How a Pressure Washer Unloader Valve Works

Updated: Jun. 23, 2023

Pressure washers have lots of parts. Discover what the unloader valve is, what it does and why it's so important.

A really good pressure washer is a delight to own. It’s useful in no small part to an important little device called an unloader valve (shown here).

Even if you’re only an occasional pressure washer user, it’s worth knowing what an unloader valve is and how it works. Pressure washer unloader valves are one of the few parts I’ve never personally seen fail, but I know people who have. Here, I’ll explain why unloader valves matter.

What Is a Pressure Washer Unloader Valve?

A pressure washer unloader valve regulates water pressure and flow rates. Factory installed on all pressure washers, this important feature keeps internal pressure from climbing too high while the machine is running. Without one, pressure washers could easily build enough water pressure to damage or destroy it within a minute or less.

Most factory-installed unloader valves I’ve seen are built-in to save space and make the machine less bulky. However, I have also seen high-quality aftermarket externally mounted unloader valves. These have a few advantages over their built-in counterparts. More on that later.

How Does a Pressure Washer Unloader Valve Work?

Whether your pressure washer has a built-in or externally mounted unloader valve, the same principle applies. While the machine runs, water pressure builds to a maximum level, determined by the unloader valve’s setting. The instant pressure reaches that level, the unloader valve switches the pump to bypass mode.

When this happens, pressurized water is redirected from the hose and spray nozzle through the pump’s pressure head in a continuous, fast-moving cycle. This prevents pressurized water from building up in the hose and nozzle, where it could cause damage and pose a serious safety hazard. It’s a great system, but it doesn’t totally alleviate the danger.

The massive friction and speed of the water as it travels through the pump in bypass mode generate lots of heat. If you run your machine too long without spraying some water to switch out of bypass mode and reduce the water speed, you’ll damage the pump. That’s why most manufacturers recommend pulling the trigger at least once every 30 seconds or so.

Built-in vs. externally mounted unloader valves

Built-in unloader valves are standard for most pressure washers. The valve mechanism inside the pump casing can’t easily be accessed or changed.

Externally mounted unloader valves, however, are self-contained devices that attach to the outside of the pump casing. This makes them much easier to find and replace if needed.

Some external unloader valves come with hoses as part of the mechanism, giving the pressurized water more distance to travel when the pump is in bypass mode. This reduces heat buildup within the pump, making heat damage less likely.

How To Adjust a Pressure Washer Unloader Valve

Pressure washer unloader valves can be adjusted by rotating the large plastic handle protruding from the valve assembly.

Before you try it though, understand that most pressure washers come with their unloader valves factory set to the optimal point — but not all. Before checking yours, it’s important to understand the goal of unloader valve adjustment: To get the greatest possible pressure from your washer to increase cleaning power.

To adjust your unloader valve properly, start by installing a pressure gauge attachment between the spray wand and the high-pressure hose of your machine. Take note of the pounds per square inch (psi) rating of your washer, then fire it up and spray some water. Watch the pressure gauge as you spray, taking note of the pressure reading.

Release the trigger, then watch the pressure gauge closely. It will spike to a higher pressure than it was while you sprayed.

Rotate the unloader valve handle in quarter-turn increments, then repeat the spray test. Aim for a spray pressure 50 to 100 psi below your washer’s max psi rating, and a spike pressure 300 to 400 psi higher than that. Achieving this means your unloader valve is set correctly. Don’t exceed the max-rated pressure or you could severely damage your pressure washer.

How To Know When a Pressure Washer Unloader Valve Needs Replacement?

A failing pressure washer unloader valve manifests itself in a few ways, the most common being your machine building little or no water pressure.

When the internal ball check valve that makes unloader valves work can’t move due to corrosion or stiffness in the mechanism, the water entering your pump will flow through a relief port and won’t be pressurized. This could be an easy fix, or your unloader valve might need to be replaced.

Either way, unless you’re comfortable pulling apart and reassembling the small intricate parts of your pressure washer, taking it to a pro is probably your best bet. I know how to do these sorts of repairs, but often I hire a pro simply to save time.

Other less common signs are steam coming from the thermal relief valve or a machine that runs poorly or stalls.