Pressure breaks the soil loose, flow carries it away - and for most cleaning work, flow is what people actually want more of.

Pressure is the force at the nozzle. It is what lifts paint, breaks a bond, or cuts into a deposit. Below the pressure needed to release a particular soil, nothing happens no matter how long you spray.

Flow is the volume of water arriving per minute. Once the soil is released, flow is what sweeps it off the surface and out of the way. Two machines at the same pressure, one at twice the flow, will not clean twice as well - they will clean roughly twice as fast.

The two are often combined into a single cleaning figure by multiplying them, which is a reasonable way to compare machines but hides the trade-off. The same figure can come from high pressure and low flow or the reverse, and those two machines suit completely different work.

The practical rule: choose pressure by what you have to remove, then buy as much flow as the water supply and the budget allow.

The supply must deliver more than the machine draws, continuously, with the machine running - and a tap that fills a bucket quickly is not proof of that.

Measure it. Time how long the supply takes to fill a container of known volume, and convert. Compare that against the machine's own consumption, with margin. A machine starved of water cavitates, and cavitation destroys pumps quickly and expensively.

Inlet pressure matters as well as volume. Most machines want a positive feed rather than having to suck water up; check the permitted inlet pressure range, since an unusually high mains pressure can damage the inlet as surely as a low one starves it.

Where the supply cannot keep up, a break tank solves it. The machine draws from the tank at whatever rate it needs while the supply refills the tank at its own pace. A break tank is also how most jurisdictions require the machine to be isolated from a potable supply, so it frequently solves two problems at once.

For work away from any supply, that is what trailer and skid-mounted units with onboard tanks exist for.

Electric where there is power and the work is indoors or noise-sensitive, engine-driven where there is no supply, battery where neither is available and the jobs are short.

Electric machines are quieter, produce no exhaust, need almost no maintenance beyond the pump, and can be used inside. They are limited by the supply available - a single-phase circuit caps the motor size, and the larger machines need three-phase.

Engine-driven machines go where there is no electricity, which is most construction and agricultural work. The cost is an engine to service, fuel to carry, noise, and exhaust that rules out enclosed spaces entirely.

Battery machines have become genuinely useful for short, remote tasks - a vehicle, a kennel, a piece of plant - where dragging a cable or starting an engine is out of proportion to the job. Runtime remains the constraint.

Where a site has power at a fixed point but work happens all around it, a stationary system with several outlets is often better than any portable machine.

Whenever the soil contains fat, oil or grease - which is kitchens, food production, workshops, vehicle fleets and most industrial floors.

Heat changes the chemistry rather than the mechanics. Fats soften and release at temperatures a cold machine cannot reach, so a hot machine removes in one pass what a cold one smears around for several. It also cuts detergent use substantially and shortens the time a surface stays wet afterwards.

For mud, dust, moss and general grime, cold water and a good flow rate do the job and the extra capital, fuel and maintenance of a burner is money not well spent.

Hot machines are heavier, need fuel and combustion air, and cannot be used in an unventilated space. They also need a warm-up and a cool-down discipline that operators have to be trained in.

A useful test: if the current process involves applying a degreaser and waiting, hot water will probably replace most of it.

Treat the jet as a cutting tool, never point it at anyone, and understand that a high-pressure injection injury is a surgical emergency even when the entry wound looks trivial.

Water at working pressure penetrates skin. The resulting injury injects water and whatever the surface carried deep into tissue, and it can look like a pinprick while causing damage that needs immediate specialist attention. Anyone using this equipment should know to seek treatment at once and to say that it was a high-pressure injection.

Eye and face protection, waterproof footwear with toe protection, and clothing that keeps the spray off the skin are the baseline. Debris comes back at the operator at least as fast as the water goes out.

Reaction force is the other hazard. A lance pushes back hard, and more so with a straight jet or on a long lance. Working from a ladder with a pressure washer is a bad idea for exactly this reason.

Ultra-high pressure work is a separate discipline with its own operator certification requirements in most jurisdictions - check the local rule before hiring or buying.

Somewhere legal, and in most jurisdictions that is not the surface water drain - the runoff carries detergent, oil and whatever came off the surface.

Surface water drains usually discharge to a watercourse without treatment. Putting detergent, hydrocarbons or paint residue into one is a pollution offence in most countries, regardless of how small the job was.

The foul sewer is often the correct destination, but discharging trade effluent to it normally requires consent from the operator of the sewer, and that consent may set limits on volume, pH, oil content and solids.

For regular washing, a bunded wash bay with an interceptor - or a closed-loop system that filters and recirculates - is the durable answer, and is often the only way to get consent at all.

For occasional work, containment mats and a wet vacuum let the water be collected and disposed of properly.

The rules differ substantially between jurisdictions. Establish what applies locally before committing to a wash-down process, because it can determine the equipment.

A jetter clears blocked pipes rather than cleaning surfaces - most of its water is aimed backwards so that the hose drags itself along the drain.

The nozzle has a small forward jet, if any, and a ring of rearward jets. The rearward jets provide thrust that pulls the hose down the pipe, and they scour the pipe wall on the way back out. That is the opposite of a lance, where all the energy goes forwards.

The selection driver is pipe diameter and length rather than pressure. A jetter that cannot generate enough thrust will not travel, and one with too much flow for a small pipe simply backs water up.

Root cutting, descaling and grease removal each use a different nozzle on the same machine, so the nozzle set matters as much as the unit.

A general-purpose pressure washer with a jetting hose kit handles domestic and light commercial drains. Main sewers and long runs need a purpose-built jetter, usually trailer or van-mounted, and usually paired with a camera.

Keep the inlet filter clean, never let the pump run dry or freeze, and follow the manufacturer's oil interval - most pump failures come from one of those three.

The inlet filter protects the pump from grit. It takes a minute to check and its neglect is the most common preventable cause of premature wear.

Running dry destroys seals in seconds because the water is also the lubricant and coolant. So does prolonged operation in bypass with the trigger closed, which heats the circulating water - a thermal relief valve exists to prevent that but should not be relied on as a working practice.

Freezing splits pump heads and is the single largest cause of winter write-offs. Either store the machine somewhere heated or flush it through with antifreeze before cold weather.

Beyond that: change the pump oil on schedule, descale the coil on hot water machines in hard-water areas, and replace nozzles when the spray pattern widens - a worn nozzle quietly drops working pressure long before anyone notices.

Choose the nozzle and the standoff distance for the surface, not for the soil, and test on an inconspicuous area before starting.

Damage is caused by concentrating too much energy on too small an area. A narrow-angle or turbo nozzle held close will erode soft mortar, lift timber grain, cut render, force water behind cladding and strip the surface layer off decking.

The fix is nearly always to widen the fan angle and back off rather than to reduce pressure at the machine. A wide fan at full pressure held further away cleans effectively and spreads the energy.

Surface cleaner attachments exist partly for this reason. The shroud holds a fixed standoff and the rotating bar sweeps evenly, which is why they leave no striping on large flat areas.

Some surfaces should not be pressure washed at all - soft brick, lime mortar, painted timber in poor condition, and most historic masonry. For those, low-pressure chemical cleaning is the correct method.

A surface cleaner for flat areas, the right nozzle set, and a hose long enough to reach without moving the machine - in that order.

A surface cleaner transforms the economics of any large flat area. It covers several times the area per hour of a lance, leaves no zebra striping, and keeps the spray contained rather than throwing it across the site.

A nozzle set is cheap and changes what the machine can do. Fan angles for different surfaces, a soap nozzle for applying detergent at low pressure, and a turbo nozzle for concentrated work cover almost everything.

Hose length decides how often the machine gets moved, which is the largest hidden time cost on a big site. A reel keeps a long hose usable rather than a liability.

Beyond those: gutter and telescopic lances for work at height without a ladder, a foam lance where dwell time matters, and a jetting kit if drains are part of the remit.