Most pressure washer pumps that fail early do not fail because of what comes out of the lance. They failed because of what went into the inlet.
A pump is a positive displacement device. It expects a full charge of water on every stroke, thousands of times a minute. Give it less, and it does not simply run slower. It runs badly, gets hot, and destroys itself from the inside.
This is the least glamorous page you will read about pressure washers, and it is the one that decides whether yours lasts two years or twenty.
The number that matters is flow, not pressure
Your machine has an inlet flow requirement, usually quoted in litres per minute. Your supply must deliver more than the machine consumes, not the same amount.
Measuring it takes two minutes. Put a bucket of known volume under the tap you intend to use, open it fully, and time how long it takes to fill. A ten-litre bucket filling in forty seconds is fifteen litres a minute.
Do that at the tap you will actually use, with the hose you will actually use, at the time of day you will actually work. All three change the answer.
Common reasons a supply underperforms:
- A long, narrow garden hose. A 12mm hose over 25 metres delivers far less than a 19mm hose over 10 metres.
- Kinks, coils left on a reel, and quick-connectors that neck the bore down.
- An outside tap teed off a small-bore pipe run.
- Shared demand, so pressure drops when something else in the building draws water.
- Partly closed isolation valves nobody has touched in years.
What cavitation actually does
When the pump cannot draw a full charge, water vaporises inside the pump and forms bubbles. Those bubbles then collapse violently as pressure rises. That is cavitation, and it is astonishingly destructive.
The collapse happens against valve seats, plunger surfaces and seals. Over time it pits metal, hammers valves and shreds seals.
How to recognise it: an irregular rattling or gravelly noise from the pump, a pulsing lance where pressure surges and drops, and a machine that sounds like it is working harder than it should. If a machine that used to be smooth is now noisy, look at the water supply before you look at the pump.
Feeding from a tank
Mobile detailers, agricultural users and anyone working away from mains water will feed from a tank. It works well, and there are three rules.
Gravity is your friend. Site the tank outlet above the machine's inlet where you can. A pump that has water pushed into it is far happier than one having to pull.
Size the outlet generously. A 15mm tank fitting will strangle a machine that wants 15 litres a minute. Use the largest outlet and the shortest, widest hose you can.
Keep the tank clean and covered. Sediment, algae and grit all end up in the pump. A covered tank stays clean; an open one grows things.
If the machine still struggles on gravity feed, a small booster pump on the tank outlet solves it, and costs far less than a pump rebuild.
Filtration is the cheapest insurance available
Water carries grit, rust flakes and scale, and every particle passes through the valves and past the seals.
Fit an inline filter at the inlet. It is an inexpensive part that catches the debris that would otherwise embed itself in a valve seat. Check it regularly and clean or replace it, because a blocked filter creates exactly the starvation problem described above.
Most machines have a small inlet strainer built in. Treat it as a last line of defence rather than a substitute for a proper filter, and pull and rinse it every few weeks.
Hard water deserves a mention. In hard water areas, scale builds inside the pump and on any heating coil. Machines used with hot water suffer most. Periodic descaling, and softened or filtered water where practical, extends life considerably.
Never run it dry, and never let it sit under pressure
Two habits that cause a large share of avoidable damage.
Running dry, even briefly, destroys seals. They rely on water for lubrication and cooling. Open the water and let the machine fill and purge before switching on, and switch off before closing the water.
Leaving it in bypass for long periods. When you release the trigger, most machines circulate water internally rather than stopping. That water heats up. A few minutes is fine. Twenty minutes of bypass while you move a ladder cooks the seals. If you are pausing, switch it off.
The five-minute pre-use check
Worth doing on any machine that earns money.
- Full-bore tap, fully open, before the machine goes on
- Hose uncoiled from the reel, not left in a loop
- Inlet filter checked and clean
- Water flowing and the machine purged of air before starting
- Pump oil level checked at the sight glass
Frequently asked questions
Why is my pressure washer pulsing or surging?
Almost always inadequate water supply. The pump cannot draw a full charge, so pressure rises and falls with each stroke. Check the flow rate at the tap with a bucket and a timer, uncoil the hose fully, check the inlet filter for blockage, and confirm no other demand is drawing on the same supply.
Can I run a pressure washer from a water tank?
Yes, and it is standard practice for mobile work. Site the tank so the outlet sits above the machine's inlet, use the largest outlet fitting and the shortest, widest hose you can, and keep the tank covered so sediment and algae stay out. Add a small booster pump if the machine still struggles on gravity feed.
What is cavitation and how do I know if it is happening?
Cavitation is vapour bubbles forming inside the pump because it cannot draw enough water, then collapsing violently and eroding valves, plungers and seals. Signs are a rattling or gravelly pump noise, pulsing at the lance, and a machine that suddenly sounds rougher than it used to.
Do I need a water filter on a pressure washer?
Yes, if you want the pump to last. An inline filter at the inlet catches grit, rust and scale that would otherwise damage valve seats and seals. It is one of the cheapest parts you can fit. Check and clean it regularly, because a blocked filter starves the pump and causes the problem it was fitted to prevent.
Is it bad to leave a pressure washer running without spraying?
Yes, beyond a few minutes. With the trigger released, the machine circulates water internally in bypass, and that water heats up. Prolonged bypass overheats and hardens the seals. Switch the machine off if you are pausing for more than a couple of minutes.
The bottom line
Everything upstream of the machine decides how long the machine lasts.
Measure the flow with a bucket, use a fat short hose, fit a filter and keep it clean, never run it dry, and do not leave it sitting in bypass. That is the whole job, and it costs almost nothing.
Kränzle UK stocks inline filters, hoses and fittings alongside the full machine range, and services what we sell. Talk to us about your water supply before you choose a machine, not after.