Pressure washer FAQ

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Pressure Washers, Jet Washers, or High-Pressure Cleaners (HPCs) as they are known in the trade, are a useful tool for all sorts of cleaning tasks around the home. Many accessories are available to extend the scope of the basic machine.

This FAQ deals primarily with domestic or DIY high-pressure cleaners.



The pressure developed is only one part of the equation. A high-pressure cleaners ability to clean depends upon a combination of pressure (measured in bar) and flow rate (in litres per minute). Both are equally important.

Beware the figures quoted in the adverts or printed on the box! They will invariably only quote pressure and then often in an exaggerated form. "Maximum Pressure 120 Bar" often means that the safety valve will open at 120 bar to protect the pump. You might find the flow-rate printed on the machine label.

The working pressure will be well below that. Commonly a machine that is advertised as 120 bar would actually have a working pressure of around 90 bar.


The flow rate in litres per minute (or litres/hour) is a very important measure of the cleaning ability. DIY machines commonly have flow rates as low as 6 litres/min. Heavy-duty industrial units vary from 10 to 40 litres a minute.

Water flow in a high pressure cleaner can be compared to several easily understood concepts, electricity and compressed air. With electricity we recognise that amps (flow) are often more important than volts (pressure). With compressed air we understand that volume (free air delivered) is often more important than pressure.

Think of it this way. A litre of water weighs one kilogram. When you look at an HPC, mentally convert the flow rate from litres to kilograms. A machine throwing 6 kilos a minute at a surface isn't going to hit as hard as one throwing 8 kilograms a minute.

Pressure & Flow

Roughly, an increase in flow rate of one litre a minute is equal to 20 bar in pressure.

If you do want to compare the performance of two machines you can use a simple calculation which will give you a figure known in the trade as the 'Cleaning Effect';

Cleaning Effect = Pressure (bar) x Flow (litres/min) / 600

This is a reliable guide when comparing two machines. However, a general guide is that the bigger the motor, the better the performance. The flow and pressure are divided by 600 just to give us a simple figure to work with. You won't find this figure on the box or in the instructions, it's a calculation you have to do for yourself.

The smallest domestic HPC's start at about 0.83 cleaning effect and go to a top of the range DIY machine at maybe 2. So a cleaning effect of 1.0 is just about OK; 1.5 is reasonable, and 2.0 is good.

Hire shop machines are going to be about 2.3, just about the most powerful motor you can run on 230 volt single phase.

A 13 HP Honda petrol machine can develop a cleaning effect of about 6.75.

A 3 phase, 415 volt, 25 amp machine, 200 bar at 30 litres/min develops a cleaning effect of 10. If you weigh less than 13 stone you couldn't physically hang on to one!

Drain Jetting machines operate from a cleaning effect of about 15. Biggest I've ever seen? 5,000 bar at 15 litres, cleaning effect of 125! Harness required and ballistic quality clothing. Used for concrete cutting.

How much power?

A more powerful pressure washer doesn't necessarily do a better job, it will do the same job in less time.



Cheap machines can perform as well as more expensive machines, but the major difference is in durability - how long the machine will last!

About two thirds of the cost of manufacturing a HPC is the motor. Cheaper machines will have universal (brush) motors, sometimes intended for vacuum cleaners! Better quality machines will have induction motors.

Typical 'total run time' on a universal motor in a HPC can be as low as 10 working hours. That means it's useless after that time!

An induction motor, even a cheap one, will run for 50/60 hours, better quality induction motors will run for 100+ hours.

How can you tell? Price is a good guide, very cheap machines will have universal motors. Weight is a good guideline, universal motors are much lighter that induction motors. Noise is another clue, because of the RPM universal motors are extremely noisy.

As a comparison, industrial machines have motors rated at 800 - 1000 hours.


Almost all DIY HPC's have axial pumps with 2, or more often 3, pistons. The material the pump is made of has a huge effect on the durability of the machine.

Very cheap DIY machines have pumps made of engineering plastics! They won't last very long and generally won't run continuously for very long without overheating.

Aluminium pumps are more durable than plastics and are found on most mid range DIY machines. Much better quality DIY machines have brass pumps, much longer lasting and resistant to most detergents.

Beware again! Some manufacturers will use brass inlet & outlet fittings on aluminium pumps to give a false impression! Peep inside the cover!


It's worth bearing in mind that a replacement High Pressure Hose often costs almost as much as the machine did. Check this out when considering a purchase. Also bear in mind that a cheap DIY machine will have a very short hose. A 5 metre hose won't get you around a normal car! Consider an extension hose.

You will also need a 1/2" garden hose-pipe to supply the machine with low-pressure water from a tap.

The water inlet thread fitting on almost all high pressure cleaners is 3/4" BSP, conveniently the same thread as a standard garden tap.

The best known & most often copied range of garden hose fittings is Hozelock. They supply 3/4" tap connectors which can be used at both the tap & the machine.

Both ends of the supply hose can then be fitted with standard Hozelock connectors for an easy way to connect up the machine.

Please be aware that Hozelock make a version of the standard connector called a 'Waterstop'. These allow you to disconnect a hose without having to turn off the tap. 'Waterstop' fittings, whilst excellent, often cause problems when used to supply high pressure cleaners.

Make sure the connectors have a completely open bore.

Most high pressure cleaners can also suck water from a drum or water butt. Cheaper machines will only suck from their own level, better machines will suck between 1 & 5 metres straight up.

In all cases a filter must be used to prevent debris & grit entering the pump. To initially prime the pump, it is best to remove the lance & operate the machine until a steady stream of water emerges.


All HPC's have a trigger gun so that you can shut off the machine. Two things can happen when you shut the trigger; either the machine stops completely (known as 'auto stop/start' or 'total stop') or the machine runs on, in which case a valve (a by-pass valve) returns the pressurised water to the inlet.

A machine without 'auto stop/start' will only idle in 'by pass' mode for 4 or 5 minutes. Any longer will damage the pump seals.


DIY pressure washers have flow rates that are very low compared to the pressure. 100bar x 6 litres/min is common. An industrial machine at 100 bar would have a flow rate of 8 - 10 litres/min.

The balance between pressure & flow is important, high pressure/low flow gives much more of an aerosol effect because the water droplets are very small, resulting in a wet operator & muck all over the place.

The next consideration is the type of nozzle. If you had a simple drilled hole in the nozzle you would get a straight 'pencil' jet of water maybe 3mm in diameter. This would have very high mechanical efficiency, but would take ages to clean any sort of area.

Most DIY machines & some lower end industrial have whats known as a 'vario' nozzle which uses two metal plates to squeeze the pencil jet into a fan shape. These can be adjusted to vary the shape of the jet between pencil & fan by turning a ring on the nozzle that moves the plates. Vario nozzles cause a large loss in efficiency and an increase in aerosol effect.

Next step up is to use a standard fan nozzle where the water stream is forced through a slit into a fan shape. These are again not that efficient and cause aerosol effect, though not as bad as vario nozzles.

The aerosol effect is caused by the very small size of the water droplets, which easily become airbourne.

Better are the fairly recent Spectrum or Powerspeed type nozzles where the inside of the nozzle is machined to spin the water stream into a fan. These produce water droplets 4 times the size of a standard fan jet & greatly reduce aerosol effect as well as improving cleaning efficiency - larger droplets hit harder.

Best of all are 'Turbo' or 'Dirtblaster' nozzles. These use a mechanical device to spin or oscillate a pencil jet into what appears to be a fan, giving large area coverage, maximum mechanical advantage and much larger (10 times) water droplets than a fan jet. Hugely reduced aerosol effect, much less splashback.

Floor cleaning tools like the Karcher Surf Cleaner have a rotating spray bar inside a shroud and reduce splashback to almost zero.

Effective use

A commercial pressure jet will work at it's best between 2" and 8" from the surface to be cleaned. 2" to 4" is probably more realistic for DIY kit. At 12" away from the surface most DIY pressure washers have little or no effect.


Many accessories are available for HPC's. Some are obvious like extension hoses or under body lances, some aren't.

Patio Cleaners

Cleaning a patio is an extremely mucky job. You and the surrounding area will get very wet & dirty. Wellies and waterproof leggings are a necessary protection (as well as goggles) unless you are happy to get soaked and bin your jeans after. If your patio is in need of re-grouting, and you hit one of the ungrouted bits between slabs, a huge spray of muddy water is likely to bounce straight back into your face.

An accessory called a Patio Cleaner (or T Racer, Surf Cleaner etc) solves this problem.

These accessories have a revolving spray bar enclosed in a circular shroud about 8" diameter and 3" deep. The pressure jets on the spray bar are made to rotate like a rotary mower blade and are extremely effective when cleaning a patio or drive. You cover the area much faster than with a lance and have virtually no splash back.

Make sure you sweep the patio first; This accessory is very susceptible to damage from stones.

Drain Cleaners

are a popular accessory and very useful for occasional cleaning of domestic drains & pipes. They work surprisingly well, but bear in mind that a domestic HPC is around 2HP and DynoRod use machines of 40+ HP.

Always work from the downstream side of the blockage if possible.


attachments draw in abrasive grit to remove rust & paint. The sand used is critical - kiln dried silica sand - same stuff used for block paving and available at your local DIY store. Make sure the sand is kept bone dry!

Expect a cleaning jet of 1/4" diameter, so the main use is for small rust spots, not large areas. Because it's a wet process there are no dust hazards to worry about, but make sure you wear eye protection and expect to find sand particles in strange places for days afterwards!

Rotary Brushes

Useful for cleaning the car, caravan or conservatory roofs, the rotary action gives a gentle scrubbing action. The rotation is easily stopped by excessive force.


Most machines have a detergent spray system of one sort or another. The detergents available in the DIY store aren't generally that effective for cleaning cars without some sort of mechanical agitation.

Always apply detergents from the bottom upwards to avoid streaks.

Where to buy

Cheaper machines are to be found everywhere, DIY stores, garden centres etc. Spares or service back up is likely to be non-existent, indeed spares for some very cheap machines simply aren't available.

For a better quality machine look in Yellow Pages under 'Cleaning Equipment Manufacturers & Suppliers - Industrial' and find your local high-pressure cleaner specialist.

Most will stock a domestic range, sometimes a brand name you have never heard of, but these companies generally know their trade and will only sell something they know and trust.

It's also worth asking about second hand commercial machines. Even a well-used commercial machine will give years of service in a domestic situation and they are usually cheap & easy to repair.

Another option is to hire a machine for the job in hand. Hire shops tend to have powerful industrial units, which will do the job faster.


Full safety instruction should come with your pressure, but a few important points.

Always wear eye protection. Pressure washers often cause grit & muck to fly about at high speeds. Keep the area you are cleaning clear or people and animals.

Electricity & water do not mix! Use a power supply protected by an RCD. A major danger point is an extension cable that may be in use. Make sure the join is off the ground and protected from accidental spray.

Almost all HPC's are IP 45 or better spec, so water splashes on the machine itself are never really a problem.

A note on extension cables, especially with induction motor machines. Induction motors demand current so if the voltage drops over a long cable, they will run on a higher amperage. This can cause serious overheating to both the machine and the cable. Make sure you use a suitable cable (check in the instruction book) and unwind it all from any drum or reel.

Don't ever aim the jet at people or animals, even in fun. The water jet could be doing over 300 mph!

If in doubt about the suitability of a surface for pressure washing, start with a wide fan-jet and stand well back. Alter the distance and jet angle once you have observed the effect.