Backflow Preventer vs Check Valve

Figure 1: A backflow preventer (up) and check valve (down).

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While having similar functions, backflow preventers and check valves are distinctly different devices by application. A backflow preventer is necessary to prevent contaminated water from flowing into potable or municipal water sources. Check valves are also one way valves, but cannot offer the high level of protection that backflow preventers do. This article overviews the primary differences between backflow preventers and check valves and describes different types of backflow preventers.

Table of contents

What is backflow?

Backflow is the reversal of a fluid's flow due to back pressure or back siphoning. If backflow occurs, it can cause issues for certain applications. For example, if contaminated water backflows into a clean water supply.

• Back pressure: The demand side pressure exceeds the supply side pressure, causing the media to move towards the supply side.
• Back siphoning: The supply-side water pressure significantly decreases, causing contaminated water to be drawn into the supply. This can happen due to a break in the water main or a discharge from a fire hydrant.

What is a backflow preventer?

Figure 2: A backflow preventer on a water main pipe.

A backflow preventer&#;s function (Figure 2) is to stop water from flowing in the opposite direction. The only purpose of a backflow preventer is to stop contaminated water from flowing into a source of potable water. Backflow preventers are installed at cross-connection points, such as where municipal drinking water enters a residential or commercial building or where the water supply attaches to appliances like a washing machine or dishwasher. Check with the local regulations to understand exactly where backflow preventers are necessary in a plumbing system.

High degree of protection

Backflow preventers offer protection through their multiple failsafe features in case of backflow. One example is the double check valve assembly (DCVA) which has two check valves and an atmospheric vent for protection, with a backup valve in case of failure. The vent creates an air gap to prevent backflow from backpressure. If the vent becomes blocked, a relief valve opens to release pressure and prevent contaminated water from flowing back into the main supply. The DCVA's combination of check valves, vent, and relief valve provides a failsafe design to protect the main water supply. Other types of backflow preventers similarly offer multiple failsafe features.

What is a check valve?

Figure 3: A brass check valve

Like a backflow preventer, a check valve stops media from flowing through it in the reverse direction. Check valves, however, do not offer the degree of protection that backflow preventers do and therefore are not suitable for protecting drinking water sources. If foreign material enters the check valve, it may block it open. In this case, there is no failsafe; the check valve will leak. Also, when check valves close too quickly, they become vulnerable to water hammer, which will damage the valve. Despite this, check valves are applicable for many applications. Read our water check valve and hydraulic check valve articles to learn more about specific applications. To learn about these valves in general, read our check valve overview article.

Backflow preventer vs check valve

The following table outlines the most important differences between check valves and backflow preventers.

Backflow preventer Check valve Main Function Stops contaminated water from flowing into potable water sources Stops fluid from flowing through it in the opposite direction Applications Protects drinking water sources Aquariums, sewer systems, plumbing systems, air lines, sump pumps, and more Protection Multiple layers of protection. For example, two check valves One layer of protection: the valve&#;s disc on the seat Installation location At the outlet of wherever drinking water flows into systems that can contaminate it (e.g., appliances, from city water to residential plumbing system) Wherever backflow can damage part of the system. For example, a check valve stops water from flowing out of a well pump, which keeps it primed. Cost 2 - 3x more expensive than check valves Less expensive

Types of backflow preventers

There are different types of backflow preventers with varying mechanisms. The best backflow preventer for an application may depend on local regulations or specifics of the plumbing system.

• Air gap: An air gap is a large non-mechanical backflow preventer that requires a relatively large amount of space but prevents contaminated water from entering the public water supply by having the discharge pipe sit at least two times its diameter above the receiving vessel.
• A double-check valve assembly (DCVA): A double-check valve assembly (DCVA) is a backflow preventer suitable for low to medium-hazard applications such as irrigation or fire sprinkler systems. It consists of two independent check valves that work together to prevent backflow.
• Hose bib vacuum breaker: A hose bib vacuum breaker is for outdoor faucets that connect to a hose. It uses a single spring-loaded inline valve to prevent back siphoning from a non-potable water source (e.g., a bucket, pool, or puddle) through the hose's end by closing and opening depending on the water pressure.
• Reduced pressure zone device (RPZD): RPZDs are applicable for high-risk applications. An RPZD has two independent spring-loaded check valves, a pressure-monitored chamber called the zone and a differential pressure relief valve. It is suitable for safeguarding drinking water and deterring health hazards.
• Atmospheric vacuum breaker (AVB): An AVB is a brass backflow preventer that uses normal water pressure to hold up a poppet valve, preventing back-siphonage by allowing air in when pressure falls; it is mainly used in irrigation systems with non-potable water supply downstream from the protected potable water source.
• Pressure vacuum breaker (PVB): A PVB is a type of backflow preventer with a spring-loaded poppet valve, allowing it to be used in systems where the non-potable water supply is upstream, and it has test cocks with calibrated gauges for proper functioning, commonly used in underground sprinkler systems.

FAQs

Is a check valve a backflow preventer?

Check valves and backflow preventers are two different devices. Backflow preventers are specifically designed to prevent contaminated water from mixing with potable or municipal water sources.

Is a check valve a safe substitute for a backflow preventer?

A check valve cannot be regarded as a safe substitute for a backflow preventer, especially when safeguarding drinking water.

What is the difference between a backwater valve vs check valve?

A backwater valve is used to prevent sewage from flowing back into a building. A check valve stops media from flowing through it in the reverse direction. These two valves differ in their application.

What is the difference between a backwater valve vs backflow preventer?

A backwater valve stops sewage from flowing back into a building whereas a backflow preventer valve prevents contaminated water from mixing with drinking water.

Foot Valve vs Check Valve

Figure 1: Foot valve (left) and check valve (right).

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In short, a foot valve and a check valve are very similar in that they automatically open or close depending on system pressure, allowing flow in only one direction and preventing backflow. Figure 1 shows a check valve on the right and a foot valve on the left, with the main visual difference being that the foot valve has a strainer on it. Given the similarities between the two valves, there are important differences. Foot and check valves have different installation locations in a system, different material requirements, and different designs. This article will focus on these key differences. Read our comprehensive articles about check valves and foot valves to learn more about each type of valve.

Table of contents

 

Operation principle

Understanding how check and foot valves work is helpful before discussing their differences. Both valve types open when the inlet pressure is above the valve's cracking pressure. The cracking pressure is the minimum amount of pressure required to open the valve and overcome the force keeping the valve closed (spring or gravity). When the inlet pressure reduces below this limit, or there is backpressure, the valve closes shut.

Differences between check valves and foot valves

Design

Foot and check valves have a variety of design types, for example, ball check valves and ball foot valves. This section, though, focuses on the design differences that exist between all check and foot valves.

• Screen: The first visual difference is the screen attached to the foot valve's inlet end, which is often called a strainer or filter. Foot valves typically sit submerged in water in a well. The screen prevents larger debris from entering the foot valve and sticking the valve's disc open and damaging other components within the system. A check valve does not have this protection and therefore is not applicable for media that has large solids in the flow.
• Threading: A check valve has threading on both sides. So, a check valve fits into any part of the piping deemed appropriate. Removing a section of the pipe and installing a check valve is a straightforward process. Foot valves, however, have threading only on one side. Therefore, foot valves are only suitable for the end of a pipe, which is the end of a pump&#;s suction line.

Material

The key material difference between foot and check valves is that foot valves are in water for the duration of their use. Therefore, whichever material selected must be corrosion resistant. Materials often chosen are PVC, heavy-duty cast iron, bronze, and stainless steel.

Check valves have a wider range of material options because they operate in a wider range of environments. When selecting a check valve material, first understand the system's pressure, temperature, and operating environment. Read our chemical resistance guide to learn more.

Installation

Because foot valves work on pump systems, this section will only cover the installation locations of check and foot valves on these systems. Both valves stop media from flowing back into the well when the pump turns off, thus keeping the pump primed.

A jet pump uses a foot valve at the very end of its suction line. In contrast, a submersible pump has a foot valve directly installed on its inlet. Both pump systems can use check valves in the same locations. However, this is not suitable for any wells that may contain solids large enough to get stuck in the valve and hinder its operation.

Deep well systems use one or more extra check valves along the suction line to protect the submersible pump and foot valve from the water column&#;s pressure. Shallow well systems may have a check valve on the suction line. Also, check valves can install directly at the jet pump&#;s inlet or between the jet pump and pressure tank. Beyond stopping backflow into the well, check valves are applicable anywhere where backflow may damage an upstream component or contaminate upstream media.

Foot valve and check valve P&ID symbols

Figure 2 depicts a check valve symbol (left) and a foot valve symbol (right).

Figure 2: Check valve symbol (left) and foot valve symbol (right).

Summary

Table 1: Comparison between foot valve and check valve

Foot valve Check valve Design A foot valve has a strainer on the inlet side. A check valve does not have a strainer. Material Foot valves have a limited selection of materials: stainless steel, heavy-duty cast iron, PVC, and bronze. Check valves have more material options because they do not rest in water. Application Foot valves are used for suction lift applications, like a well pump. Check valves are applicable for pump systems and any system that requires backflow prevention. Installation Foot valves only go at the end of a pump&#;s suction line. Check valves can go at the end of a suction line, in the middle of the suction line, and anywhere else in the system where necessary. Threading Foot valves have threading on the outlet side only. Check valves have threading on both sides.

FAQs

What is the difference between a foot valve and a check valve?

A foot valve has a screen on its inlet side to prevent large solids from entering the valve. Also, it fits at the end of a suction line in a pump system. A check valve is suitable for any system, including pump systems, that require media to flow in only one direction.

Can a check valve be used in place of a foot valve?

Yes, a check valve can be used in place of a foot valve. However, check valves do not have a protective screen and any large solids in the media can stick them in the open position.

 

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