A buffer tank is a water storage container that sits between your water source and your pressure washer pump, ensuring the machine always has enough water to operate safely. Most professional pressure washers consider it essential equipment because residential water supplies rarely deliver enough flow to keep up with a commercial machine’s demand.
Why Your Water Supply Isn’t Enough
Pressure washers are rated by how many gallons per minute (GPM) they consume. A professional-grade machine typically runs between 4 and 8 GPM. The problem is that most residential outdoor spigots only deliver around 3 to 5 GPM, and that number drops further with long garden hoses, older plumbing, or multiple fixtures running inside the house. In some areas, supply can be even lower and wildly inconsistent from one job site to the next.
Without a buffer tank, your pump tries to pull water faster than the spigot can deliver it. This creates a deficit that starves the pump of water, and the consequences range from annoying pressure drops to serious mechanical damage. A buffer tank solves this by collecting water from the spigot continuously and holding a reserve. Your pump draws from that reserve instead of directly from the hose, so even if the spigot only delivers 3 GPM and your machine needs 5, the tank covers the gap for as long as the stored water lasts.
How a Buffer Tank Protects Your Pump
The most important reason to use a buffer tank is preventing cavitation. When a pump can’t get enough water, the pressure inside the pump head drops so low that the water essentially begins to boil at room temperature, forming tiny vapor bubbles. Those bubbles collapse violently when they hit higher-pressure areas inside the pump, releasing concentrated bursts of energy against internal surfaces. Over time, this pits and erodes the pump’s components. Cavitation is one of the leading causes of pump failure, and it’s entirely preventable by keeping the inlet side of the pump consistently fed with water.
A buffer tank also helps with heat management. When you release the trigger on your spray gun, most pressure washers recirculate water through an internal bypass loop. That recirculating water heats up quickly. With a buffer tank plumbed correctly, the bypass line routes water back into the tank instead of through a tight internal loop. The larger volume of water in the tank absorbs that heat, keeping the pump from overheating during pauses between spraying.
What Size Tank You Need
Tank sizing depends on two variables: your machine’s GPM rating and how much water the job site’s spigot actually delivers. The difference between those two numbers is your deficit, and the tank needs to hold enough water to cover that deficit for however long you plan to spray continuously.
Here’s a practical example. Say you run an 8 GPM machine and the spigot provides 4 GPM. That’s a 4 GPM deficit. A 200-gallon tank would give you about 50 minutes of continuous trigger time before running dry, which is more than enough for most tasks since you’re rarely holding the trigger nonstop. If the same spigot delivered 7 GPM, that deficit shrinks to 1 GPM, and the same 200-gallon tank would last over three hours.
Most professionals running machines in the 4 to 8 GPM range use tanks between 100 and 225 gallons. A 225-gallon “leg tank” (a flat, rectangular tank designed to fit in a truck bed or on a trailer) is one of the most popular choices. Some operators find that 100 gallons can be tight for an 8 GPM machine, especially when pulling water through a long supply hose. If you’re running 4 GPM or less, a smaller tank in the 35 to 65 gallon range may be sufficient.
Key Components of a Buffer Tank System
The tank itself is typically made from thick polyethylene plastic. It’s durable, lightweight relative to metal, and resistant to the chemicals commonly used in pressure washing. Beyond the tank, a few components make the system work:
- Float valve: This sits inside the tank and works like the fill valve in a toilet. As the water level drops, a buoyant float descends, opening a valve that lets fresh water flow in from the garden hose. When the tank refills, the float rises and shuts the valve off. This keeps the tank topped up automatically so you never have to stop and wait for it to fill manually.
- Bulkhead fittings: These create leak-proof pass-throughs in the tank wall for your inlet and outlet plumbing. They typically consist of two threaded or slip-fit pieces that clamp on either side of a drilled hole. Uniseal rubber grommets are a popular alternative, especially on round barrels, because they flex to conform to curved surfaces.
- Suction hose: This connects the tank’s outlet to your pump’s inlet. The hose diameter matters. Machines running 4 GPM or more need at least a 3/4-inch or 1-inch suction hose to avoid restricting flow. A hose that’s too narrow creates the same starvation problem you’re trying to avoid. Keep this hose as short as possible.
- Bypass line: A hose running from the pump’s bypass port back into the tank. This returns recirculated water to the tank when you’re not pulling the trigger, keeping the pump cool.
Weight and Transport Considerations
Water weighs about 8.3 pounds per gallon. A full 225-gallon tank adds roughly 1,870 pounds to your truck or trailer before you count the weight of the tank itself, your machine, hoses, and chemicals. That’s a significant load, so you need to know your vehicle’s payload capacity before choosing a tank size. Many operators mount the tank low on a trailer to keep the center of gravity stable, or they use leg tanks that distribute weight across the truck bed.
Some professionals fill the tank on-site from the customer’s spigot and travel with it empty or only partially full. Others pre-fill at home or at a fill station for jobs where spigot access is limited, like commercial properties or post-construction cleanups where the water isn’t turned on yet. Having a full buffer tank also means you can work at locations with no water source at all, though your working time is limited to however many gallons you brought.
When You Can Skip the Buffer Tank
If you’re using a small residential pressure washer rated at 2 GPM or less, most garden hoses will keep up without any trouble. Light-duty electric machines and entry-level gas units fall into this category. You can connect the garden hose directly to the pump inlet and work without issues, assuming the hose isn’t excessively long or kinked.
Once you move into machines rated at 3.5 GPM and above, a buffer tank goes from optional to strongly recommended. At 4 GPM and higher, it’s effectively mandatory for reliable operation. Even if a particular job site has strong water pressure, the next one might not. A buffer tank removes that uncertainty entirely and protects a pump that likely cost you several hundred to several thousand dollars to replace.