# What is PSI?

Water pressure from the pressure washer is measured in pounds per square inch or PSI. This pressure is the amount of force delivered, and in the case of a pressure washer, to the surface being cleaned and is the critical factor in breaking debris from that surface.

## How is PSI Calculated?

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. Nozzle selection depends on the work, and the amount of pressure a surface can withstand before it is damaged.

If you are looking to rent a pressure washer from us, please don’t hesitate to ask what nozzle you should be using! Our team of experts can give you recommendations for washing your driveway, siding or event outdoor furniture.

### How Close Should I Stand to the Surface I’m Power Washing?

While PSI is a constant at the tip of the nozzle, the pressure decreases as its distance from the targeted surface increases. Experienced power wash contractors understand how to manipulate these distances for maximize both cleaning power and their time. The pressure that contractors need varies a great deal, so 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.

### Does Temperature Matter?

Choosing a hot water washer or a cold-water washer will depend on the surface you are looking to clean. Heat provides a tremendous advantage when cleaning grease and oil. A few Vent-a-Hood cleaners will use steam heated to 310⁰ F, while others will use units that deliver 3,000 PSI at 200⁰ 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).

### What PSI Is Needed for My Project?

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 sandpaper 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 do the work helps avoid furring. Test the effect of pressure on the underside of the deck.