks. You would not use a MEG nozzle, considering the time it takes to get out the wrenches and change it. 

QUICK CONNECT
Quick Connect, or QC, nozzles are very convenient.  They give the operator the ability to change pressures and spray patterns in a few seconds without a wrench.  QC nozzles come in a variety of sizes and spray patterns, which are color coded for ease of identification.  With a QC nozzle you need a QC fitting, which you fit to the end of your wand.  The QC nozzle then fits into the QC fitting.  Occasionally the o-ring in the fitting needs to be changed.

The above are the basic nozzles available for the pressure washer.  However here is a list of other nozzles to make the the go easier and faster:

ADJUSTABLE NOZZLE
This nozzle allows you to adjust the spray pattern by simply turning the housing clockwise or counterclockwise.  Most allow the patter to be changed from 0 degree to a flat spray.  This works by squeezing together, or pulling apart two plates which interrupts the flow of pressurized water.  

VARIABLE NOZZLE
Also known as HI-LOW nozzle.  The variable nozzle works the same way as the adjustable nozzle to change the spray pattern plus you can change the pressure, from high to low, with a push-pull action. These nozzles work well for lowering pressure to activate a high pressure soap injector.

ROTARY NOZZLE
One of the most effective nozzles you can use.  The rotary is basically a zero degree nozzle that spins in a circle and is very aggressive. It needs to be used with caution.  It can rip apart unstable concrete, remove paint that you don't want removed and drill holes in soft wood decks or fences.  However, used in the right circumstances, these nozzles will save you lots of time and do a great cleaning job.  Usually, on quality models, the internal moving parts are made of ceramic. While ceramics are very hard and resist wear from the fast moving parts, the are brittle.  Always point the nozzle down before pulling the trigger.  That way the ceramic pieces are together when the pressure hits, saving the pieces from slamming together.  Replacement ceramic parts are usually available but can be expensive. Rotary nozzles come in a variety of styles to fit the job.  Some just spin but others are adjustable and/or  variable (see above).

SAND BLAST NOZZLES
These nozzles pick up sand from a bag or bucket, using the venturi-effect, and mix that sand with the high pressure water. If you want to strip paint or rust from a metal object, this is the way to do it.  Not recommended for wood or any soft substrate unless you want it to go away!  To work properly the sand needs to be dry and sifted to an even size. Quartz or silica sand works great. Play sand is cheap but sometimes the sizing is a problem. There are two models that are generally available.  One is cheaper but has no sand flow adjustment and limited life.  The more expensive unit has adjustable sand flow plus the exposed internal parts are made from hard wearing tungsten carbide.

DRAIN CLEANING NOZZLES
These specialty nozzles are made to be put on the end of a drain hose and shoved into a plugged drain.  The principle is to break up a blocked drain with one stream shooting forward and two or three streams shooting back.  As the stream shooting forward breaks up the blockage the streams shooting back flush the loosened debris from the line.  Some are made to spin inside the pipe. Drain nozzles range from very small to quite large to fit into various diameter pipes. It is important to size the nozzle for the job. Too large a nozzle will jam in the pipe or not be able to follow the bends.  Too small a nozzle and it will not do an effective job on the sides of the pipe.  A hot water pressure washer is extremely effective If cleaning grease filled drains.

SOAP NOZZLES 
Soap nozzles work in a few ways to inject soap or chemicals into the wash water stream.  Most pressure washers come with a soap injector.  There are two basic type of soap injectors.  High pressure and low pressure.  The low pressure injector works with virtually any type of nozzle.  It injects soap at the low pressure side of the pump making for continuous flow of soap laden water to the nozzle.  A low pressure soap injector works by injecting soap on the high pressure side of the pump.  However, the pressure in the line needs to be lowered for this soap injector to work.  To lower the pressure click a soap nozzle on the wand. Now you have low pressure soap.  Variable nozzles allow you to lower pressure without changing anything.

Other nozzles produce foam at the end of the wand, drawing soap from a small jug attached to the nozzle.  These can be very effective by producing a thick foam, like shaving cream, which will stick to almost any surface.  This increases dwell time, making good use of your soap.