20 years in the pressure washer industry
What is a "GOOD" pressure washer?
I won't call out a specific brand, but I will tell you what to look for.
First off price is not a qualifier, if your shopping price only, you will get a throw-a way pressure washer, may last a season, may not last through the task you purchased it for.
Second, high PSI numbers should not be the main goal, you need to understand the relationship between PSI & GPM.
Third, the pump is the "hart" of the unit, no matter what the other bells & whistles are, it is the pump that makes it a pressure washer.
So if your with me so far, let's talk about the pump first.
You may think that the pump "compresses" the water, it dose not, the pump is known in the industry as a "positive flow pump"
O.K. you say but what dose that mean? think of a sump pump or even the water pump in your car, and how it's constructed,
that kind of pump has a impeller (like a fan) that spins inside of a volute (a cavity that directs the flow).
This kind of pump can be dead headed (stopping the flow at the exit/discharge port) with no damage to the pump, the water will just circulate around the impeller and not exit the discharge, however if this conduction is maintained for a period of time, due to friction the water will heat up and eventually destroy the pump.
A pressure washer pump has pistons, seals and check valves, to move a constant flow of water, and by restricting the flow pressure is raised in side the pump, if this kind of pump is "dead headed" it will (provided you have enough horse power) blow the pump apart.
Still with me? good, so now you might ask well I have a trigger gun on the end of the hose, it stops the flow of water so why doesn't the pump blow up?
Good question indeed, when the trigger gun is released the flow of water(depending on the manufacture) is either stopped or redirected. On electric units it is common to have a pressure switch on the pump to shut off the electric motor driving the pump when the trigger is released. The down fall of this system is first, it's hard on the electric motor, the hardest thing for an electric motor is the "start" and excessive starting will shorten the life of the motor. Also because this type of system traps pressure in the pump and hose, any leaks or loss in pressure will restart the motor (short cycling).
On gas/diesel driven motors it's unpractical to stop/stare the motor every time the trigger gun is released, so on the pump an "unload-er" valve is installed, when the trigger is released the pressure in side the pump increases to the point where the valve opens to redirect the flow back into the input of the pump. The down side of this system is that first off most operators do not understand this valve and it's the first thing they go for when a loss of pressure is experienced. This valve needs to be set so working pressure is less than the unloading pressure, if incorrectly set the pump will not unload and result in pump damage. ( I have seen many pumps will the head bolts torn off, broken pistons etc.) Also with this type of unload-er system when the trigger is released the pump is circulating water from the output back to the input, this is a small amount of water and due to friction will heat up fast, because the pump only wants to see cold water ( hot water is always made after the pump) damage will occur in a short period of time, to minimize the potential of pump damage due to high water temperature a thermostatic valve is installed on the input side of the pump, so in a perfect world when water temperature got to high it will open releasing the hot water and "new" cold water enters the pump as the temperature cools down the valve should close. The drawback of this is first off, the seals may be damaged by the hot water before the valve opens, ceramic pistons that got hot may crack when the cold water rushes in, and the valve may not totally seal on reset allowing air to enter the pump (cavitation)
Just a few words on cavitation, it's the formation of bubbles or cavities in liquid, developed in areas of relatively low pressure around an pistons. The imploding or collapsing of these bubbles trigger intense shockwaves inside the pump, causing significant damage to the pump housing and internal parts.
So the take away is do not let the pump run without keeping the trigger pulled for any long period of time, how long?
the shorter the better, shut it off when not working it, typically two minutes or less is recommended.
So now you know more about the pump then you want, (there's plenty more but for now) lets talk PSI vs GPM.
The sales department will sell you on PSI, but you really need to understand the relationship between the two.
I can let you hold a 3000 PSI wand with 2 GPM, and you can cut a 2 X 4 in half with it, but try and wash your wall or driveway and you'll be there all day.
If I gave you a 1000 PSI wand at 5 GPM it will feel like a cannon, you will not cut the 2 X 4 but you will wash like crazy.
You need the flow of water to move away dirt as much as pressure.
A car automatic wash will typically use 700 PSI and 10 GPM to clean a car without removing/damaging decals and paint.
A body/paint shop that is prepping cars/trucks for paint wants to remove loose paint, rust, decals, and the like will use 2000- 3000 psi and 4-5 GPM.
A professional house washer is looking for 1000-1500 psi and 4-10 GPM.
Now that you got a idea of PSI vs GPM, the next issue is how much water do you have available?
A typical garden hoes is hard pressed to deliver 2-3 GPM, depending on the length and diameter of the hose. So for larger GPM models a holding tank is necessary, often referred to as a float tank, because it had a float valve in it (like a toilet valve) in it, this gives you a greater volume of water to work with also the best units direct the bypass water back into the float tank so the pump can run for a long time in bypass/unload-er and because of the large amount of water in the tank will not heat up.
So if you made it this far, you might have noticed a reoccurring thyme, heat is the killer of the pressure pump. Not only the heat of the water, but the pump and oil in the pump, Weather electric motor or gas/diesel driven, the cheaper pumps are direct drive, attached directly to the motor and spinning at the motor speed (typically 3450 RPM) the pump will have fins on the casting in an effort to disperse the heat.
The best pumps will be belt driven under 1000 RPM and have no need for cooling fins.
Things to remember,
You need horse power to move the pump, the most H.P. you will get from a 120v outlet is 1-1.5 that if really squeezed may get you 1500 psi @ 2 GPM. playing the numbers 2000 @ 1 GPM.
To get real washing power you need H.P. a gas/diesel motor, for 3000PSI @ 4-5 GPM 20 H.P.
All this can be easily found, better manufacturers have charts show the relationship between H.P. GPM & PSI.
Also this is a very price driven industry, you get what you paid for. Manufactures use tricks like fooling with PSI/GPM numbers or the new nomenclature " cleaning units" to confuse and deceive.
I hope this long winded message is of some use, and I will be happy to answer any questions.