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IAPMO
The Ins and Outs of Backwater Valves 
 
 

A backwater valve is a plumbing component used to prevent the backflow of sewage into a building. Backwater valves are also used to protect against backflow into fixture drains and branches of the building drain, as well as in the storm drainage system. A backwater valve can be an integral part of a waste drainage system and may be required by local building codes. Commonly used backwater valves include normally open backwater valves and normally closed backwater valves. 

The plumbing industry classifies backwater valves into four types. Type I backwater valves are used in horizontal pipe runs, such as a building drain or building sewer, up to a maximum burial depth of 24 inches. Type II backwater valves are an integral part of a floor drain, or they attach to a floor drain. Type III backwater valves are designed for use in vertical piping.

Type IV backwater valves have the same intended use as Type I valves. However, they can be buried deeper than 24 inches because they are extendable. This means they can be equipped with a valve extension and extension pipe (lifting device). The valve extension allows ground-level access to the valve, and the extension pipe allows the valve seat and/or gate (flapper) to be removed for service and inspection.

Some type IV backwater valves are equipped with a removable cassette, while others are equipped with a removable gate. A cassette contains the valve seat and the valve gate, which are the working components of the valve. A gate is just the flapper portion of the valve. Backwater valves with a removable cassette are more versatile because they can be adapted to different applications. For example, a cassette with a normally closed gate can be replaced by a cassette with a normally open gate.

Normally Open Backwater Valves
According to Section 710.6 of the UPC, a backwater valve installed in a building drain or building sewer must be of the normally open design to allow the free circulation of air. There are three types of normally open backwater valves: flapper type backwater valves, ball float backwater valves, and automatic knife backwater valves. Each type of valve has a different method of operation. All three valves are type I backwater valves.

A flapper-type backwater valve is a valve that has a hinged flapper that rests on the bottom of the valve in the normally open position. It floats up and closes only when a reversal of flow occurs. Some normally open flapper-type backwater valves have a clear plastic cover to allow visual inspection.

A ball float backwater valve is a valve that has a gate attached to a counterweight that rests on a float. The counterweight holds the gate in the open position. During a reversal of flow, the float rises and pushes up on the counterweight, which closes the gate. Commonly known as the ML-FR4, the ball float backwater valve is ideal for existing systems because it is easy to install and requires less concrete breaking compared to other backwater valves. However, it can also be used in new installations.

An automatic knife valve is a backwater valve that contains a steel knife gate and an expansion chamber. The knife gate has an opening aligned with the drain pipe to allow liquid to flow freely. When backflow occurs, trapped air is forced into the expansion chamber. Air pressure in the expansion chamber causes the knife gate to rise, preventing water from flowing back up the drain.

A counterweight on top of the knife gate forces the gate down when pressure inside the expansion chamber returns to normal. An automatic knife valve can be equipped with an alarm to signal a backwater condition. However, automatic knife valves are more expensive than other backwater valves and require routine inspection and maintenance.

Normally Closed Backwater Valves
The plumbing code permits normally closed backwater valves in parts of the sanitary systems serving fixture drains and in branches of the building drain. They are also permitted for use in the storm drainage system. Normally closed backwater valves are not allowed in the main building drain or the building sewer because they prevent the free flow of air throughout the System.

In a normally closed backwater valve, waste water pushes the hinged gate open, allowing flow through the valve. In a backwater condition, the gate is in the normally closed position, stopping the reversal of flow.

Local plumbing codes may vary regarding the types of backflow devices allowed on fixture drains and branches. Therefore, the authority having jurisdiction should be referenced concerning the use and installation of backwater valves.