Sucking the mains...

AIUI the reason for banning pumps has nothing to do with concern for your health and everything to do with not sucking the sides of the water main in.

Rgds

Andy R

They are used (or should be) on the end of a hose with a double check valve.

Secondly, the amount of water used is relatively small and the hose is still under positive pressure.

.andy

To email, substitute .nospam with .gl

I suppose it would be reasonable, as long as you've got a check ensuring that the pressure at the rising main does not drop below 14PSIA?

I've always wondered about this. I can say, though, that for me the flow rate is *lower* with the pressure washer than just the hose, which suggests that the pressure in the hose will still be postive (and, indeed, higher than just the normal trigger head). The pump isn't to increase flow rate by sucking more water, it is just to produce a pressure jet. This doesn't necessarily lead to much reduction in pressure.

In fact, you could get a working system with a complete airbreak (i.e. ballcock tank fed by hose, outlet to pressure washer).

Christian.

On a (fairly) related note is there such a thing as a pressure washer that can be run from a bucket (or built-in tank)?

Mine was perfectly happy to do so when I desparately needed to clean something and didn't have my garden tap fitted. It was a 39.99 from B&Q and didn't advertise this as a particular feature. The main problem was keeping the end of the hose in the bucket with no additional help or string, gaffer tape or anything.

Christian.

I run mine from a water butt.

Doesn't stop the fire brigade...

Flow rates from a pressure washer are fairly low, well below that which the mains will supply. Until you start to demand more flow than can be provided the pressure stays positive.

Yes, it's called a Stirrup Pump ! Used in WW2 for putting out fires or trying to !

Andrew Mawson

Most pressure washers have a small tank that is filled before the water reaches the pump. Try turning off the water supply to the washer and you'll that you get a flow water for a few seconds before it shuts down. They never, usually, suck water directly from the mains pipe.

I was going to say that the DC valve is just to stop any possibilty of back flow by having two valves so if one leaks a bit it still has to get past the second.

But the realised that I don't really *know* how DC valve works.

Obviously there is the anti back flow element from one half but could the other half be an anti flow? The anti flow being set to cut in when the pressure difference across that part is greater than x bar?

My curious mind wants to know...

The simplest double check valves work with a little light ball inside them. When the pressure is sort of equal on either side because of the pipework, the little ball sits in its own little cup (groove actually) in the body of the valve. If a major leak occurs on the flow side of the valve, the little ball is literally blown out of its cup by the increase in pressure on one side of it, and it blocks the hole that the water comes out of.

There are other designs of double check valve that work the same sort of way but they use a flap or disk rather than a little ball.

That doesn't sound as if it'll stop back siphonage which I thought was the main reason for fitting DC valves... The ones I've played with you can easily blow through one way but not at all the other.

A double check valve is used on the mains pipework and an anti-siphon valve is used on an appliance.

The same principle is used in both double check and anti-siphon valves, only in reverse. The only difference is that a double check valve stops your house from flooding if the new poly pipes blow off the consumer side of your mains stopcock. An anti-siphon valve is there to stop gravity or increased pressure from a pump on an appliance from blowing muck back down the pipework the wrong way if the mains is turned off in the street and the valves in the house are all open.

I was going to comment that pressure washers don't really have much pulling capability on their inlets as they can't even operate from a bucket of water with no head (pressure that is!) but now I see your post...... I couldn't get my cheapo washer to work from a bucket, but maybe it just very fussy about priming. In any case, I still doubt that pressure washers could create any real negative pressure on their inputs - mines certainly can't even collapse a garden hosepipe when you shitch off the water tap.

The stirrup pump I used a few years ago was designed for high volume at low pressure - not really the thing for cleaning paths/cars etc.

I think we have a terminolgy problem. I was under the impression that outside taps etc had to be fittted with "double check valves" to prevent water from the hose getting drawn back into the system. Not seen or heard of an specific part called an "anti-siphon valve" before.