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OT - Why would backpressure occur in a sprinkler system?

Thanks everyone.

Yes, I meant back flow, not pressure.

The issue is the neighbor had had this house for 5 years and over the winter the back flow valve froze and cracked. He replaced it with a new one, same kind, and now he is getting water hammer noises. I have not heard the noises, but he also says the backflow valve spurts out water momentarily when he runs the sprinkler system so I am guessing that is tied to the water hammer noises. But not sure. I looked inside the new valve and it looks fine...though I am not 100% familiar with what it should look like.

It's this model

I worked as a consultant on backflow supplying valves and IP to companies like Watts. Sometimes you need to open the tap at the end of your line and bleed the air out of the line. Air will migrate to the highest point and act like a spring causing hammer.
 
I worked as a consultant on backflow supplying valves and IP to companies like Watts. Sometimes you need to open the tap at the end of your line and bleed the air out of the line. Air will migrate to the highest point and act like a spring causing hammer.
so someone paid you to tell them that air floats on water? what is their phone number?
 
In a former life I was paid (quite well actually) to explain to people that water will not flow uphill unaided
Soon after I went to work at a large engineering facility, I learned the value of listening to the older skilled trades guys as they imparted wisdom that has helped me maintain my own homes over the decades. One of the plumbers was called Torchy (he soldered copper pipes), though I don't recall his real name. His lesson in the basics of plumbing was, "Hot is on the left and sh...t don't run up hill." Words to live by, and not included in my fluids engineering classes.

Larry
 
Plumbing was the trade apprenticeship I served, (family business and no choice what I was going to do!!)

My favorite was Arthur Roe ''It might be s*** to you but it's our bread and butter''
 
I'll toss out an idea. As you know, a back flow valve reacts when the pressure on the downstream side exceeds the pressure on the supply side. If there are low spots in the sprinkler lines water will collect and remain there when the system is off. When the water flow is turned back on back pressure is "low" and flow is "high" until the system becomes charged (pressure and flow reach a steady state). While filling, water velocity in the lines is faster than while charged. When the faster "filling" water flow hits the stationary water in a low spot the impact will create a pressure spike (water hammer) that the back flow valve would interpret as a reason to vent. Hence water hammer and a momentary spurt of water.

Why it didn't happen with the previous valve is open to question. Possibly, this was not the first time the valve had frozen. Just the first time it was damaged enough to be noticed.

Just a guess.

Thank you,
Mr.Smith
 
Most irrigation valves have built in breaker.

The valve itself uses Linnea pressure with a spring to close.

So if one is using a low flow circuit like drip tape the line will be pressurized and hold pressure for some time.

When the valve closes it just closes the bleed port so the spring can push the diaphragm down with assistance from line pressure seeping around the guide pin.

If someone flushes a toilet the lone pressure can drop below the pressure in the drip tape causing the water to flow back.

The breaker is a simple disk that pops up to close a vent with pressure, it falls when pressure drops.

There are check valve type and larger breakers as well, local codes specify which is used.
 
Close, but not a winner.

If you are uphill and there is a break in the water main downhill from your sprinkler system, then the water will try to flow DOWNHILL and out the break. This will create a SUCTION, a LOW PRESSURE, even a NEGATIVE PRESSURE in the line at your, uphill location. It is that negative pressure in the main that can suck the sprinkler water and any chemicals or algae back into the main where it can be mixed with the city water when the leak is fixed.

Of course, if you also have a well with a pump feeding the sprinklers, then that can create a higher pressure than the city water pressure and, again cause a back-flow.



Imagine you are on top of a hill, and the water main breaks at the bottom. Air can work its way up the main from the break, water can then flow back from the sprinkler lines down the hill into the main and contaminate drinking water. Most homes now have expansion tanks on the water heater, and BP's on the water line into the house too.
 








 
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