Petrol for lighter, portable machines used intermittently; diesel for heavy, sustained duty and for sites that already stock diesel.

Petrol engines weigh less for the same output, cost less to buy, and start readily from cold. That suits a machine that gets lifted in and out of a vehicle and used for an hour at a time.

Diesel engines are heavier and more expensive but handle continuous full load better, run longer between overhauls and use less fuel per hour of work. On trailer and skid-mounted units they are effectively the default.

Fuel logistics often decide it. A site with a diesel bowser for plant does not want a petrol can as well, and the reverse is true for grounds maintenance fleets running petrol equipment.

Storage rules differ by jurisdiction and petrol is generally the more tightly controlled of the two - worth checking against site rules before committing.

Because the exhaust contains carbon monoxide, which is odourless, and it accumulates faster in an enclosed space than people expect.

Carbon monoxide poisoning from petrol equipment used inside sheds, barns, basements, garages and partially enclosed structures kills people every year. The gas gives no warning and the early symptoms are easily mistaken for fatigue.

Opening a door or a window is not adequate ventilation. Neither is a fan. The rule most regulators apply is that combustion engines do not operate in enclosed or partially enclosed spaces at all.

The practical workaround is to leave the machine outside and run the high-pressure hose in. Hose is cheap and long runs cost only a little pressure.

Where indoor washing is routine, an electric or electrically heated machine, or a stationary system with the plant room outside the work area, is the correct answer.

Air filter, oil and fuel are the whole story for most failures - and the air filter is the one that gets neglected.

Pressure washers work in dust. A clogged air filter starves the engine, causes it to run rich, fouls plugs and eventually damages the bore. Checking and cleaning it should be routine rather than scheduled.

Oil changes at the stated interval matter more on small engines than on vehicles, because oil capacity is tiny and the oil works hard. Many small engines have a low-oil cutout, which prevents catastrophic damage but does not substitute for changes.

Stale fuel is the classic seasonal failure. Petrol left standing degrades and gums carburettors, which is why a machine that worked in autumn will not start in spring. Either drain it or use a stabiliser.

Beyond that: spark plug, valve clearances at the stated interval on four-strokes, and keeping the cooling fins clear of debris.

By siting the machine away from the sensitive boundary, working within permitted hours, and choosing a quieter machine where the limits are tight.

Engine noise is a common source of complaints on urban and residential work, and many jurisdictions impose permitted working hours or noise limits at the site boundary. Those rules are local and worth checking before quoting.

Position helps more than anything else. Distance, screening and putting solid structures between the machine and the boundary all reduce received noise substantially, and the hose lets the machine sit well away from where the work is.

Machine choice matters. Enclosed and acoustically treated units exist, and diesel engines running at lower speeds are often quieter than small petrol engines at full throttle.

Operator hearing protection is a separate obligation and applies regardless of the boundary position.

It sets a ceiling, but the pump determines what actually arrives at the nozzle - and the two are often badly matched.

Engine power has to be sufficient to drive the pump at its rated pressure and flow, with margin. An undersized engine bogs down under load and the machine never reaches its quoted figures in practice.

An oversized engine on a light-duty pump gives no benefit at all. The pump is the limiting component, and this combination is common in cheaper machines because a large engine looks impressive on a spec sheet.

Look at the pump: crankshaft-driven with a forged or brass head and serviceable seals for professional duty, direct-drive axial for lighter work. Direct drive at engine speed shortens pump life considerably.

Belt or gear reduction between engine and pump is a good sign - it lets the pump run at the speed it was designed for.

Carry enough fuel for the shift, store it legally, and expect noticeably reduced output at altitude unless the engine is jetted for it.

Consumption is significant on a machine running all day, and running out mid-job is disruptive. Naturally aspirated engines lose power as air density falls, which becomes obvious at high-altitude sites - the machine will not reach its rated output and may run rich, foul plugs and smoke.

Some manufacturers supply high-altitude jetting kits; where work is routinely conducted at elevation, this is worth specifying rather than tolerating.

Fuel storage and transport are regulated in most jurisdictions, particularly for petrol. Approved containers, quantity limits and separation from ignition sources apply on most sites.

Refuelling a hot engine is a fire risk. Stop, let it cool, and refuel away from the work area.

Wheeled for machines that move by hand around a site, frame or skid for machines that are lifted, transported and set down for the day.

A wheeled chassis with pneumatic tyres is what makes a machine usable across rough ground. Solid castors are fine on a yard and useless on a construction site.

A roll frame protects the engine and pump from the treatment machines receive in the back of a vehicle, and provides lifting points. On sites where everything is craned into position, certified lifting points are a requirement rather than a convenience.

Skid mounting suits machines that live on a vehicle bed or in a container and are used where they sit, with a long hose doing the travelling.

Weight decides whether one person can handle the machine. Anything much beyond comfortable two-person lifting needs wheels, a tail lift or a crane, and that has to be planned rather than discovered.