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I understand that most pressure washers need at least 20 PSI of water flow at the inlet and at least the same GPM that the pressure washer will output, e.g., pressure washer rated at 4GPM should have at least 4GPM water supply.

However, I'm curious about the maximums as I don't see any of that in the manuals.

What happens if the water supply is 12GPM on a 4GPM power washer? and if the water supply is pushing 100 PSI instead of a typical garden hose of 45 to 80 PSI? Thanks

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    It will only take as much gallons as it needs. You do ot have 100 PSI incoming. Max is 60
    – Traveler
    Jun 22, 2022 at 4:01
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    If you had 100PSI on the inlet hose, it would blow up.
    – Traveler
    Jun 22, 2022 at 4:06
  • @knowitall Thanks. Yeah, I did not put actual values, however, I assume it's possible for someone to supply similar values to a pressure washer, especially since pretty much every manual I have read does not provide any of this data. Jun 22, 2022 at 4:17

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So I understand that most pressure washers need at least 20 PSI of water flow at the inlet and at least the same GPM that the pressure washer will output

I've seen plenty that will feed from a bucket, or unpressurised cistern.

What happens if the water supply is 12GPM on a 4GPM power washer?

How are you going to do that? water is not compressible, if 4GPM is coming out there's only room for 4GPM going in.

and if the water supply is pushing 100 PSI instead of a typical garden hose of 45 to 80 PSI?

At some point (maybe 200PSI) the pressure will exceed the capability of the machine and it will break. until that pressure it will continue to work. And may give extra pressure.

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  • gpm is pressure * diameter. i like to think of pressure as voltage, diameter as current, and GPM as watts. I think the OP meant to imply the supply being capable of many GPMs.
    – dandavis
    Jun 22, 2022 at 9:34
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I sometimes have to dangle mine over the edge of a pontoon, so it's a foot or so above the water level in the lake. That means there's no head, no pressure going in. It seems to work as well that way, might take a moment or two longer to prime.

So, the machine is taking as much water as it needs, and delivering at the pressure stated, as long as the inlet isn't restricted. As far as over pressurising the input, it seems a crass thing to do, and probably won't increase the output pressure.

Best really is to ask the manufacturers, who will have tested their machines to destruction, expecting users to be abusers on occasions.

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