PSI versus flow rate, when high pressure is appropriate, and when high pressure causes damage you cannot undo.
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Pressure breaks the bond between dirt and surface. Flow carries it away and spins the surface cleaner. A 4000 psi machine at 8 LPM is slower in real life than a 3000 psi machine at 15 LPM.
It falls off rapidly with distance and spreads with nozzle angle. A 25-degree tip at 100mm is aggressive; the same tip at 400mm is gentle. Your hand controls the pressure more than the dial does.
Concrete, brick pavers, aggregate, stone flatwork, hardstand and steel. These are hard, homogeneous and forgiving — as long as you keep the distance consistent and the deck moving.
Render, painted surfaces, roof tiles, timber, sandstone, fibre cement, windows and seals. All of these are soft, coated or brittle — chemistry and time do the work, not pressure.
Etching and lines on concrete, stripped coating on render, forced water into a wall cavity, cracked or dislodged roof tiles, furred timber, and blown window seals. Every one is permanent and every one is on you.
Somewhere inconspicuous, at the gentlest setting that might work, then escalate. Thirty seconds that prevents the only kind of mistake a customer never forgives.
This is the whole philosophy of professional exterior cleaning. Pressure is the last resort, not the first.
Cleaning power is a function of pressure and flow together. Beginners buy pressure. Professionals buy flow.
Pressure cuts. It breaks the bond between the dirt and the surface. Beyond a point, more pressure just damages the substrate — it does not clean faster.
Flow carries. It moves the loosened dirt away and rinses the surface. Flow is what makes a machine fast.
Two machines, same 3000 psi: one at 8 L/min, one at 15 L/min. The 15 L/min unit cleans a driveway in roughly half the time. Same pressure, double the productivity — that is your hourly rate.
A rough comparison figure: PSI x LPM = cleaning units. It lets you compare machines on one number. - Domestic big-box unit: 2000 psi x 7 L/min = 14,000 - Entry prosumer: 3000 psi x 10 L/min = 30,000 - Working commercial: 4000 psi x 15 L/min = 60,000
Petrol for mobile work — you cannot rely on a customer's power, and 15 L/min needs more than a domestic circuit can give. Electric only for indoor, enclosed or noise-restricted commercial work.
Hot water dissolves grease and halves the chemical needed on organic growth. It is also expensive, heavy and only necessary for commercial food, grease and industrial work. Not a first purchase.
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