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OT - Why would backpressure occur in a sprinkler system?


Jan 1, 2005
I see most sprinkler system are installed with a BP valve of some sort. The concept is the valve prevents 'backpressure' from allowing water to flow backwards...in other words, once the water has entered the sprinkler system, you don't want it flowing back into the 'regular' water supply. That's because, of course, the sprinkler system contains very toxic and deadly chemicals....

Anyhow....why would water want to flow backwards? I can see that happening IF the sprinkler system were uphill of the water supply. But, in most cases, it is not. It's usually a few feet or more below the source of water. It seems the water would flow out of the sprinkler heads, under pressure, and once the valve in the sprinkler system shuts off, the pressure in the sprinkler system's pipes would lose any residual pressure out of the sprinkler heads and all is well.

I understand a lot of sprinkler system are designed 'one size fits all' but it seems the BP valve is doing nothing in most systems. What am I missing?
sprinkler systems have a mix of air and water in the system and any reduction or variation in pressure at the water mains will cause flow.

every year or so some kid in my former highschool will hang something off a sprinker and the water that comes out for the first few minutes is pretty nasty.
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.
I have never seen a BP valve in a home irrigation system. Just vacuum breakers. My father had to install a bp valve after they found he had well water system for irrigation only. the well pump could have pumped well water into the a main line if they lost pressure in the street. Only if they were connected. they were not .
Bill D
The required valve is a backflow prevention device, not a back pressure valve.

An extreme example of an event that could cause contamination from an irrigation system into the clean water supply is a fire. If a fire pumper sucks water out of a fire hydrant, the pressure in the water main is locally reduced and could cause nearby irrigation water to be sucked into the main. The backflow preventer does what its name says.

My backflow preventer is a Febco 765 Pressure Vacuum Breaker.

My city requires the valve to be tested ($75) by a professional annually, even if the irrigation system is not connected to the water supply.

Larry is correct. Not back pressure, back flow.

Siphoning is a real concern. If the main loses pressure, contaminated water can be siphoned into the potable supply.

I sure don't know all the rules. My system requires a double check valve backflow prevention device. Sometimes a vacuum breaker is enough.

The same siphon issue can occur with a garden hose. Hose bibs are made with a built in vacuum breaker, or an add-on can be screwed on to an existing hose bib. The add-ons have a breakaway head setscrew so they "cannot" be removed once installed.
Is it a recent issue?
By chance was a hose bib or something that taps off you water supply line been added recently?
Can you detect discolored or smelly water in your taps?
Any way...maybe time to call a licensed contractor to do what is usually a required test on your back flow preventers. Check your code/insurance requirements.
On a cattle ranch if the cattle water troughs are hooked up to the water supply back flow valves are required (think cattle slobber, algae, dead critters, and mosquito larva in the troughs). At one of the water supply meetings the cost of testing was brought up and I asked why a person could not just install a new device yearly instead of testing. The idea was frowned on. Our cattle watering system was independent with water being pumped out of our irrigation reservoir, so it was a nonissue for us.
Best fitting description is an anti syphon valve. Municipal code just about everywhere. The city of troy mi wants it certified as functioning every 3 years by a licensed plumber.
You only get back pressure from a blockage or a restriction, high blood pressure, blocked arteries.
Closing a valve, or throttling = back pressure.
Throttling is a constant energy process, pressure decrease. Velocity increase.
That Bernoulli guy was smart
Even if it's just a sprinkler system with plain water, you don't want backflow. How often is a sprinkler system used? The water often just sits in there and stagnates. Still water in pipes is a breeding ground for any bacteria present... Heard of Legionnaire's Disease?
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 didn't know it got that cold in Houston. Here in Indiana, we have plenty of freezing weather, so every October we shut down the irrigation system. The city shuts off the curb valve and disconnects the dedicated irrigation meter so I do not get charged the minimum monthly fee until the following June when the system is hooked up again. I pay my irrigation service company to connect a huge portable air compressor to the supply pipe and blow out each of the six branch circuits. That gets most of the water out of the pipes and prevents freeze damage.

My system was installed in 1991. About five years ago, the original back flow valve failed the annual test and was replaced with a new one. I suspect the failure was caused by a spider nest in the breather cap. The service guy said it was better to install a new valve than try to repair the old one and it was not very expensive. These valves are heavy bronze castings with bronze ball valves and should have a few bucks of scrap value.

The same valve Larry described is required here for a business. It has to be tested yearly and the fee here is also $75.
About 2 years ago the water service was forced to install a smaller version at all residential meters. I already had one at my meter. Now I have 2.
They also had to install a blow off valve at the dead end of each water line. It's just a 2" ball valve to flush out the line. It's supposed to be flushed each month. My meter is about 5 feet from the end of the line on my road. To my knowledge the only time it's been flushed is the day it was installed.
One place I worked at had a huge boiler shaped tank for the works sprinkler system. It was half filled with water and then pressurised with compressed air. It started to lose water from somewhere. That meant we had to let off the compress air, pump the water level back up, then re pressurise the system. It was all a bit of a ball ache.

We looked high and low to find the source of the leak without any success. Then one day a family car sized hole appeared in the works yard. We’d found the leak !

Regards Tyrone