Water pressure from the pressure washer is measured in pounds per square inch or psi. This pressure is the amount of force delivered to the surface being cleaned, and is the critical factor in breaking debris from that surface. PSI is determined by the orifice size of the nozzle tip and the flow rate (gallons per minute). For labeling purposes, the standard nozzle size for measurement is a #4 orifice, which delivers 4.0/gpm at 4,000psi. As nozzle sizes increase or decrease, psi fluctuates accordingly. The nozzle chart in our catalogue lists the psi with different flow rates and nozzle sizes. Nozzle selection depends on the work, and the amount of pressure a surface can withstand before it is damaged.
While psi is a constant at the tip of the nozzle, the pressure decreases as its distance from targeted surface increases. Experienced power wash contractors understand how to manipulate these distances for maximize both cleaning power and their time.
Because the pressure that contractors need varies a great deal, they get the most out of their equipment by adjusting the distance between the nozzle and the surface being cleaned. For less pressure and heat in the application, they hold the nozzle back, thereby increasing the distance from the surface. For increased pressure and heat, they hold the nozzle closer. Larger contrast cleaners will use pressure washers that produce 4 to 6 GPM at 3,000 to 3,500psi at 200°F for concrete cleaning. Vehicle washing can be accomplished using 1,500 to 2,000psi.
Heat provides a tremendous advantage when cleaning grease and oil. A few Vent-a-Hood cleaners will use steam heated to 310°F, while others use units that deliver 3,000psi at 200°F. However, some vent cleaners use electric, 1,000 PSI cold water washers hooked up to hot water (they must hand scrape more for this to be effective).
The PSI for decks and other wood surfaces varies from 200 to 3,000 and must be carefully tested first, as too much pressure will cause the wood to fur. If this happens, you will need to sand the affected areas with fine sand paper or steel wool to knock off the furred surface. Many professional deck cleaners use a variable pressure wand (like the ST-54 36" Double Lance Wand) so they can adjust the pressure as necessary. Using low pressure, and letting the chemical (like Powerwash.com's DSR-49 Deck and Sliding Restorer or DSR-50 Deck and Sliding Restorer plus Stripper) do the work helps avoid furring. Test the effect of pressure on the underside of the deck.