Irrigation Backflow Preventer: PVB, RPZ and Winterizing
The brass assembly sticking out of the ground beside many Front Range homes, usually near the foundation by a hose bib, is easy to overlook until it cracks in a cold snap or a water utility sends a testing notice. That device is the irrigation backflow preventer, and it has a straightforward job: keeping water from the sprinkler system from flowing backward into the home’s drinking water. Sprinkler lines sit in soil that can contain fertilizers, pesticides, pet waste and bacteria, and the heads sit at ground level where puddles form. Without protection, a sudden drop in water pressure can pull that water back toward the household supply. This guide explains why irrigation systems need backflow protection, how the common device types differ, how testing works, why Colorado winters are hard on these assemblies, how water pressure plays in, and what repairs and replacements tend to cost.
Part of our Plumbing & Water guide — start there for the full picture →
What an Irrigation Backflow Preventer Does
An irrigation backflow preventer is a mechanical assembly installed between the drinking water supply and the sprinkler system. It allows water to flow out to the sprinklers but is designed to stop water from flowing back the other way.
The U.S. Environmental Protection Agency defines cross-connections as actual or potential connections between non-potable sources and potable water that could allow non-potable materials to enter premise plumbing or distribution systems through backflow. EPA defines backflow as any unwanted flow of used or non-potable water, or other substances, from a piping system back into the potable water distribution system. It explains that when cross-connections are present and pressure in the potable system drops below the pressure in the non-potable source, contamination can enter the distribution system.
A lawn sprinkler system connected to household plumbing is a classic cross-connection. Backflow can happen in two basic ways:
- Back-siphonage. Pressure in the supply drops, for example during a water main break, firefighting nearby or heavy demand, and creates suction that pulls water backward from the sprinkler lines.
- Backpressure. Pressure on the irrigation side rises above supply pressure, for instance from a booster pump or elevation differences, and pushes water backward.
EPA’s distribution system resources describe cross-connection control and backflow prevention programs as measures that can help sustain water quality. In practice, water utilities and local plumbing codes along the Front Range generally require an approved backflow device on irrigation systems connected to the public supply.
Why Sprinkler Systems Are a Contamination Risk
Irrigation water does not stay clean once it leaves the pipe. Heads sit at or below grade, where water collects after each cycle. Soil and mulch around heads and drip emitters can contain fertilizers, herbicides, pesticides, animal waste and bacteria. Some systems have fertilizer injectors plumbed in, and some homeowners attach chemical applicators to hose bibs. When the system shuts off, water drains from low heads, and dirty water can sit in the lateral lines.
EPA’s WaterSense watering tips note that overwatering can produce runoff that carries fertilizers and pesticides into local waterways. The same materials are present in the soil around sprinkler heads. A backflow preventer is what keeps that soil-side water from being drawn into the pipes that feed kitchen faucets.
Types of Irrigation Backflow Preventers
Several device types are used on residential irrigation. Which one is required depends on the local water provider, the plumbing code adopted in the jurisdiction, the degree of hazard and the system design. The descriptions below are general; installation requirements should come from the local utility or building department.
Pressure Vacuum Breaker (PVB)
A pressure vacuum breaker is the most common device on residential irrigation systems along the Front Range. It has a spring-loaded check valve and an air inlet valve. If supply pressure drops, the check closes and the air inlet opens to break any vacuum, preventing back-siphonage. PVBs are generally installed above ground and above the highest sprinkler head on the system, because they protect against back-siphonage but not against backpressure. They have test cocks for annual testing and two shutoff valves.
Double Check Valve Assembly (DCVA)
A double check valve assembly uses two independent check valves in series. It can protect against both back-siphonage and backpressure and can often be installed below ground in a vault in some jurisdictions. DCVAs are generally accepted for lower-hazard applications, and whether they are allowed for irrigation varies by water provider.
Reduced Pressure Zone Assembly (RPZ or RP)
A reduced pressure zone assembly has two check valves with a relief valve between them. If either check fails, the relief valve discharges water to the outside rather than allowing backflow. RPZ assemblies are generally considered the highest level of mechanical protection and are often required where chemicals are injected into irrigation, such as systems with fertilizer injectors. They must be installed above ground where the relief valve can discharge freely, and they will dump water when something is wrong, so placement and drainage need thought.
Atmospheric Vacuum Breakers and Hose-Bib Devices
Atmospheric vacuum breakers (AVBs) are simpler devices sometimes used on individual zone valves, often called anti-siphon valves. They are generally not testable and cannot be under continuous pressure. Hose-bib vacuum breakers screw onto outdoor faucets and protect against back-siphonage from hoses and hose-connected drip kits.
Comparison at a Glance
| Device | Protects against | Typical location | Testable |
|---|---|---|---|
| PVB | Back-siphonage | Above ground, above highest head | Yes |
| DCVA | Back-siphonage and backpressure | Above or below ground (where allowed) | Yes |
| RPZ | Back-siphonage and backpressure | Above ground, with drainage for relief valve | Yes |
| AVB / anti-siphon valve | Back-siphonage | Each zone valve, above highest head | Generally no |
| Hose-bib vacuum breaker | Back-siphonage | Outdoor faucet | No |
Annual Backflow Testing
Testable backflow assemblies have test cocks that allow a certified tester to measure whether the check valves and air inlet or relief valve are working within acceptable limits. Many water providers require testing when a device is installed, after it is repaired or relocated, and periodically after that, frequently every year. Requirements vary, and some providers send reminders, keep records and expect testers to submit results directly.
A typical test involves:
- Visual inspection of the device for cracks, leaks and damage.
- Connecting a differential pressure gauge to the test cocks.
- Closing shutoff valves in a specified sequence and measuring pressure differentials across each check valve.
- For a PVB, checking that the air inlet opens at the right point and that the check holds.
- Recording results and submitting them to the water provider where required.
If a device fails, it usually needs cleaning, a repair kit with new seals and springs, or replacement, followed by a retest. Our guide to backflow testing covers what the tester looks for, who can perform the test and what to do with a failed result.
Freeze Damage and Winterizing in Colorado
An above-ground PVB or RPZ is the most freeze-exposed part of a sprinkler system. Brass bodies, bonnets and internal parts crack when trapped water freezes, and a cracked device can spray water the next time the system is turned on. In much of the Front Range, early October cold snaps and late spring freezes catch systems that have not been shut down or that were started too early.
Winterizing the backflow preventer is part of the overall fall shutdown:
- Shut off the irrigation supply. Close the main irrigation shutoff, usually inside the house in the basement or crawl space, and open the drain on that line if one exists.
- Blow out the system. Compressed air is used to push water out of lateral lines and heads, zone by zone. Air should not be forced through a backflow preventer in a way that damages it; contractors typically connect downstream of the device.
- Drain the device. Open the test cocks and set the shutoff valves partly open, often at a 45-degree angle, so trapped water can escape. Exact steps vary by device.
- Insulate if recommended. Some homeowners cover the device with an insulated pouch, though draining is what really protects it.
In spring, open the supply slowly with the shutoff valves on the device in the correct position. Opening a valve too fast can slam water into the assembly and damage it. Our guide to sprinkler winterization covers timing and the full blowout sequence.
The hose bib next to the backflow device can freeze too. A frost-free hose bib reduces the risk, but only if hoses are disconnected in winter. Our guide to the frost-free hose bib explains how those work and how they fail.
Water Pressure and Backflow Devices
Backflow devices create some pressure loss as water passes through them, and they are affected by both very low and very high supply pressure. Very high pressure can stress seals and springs and contribute to leaks at the device and throughout the irrigation system. EPA’s WaterSense Sprinkler Spruce-Up guidance notes that leaking joints can be a sign of too much water pressure or particles in the water and suggests checking that the pressure regulator is installed properly.
Checking static pressure at a hose bib with an inexpensive gauge is a quick way to know whether household pressure is in a normal range. Our guide to the water pressure test explains how to do it and what readings mean. If pressure is high, the home’s pressure-reducing valve may need adjustment or replacement.
Very low pressure, on the other hand, can make a system look broken when the backflow device is simply partly closed, clogged with debris or has a stuck check. A tester or irrigation contractor can usually sort that out during a service visit.
Signs of a Failing Backflow Preventer
Common symptoms worth investigating include:
- Water spraying or dripping from the bonnet, canopy or test cocks when the system is pressurized.
- A PVB that spits water every time a zone starts and does not stop.
- An RPZ relief valve that discharges continuously or repeatedly.
- Visible cracks in the body or bonnet, often after a freeze.
- Weak pressure at every zone when the supply is fully open.
- A failed annual test.
A small amount of discharge when a PVB first pressurizes can be normal. Continuous leaking is not. Because water running at the device sits next to the foundation in many installations, leaks there should not be left running. Water dumping against a foundation wall all summer is a fast way to saturate the soil where it does the most harm.
Installation and Placement Considerations
Where and how a backflow preventer is installed affects how well it works, how easy it is to test and how likely it is to be damaged. Installation is typically done by a licensed plumber or irrigation contractor, and many jurisdictions require a permit and an initial test. A few placement issues come up repeatedly on Front Range homes.
Height Above the Highest Head
Pressure vacuum breakers and atmospheric vacuum breakers rely on air entering the device to break a siphon. Manufacturers and plumbing codes generally require them to sit a specified distance above the highest downstream head or outlet. On sloped lots, where part of the yard sits above the house, a PVB mounted at the foundation may end up lower than some heads, which defeats its purpose. That situation may call for relocating the device or using a different type.
Clearance and Access
Testers need room to attach gauges and turn shutoff valves. A device boxed in by shrubs, buried behind a window well cover or mounted tight against siding is harder to test and maintain. RPZ assemblies also need somewhere for relief-valve discharge to go without flooding a window well or soaking the foundation.
Protection From Damage
Above-ground assemblies are exposed to lawn equipment, snow shovels, vehicles and theft of brass parts. Locking covers and insulated enclosures are available, though any enclosure must still allow testing and, for RPZ devices, drainage. Enclosures do not replace draining the device in fall.
Repairs After Freeze Damage
When a freeze-cracked device is replaced, it is a good time to confirm that the new assembly is the type the water provider currently accepts, that it is high enough, and that it is placed where water from leaks or discharge drains away from the house. A retest after replacement is commonly required.
Irrigation Backflow Preventer Costs
Costs vary by device type, size, local requirements and labor rates. The ranges below are broad and hedged.
| Item | Rough range | Notes |
|---|---|---|
| Annual backflow test | Often under $150 | Some providers bundle with startup or blowout |
| Repair kit installed (seals, springs, bonnet) | Often around $100 to a few hundred dollars | Depends on parts and device |
| Replace residential PVB | Often a few hundred dollars and up | Includes parts, labor and retest |
| Replace or install RPZ | Often more than a PVB | Larger assemblies and drainage needs add cost |
| Fall blowout | Often under $150 for a typical home | Varies by system size and provider |
A freeze-cracked PVB replaced in spring is one of the most common irrigation repairs along the Front Range. Compared with that cost, a properly done fall blowout and draining is usually inexpensive.
Backflow Preventers and Home Inspections
A standard home inspection usually does not test the irrigation backflow preventer. The ASHI Standard of Practice states that inspectors are not required to inspect landscape irrigation systems. Some inspectors note visible damage to the device, missing test records or an obviously improper installation, and may recommend that a certified tester evaluate it. When a home is sold in winter, the system and device are typically shut down, so ask the seller for recent test records and consider asking for a spring test or a credit.
A missing backflow preventer on an irrigation system connected to public water is worth flagging with the local water provider. For more on drip zones and their own backflow needs, see our guide to drip irrigation. For more on what a general inspection covers and how to choose an inspector, see our guide to hiring a home inspector and the plumbing and water hub.
References
- Drinking Water Distribution System Tools and Resources: Cross-Connections and Backflow — U.S. Environmental Protection Agency
- Sprinkler Spruce-Up — U.S. Environmental Protection Agency, WaterSense
- Watering Tips — U.S. Environmental Protection Agency, WaterSense
- ASHI Standard of Practice for Home Inspections — American Society of Home Inspectors
Frequently asked questions
Do all sprinkler systems need a backflow preventer?
Irrigation systems connected to a public drinking water supply generally need an approved backflow device under local plumbing codes and water provider rules. The specific type required varies, so check with the local water utility or building department.
What is the difference between a PVB and an RPZ?
A pressure vacuum breaker protects against back-siphonage and must sit above the highest sprinkler head. A reduced pressure zone assembly has two checks and a relief valve, protects against back-siphonage and backpressure, and is often required where chemicals are injected.
How often does an irrigation backflow preventer need testing?
Many water providers require testing at installation, after repairs and periodically after that, often annually. A certified tester uses a differential pressure gauge on the test cocks and submits results where the provider requires it.
Why is my backflow preventer leaking?
Common causes include freeze damage, worn seals or springs, debris on a check or air inlet seat, and high water pressure. A brief spit when a PVB pressurizes can be normal, but continuous leaking needs repair or replacement and a retest.
How do I winterize a sprinkler backflow preventer?
Shut off the irrigation supply, blow out the system, then open the test cocks and set the device's shutoff valves partly open so trapped water drains. Exact steps vary by device, so follow the manufacturer's or contractor's instructions.
Looking at a Front Range home with a backflow device that has not been tested in years? Contact our team and we can help you find an inspector who will note visible damage, water against the foundation and pressure concerns so you know what to ask the seller.