What Is a Double Check Valve: A Practical Guide to Backflow Prevention

In plumbing and water supply systems, safeguarding drinking water from contamination is essential. A double check valve is one of the most widely used devices for backflow prevention in domestic, commercial, and industrial settings. This guide explains what is a double check valve, how it works, where it is used, and what to consider when selecting and maintaining one. It also explores common issues and how to test and certify double check valve assemblies in the UK and beyond.

What is a double check valve and why it matters

What is a double check valve? Put simply, it is a backflow prevention device designed to stop water from flowing backwards through a supply line when pressure conditions change. It achieves this by employing two independently operating check valves in series, each of which opens to permit forward flow and closes if backflow threatens. The result is a reliable barrier against back-siphonage and backpressure, protecting the purity of the potable water supply.

Unlike a single check valve, which can fail if debris or wear prevents a seal, a double check valve provides an additional layer of protection. It is typically used where the potential hazard posed by backflow is considered moderate. In many jurisdictions, including the UK, double check valve assemblies are a recognised and regulated solution, often preferred for their balance of reliability, cost, and ease of maintenance.

How a double check valve works: the basics

To understand what is a double check valve, it helps to picture two check valves arranged in series within a compact body. Water flows in one direction, and both valves open to allow it to pass. If pressure reverses or backflow occurs, the internal forces push the seats onto their seals, stopping flow. The arrangement typically includes test cocks and a small internal chamber that allows testing and isolation without removing the device from the pipework.

Key components

• Two independently operable check valves in series
• A test cocks assembly for periodic testing and commissioning
• A bonnet or cover housing the internal components
• In some designs, a small chamber between the two checks and/or a vent path for testing

What makes the difference in performance

The two check valves share the workload of preventing backflow. Should debris or mineral build-up cause one valve to stick slightly, the second valve can still provide a barrier to backflow. This redundancy is a primary reason for choosing a double check valve over a single check valve in many installations.

Where and when to use a double check valve

Double check valve assemblies are well-suited to a range of applications where backflow hazards are present but not considered high risk. Typical uses include:

• Domestic cold and hot water lines where reduced risk of contamination from irrigation or appliances is acceptable
• Industrial facilities with moderate hazard processes that could create backflow into the potable water supply
• Commercial buildings such as offices, schools, and hotels where multiple fixtures share a common water supply
• Irrigation systems and landscaping where contamination sources might include fertilizers or chemicals, but where the local risk is managed

In the United Kingdom and many other countries, double check valve installations must comply with local regulations and be approved by the relevant authorities. In the UK, WRAS (Water Regulations Advisory Scheme) approval is commonly required, and installations should align with BS EN 1717 guidelines for backflow protection.

When a double check valve is preferable to other options

• Cost and simplicity: Double check valve assemblies are generally more affordable and easier to install than higher-hazard devices.
• Maintenance: They offer straightforward testing and servicing, often with readily available replacement parts.
• Adequate protection: For many commercial and domestic scenarios, a DCV provides reliable protection against backflow without the complexity of more advanced devices.

Comparisons: double check valve vs other backflow prevention devices

Understanding how what is a double check valve compares to other backflow devices helps in making an informed choice.

Double Check Valve Assembly vs. DCV alone

A double check valve assembly (DCVA) combines a double check valve body with a service line shut-off and test cocks. It is designed for ease of isolation and testing, fulfilling installation and maintenance requirements for backflow prevention programs. A simple DCV may refer to the two-check design without the more formalised assembly features. In practice, most installations in building services use the DCVA configuration.

Double Check Valve vs. Reduced Pressure Zone (RPZ) devices

RPZ devices provide a higher level of backflow protection and are used for high-hazard cross-connection scenarios. They incorporate a relief valve and are more complex and expensive. A double check valve is suitable for moderate hazard contexts, while an RPZ is selected where backflow could introduce significant contamination risk to the potable water supply.

Double Check Valve vs. air gap and backflow preventers

Air gaps and backflow preventers offer robust protection in certain installations, particularly where physical separation from potential contaminants is essential. These solutions are often used for fixtures like dishwashers or sinks where backflow risks are higher or where plumbing codes require an air gap. The double check valve remains a practical choice where space, cost, and maintenance considerations favour a mechanical barrier within the supply line.

Installation considerations: key factors for success

Proper installation is critical to the effectiveness of what is a double check valve. Poor placement or incorrect orientation can undermine protection and lead to performance issues or failure of the device.

Location and orientation

Double check valve assemblies are typically installed on the cold water supply to the building or to a specific branch serving critical fixtures. Orientation generally does not affect the fundamental operation, but manufacturers often specify a preferred alignment to accommodate test cocks and ensure accessibility for maintenance. It is important to provide enough clearance around the unit for testing and replacement of components.

Connections and fittings

Use appropriate pipework connections, such as unions or nipples that are compatible with the valve body. Avoid using rigid pipework that could stress the device during water hammer or thermal expansion. Ensure that the assembly is installed with a straight run into and out of the device to minimise turbulence and wear on the seals.

Testing and commissioning

Following installation, the double check valve should be tested by a qualified professional to verify that both check valves seal properly and that backflow conditions are not able to overcome the barrier. The test typically involves pressurising the test ports and applying a defined test sequence to prove closure of each valve. In many regions, certification of the test is required and must be documented for compliance records.

Testing, maintenance and signs of wear

Regular testing and maintenance are essential to ensure what is a double check valve continues to protect the water supply effectively. Without periodic testing, gradual wear or debris could compromise the performance of one or both check valves.

Routine testing procedures

Annual or more frequent tests may be mandated by local regulations or the building’s water supplier. A qualified backflow tester will usually perform:

• Shut-off of the DCVA using test cocks
• Measurement of pressure across each valve to confirm proper seating
• Assessment of the intermediate chamber and test ports for blockages or leaks
• Documentation of test results and issuance of a certification for asset records

Common issues and maintenance tips

• Sticking or leaking valves due to debris or mineral buildup
• Wear to the seat diaphragms or discs reducing seal integrity
• Corrosion or damage to the body or test cocks
• Improper installation leading to backflow risk from elevated pressures or adverse piping configurations

Maintenance strategies include regular cleaning, replacing worn parts, and verifying that test ports are unobstructed. If a device fails a test, it should be repaired or replaced promptly to restore protection.

Standards, approvals and regulatory context

Regulatory frameworks around what is a double check valve emphasise safety, reliability, and traceability. In the UK, compliance often requires WRAS approval for the device itself and adherence to BS EN 1717 guidelines for backflow prevention. This standard outlines general principles for preventing contamination of potable water through backflow and provides guidance on selecting and installing backflow prevention devices. Local water suppliers may have additional requirements for testing intervals, documentation, and servicing by approved contractors.

On a broader scale, backflow prevention devices like the double check valve are part of an integrated approach to water safety. Building owners and facilities managers should maintain records of installation dates, test results, and any maintenance actions. The aim is to provide a clear audit trail that demonstrates ongoing protection of the drinking water supply.

Choosing the right backflow protection: a practical decision guide

When deciding what is a double check valve and whether it is the right choice for a given installation, consider the following factors:

• Hazard level: Is the potential contaminant risk class moderate or high? For moderate hazards, a double check valve is often appropriate; for higher hazards, an RPZ or other robust device may be necessary.
• Water pressure and dynamics: Will the system experience significant pressure fluctuations or backpressure scenarios? Ensure the device can tolerate the expected operating conditions.
• Maintenance commitments: Are there qualified personnel available to perform regular testing and maintenance? DCVA installations benefit from accessible test cocks and straightforward servicing.
• Code and supplier requirements: Do the local regulations specify a particular device type or testing frequency? Always align with WRAS, EN standards, and the local water supplier’s guidelines.
• Financial and logistical considerations: Compare initial costs, ongoing maintenance, and parts availability when selecting between a double check valve assembly and higher-hazard alternatives.

Practical tips for installers and facility managers

To maximise reliability and compliance, keep these practical guidelines in mind when dealing with what is a double check valve in real-world projects:

• Choose a DCVA with accessible test cocks and clear installation instructions.
• Plan for future maintenance by ensuring adequate space for service access and parts replacement.
• Label the device clearly in the building’s water system map and maintain up-to-date test certificates.
• Coordinate with the building’s water authority or supplier to confirm testing requirements and acceptable device models.
• Educate site staff about the purpose of backflow prevention and the importance of not bypassing the valve or interfering with the device.

Frequently asked questions about what is a double check valve

Q1: What is a double check valve used for in a domestic setting?

A double check valve is used in domestic plumbing to prevent backflow from appliances or irrigation systems into the potable water supply. It helps ensure that contaminants do not enter the drinking water network when other parts of the system experience pressure drops or sudden changes in flow direction.

Q2: How often should a double check valve be tested?

Testing frequency varies by jurisdiction, but annual testing is common in many regions. In some cases, more frequent checks may be mandated, especially for commercial buildings or systems with higher risk of backflow. Always follow local regulations and the recommendations of the water supplier.

Q3: Can a double check valve be installed in any position?

Most double check valve assemblies are designed to operate in any orientation, but installation guides should be followed precisely. Some orientations may affect maintenance access or drainage of the test ports, so it is best to install per the manufacturer’s instructions and local codes.

Q4: How does a double check valve differ from a backflow preventer with an air gap?

A double check valve provides a mechanical barrier within the plumbing line. An air gap is a physical separation between the end of a water supply and a potential source of contamination, such as a drain or appliance. In some installations, both approaches may be used in conjunction to meet code requirements or specific hazard assessments.

Sustainability and long-term reliability

Investing in reliable backflow prevention, including what is a double check valve, supports public health and reduces the risk of costly contamination incidents. The long-term value comes from predictable performance, straightforward maintenance, and compatibility with standard testing and certification procedures. Durable materials, proper sizing, and timely replacement of worn parts all contribute to the overall resilience of the water system.

Conclusion: what is a double check valve in summary

In the landscape of backflow prevention, what is a double check valve stands out as a practical and dependable solution for many moderate-hazard applications. Its dual-valve design provides redundancy, ease of testing, and reasonable maintenance requirements, making it a favourite for domestic and commercial installations. By understanding how a double check valve works, where it should be applied, and how to test and maintain it, engineers, plumbers, and building managers can protect precious water supplies while meeting regulatory expectations.

Glossary: quick definitions you may encounter

Backflow prevention: Techniques and devices that stop water or other liquids from flowing backwards in a supply line, protecting the potable water network.

DCVA (Double Check Valve Assembly): A complete backflow prevention assembly that uses two check valves in series and includes test cocks for testing and maintenance.

Test cocks: Accessible ports on backflow prevention devices used during testing to measure valve seating and performance without removing the device from the line.

WRAS approval: Certification indicating that a product complies with water supply regulations in the UK and is approved for use in contact with potable water.

BS EN 1717: A European standard providing guidelines for backflow prevention and pressure and temperature considerations to protect drinking water supplies.

Final thoughts: implementing what is a double check valve responsibly

Choosing and installing a double check valve is more than a technical decision; it is a commitment to safe drinking water and regulatory compliance. By selecting the right device, ensuring proper installation, and maintaining ongoing testing and documentation, property owners and managers can foster a reliable, compliant, and sustainable water system that serves occupants well for years to come.