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Is there such a thing as a pressure relief valve which can be attached to garden hose bibb threads that would relieve backpressure from a pressure washer, in order to protect old pipes and reduce the chances of backflow into the potable water supply?

I have a pressure regulator on the garden hose spigot reducing the mains supply pressure from 80psi to 60psi. When the user releases the hand-grip of the pressure washer, its motor cuts off instantly, by design, creating a backpressure situation. The dial on the pressure regulator jumps to 130psi.

Is there such a thing as a pressure relief valve which could be threaded onto the hose bibb, that would release backpressure by expelling a little jet of water, analogous to those that are used with hot-water heaters but which could be set by the user to a particular desired pressure? If I could set it to 80psi, anything over that threshold would send a jet of water onto the grass.

BTW, the pressure washer's manual warns against using a one-way check valve on the supply line.

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  • $\begingroup$ You may also look at an air chamber or anti surge device, that add "cushon" and can reduce the magnitude of pressure spikes even without a pressure relief valve $\endgroup$
    – RC_23
    Jun 28, 2023 at 2:25

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The proper device outside is a backflow preventer. These are required to keep bad stuff from outside from getting inside. It will vent if the outside pressure is too high. You or a previous owner probably took yours off.

Yes overpressure reliefs exist. Search for Pressure Relief Valve. You'll need a set of adapters to be able to pipe it in.

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  • $\begingroup$ As I say at the bottom of the question, the manufacturer specifically warns against using a backflow preventer. I assume it's because the check valve could close, cutting off flow to the pressure washer, and since it's water-cooled, the pump motor could be harmed? $\endgroup$
    – Tim
    Sep 30, 2022 at 14:20
  • $\begingroup$ I have searched for Pressure Relief Valve but haven't found any that can be set by the user to a desired pressure and attached to the outdoor hose bibb. That search also brings back hammer arresters. $\endgroup$
    – Tim
    Sep 30, 2022 at 14:20
  • $\begingroup$ Backflow preventers are required by code, the pressure washer manufacturer is bad. Pressure washers are one of the specifically problematic items that backflow preventers are there for, since you often dip a line into stuff you wouldn't want in your pipes. The cheapest one at Amazon is for 75 PSI. Grainger has a "Calibrated Adjustable Relief Valve: Brass, MNPT, FNPT, 3/4 in Inlet Size, 1/2 in Outlet Size" for $45. None will connect to a hose bib directly, you'll have to buy adapters to connect it. $\endgroup$
    – Tiger Guy
    Sep 30, 2022 at 14:28
  • $\begingroup$ Thanks for the Grainger info. The manufacturer is a major brand, always advertising themselves as the most trusted name in backup generators on TV commercials. I've been trying to get some specific answers from their tech support about the problems that using a one-way check valve would cause for the unit, but without success. All they will say is that the unit is working properly and the backpressure is normal. $\endgroup$
    – Tim
    Sep 30, 2022 at 14:53
  • $\begingroup$ Most RVs and Boats use a 75psi HWH blowoff - so these are readily available. Your home HWH probably has a 150psi unit. For my money, I put a nick in the hose washer and let it drip a little. $\endgroup$
    – Phil Sweet
    Sep 30, 2022 at 15:22

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