Understanding the Differences - Type I vs. Type II Backflow Detector Assemblies

By "Apollo" Valves
August 1, 2017

All buildings must comply with the water utilities requirement to protect the water distribution system. This term is commonly referred to as a Cross-Connection Plan. If your state has mandated the water utility to implement a Cross-Connection Plan, most likely they are using one of two methods.

The most popular plan is referred to as “Containment Protection”. Once the water enters the building through the backflow protector, the assembly makes sure that it does not return back into the water distribution system. This method is the water utilities preferred method, because it brings redundancy protection into their system. While the plumbing code protects cross-connections within the building, the water utilities containment assembly makes sure that none of that water can return back into the water distribution system.

The second program is commonly referred to as “Isolation Containment Protection”. It has no redundant containment backflow assembly protecting the water distribution system. It relies solely on making sure every cross-connection within the building has been identified, controlled or eliminated. For obvious reasons these programs are more difficult to manage, invite jurisdictional wars between water utilities and the plumbing code enforcement and offer no greater safeguards to the water distribution system.

Since containment service protection offers the greatest protection to the water utilities, every building will require a backflow assembly immediately downstream of the water meter. All backflow assemblies must conform to a specific American Society of Sanitary Engineering (ASSE) Standard. Some assemblies protect only against backsiphonage, some for low hazard applications. The ASSE 1013 Reduced Principal Backflow Assembly (RP) is used in buildings that must protect against both backpressure and backsiphonage high hazard applications. While the ASSE 1015 Double Check Valve Assembly (DCVA) also protects against backpressure and backsiphonage, it only protects against low hazard applications because it has no relief. These are the only two standards that can be used to protect domestic and fire containment water protection.

Fire sprinkler systems must also adhere to these same above mentioned regulations. Because water meters are frowned upon on fire sprinkler systems (Pressure loss across the water meter) some utilities require a Detector Assembly (DA). The purpose of the Detector Assembly is to capture water losses up to 2-GPM in the system by either theft or leak in the system. Because these assemblies must be designed and built differently, they must follow a different standard. The ASSE 1047 RPDA and ASSE 1048 DCDA with Type l and Type ll configurations.


Both the Type l and Type ll must offer the same by-pass protection. Type l backflow assemblies offer the protection around the mainline, before the first check and after the second check. Since there are two independent backflow assemblies (mainline and by-pass with meter), each assembly has its own unique ID tag (Make, Model, Serial Number, etc.)

Type ll assemblies offer the same type of protection as Type l, only they share the mainlines first check and relief and have a single independent second check positioned downstream of the mainlines second check. Since all detector checks require two unique ID’s, the location of the Type ll ID Tag is positioned on the single by-pass second check.


Through better design, engineers have realized that if they share the first check and relief from the main line assembly, a lower head loss pressure can result across the entire detector check assembly. This engineering design has a lower cost associated against the Type l, because redundant backflow assemblies are more expensive than a single check valve.

Since this assembly is fairly new to some backflow testers, it is worth noting that when testing Type ll Detector Assemblies, you must test the mainline backflow assembly then restart the process all over again by testing the by-pass first check, relief valve again, then obtain a numerical number value for the single by-pass check assembly.


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