The short answer
Hot water is worth it when the thing you are removing is fatty, oily or sticky. Cooking grease on a restaurant patio, years of engine drips on a garage floor, chewing gum on a storefront sidewalk and the greasy film on a dumpster pad all come off faster and more completely with heat.
For most residential work it adds little. Dust, pollen, tree litter, mildew and algae respond to chemistry and dwell time, not temperature. A house wall, a stucco courtyard or a shaded walkway with dark organic staining is a cold water job done right.
Why heat works on grease
Think about washing a greasy pan. Cold water slides over the fat. Hot water softens it so soap can surround it and carry it away. Pressure washing works the same way. Heat lowers the stickiness of oils and waxes, loosens gum from the surface, and speeds up the reaction of degreasers so they break the soil down in less time.
Organic growth is different. Mildew and algae are living films. They are killed and lifted by a cleaning solution applied at low pressure and given time to work. Heating the rinse water does not change that much, which is why a soft wash on a house usually runs cold.
Where hot water earns its keep
Commercial food areas are the clearest case: restaurant patios, kitchen back doors, drive-thru lanes and dumpster pads. Grease gets tracked out and baked in by the sun, and cold water tends to smear it. Gas station aprons, loading docks, fleet vehicles and parking spots with heavy oil drips are similar.
Gum is another. Heat softens it so it releases from concrete with less pressure, which also means less risk of etching the surface around it. On a home, hot water can help a garage floor or a driveway spot where a car has leaked for years, though deep oil that has soaked into concrete may only lighten, hot or cold.
Fleet washing is a mixed case. Road film on trucks and vans is oily, so warm water and the right detergent help, but painted panels and decals still need a gentle, wide spray held at a safe distance.
Where it adds risk instead of results
Heat does not belong on every surface. Painted trim, vinyl siding, some sealers and coatings, and older or soft wood can be damaged or dulled by hot water with little cleaning benefit. Roofs and stucco are cleaned with a low pressure soft wash, and temperature is not what makes that work.
Hot water can also flash dry on a warm surface. In a Valley summer, concrete in full sun can be hot enough that cleaner and rinse water evaporate quickly, which can leave residue or uneven patches. Working in sections, keeping surfaces wet and scheduling around the afternoon heat matter more than water temperature.
The tradeoffs in cost and time
Hot water units burn fuel to heat the water, weigh more and take more upkeep than cold water machines. That is why not every job uses one and why a grease-heavy commercial job is often priced differently from a house wash. Heat can shorten the time on a greasy surface, but it never replaces pre-treatment, dwell time and a careful rinse.
Grease also raises a water handling question. Los Angeles Municipal Code section 64.70.02 prohibits discharging untreated wash water from paved surface washing and mobile operations into the storm drain system unless a permit allows it. Greasy runoff should be contained and handled, not rinsed into the gutter, regardless of its temperature.
Heat also does not change the result on stains that are really discoloration rather than residue. Rust, fertilizer marks and tannin from leaves are chemical stains. They need a cleaner made for that stain, and hot water alone will not lift them.
What to ask a contractor
Ask whether the job calls for hot water and why. A good answer names the soil: grease, oil, gum. Ask what degreaser or cleaner they use, how long it dwells before rinsing, and how they control runoff near drains. Ask what result to expect on old stains, because some will lift and some will only fade.
At Say Wash It we match the method to the surface and the stain instead of using one setup for everything. If you want a starting number for your property, the instant quote on this page gives you one, and the flat price is confirmed with you before work begins.
Key points
The short answer
Hot water is worth it when the thing you are removing is fatty, oily or sticky. Cooking grease on a restaurant patio, years of engine drips on a garage floor, chewing gum on a storefront sidewalk and the greasy film on a dumpster pad all come off faster and more completely with heat.
Where it adds risk instead of results
Heat does not belong on every surface. Painted trim, vinyl siding, some sealers and coatings, and older or soft wood can be damaged or dulled by hot water with little cleaning benefit. Roofs and stucco are cleaned with a low pressure soft wash, and temperature is not what makes that work.