Can You Use Rainwater in a Pressure Washer?

Yes, you can run a pressure washer on rainwater, and plenty of people do it through a hosepipe ban or simply to keep the water bill down. The catch is that it only works reliably if you sort out two things first: how clean the water is, and how it gets into the machine. Get either one wrong and the pump pays for it.

What actually goes wrong with rainwater

A pressure washer pump is not a rugged bit of kit inside. It relies on ceramic or steel plungers moving through seals at high speed, plus a set of small inlet and outlet valves. Everything in there is machined to tight tolerances, and it depends on clean water for both lubrication and cooling.

Rainwater off a roof carries grit, moss fragments, silt, and whatever the gutters have collected since you last cleaned them. Once that gets into the pump it behaves like a mild abrasive paste. You will not notice on the first job. You notice months later when the pressure has quietly dropped off and the seals start weeping.

The second issue is supply. Most domestic pressure washers are designed to be fed by mains pressure, which pushes water into the inlet. A water butt gives you almost nothing by comparison, so the pump has to suck the water up itself. Some machines will do that within limits. Many will not, and running one dry or half starved will cook the seals in minutes.

Check whether your machine can self prime

Before anything else, look up your model. Manufacturers usually state a minimum inlet pressure or a maximum suction lift in the manual, and some sell a dedicated suction hose kit with a foot valve and strainer on the end. If that accessory exists for your machine, that is a strong signal it can draw from a tank or butt.

If the manual specifies a minimum inlet pressure of, say, 1 bar and says nothing about suction, treat it as mains only. Petrol machines and higher end electric ones are generally more tolerant. Cheaper entry level electric washers often are not, and using one this way can void the warranty.

Setting up a rainwater feed properly

1. Raise the butt. Height gives you gravity, and gravity does the work the pump would otherwise struggle with. Standing the butt on blocks so the outlet sits well above the machine's inlet makes an enormous difference.
2. Keep the machine low and close. Long runs of hose and any uphill section cost you pressure. Put the washer next to the butt, not at the far end of the garden.
3. Use a wide bore hose. A standard thin garden hose throttles the flow. A 3/4 inch or larger feed hose, kept short and free of kinks, delivers noticeably more.
4. Fit a proper inline filter. This is the part people skip. A mesh inline filter between the butt and the machine catches the grit that would otherwise reach the pump. Cheap insurance.
5. Prime before you start. Fill the feed hose with water and let it run through so the pump never starts dry. Squeeze the trigger with the machine off to purge trapped air.

Filtering: build it in layers

One filter is better than none, but a staged approach works far better and saves you cleaning the fine filter constantly.

• At the gutter: a leaf guard or downpipe filter stops the bulk of the debris ever entering the butt.
• At the butt outlet: a coarse mesh strainer or a strainer foot on the end of the suction hose.
• At the machine: the pressure washer's own inlet filter, plus a separate inline filter if you can fit one.

Check and rinse these regularly. A clogged filter starves the pump just as effectively as an empty butt, and starved pumps are where the expensive damage happens.

Will you have enough water?

This surprises people. A typical domestic pressure washer uses somewhere around 6 to 8 litres per minute at full flow. A standard 200 litre water butt therefore gives you roughly 25 to 30 minutes of continuous trigger time, and realistically longer since you are not holding the trigger the whole while. Still, a full driveway clean can empty a butt before you are done. If you are planning larger jobs, either link two butts together or accept that you will be working in stages.

Signs it is not working

Watch for pulsating output, a pump that sounds like it is labouring or rattling, or pressure that surges and drops. All three usually mean air is getting into the system or the feed cannot keep up. Stop straight away rather than pushing on. Running a starved pump for a few minutes does more damage than a whole season of normal use. And if the water in your butt is visibly green, deal with that first. A dark coloured butt with a proper lid keeps algae down, since algae needs light.

The short answer

Rainwater is perfectly usable, and for patios, paving, fencing, and general outdoor cleaning it does the job just as well as mains water. It is not the right choice for washing a car if there is any sediment in it, since fine grit and paintwork are a bad combination. Filter it properly, raise the butt, keep the feed hose short and wide, and confirm your machine is happy drawing water rather than being fed it.