Pressure washer backflow preventer?

13 Jun 2019
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United Kingdom
I just bought my first pressure washer and in reading the manual it has a dire warning about not connecting it to the regular water supply without a specialist backflow preventer. I did a lot of research before I finally bought a washer and this is the first I've heard of this. Is it something that is strictly necessary? I've just read that a non return valve on the tap would have the same function. I'm wondering if the tap probably has built in as it was only installed a month ago.
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Technically speaking a pressure washer should have Category 4 or Category 5 backflow prevention according to WRAS, dependent on usage. Double check valves only protect up to Category 3, and outside taps with built in check valves are not approved for use at all (apart from the ones sold by Travis Perkins and stamped TPK). Outside taps should have a separate double check valve on the pipework inside the property
So is that a separate gizmo I need to buy in order to use the thing? I'm irritated that none of the information I read before buying it mentioned anything about this.
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So is that a separate gizmo I need to buy in order to use the thing? I'm irritated that none of the information I read before buying it mentioned anything about this.
To the letter of the law you should have this backflow prevention in place, but it costs more than your pressure washer cost you and needs to be properly calibrated every year by an authorised person, so most people don't bother
Change the outside tap for one with the valves in it job done. Bob
Really depends on how careful you want to be. With these appliances, there is a risk (however small) that if there was a drop in your mains pressure then contaminated water from the pressure washer could seep back into the mains drinking water.

As suggested previously the water regs state to remove that risk then a backflow preventer should be installed. They specify a higher spec'd backflow device as pressure washers can use detergents/soaps etc. At an absolute min there should be a double check valve installed internally onto the taps supply pipework.

Again as suggested don't use the ones that have the check valve built into the tap, they are no longer WRAS approved and are known to fail in the winter if the tap freezes.
is that a separate gizmo
Tap normally has one fitted. (Note that if tap does not work after freezing weather, freezing water that you have not emptied out has pushed it closed).

Good hose pipe connectors normally have one included - see link that shows water flow direction shown with an arrow).

And if you are disconnecting hose from tap when not in use, then there is no hose and no sitting water to be pulled back into system.

So all in all you are in a safe position compared to say tripping over hose and breaking arm.
Tap normally has one fitted
Only if it is a double check type, could just as easily be a normal outside tap or if it needed to be replaced (as it has frozen) it could be replaced with a normal tap therefore fitting an internal inline type double check valve provides a semi permanent fixture.

Good hose pipe connectors normally have one included
None of those attachments has any backflow protection, the arrow just indicate the water flow direction. The grey/yellow connector is a straight through type and the red/yellow one is a water stop. All that does is shuts off the outflow water when the hose spray head is disconnected

I have found that invariably people use hose reels, and they are more of often than not left connected all the time to the outside tap.

I do agree the risk is minimal but it isn't risk free hence the rules being in place.
The outside tap was only fitted in June, so I don't know how it will fare in cold weather. I ended up just deciding to take the risk since at this point I don't intend to use any detergent. I'll be removing the hose when I'm done with it so maybe I shouldn't be too worried. I just thought it might be one of those things that every idiot knows about so they don't bother to tell you that you need it.
I'm in the US. There are national laws governing use of cleaning agents used with pressure washers, and local ordinances here require one-way valves for sprinkler systems, but I don't believe there are any ordinances governing the connection of pressure washers to the domestic water supply. The user manual for the pressure washer I just purchased actually warns the customer not to use a backflow preventer of any kind:

"DO NOT use a one-way valve, vacuum breaker, or check valve in any part of the water supply."

The unit I bought has a water-cooled electric motor that automatically comes on when the wand handle is squeezed and turns off when it's released. In the course of shopping for one, I read on the website of one manufacturer that mains pressure above 75psi could cause problems for their units. Since our mains pressure is 80psi, I bought a pressure regulator and set it to 60psi, which was said to be the optimal pressure for the unit I bought.

The pressure regulator's dial rises to 130psi after the pressure washer has been in use and then automatically turns off when I set the spray wand down. I've written to the manufacturer's tech support department seeking an explanation for the backpressure -- are the unit's seals defective? -- and asking for an explanation for the warning not to use a check valve. I'm no engineer but I suspect that a check-valve could close because of the backpressure and thus cut off flow of water to the unit, which would cause it to overheat.

Is such backpressure typical of consumer class pressure washers?

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