Water expands by roughly nine per cent when it freezes. A pump head is a rigid, sealed space with water trapped inside it.
That is the entire problem, and it explains the single most common avoidable pressure washer failure in the UK. Not wear. Not misuse. One unexpectedly cold night in an unheated van or an outbuilding, with a few hundred millilitres of water still sitting in the pump.
The repair is expensive and it is very often not covered by warranty, because it is preventable.
Where the water hides
After use, a machine looks empty. It is not. Water stays in:
- The pump chambers, which is where the damage happens
- The inlet and outlet manifolds
- The unloader valve, a complex casting with small internal passages
- The hose, the lance and the gun
- The detergent injector and any chemical lines
- The heating coil, on hot water machines, which holds a considerable volume
- The inline filter housing
Draining the hose and standing the machine upright does not empty the pump. That is the bit people get wrong.
Protecting it properly
There are two approaches, and one of them is far more reliable.
The reliable method: antifreeze through the pump
This is what any commercial user should do, and it takes about five minutes.
- Finish the job, then release the pressure by squeezing the trigger with the machine off and the water disconnected.
- Disconnect the water supply and the high-pressure hose.
- Put a container of pump antifreeze, or a suitable non-toxic glycol solution, at the inlet.
- Run the machine briefly so it draws the antifreeze through until it appears at the outlet.
- Stop, and store it.
The antifreeze occupies the space water would have, and it also leaves a film that protects seals from drying out.
Use a proper pump antifreeze. Automotive coolant is not always compatible with pump seals, and screenwash is mostly water.
The basic method: drain and blow through
Better than nothing, less reliable.
Disconnect everything, release the pressure, tip the machine to drain what will come out, and blow low-pressure compressed air through the inlet to clear the passages. Drain the hose, the lance and the gun separately.
It works if it is done thoroughly. It fails when somebody is in a hurry, which in November is most of the time.
The simplest method of all
Keep the machine somewhere that does not drop below freezing. A heated workshop, a utility room, an insulated garage. If that is genuinely available, it beats both of the above.
A van is not frost protection. Vans get colder inside overnight than most people assume, and they are where the majority of frozen pumps happen.
The winter storage routine
If the machine is going away until spring, do a little more than frost protection.
Change the pump oil first. Old oil holds moisture and acids, and leaving that sitting against internal surfaces for four months causes corrosion. Fresh oil before storage is better than fresh oil in March.
Clean and dry the nozzles. Deposits harden over a winter and are far harder to shift later.
Rinse the detergent system through with clean water so chemicals are not sitting in the injector for months.
Coil the hose loosely, in large loops. Tight coils in the cold set a permanent memory into the hose.
Store it off the floor, covered but not sealed in plastic. Sealed plastic traps condensation.
Leave the trigger unlocked and pressure released so the gun's internal spring is not compressed all winter.
If it has already frozen
Do not switch it on. This is the important one. If ice is inside the pump and you run the motor, you turn a frozen pump into a broken pump, and you may take the motor with it.
Do not apply direct heat. A blowtorch or heat gun on a pump head cracks castings through thermal shock and can damage seals.
Instead, move the machine somewhere warm and let it thaw slowly and completely. Several hours, or overnight for a large machine. Then, before starting, connect water, turn the trigger and check that water flows freely through. Start it and listen: rattling, pulsing or a lack of pressure means something inside has been damaged and it needs looking at.
Check for external cracks and weeping around the pump head and the unloader while it is running.
Frequently asked questions
Will freezing damage a pressure washer?
Yes, frequently and expensively. Water expands as it freezes and the pump head is a rigid sealed casting, so trapped water can crack the housing, split valves and destroy seals. It is one of the most common pressure washer failures in the UK and it is usually excluded from warranty because it is preventable.
How do I winterise a pressure washer?
Release the pressure, disconnect the water and hose, then draw pump antifreeze through the machine until it appears at the outlet, and store it. Drain the hose, lance and gun separately. If you are storing it for the season, change the pump oil first, rinse the detergent system, and keep it off the floor and covered but not sealed in plastic.
Can I use car antifreeze in a pressure washer?
Use a purpose-made pump antifreeze rather than automotive coolant, because coolant is not always compatible with pump seals and some formulations attack them. Screenwash is not suitable either, since it is mostly water and offers little protection at the concentrations sold.
What do I do if my pressure washer has frozen?
Do not switch it on, and do not apply direct heat. Move it somewhere warm and let it thaw slowly and completely, which can take several hours. Then connect water, confirm it flows freely through the machine, and start it while listening for rattling or pulsing and checking for weeping around the pump head.
Is it enough to just drain the hose?
No. Draining the hose leaves water in the pump chambers, the manifolds and the unloader valve, which is where freezing does its damage. The hose is the easy part. Antifreeze drawn through the pump, or thorough compressed-air purging, is what actually protects the machine.
The bottom line
Five minutes with a container of pump antifreeze in October is the cheapest maintenance you will ever do.
Frost damage is not bad luck. It is a machine that was put away wet, once, on a night nobody was expecting.
Kränzle UK stocks pump antifreeze, pump oil and every spare in the range, with in-house servicing if the worst has already happened. Talk to us before the first frost, not after it.