On greasy soils, the difference is between removing it and moving it - hot water typically cuts both the time and the chemical needed by a large margin.

Fats and oils are held to a surface by a film that softens with temperature. Once softened, ordinary working pressure lifts it cleanly. A cold jet has to rely on detergent and mechanical force alone, and the usual result is a thin film spread over a wider area.

On non-greasy soils - mud, moss, dust, mortar - hot water buys very little. The adhesion is mechanical and pressure alone deals with it, so the extra capital and fuel return nothing.

There is a secondary benefit that matters in food and healthcare settings: heat contributes to reducing surface bacterial load, though it does not replace a validated disinfection step.

Surfaces also dry noticeably faster after hot washing, which shortens the time a floor is a slip hazard and out of use.

Warm water for light grease, near-boiling for heavy oil and food fats, and steam-range output only for baked-on carbon and the worst industrial deposits.

Light films - traffic grime, cutting fluid mist, kitchen surfaces cleaned regularly - release at modest temperatures well below boiling, and running hotter than necessary simply burns fuel.

Heavy grease, animal fat, engine oil and long-neglected workshop floors want the machine near the top of its normal range. This is where most industrial hot machines spend their working lives.

Steam-capable output, where the water flashes to vapour at the nozzle, is a specialist setting for carbon deposits, heavy bitumen and tenacious residues. Flow drops substantially in steam mode, so it cleans a small area intensively rather than covering ground.

Match the setting to the job. Operators left to their own devices run everything at maximum, which costs fuel and can damage heat-sensitive surfaces and coatings.

Diesel for anywhere with ventilation and no huge electrical supply; electric heating for food production, enclosed spaces and anywhere combustion products are unacceptable.

A diesel or paraffin burner produces a great deal of heat from a modest fuel supply, which is why it dominates. It needs combustion air, produces exhaust, and requires the flue to discharge safely - so the machine cannot simply be wheeled into a closed room.

Electrically heated machines have no exhaust and no fuel handling, and can be used in clean rooms, food halls and basements. Heating water electrically at any useful flow rate takes a very large supply, which is the practical limit and usually confines them to fixed installations.

Some sites solve it differently: a stationary system heats water centrally and feeds cold-machine outlets around the building.

Local emissions and indoor air quality rules may decide this for you, particularly in food manufacturing - check before specifying.

Descale the coil on a schedule set by water hardness, service the burner and its nozzle annually, and never shut down hot without running water through the coil.

Scale forms inside the heating coil wherever hard water is heated. It insulates the coil, so the burner works harder for less output, and eventually blocks flow and splits the tube. Descaling is a routine chemical procedure and its interval depends entirely on local water hardness - soft-water sites may go years, hard-water sites months.

The burner has a fuel nozzle, electrodes and a filter, all of which foul. A burner running rich sooty deposits onto the coil, which insulates it further and shows up as smoke and poor temperature.

Shutting down matters. Stopping the machine while the coil is hot with no flow bakes residue inside it. Run cold water through for a minute or two before switching off - many machines have a cool-down cycle for this.

Fuel quality and cleanliness matter more than on a vehicle; water or dirt in the fuel shows up as a burner that will not light.

An electrically heated one, yes. A fuel-burning one only with proper ventilation or flue extraction, never in a closed space.

A burner consumes oxygen and produces carbon monoxide and other combustion products. In an enclosed room this is a poisoning risk that develops faster than most people expect, and it has killed operators.

Where a fuel-fired machine must be used inside, the usual arrangement is to site the machine outside or in a plant room and run the hose in, or to duct the flue to outside air.

Steam and heavy water vapour indoors bring their own problems - condensation on cold surfaces, reduced visibility and slippery floors.

Electrically heated machines sidestep all of this and are the standard answer in food factories, commercial kitchens and any space where combustion is not acceptable.

It can - paint, some plastics, sealants, vinyl and heat-sensitive coatings all react to temperature in a way they do not react to pressure.

Coatings soften. A paint film that survives a cold jet may blister or lift under hot water, particularly where the coating is already aged or poorly bonded.

Plastics distort. Trim, signage, guttering, some cladding panels and vehicle components can deform, and adhesive-fixed items let go when the adhesive softens.

Thermal shock matters on glass and on cold masonry. Hitting a cold surface with a hot jet, or the reverse, can crack it.

The mitigation is the same as for pressure: test an inconspicuous area first, use the lowest temperature that works, and keep the lance moving. Operators trained only on cold machines routinely underestimate this.

Substantially higher per hour on fuel and maintenance, and frequently lower per job once chemical, labour and repeat passes are counted.

The direct costs are unavoidable: burner fuel, coil descaling, burner servicing and a higher purchase price. On a like-for-like hourly basis a hot machine is clearly more expensive to run.

The offsets are in the work rather than the machine. Greasy jobs that took three passes take one. Detergent consumption falls sharply. Surfaces dry faster, so areas return to service sooner - which in a commercial kitchen or a production hall is often the largest number of all.

The calculation flips on the proportion of greasy work. A fleet washing operation or a food plant recovers the difference quickly; a groundworks contractor washing mud off plant does not.

Where greasy work is occasional, hiring a hot machine for those jobs is usually cheaper than owning one.