Learn about Siamese fire department connection components, uses, and accessories
The fire industry has lingo for all kinds of equipment, and one term that’s often used is “Siamese connection.” It really just describes a fire department connection (FDC) featuring at least two twin female inlets that are attached at an angle to the body (hence the term “Siamese”), and it plays an essential role in providing water to fire sprinkler and standpipe systems. Fire departments typically supply water to standpipe FDCs and then access internal hose connections to fight fires within larger buildings. Or they supply water through FDCs to supplement flow and pressure in sprinkler systems. The two Siamese-like couplings connect to supply hoses that pump water from fire engines or hydrants.
Know enough already and need to buy equipment? Browse QRFS’s FDC Siamese Connections and accessories here.
Keeping Siamese connections code-compliant
FDCs are one of the most important tools for firefighters at the scene of a fire, and there are several placement requirements within NFPA 14: Standard for the Installation of Standpipe and Hose Systems that guarantee first responders can use them quickly and easily. Essentially, FDCs must be visible, accessible, unobstructed, and within an acceptable distance from fire hydrants:
From the 2019 edition of NFPA 14
6.4.5.1 Fire department connections shall be visible and recognizable from the street or nearest point of fire department apparatus accessibility or on the street side of buildings.
6.4.5.1.1 Fire department connections shall be located and arranged so that hose lines can be attached to the inlets without interference from nearby objects, including buildings, fences, posts, landscaping, vehicles, or other fire department connections.
6.4.5.4* Fire department connections shall be located not more than 100 ft (30.5 m) from the nearest fire hydrant connected to an approved water supply.
FDCs also have signage requirements that vary based on the system they serve:
6.4.5.2 Each fire department connection shall be designated by a sign, with letters at least 1 in. (25.4 mm) in height, that reads “STANDPIPE.” For manual systems, the sign shall also indicate that the system is manual and that it is either wet or dry.
6.4.5.2.1 If automatic sprinklers are also supplied by the fire department connection, the sign or combination of signs shall indicate both designated services (e.g., “STANDPIPE AND AUTOSPKR” or “AUTOSPKR AND STANDPIPE”).
6.4.5.2.2 A sign also shall indicate the pressure required at the inlets to deliver the standpipe system demand.
6.4.5.2.2.1 The pressure required sign shall not be required when the pressure required is 150 psi (10.3 bar) or less.
6.4.5.3 Where a fire department connection services multiple buildings, structures, or locations, a sign shall be provided indicating the buildings, structures, or locations served.
In addition, FDCs must be maintained according to rules in NFPA 25: Standard for the Inspection, Testing, and Maintenance of Water-Based Fire Protection Systems. Among the key tasks is a quarterly inspection:
From the 2023 edition of NFPA 25
13.8.1 Fire department connections shall be inspected quarterly to verify the following:
(1) Fire department connections are visible and accessible.
(2) Couplings or swivels are not damaged and rotate smoothly.
(3) Plugs or caps are in place and undamaged.
(4) Gaskets are in place.
(5) Identification signs are in place.
(6) Check valve is not leaking.
(7) Automatic drain valve is in place and operating properly.
(8) Fire department connection clapper(s) is in place and operating properly.
(9)* Interior of the connection is inspected for obstructions.
(10) Visible piping supplying the fire department connection is undamaged.
For a far more in-depth look at FDC upkeep, including various testing requirements, read our blog: “Fire Department Connections: A Guide to FDC Maintenance.”
The basic components of a Siamese connection
- The first thing people notice about the Siamese connection is the female inlets, which come in several sizes. Most buildings require 2½” inlets. However, 3” connections are used in New York City and San Francisco and 4″ and 5″ “Storz” FDCs are becoming common replacements for traditional threaded models due to their simple, quarter-turn connections.
- Siamese FDCs will contain either a single or double clapper to channel the flow of water coming into the system. With a single clapper model, a lone clapper swings freely to cover any unused inlets via water pressure. If one hose is connected, the other side is kept shut by pressurized water. If two hoses are connected, the equal pressure will cause the clapper to sit in the middle of the body and let water flow in from each side. In contrast, double clappers have one clapper per inlet and provide greater control and durability. Aside from how they function, the biggest difference between single- and double-clapper models is the price; double-clapper FDCs are more expensive.
This FDC contains a single clapper. - The attached, threaded brass ring that connects fire hoses, breakable caps, and plugs is known as the swivel. The swivel features lugs on the outer ring to support breakable caps.
A Siamese FDC with one swivel (left side) attached. - The next major visible components are FDC plugs or breakable caps. The inlets in Siamese connections need to be covered. Exposure can lead to the accumulation of foreign material which can clog waterways. FDC plugs, which come standard with most purchases, feature a chain for quick retrieval when removed. But other FDCs, local rules permitting, use breakable caps that alleviate worries about stolen plugs. For more information on which to choose, read “FDC Caps or Plugs: What Are My Options?”
A 2.5″ FDC plug with a chain. 
Breakable plastic caps can be used to cover siamese connection inlets. - As mentioned above, siamese connections must have minimum signage. First responders need quick, specific system information when arriving on the scene — whether it’s what type of standpipe or sprinkler system is involved or if an FDC-connected system reaches throughout an entire building. While FDCs are often marked with stamps, signage also takes the form of an escutcheon that surrounds the FDC body’s entry into the building. Examples of common escutcheon signage include: “Standpipe & Auto-Sprinkler”, “Automatic Sprinkler”, “Test Connection”, or “Standpipe System.”
A common 4” inner-diameter FDC escutcheon can be found here.
Things to consider when browsing for a Siamese connection
The number of female inlets and clappers, as well as the thread type, are the most important features to make sure are correct. To do this, check with local authorities; jurisdictions can differ in preferences. Since they are outside, the inlet coverings to Siamese connections can also frequently ‘go missing,’ which means that building owners must purchase replacement plugs or caps. Again, these components are vital to prevent foreign material from clogging the FDC and a system’s waterways.
QRFS.com is proud to offer Siamese connections, other variations of FDCs, and all the accessories needed to keep these devices working and compliant:
Shop FDCs, signs, caps and plugs, and other parts here!
This blog was originally posted by Jason Hugo and Cameron Sharp at blog.qrfs.com on January 2, 2019, and updated on Dec. 28. 2022. If you like what you’ve read, check us out on Facebook or Twitter @QuickRsponseFS.
Im looking for a price and availability on a 4″ female thread siamese wye fire dept. connection
Brad — thank you for reaching out. We will have someone from our QRFS Customer Support Team contact you, and you can also email your request there!
Dear Sir,
I need your advice,
what are the basis for selection of the Siamese connection (two way or four way)?
also, kindly advice, what are the recommended accessories which should be installed/provided with the Siamese connection (i.e.: non return valve and else).
Thanks is advance
Taha — the obvious accessories for an exposed FDC are caps or plugs to keep debris out of the inlets. As for selecting an FDC with a different number of inlets (siamese refers to a specific exposed FDC with 2), whether a building has 1, 2, 4, etc. is typically determined by the standpipe design requirements based on the size of the building, the specific standpipe/sprinkler system, and the possible needed water supplementation by the fire department. Building owners often buy a direct replacement for what they have, but the best course of action is to consult the system design documents or a professional water-based system designer or maintenance professional.
if i have 3 standpipes in a building is it required to have 3 siamese connections at the exterior of the building?
Mark — A siamese connection is really just a specific type of fire department connection (FDC) that has water inlets at an angle. Some buildings/standpipe systems have multiple FDCs or a single FDC between a water supply and multiple standpipes, and the number of inlets on FDCs can vary based on pressure/flow requirements and building size, etc. You should consult a qualified fire protection professional to determine the exact needs. You can also try our Ask a Fire Pro service for a more detailed answer to the basic question.
could i block the Siamese connection with stables and chairs
Jay — We are not sure exactly what you mean by the question — but FDCs cannot and should not be blocked by anything that restricts their visibility and access for firefighters who must use them in an emergency. Thanks for reading!
Is it required to provide a “RED LIGHT” above the Siamese Outlets? if so, how high above the Siamese Outlets, shall be mounted.
Gerzan — We are unaware of any rules in model codes or NFPA standards that require a “red light.” Clearly marked, visible, and accessible are the basic requirements. But local codes could require a red light, such as this mention of a possible new township code in New Jersey. Thus, check with your local ‘authority having jurisdiction’ (code official or the fire department). Thanks for reading!
I am looking for a 5″ x 3″ x 3″ siamese connection for NYFD
Renee — We will have someone from customer support contact you, or you can email support@qrfs.com for faster service!
Question (because I am one of those enquiring firefighters that likes to understand the operation of my equipment): On a single clappered siamese LDH connection, is there an NFPA standard for the pressure differential to activate full closure of the clapper?
In our operations, the first due Pump Rated Tanker/Tender pumps through the siamese at 100 PSI to the 1st due (attack) engine, with the 2nd due Engine arriving to connect and pressurize their LDH to the siamese at 90 PSI. The theory being, once the first Tanker runs out of water, the pressure will drop, and the 2nd due Engine overtakes pumping to the attack engine.
It came to mind the other evening if both the tanker and 2nd due engine have only a differential of 10 PSI, does this mean the clapper is fully closed to the pump with the lesser pressure, or, are both flowing through the siamese, with the clapper closing off to the pump running out of water, and pressure decaying well below the 10 PSI differential?
Since there wouldn’t be much in the way of friction loss from the Siamese to the Attack Engine, with both feeding pumps at nearly 100 PSI the flow would be HUGE until resistance of the Attack Engine Pump slowed the flow.
I’ve looked to several manufacturer websites, but the clapper dynamics or specifications is not listed.
Any information would be greatly appreciated
Murray — Unfortunately, we don’t know the answer to your question, offhand, though it is an interesting one. If you’d like to try our Ask a Fire Pro service, however, we can run this by a fire service AHJ and NFPA committee member. Thanks!