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A boss fitting and a bulkhead fitting can both provide a connection point on a water tank, but they usually attach to the tank in different ways.
In common tank terminology, a boss fitting is built into or permanently attached to the tank, often during manufacturing. A bulkhead fitting is a separate fitting installed through a hole in the tank wall and sealed with a gasket and nut.
That difference matters when you are choosing a rainwater tank, adding an outlet to a barrel, or repairing an existing connection.
Boss Fitting vs. Bulkhead Fitting at a Glance
| Feature | Boss fitting | Bulkhead fitting |
|---|---|---|
| Part of the tank? | Usually molded, fused, or permanently installed | Separate removable fitting |
| How it seals | Depends on how the boss is made into the tank | Gasket compressed against the tank wall |
| Requires a drilled hole during installation? | Usually installed during tank manufacturing | Usually yes |
| Easy to add later? | Usually no | Often yes |
| Easy to replace? | Can be difficult | Usually easier |
| Can it protrude inside the tank? | Often little or no projection | Usually some projection |
| Common use | Factory tank inlet, outlet, or drain | Added tank outlet, overflow, pump connection, or drain |
The exact meaning of boss fitting is not as standardized as bulkhead fitting. Always check the tank manufacturer's drawing before assuming how a fitting is constructed.
What Is a Boss Fitting?
A boss is a raised or reinforced area designed to provide a connection point.
On many plastic tanks, a boss fitting is formed or installed as part of the tank. The boss may contain female pipe threads, male pipe threads, or a smooth connection for a hose.
For example, some molded tanks use an NPT threaded boss formed directly into the bottom of the tank. NPT means National Pipe Thread, a common tapered pipe-thread system. Because the outlet is part of the tank rather than a separate fitting pushed through it, the connection may sit close to flush with the inside bottom.
This can be useful for a drain because less water may remain trapped around the fitting.
Other tanks use heat-fused or otherwise permanently installed fittings rather than molding the threads directly into the tank. The important point is that the connection is normally treated as part of the tank itself.
Advantages of a Boss Fitting
A factory-installed boss can offer a few useful features.
There may be no separate bulkhead gasket and locknut inside the tank. The fitting can also be placed where the tank manufacturer has reinforced or shaped the wall for it.
A bottom boss may also allow better drainage than a bulkhead fitting that projects above the tank floor. One tank manufacturer, for example, distinguishes its molded boss outlet from a bulkhead outlet because the bulkhead leaves a small raised edge inside the tank.
The main limitation is flexibility. You normally cannot move a molded boss or simply add another one with ordinary plumbing tools.
What Is a Bulkhead Fitting?
A bulkhead fitting is a separate fitting that passes through the wall of a tank, rain barrel, reservoir, or similar container.
A typical bulkhead has:
- A fitting body
- A wide flange
- A gasket
- A threaded barrel
- A large locknut or backnut
The body passes through a correctly sized hole in the tank. Tightening the nut clamps the tank wall between the two sides of the fitting. The gasket creates the water seal.
This makes bulkhead fittings useful when you want to add a connection to an existing rainwater container.
You might use one for a:
- Garden-hose outlet
- Pump supply line
- Tank-to-tank connection
- Overflow
- Drain
- Irrigation line
Why the Gasket Matters
The gasket is what stops water from leaking around the hole.
Its exact position depends on the fitting design, so follow the manufacturer's instructions. On many common bulkhead designs, the gasket sits between the fitting flange and the tank wall while the nut provides the clamping force.
Do not assume that adding thread tape or sealant around the outside of the fitting will fix a poorly installed tank-wall seal. Pipe threads and the tank-wall gasket are two different sealing points.
The Main Difference Is How the Tank Connection Is Made
The easiest way to remember the difference is this:
A boss is usually part of the tank. A bulkhead fitting clamps through the tank wall.
With a molded boss, the tank manufacturer has already created the connection point. You connect your valve, adapter, hose, or pipe to that point.
With a bulkhead fitting, you create a new penetration through the tank wall and install the fitting into it.
That makes bulkheads much more useful for DIY modifications.
It also means the installer has more ways to create a problem.
A bulkhead can leak if the hole is the wrong size, the sealing surface is uneven, the gasket is damaged, or the fitting is tightened incorrectly. Hole dimensions vary between fitting models, so the pipe size alone does not tell you what size hole saw to use.
Which Is Better for a Rainwater Tank?
Neither type is automatically better.
If you are buying a tank with a suitable factory boss exactly where you need the outlet, using that connection is usually simpler than drilling another hole.
For example, suppose your rainwater tank already has a 1-inch threaded outlet near the bottom. If that outlet provides the flow and connection you need, there may be little reason to install another fitting.
A bulkhead makes more sense when you need a connection that the tank does not already have.
You might want to add an overflow higher on the tank or create another outlet for a garden irrigation line.
Before drilling, check that the tank manufacturer allows modifications in that location. Tank walls can curve, change thickness, or flex when the tank fills. A fitting needs a suitable sealing surface, and unsupported pipes or heavy valves can put extra stress on the wall.
Bulkhead Fittings Are Usually Easier to Retrofit
For DIY rain barrels and many small water-storage systems, this is the biggest advantage of a bulkhead fitting.
You can choose the location, drill the specified hole, install the fitting, and connect your plumbing.
That does not mean any part of the barrel is suitable.
A bulkhead needs enough flat or properly shaped surface around the opening for its gasket and flange. Ribs, seams, sharp curves, and molded details can prevent a good seal.
You also need to check access.
Examine bulkhead fitting installation steps to evaluate long-term maintenance priorities for the installation.
Some bulkhead designs must be inserted from inside the container. That can be easy on an open rain barrel but difficult on a closed cistern with only a small inspection opening.
Do not enter a tank or cistern to install a fitting. Tank interiors can be confined spaces and can present serious hazards. Use a fitting and installation method that can be completed safely from accessible areas, or have the work done by a qualified installer.
A Boss Can Be Better for Complete Drainage
A bulkhead fitting often extends slightly into the tank.
If it is installed near the bottom, that raised edge can leave a shallow layer of water behind. A molded bottom boss can sometimes avoid this because the drain connection is formed into the tank itself.
This difference matters more when you need to:
- Drain a tank before freezing weather
- Remove sediment during cleaning
- Empty a tank for maintenance
It matters less when the fitting is an overflow or a normal supply outlet where complete drainage is not required.
Do not assume every boss is flush, however. Tank shapes and fitting designs vary.
Check the Connection Type Too
Knowing that a tank has a boss or bulkhead fitting is only part of the job.
You also need to know what connects to it.
A fitting may have:
- Female pipe threads
- Male pipe threads
- A slip socket for pipe
- A hose barb
- A smooth hose connection
The nominal pipe size also does not always equal the outside diameter of the entire fitting.
Before buying valves or adapters, check the manufacturer's drawing or specifications for both sides of the connection.
For threaded connections, also confirm the thread standard. Parts that look close in size can still have incompatible threads.
Do Not Let the Pipe Hang From the Tank Wall
This matters with either type of fitting.
A long pipe, pump, heavy valve, or filter housing can act like a lever against the tank connection. Movement from pumps or changing water levels can add more stress.
Support heavy plumbing independently rather than expecting the tank fitting to carry its weight.
For larger tanks and pressure systems, follow the tank and fitting manufacturers' requirements for flexible connectors, supports, valves, and allowable fitting locations.
Be Careful With the Term B.O.S.S. Fitting
There is one source of confusion worth knowing about.
B.O.S.S. Fitting® is also the name of a specific fitting made by Poly Processing for industrial chemical-storage tanks. In that name, B.O.S.S. stands for Bolted One-Piece Sure Seal.
That product is installed through a tank wall and uses a gasket and external hardware. In other words, it should not be confused with the more general use of boss for a molded or permanently formed tank connection.
If a tank specification writes the name as B.O.S.S. Fitting®, check the manufacturer's information rather than treating it as a generic molded boss.
What Should You Use for a Rain Barrel or Cistern?
Start with the tank you already have.
If it has a factory outlet in the right location and the connection is suitable for your plumbing, use that connection when possible.
If you need to add an outlet or overflow, a properly sized bulkhead fitting is often the more practical choice for a DIY rainwater system.
Before cutting the tank, confirm:
- The fitting's required hole diameter
- The tank wall is suitable for the fitting
- The gasket is compatible with the stored water and tank material
- You can reach the fitting well enough to install it correctly
- The connected pipe and valves will be supported
- The fitting will not interfere with cleaning or complete drainage
Cutting a hole that is too large is difficult to correct. Use the hole size specified for the exact bulkhead fitting, not a generic chart for the nominal pipe size.
For drinking-water systems, fitting type is only one small part of water safety. A boss or bulkhead fitting does not make collected rainwater potable. Potable means suitable for drinking. Drinking-water use requires suitable collection materials, treatment, maintenance, current water testing, and compliance with applicable local requirements.
Frequently Asked Questions
Is a boss fitting the same as a bulkhead fitting?
Usually not. A generic boss fitting is normally molded, fused, or permanently built into the tank. A bulkhead fitting is a separate fitting that passes through a hole and clamps against the tank wall with a gasket and nut. Some manufacturers use the word boss differently, so check the tank specifications.
Can I replace a boss fitting with a bulkhead fitting?
Sometimes, but not simply by unscrewing one and installing the other. A molded or fused boss may be a permanent part of the tank. Modifying it could damage the tank. If a factory connection is damaged, ask the tank manufacturer about approved repair options before cutting it away.
Can I add a bulkhead fitting to a rainwater tank?
Many tanks can accept additional bulkhead fittings, but not every location is suitable. The wall must provide the flat area, thickness, and strength required by the fitting. Check the tank manufacturer's instructions before drilling.
Does a bulkhead fitting need sealant around the tank wall?
The tank-wall seal is normally created by the fitting's gasket rather than by caulk or ordinary pipe-thread sealant. Follow the fitting manufacturer's instructions for gasket placement and tightening. Sealant requirements for the pipe connection itself may be different.
Which fitting is better for draining a tank completely?
A bottom boss that is designed flush with the tank floor can sometimes drain more completely. A bulkhead fitting may project slightly above the inner surface and leave some water behind. The actual result depends on the tank and fitting design.
Can I overtighten a bulkhead fitting?
Yes. More force does not always create a better seal. Excessive tightening can deform the gasket, fitting, or plastic tank wall. Follow the tightening instructions supplied with the fitting.
Can either fitting be used for drinking water?
The shape of the fitting alone does not determine whether it is suitable for potable water. The tank, fitting materials, gaskets, collection surface, treatment system, plumbing, maintenance, and water quality all matter. Verify that all wetted components are suitable for the intended use and follow current local drinking-water requirements.
