Pressure washers - useless, or less than useless?

[Hugo Nebula]

Reading car forums, the overwhelming opinion is that a pressure washer is needed to wash one's car properly. So I went and bought a cheap Halfords one, complete with snow foam attachment. The results were disappointing to say the least.

I know it's not a mega powerful one, but I was amazed at how low the pressure was out of the nozzle. I was certain my hose gave a more powerful flow, so I did an experiment and timed how long it took to fill a bucket, and therefore what the flow rate is from my tap. It amounts to 1,000 litres per hour, nearly three times the flow rate of the PW. Some of the best pressure washers only appear to give half of that.

Am I missing something here? Is pressure a function of the flow rate, and if so, what's the point of a pressure washer if it gives less than mains pressure?

[Lobster]

There's certainly no need for them to shift vast volumes of water to be effective. I think the point is that they generate (or should do) a very high pressure by pumping water through a very small orifice. Certainly my Karcher blasts out a very effective jet, but the actual flow rate is way less than provided by a mains-fed tap.

David

[Hugo Nebula]

So would a hose with a small nozzle not achieve the same effect?

[Bill]

In message , Hugo Nebula writes

They, as the name suggests, provide extra pressure and not flow rate, so I would expect an open hose to provide a higher flow rate, otherwise they would be sucking water out of your mains.

I use a middle of the road Karcher and it is marvellous, fetches paving slabs and concrete back to a nice crisp finish and clears most of the dirt off the car. If you have it on a pencil jet and it cuts the rubber trim around the windows then it is OK. A problem I had when I first used mine years ago. Quite good at taking paint off painted bumpers too if there are any stone chips.

[Mentalguy2k8]

Those are mainly what I use mine for, plus with not-so-great drainage in my back garden, I can clean it all without flooding the garden because it uses less water than a hose. The patio looks like new at the moment, I don't think anything else would bring it up that clean.

I'm assuming the best way to compare tap versus jetwasher is the bar measurement rather than flow rate. I think my washer is about 120bar, that's way more than a tap. You can stop a tap running with your thumb over it, you ain't stopping a jet from these machines.

[Davey]

No, because you can't generate more pressure than you start with without some extra force, such as a pump.

[harryagain]

Pressure washers are a positive displacement pump. They deliver a fixed amount of water.

pressure you get depends on how small the outlet orifice is.

So to get a higher pressure you adjust the outlet down. If the outlet is wide open, you just get a miserable dribble. (But it is the same volume of water.

There is a relief/bypass valve inside opens if over pressure ocurrs. eg when the operating valve on the lance is closed.

[Andrew Gabriel]

I thought a problem with them for this purpose is the pressure can take the paint off.

I do use one for washing the car, but the main purpose is to pump the water from the water butt. I never point a high pressure jet straight at the paintwork, or elsewhere where it might cause damage. I use a soft brush attachment for washing, and a misting spray for rinsing.

The flow rate from most is quite low.

To a first approximation, pressure x flow rate will be proportional to the power consumption, but that ignores that some have efficient induction motors, and others apparently have less efficient (and probably shorter life) universal motors.

They are much higher pressure than mains.

[Martin Brown]

There is a limit to the flow rate through a small nozzle unless you apply a much larger pressure (ie pump) to the back side of it. People make terrible mistakes with leaking high pressure hydraulics systems.

A small amount of water carrying a large amount of kinetic energy can do a lot of damage. I would not use one on my car as I don't want to grit blast the surface or strip the paint. YMMV Fine for use on dirty patios though and stone doesn't suffer much from additional scratches.

[Dave Liquorice]

Road film is remarkably resistant to simply being blasted off. You still need some form of rubbing to get it off, either a brush or cloth. If just pressurised water was enough do you think automatic car washers would have great big rotating brushes?

[The Medway Handyman]

The dirt on cars is known as 'traffic film' and comprises of unburnt fuel residue, oil/grease residue, small amount of rubber & general grime. It's basically oil based & forms an extremely good bond to vehicle paintwork. In addition, since the vehicle is moving while it gets dirty, a static bond is also generated.

It short, it's a bugger to remove without mechanical agitation.

Pressurised water alone will not shift traffic film. A specific detergent - known as a Traffic Film Remover or TFR is needed to remove it. You won't find a decent product in a DIY place, proper TFR is pretty nasty stuff - pH of around 13. You need to go to a specialised pressure washer dealer.

Pressure washers were developed in Denmark & Germany when water & it's disposal has always been very expensive. The concept was that you could do the same amount of cleaning with much less water. In the UK, where water has been cheaper, they were sold as labour saving, not water saving.

You are confusing flow & pressure. A household tap may give 5 bar at 15 litres a minute. Connecting a pressure washer to that tap gives say 100 bar at 10 litres/min - which is a tenfold increase in cleaning power.

The balance of pressure/flow is important. 120 bar @ 6.5 l/min is less powerful than 100 bar @ 8 l/min.

Have a look here;

[Andy Burns]

Not for blasting the dirt (and paint) off, but for applying the snow-foam ...

[The Medway Handyman]

Urban myth.

[Bill]

Tell that to the painted plastic bumper on a Mondeo I had! :-)

There again it didn't harm the painted metalwork.

>

[Andy Burns]

Ditto bumper on an old car of mine.

[grimly4]

Only time I've ever done that was when the paint was pretty shittily applied in the first place and deserved to come off. In fact, it was waiting for the first heavy rain to take it off.

[Another Dave]

I believe they were invented as a water SAVING device. They don't use much water but should produce a high pressure jet. Mine certainly does.

Is there a leak at the junction of the wand and the machine?

Another Dave

[grimly4]

Traffic Film Remover - the real stuff, not the weak as piss stiff in Halforts, is really good at that.

[ss]

Ok a non tech answer :-) And I dont recommend this. I have a cheap one but was more than happy with the patio results to the point I got carried away and decided to clean the plastic patio table....It now has grooves cut into the table top where I used the PW, this wouldnt have happened with a garden hose, I could have ripped through it given a few more minutes. Last year in hot weather (bare feet) cleaning the slabs.... I accidently blasted my foot with the jet....damn painful it was, again much more pressure than a garden hose. In both instances it used much less water than a hose.

[Roger Mills]

YES!

A pressure washer blasts a relatively small amount of water through a small nozzle at very high *velocity* - thus imparting a very high force on any surface impacted by the jet. This is great for cleaning patios etc. (been using one myself, today!). Many lances have adjustable nozzles - so you can have a very narrow very fast jet, or a wider 'fan' at lower velocity. The wider fan is much faster, but you sometimes need to reduce the size of the jet to remove very stubborn dirt.

When washing cars with a foaming device, the water is usually delivered through a brush or a fairly wide nozzle, so far less back pressure is produced. That's really a two stage operation: Stage 1 to spread the foam and loosen the dirt, and Stage 2 to blast the loosened dirt off with a small-bore high pressure nozzle.

I wouldn't bother with a pressure washer for washing cars - a bucket of soapy water applied with a hand-brush, and then washed off with a garden hose, is just as good. But patios are a different matter!