Your pressure washer has a safety device known as an "unloader". This device is located on the high-pressure side of the pump and usually has a large spring or black plastic handle or adjustable nut on it.
The function of the unloader is to relieve excess pressure before your pump blows up. While that sounds dramatic, think about your system for a moment. You have lots of horsepower driving a pump that is creating three to four thousand PSI of pressure. When the gun is open, the water flows and it does its work. When you close the trigger gun, where does all that pressure go? The unloader is the automatic valve that opens and allows the water to move within the system even when the gun is closed.
Depending on the design of the unloader valve, it will sense either the build-up of pressure or the lack of water flow. When the valve opens, the water is allowed to move to the low-pressure side of the pump. The valve stays open until you pull the trigger ? which releases the pressure or restarts the flow. With the valve is open (when it is in by-pass mode) the water simply recirculates from the high-pressure side (outlet) to the low-pressure side (inlet). This is a short-term fix for a problem, and the unloader will only protect the equipment for a reasonably short period of time. Left to idle with the trigger gun closed, the water inside your pump keeps getting hotter and hotter. If you are lucky, your machine will also have a thermal relief valve, which will blow and let the water escape ? allowing cool water to once again enter your pump.
Enough damage to the pump occurs in just a few minutes of letting your pressure washer idle with the trigger gun closed to put a big dent in your paycheck. Three minutes of idling is enough to cause extensive damage to seals and packings. Five minutes can even cost you the pump itself.
The thermal shock of allowing cold water into a pump that has become super-heated this way will often crack the ceramic pistons and generally damage the wet end, and replacement parts and labor can be as expensive as buying a whole new pump. (It?s the same story as a car, where you can buy a car for $20,000 or build it from parts for $150,000. That?s just what buying all of the replacement parts can seem like.)
I am often asked to suggest the "right" unloader for a contractor?s machine. This is one of those personal preference questions, and I hate to suggest one brand or type over another. I prefer that the operator select the type and brand of unloader he wants to work with. Here is some information to help you make an informed decision.
Pressure-type (or "trapped pressure") unloaders sense the pressure in the system and open when pressures hit a certain point. It is called a trapped pressure unloader because the pressure is trapped throughout the system. The hose and trigger gun are still under full pressure even when the unloader valve opens. When you pull the trigger, there is an instant burst of highly-pressurized water ready to go. Operators feel a lot of shotgun-style kick-back when the trigger is pulled on one of these systems. Trapped pressure unloaders are by far the most common found, and a typical example is the YU-2140 by General Pump ? a.k.a. the "green spring" unloader.
A variation on the trapped pressure design is the compensating type unloader. This is essentially a trapped pressure unloader with a compensating design that limits the burst of pressure when the trigger gun is opened. Typically, a compensating unloader limits pressure in the system to between 10% and 25% of full pressure. One of the most popular compensating unloaders is the VB10 by PA, which is rated at 3600 PSI but drops the pressure in the system to around 400 PSI when in bypass.
Flow unloaders sense a lack of water flow and respond by opening the unloader valve. In this style of unloader, no pressure is trapped in the system with the trigger gun closed. There is no big burst when the trigger gun is opened, either. Pressure usually comes back slowly over several seconds. The most common flow unloader seen today is the K-7 by General Pump.
Unloaders on fixed equipment (like a skid unit on a trailer) are often plumbed right back to the water tank. Equipment that is fed from a jobsite source (like a water hose) doesn't have that luxury. Most unloaders installed by the factory on portable pressure washers will have a short (12" or so) hose running from the high-pressure side to the low-pressure side. That 12" hose cannot hold very much water, and cannot keep the pump from being damaged in a continuous bypass situation for very long.
A friend of mine who builds and sells equipment on the west coast showed me an interesting twist on unloader installation. He re-plumbs the unloaders for his custom-made machines with a 5? or 6? length of hose that loops on the underside of the chassis. This is just one better way to protect your equipment from having a very bad day.