It removes anything stuck on mechanically and struggles with anything bonded chemically - which in practice means it handles dirt and fails on grease.

Mud, dust, sand, mortar splashes, moss, algae, loose and flaking coatings, and general atmospheric grime all come off with pressure and flow alone. This covers the majority of construction, agricultural, municipal and grounds maintenance work.

Fats, oils and greases do not. They soften with heat and release with surfactant, and a cold jet mostly relocates them - which is why a cold-washed workshop floor often looks worse afterwards, with the oil spread thinly over a larger area.

Protein soils in food environments behave the same way, and baked-on carbon deposits resist everything short of heat and chemistry together.

A cold machine plus a detergent applied through a low-pressure nozzle and given time to dwell closes part of the gap and is the usual compromise where hot water cannot be justified.

Match the pump type to weekly running hours: axial pumps for occasional use, crankshaft-driven pumps for anything approaching daily work.

An axial (wobble plate) pump is compact, cheap and runs at full motor speed. It is entirely adequate for short, intermittent tasks, and its seals are usually not designed to be replaced - when it wears, the machine is replaced.

A crankshaft pump runs at reduced speed through a gearbox or belt, with a forged head, ceramic plungers and serviceable seals. It costs several times as much and lasts correspondingly longer under continuous use.

The honest test is hours per week, not the size of the site. A contractor washing for two hours a day will destroy a domestic machine within a season and never notice the difference on a professional one.

Where the machine will be shared across a crew, assume the harshest possible treatment and specify accordingly - shared equipment is not maintained by anybody in particular.

Single-phase for portability and general access, three-phase where the flow rate needed exceeds what a standard socket can support.

A single-phase supply limits motor power, which limits the combination of pressure and flow available. That ceiling is well above domestic requirements and below what sustained industrial washing wants.

Three-phase machines deliver substantially higher flow, run cooler, and tolerate continuous duty. The cost is that they can only work where a suitable supply exists, which usually means a fixed position or a site with distribution boards.

Cable runs matter more than people expect. A long extension on a single-phase machine drops voltage, which raises current and overheats the motor - use the shortest heavy cable that reaches, fully unwound.

Where a site needs high flow at several points, one three-phase stationary system with multiple outlets usually beats several portable machines on both cost and reliability.

No - past the pressure needed to release the soil, extra pressure buys nothing except a greater risk of damaging the surface.

Cleaning happens when the jet exceeds the adhesion holding the soil on. Beyond that threshold the surplus energy goes into the substrate, which is how render gets cut, mortar gets washed out and timber gets furred.

The number that changes productivity is flow. Doubling flow at the same pressure roughly halves the time for a given area, because the water is carrying more away per minute.

Very high pressure has a legitimate role in coating removal and surface preparation, but that is a different class of machine with different controls and training.

For general cleaning, buy the flow and specify the pressure - and expect to spend most of the working day at a wide fan angle well below the machine's maximum.

A professional machine will run all day; the limits are operator fatigue, water supply and time spent in bypass with the trigger shut.

Continuous operation is what a crankshaft pump is designed for, and duty is limited by cooling and lubrication rather than by any timer. Domestic axial-pump machines typically specify a duty cycle and overheat if it is exceeded.

The hidden constraint is bypass. When the trigger is released the pump keeps turning and recirculates water, which heats up. Long periods in bypass cook the seals; most machines fit a total-stop system that shuts the motor down after a delay for exactly this reason.

Water supply is the other ceiling. A machine drawing continuously will outrun a marginal tap within minutes and start cavitating.

Operator fatigue is real and often the true limit. Lance reaction force over several hours is tiring, which is another argument for surface cleaners on large areas.

Detergents formulated for pressure washers, applied at low pressure through a soap nozzle or injector, and given time to work before rinsing.

Detergent is drawn in either through a downstream injector after the pump or from an onboard tank. Downstream injection only works at low pressure, which is why a wider soap nozzle must be fitted to apply it.

Dwell time does the work. Applying chemical and rinsing immediately wastes it; applying, waiting several minutes without letting it dry, then rinsing at pressure is the method.

Household cleaners are a poor choice. They foam unpredictably, may attack pump seals, and are not designed for the dilution ratios involved. Alkaline degreasers, traffic-film removers and acidic descalers are formulated for this equipment.

Always flush the injector and lines with clean water afterwards, particularly with acidic products, and confirm that whatever is used is compatible with the wash-water disposal route.

Electric wherever a supply exists, petrol for portable off-grid work, diesel for heavy continuous off-grid duty.

Electric is quieter, cleaner, cheaper to run and needs almost no attention beyond the pump. It also allows indoor use, which no combustion engine does.

Petrol engines are lighter and cheaper to buy, start easily, and suit machines that move around a site all day. They need fuel handling, servicing and a warm-up, and they are noisy.

Diesel engines cost more, last longer under sustained load, and are the norm on larger trailer and skid units. Fuel is often already on site for other plant.

Whatever the drive, the pump is a separate component with its own duty rating. A good engine on a domestic-grade pump is still a domestic-grade machine, and that combination is common at the cheaper end of the market.