Elbow, tee or reducer: which fitting belongs where in an installation

We are still preparing products.

But you can look at other categories.

Elbow, tee or reducer: which fitting belongs where in a PEX/AL/PEX installation

When you're planning the route of a heating system or a hot and cold water distribution, a seemingly simple question - "what fitting do I actually need here?" - quickly turns out to be something that deserves proper thought. An experienced installer does this almost automatically, but anyone encountering PEX/AL/PEX press fittings for the first time, or setting up a distribution system in a family house on their own, can easily lose track among the many types, sizes and configurations.

In this article, we'll systematically go through all the basic types of fittings - elbows, tees, couplings and reducers - explain where and why each one is used, show real-life practical situations and help you build an installation without unnecessary mistakes and rework. If you haven't yet read the article How to choose PEX/AL/PEX press fittings: type, size and compatibility, we recommend starting there - here we build on the basic selection logic and go deeper into specific fitting types.

The basic logic of fitting classification: what does a fitting actually do?

Before diving into individual types, it's useful to think for a moment about what a fitting in a distribution system actually solves. Each fitting performs one or a combination of the following functions:

Changing the direction of the route - for example when the pipe hits a wall, ceiling or a built-in element
Branching the flow - when one supply must feed multiple outlets or mix supplies
Changing diameter - when the size of the distribution changes, for example from a main line to a secondary branch
Extension or joining - when two sections of pipe of the same diameter are joined into one line
Transition to a different thread type or system - when connecting a tap, valve, boiler or another element with a thread

Each of these purposes has its own dedicated group of fittings. Confusion arises when, for example, an installer tries to use a plain elbow where a reduced elbow would be the correct tool, or when they cannot decide between a straight tee and a reduced tee. These are mistakes that cost time, money and, in the worse case, lead to leaks or restricted flow.

Elbow: when to bend the route and what is the right choice

The elbow is probably the most numerous fitting in every installation. Wherever a pipe changes direction - and in real practice this happens very often - you need an elbow. But "elbow" is not just one thing. In the world of PEX/AL/PEX press fittings there are several variants, each for a different context.

Standard 90° elbow PEX/PEX: the basis of every route

The simplest case - both ends are press PEX ends of the same diameter. It's used wherever the pipe route changes direction by 90° and both sides are PEX/AL/PEX pipe of the same size. Typically when transitioning from a horizontal run to a vertical one (e.g. descending to underfloor heating), when going around a load-bearing wall, or when changing direction at a manifold.

Practical example: when installing a supply to a radiator assembly in a corner by a window, you will almost certainly need at least two 90° elbows - one at the branch from the riser and another at the approach to the valve assembly. The Press elbow 90° PEX/PEX (16) is the fitting most commonly chosen in this scenario for size 16, i.e. for standard supplies to radiators or underfloor heating in a family house.

Elbow with female thread: connecting fittings and taps

The situation changes when you need a thread on one end of the elbow - for example when connecting to a ball valve with a threaded end, connecting to a radiator valve assembly, or feeding a corner valve. This is where the Press elbow 90° PEX/FEMALE THREAD (16x1/2") comes in.

A female thread (IG - Innengewinde) means you screw the counterpart's male thread into the thread. Typical configuration: a radiator corner valve has a 1/2" male thread, and a press elbow with a 1/2" female thread catches it directly. This type of elbow saves one connection point - you don't need to combine a plain press elbow with a threaded coupling.

Important practical note: when screwing the threaded part, always use sealing tape (Teflon) or liquid sealant. The press joint on the other side is self-sealing, but the threaded part is not. This point is discussed in more detail in the article Pressing fittings with female and male thread: how to properly connect a distribution system.

Wall elbow: an anchor point directly in the wall

A specific category that many buyers initially overlook: the Press wall elbow PEX/FEMALE THREAD (16x1/2"). Unlike a regular elbow, this one has a mounting lug or plate that is pre-installed into the wall, and the threaded outlet points perpendicularly out of the wall - exactly where a tap, shower head or similar fitting will be connected.

Where to use it: when installing a basin tap, sink, shower mixer or bath mixer. The pipe comes from behind (from masonry or plasterboard), the fitting is fixed into the wall, and the threaded protrusion sits at exactly the prescribed distance from the wall surface. This allows the tap to be connected directly without further bending of the pipe.

When installing a wall elbow, placement accuracy is critical - the mutual distance between the two outlets must be exactly 150 mm (the standard for a two-hole tap) and perpendicularity to the wall must be maintained, otherwise the tap connections will be mechanically stressed. Most mistakes we encounter when installing bathroom taps stem precisely from a poorly placed wall elbow.

Elbow with male thread: less common, but sometimes irreplaceable

The Press elbow PEX/MALE THREAD (16x1/2") is suitable where the counterpart has a female thread - for example some types of valves, manifold mounting fittings or special adapters. The male thread (AG - Außengewinde) is screwed into the fitting's female thread. In typical residential installations, this type is less common than the female-thread variant, but when connecting some types of underfloor manifolds or threaded boiler connections, it's indispensable.

Reduced elbow: change of direction and diameter at once

A clever fitting that combines two functions. The Press elbow 90° reduced PEX/PEX (20x16) allows you not only to bend the route, but also to change the diameter from 20 to 16 mm (or vice versa). Instead of using a separate elbow and a separate reducer - which would mean one extra joint - a single fitting handles it.

Typical scenario: the main distribution runs at 20 mm, from which you branch off to individual appliances at 16 mm. When this branch also needs to be bent (for example to go around an obstacle), a reduced elbow is an elegant solution. It saves space in the installation shaft or ceiling void, reduces the number of press joints and thus also the number of potential leak points.

Tee: the heart of every branching

If the elbow is "direction", the tee is "logic". Wherever a distribution branches into two lines, or wherever two lines join into one, you need a tee. In practice, the tee is the most important fitting when designing any distribution system.

Equal tee: even branching

A straight (equal) tee has all three outlets of the same diameter. It's used in ring distributions, where you branch off to several appliances of the same output from one riser or distribution pipe - for example to several radiators on the same floor, if each one is connected directly from the distribution line.

Practical example: in older apartment buildings, where the heating pipe is being renovated and the original layout is series-connected (one radiator after another), equal-diameter tees are used extensively - each radiator is connected via a branch from the main line using a tee.

Reduced tee: main line continues, branch is smaller

This is one of the fittings where the wrong choice is most often made. A reduced tee has two outlets of the same (larger) diameter in one line and a third, smaller diameter as the branch. The logic is simple: the main distribution line, for example DN20 or DN25, continues onward, and from it a smaller branch is taken - for example DN16 - to an individual appliance.

Practical scenario: a DN20 distribution pipe runs along the floor of the whole apartment. From it, at every point where there is a radiator, a DN16 branch goes upward. Each such branch is a reduced tee 20x20x16. Inexperienced installers sometimes use an equal DN20 tee and then add a separate reducer - this is unnecessary, more expensive and adds one extra joint. This topic is covered in more detail in the article Reduced tees and elbows: when and why to use them in a distribution system.

How to correctly orient a tee in the route

An important detail that is often forgotten on site: in heating (not in cold water), the orientation of the tee matters. The branch should be designed so that the flow direction in the main line supports natural flow into the branch. If the tee is turned incorrectly - for example the branch is oriented against the flow direction - it can lead to increased hydraulic loss or insufficient heat supply to the branched section.

In practice, for smaller distributions (DN16-DN20) this may not be a dramatic problem, but for larger distributions or gravity heating (without a circulation pump) the orientation of the tee is important. Correct: the main flow goes into the longer pass-through of the tee (the so-called "through end"), the branch goes perpendicularly.

Reducer (straight coupling with reduction): when the diameter changes in a line

A reducer is straightforward - it joins two pipes of different diameters in one line, without a change of direction. In PEX/AL/PEX systems, reducers are used less often than one might expect, because most reduction combinations are handled directly via reduced tees or reduced elbows. Nevertheless, there are situations where a straight reducer is essential.

Where a straight reducer is really useful

Typical situation: extending an existing larger-diameter distribution with a new smaller-diameter section, when the route doesn't change. For example, a DN25 riser transitions to DN20 when entering an apartment unit, or a DN20 distribution pipe narrows to DN16 at the end of the route for the last radiator, where hydraulic calculations show that the larger diameter is not needed.

Another case: when connecting a boiler or tank, where the manufacturer specifies a certain connection diameter, but your distribution has a different diameter. A straight-line reducer is a cleaner solution here than combining several fittings.

What to remember when choosing the reducer size

The reducer size is always given in order of larger/smaller diameter - for example 20/16 means the larger end is 20 mm, the smaller is 16 mm. Never forget that the press diameter of the pipe is not the same as the bore - the internal diameter of a 16 mm PEX/AL/PEX pipe is typically around 10-11 mm, the outer diameter is 16 mm. All press fittings are sized according to the outer diameter of the pipe. If in doubt, read the article What diameter of press fittings do I need for my distribution.

Straight couplings: extending a line or connecting two sections

A straight coupling (also called a "sleeve" or "pipe joiner") is the simplest type - it joins two sections of pipe of the same diameter in one line. Although it may seem trivial, it has several important practical uses:

Extending pipe: when one length isn't enough and two parts need to be joined
Repair joint: when replacing a damaged section of pipe - the faulty section is cut out and replaced with a new one, with both ends joined using couplings
Installation tolerance: when the route is long and small measurement deviations require correction
Connecting to an existing distribution: when extending a distribution, when a tee is inserted into the existing route and both ends are connected via couplings

Practical note: when repairing an existing distribution by inserting a tee, you will typically need not only the tee itself, but also two straight couplings (or one elbow, depending on the route of the new branch). This is the situation where people ask why they bought a tee but the installation still doesn't come together - they're missing the fact that the shortened existing distribution needs to be reconnected via couplings.

Threaded transitions: where PEX ends and the rest of the world begins

In every real distribution system there are places where the PEX/AL/PEX system must connect to a different world - fittings, sanitaryware, boiler, tank, manifold. This is where threaded fittings (female or male) and their variants come in - elbows with thread (see above), tees with a threaded branch, straight threaded couplings.

It's important to know where the female thread is and where the male thread is. Rule: a female thread (abbreviation IG or F - Female) receives the counterpart's male thread. A male thread (AG or M - Male) is screwed into the counterpart's female thread. Before buying, always check what type of thread the fitting you're connecting to has, and choose a fitting with the opposite thread type accordingly.

In practice, in heating distributions we most often encounter 1/2" threads (for individual appliances and valves), 3/4" (for manifolds and larger fittings) and 1" (for boiler and tank connections). Always measure the existing thread or check the technical data sheet of the connected appliance before ordering.

Practical examples: how to solve typical situations

Case 1: Renovation of hot and cold water distribution in a bathroom

Job: an apartment in a panel building, complete bathroom renovation. New pipework from the riser (DN20), connecting to a basin tap, bath mixer, WC filling valve and washing machine. Each of these points has different fitting requirements:

Basin tap: two wall elbows 16×1/2" female thread, built into the wall at a mutual distance of 150 mm, connections exactly perpendicular to the tile surface. The distribution comes from behind, from a plasterboard false wall.
Bath mixer: also two wall elbows, this time at a height according to the design (typically 900-1000 mm from the floor). Here an accuracy of ±5 mm is required to maintain the 150 mm spacing.
WC filling valve: corner elbow 16×1/2" female thread, because the supply comes from the wall and the valve is perpendicular - a classic corner elbow is sufficient, a wall elbow is not needed.
Washing machine: a tee in the cold water line (branch for the washing machine) and, at the end of the branch, a ball valve + threaded coupling for the washing machine hose.
Transition from DN20 to DN16: at the entrance to the bathroom from the hallway, a straight reducer 20/16, because DN20 piping isn't needed in the bathroom.

Case 2: Underfloor heating installation in a new building

Situation: a family house, underfloor heating in all rooms, a central manifold in the boiler room. DN20 supply from the boiler to the manifold, from the manifold DN16 or DN17 loops directly into the floor.

In this case, the fittings are simple and their number is minimized because most of the pipe runs in unbroken loops from the manifold and back. Fittings are used only at:

The transition from the boiler to the DN20 supply line - a 3/4" or 1" threaded coupling
The transition from the distribution line DN20 to the manifold - manifold threaded connections
Any extension of the DN20 distribution line - a straight coupling
Passages through walls to another floor - elbows or bend guides (not fittings)

Case 3: Connecting a new radiator to an existing distribution

Job: the customer wants to add a radiator to a new room created by a remodel. The existing DN16 distribution runs through the hallway. Procedure: a reduced tee is inserted into the existing distribution (or an equal DN16 tee, if the branch diameter is also DN16), and a new branch runs from the tee to the radiator. At the end of the new branch there is an elbow with a 1/2" female thread to connect to the radiator's valve assembly. The whole job requires: 1× DN16 tee, 1-2× DN16 elbow (depending on the route), 1× DN16×1/2" female thread elbow at the radiator, possibly straight couplings where the existing line is interrupted.

Choosing the right fitting: a systematic approach

When planning a distribution and needing to choose fittings, we recommend the following procedure:

Draw the route: Even a simple sketch on paper will show you where the direction changes (elbows), where the branches are (tees) and where the diameter changes (reducers).
Identify connection points: To the boiler, tank, taps, radiators - anywhere you connect to a thread, you need an elbow or coupling with a thread (female or male, depending on the counterpart).
Check diameters: Don't buy fittings without knowing the pipe diameter. If you have a combination of DN20 and DN16, you need reduced fittings at the transition between them.
Prefer multifunctional fittings: A reduced elbow instead of an elbow + reducer, a reduced tee instead of a tee + reducer. Fewer joints = greater reliability.
Check tool availability: Every press fitting requires a press tool with the correct jaw for the given diameter. If you don't have a press tool, rent one or call a professional. More on this in the article Installing PEX press fittings step by step: tools and procedure.

The most common mistakes in fitting selection and installation

Over years of practical work, we keep seeing the same mistakes. Let's summarize them so you can avoid them:

Confusing female and male threads: Buying an elbow with a female thread where the counterpart also has a female thread. These two parts won't connect - they need an adapter or a fitting with a male thread.
Using an equal tee instead of a reduced one: Result - a reducer has to be added after the tee, an unnecessary joint, unnecessary costs.
Guessing sizes: Buying DN20 fittings, but having DN16 pipe. Or vice versa. Always measure the outer diameter of the existing pipe with a caliper.
Omitting a wall elbow for taps: Instead of a wall elbow, combining a plain elbow and a threaded coupling protruding from the wall - the result is a mechanically stressed joint and a tap that eventually starts to rattle or loosen.
Insufficient thread sealing: The press joint seals itself, but the threaded part of the fitting doesn't. Forgotten Teflon = a guaranteed drip.

A detailed analysis of these mistakes, with photographic examples, can be found in the article Common mistakes when pressing PEX fittings and how to avoid them.

Material and construction quality of press fittings

Not all press fittings are the same. In the world of PEX/AL/PEX systems, we most often encounter fittings made of cast brass (CW614N or CW617N) with an internal sealing rubber (EPDM or NBR), or stainless steel for special applications. For standard heating distributions and drinking water systems, a brass fitting with a stainless steel press ring is the standard.

An important parameter is the maximum working pressure (usually 10 bar for heating, 16 bar for cold water) and the temperature range (typically -20 °C to +95 °C). These values can be found in the technical data sheet of every fitting. For standard household heating (working pressure 2-4 bar, temperature up to 80 °C), these limits are met with a large margin.

Regarding system compatibility: fittings from different manufacturers can only be combined if they are geometrically identical and use the same press profile (most often profile M or U according to the standard). Before mixing systems from different manufacturers, check compatibility or consult a professional. This topic is covered in the article How to choose PEX/AL/PEX press fittings: type, size and compatibility.

Overview: which fitting goes where

Fitting type	Function	Typical use
Elbow 90° PEX/PEX	Change of route direction	Every direction change of pipe with the same diameter
Elbow 90° PEX/female thread	Direction change + connection to male thread	Radiator corner valve, ball valve, fittings
Wall elbow PEX/female thread	Built-in supply into the wall + threaded outlet	Bathroom taps, basins, baths, showers
Elbow PEX/male thread	Direction change + connection to female thread	Some types of valves, manifolds, boilers
Reduced elbow PEX/PEX	Direction change + diameter reduction	Transition from main line (DN20) to branch (DN16) with a direction change
Equal tee	Branching into three directions of the same diameter	Series installation, ring distributions
Reduced tee	Branching with reduction on the branch	Branch from a DN20 main line to a DN16 appliance
Equal straight coupling	Extension or joining of the same diameter	Pipe extension, distribution repair
Straight reducer	Diameter change in a line	Transition from a larger to a smaller size without a direction change

Frequently Asked Questions

Can I use an elbow with a female thread instead of a wall elbow for a bathroom tap?

Technically it would be possible, but not practically. The difference is that a wall elbow has a mounting plate (foot) that is attached to the rough wall structure before plastering or tiling. The threaded outlet is thus fixed in the exact position. A regular elbow with a female thread will indeed create a threaded connection, but the pipe and fitting are not anchored in the wall - the tap will have mechanical support only through the pipe, which over time leads to loosening and leaks. The wall elbow is the technically correct solution for tap connections.

What is the difference between a reduced tee 20×20×16 and 20×16×20?

The tee designation gives the diameters in order: first through-end × second through-end × branch. So 20×20×16 means both ends of the run are DN20 and the branch is DN16. 20×16×20 would mean one through-end is DN20, the other DN16, and the branch is DN20 - which is an unusual configuration. In practice, a symmetrical reduced tee is almost always used, where the run has the same diameter and the branch is smaller.

How many press joints in an average 3-room apartment is "normal"?

In a standard renovation of hot and cold water distribution in a 3-room apartment (bathroom, kitchen, WC), we typically see between 40-80 press joints, depending on the complexity of the route, the number of appliances, and whether the distribution is run in lines with tees (more joints) or a manifold system is used (star wiring, fewer joints). For a heating distribution with 8-10 radiators and a central manifold, there may be 30-60 joints.

Can I mix PEX press fittings from different manufacturers?

It depends on whether the manufacturers declare compatibility and whether they use the same press profile. Most European manufacturers (Viega, Geberit, Herz, Giacomini and others) use the M or U profile, but always check the technical documentation of both systems before mixing. In practice, it's safest to work with one system. If that's not possible, check that the press tool and jaws are suitable for both systems.

Why is the 90° elbow more common than the 45° elbow?

PEX/AL/PEX pipes are stiffer than soft copper or plastic hoses - they can be bent, but only to a