How to split the heating pipe using a T-piece without pressure loss
How to split the heating pipe using a T-piece without losing pressure
Everyone who has ever dealt with branching a heating system – whether during a renovation, adding a new room, or installing a new radiator – has encountered the same problem: how to correctly split the existing pipe so that the new loop functions just as well as the original one. And it is precisely here that the T-piece comes into play – a seemingly simple pipe fitting that, when incorrectly selected or improperly installed, can cause more damage than benefit. Air in the system, unbalanced radiators, a cold leg of the heating loop, or a humming pump unit – all of these are consequences that can be directly traced back to a poorly designed T-connection.
This article will guide you through the entire issue: from the basics of hydraulics when branching a pipe, through the selection of the correct diameter of the T-piece, to real installation procedures and practical configurations. We will focus exclusively on copper piping and Hepworth T-pieces, which are the standard for central heating in family homes and apartment buildings in Slovakia.
Why splitting the pipe is hydraulically critical
Most people think that inserting a T-piece into a pipe is like adding another branch to an electrical circuit – simply connect it and you're done. In electricity, this roughly works. In hydraulics, absolutely not.
When a fluid flows in a closed heating loop, it follows the Bernoulli equation and the laws of energy conservation. The key parameter is dynamic pressure (kinetic energy of the flowing water) and static pressure (pressure energy). At every point where the flow is split – that is, at every T-piece – the redistribution of both components of pressure occurs. If the T-piece is incorrectly designed or improperly installed, energy is converted into heat (vortex losses), the pressure gradient is disrupted, and one of the branches may receive significantly less flow than planned.
In practice, this looks like this: a radiator at the end of a long pipe is cold, while a radiator right after the split is hot. Or: at a higher boiler output, the entire system destabilizes and the pump starts humming. These problems have nothing to do with the boiler's performance – they are related to the hydraulic balance, which has been destroyed by an incorrectly dimensioned or incorrectly oriented T-piece.
The diagram above illustrates the basic principle: the total flow Q₁ entering the T-piece is divided into Q₂ (continuing straight) and Q₃ (branching into a side branch). The sum of Q₂ + Q₃ must equal Q₁ – this is the law of conservation of mass. The pressure drops in each branch – slightly in the straight direction, a bit more in the branch, where the flow changes direction. It is precisely this pressure difference that must be considered in the system design.
Pressure loss in a T-piece: what causes it and how to minimize it
Pressure loss in a T-piece has two sources. The first are friction losses – these are determined by the length and diameter of the pipe and are minimal in the T-piece itself, since the fitting is short. The second and much more significant source are local (dynamic) losses, which arise from the change in flow direction, turbulent vortex zones behind the bend of the branch, and contraction/expansion of the flow at the T-connection.
Local losses are expressed using the local resistance coefficient ζ (zeta) and the calculation formula:
Δp = ζ · (ρ · v²) / 2
where ρ is the density of water (at 70 °C ≈ 978 kg/m³) and v is the flow velocity in m/s. The coefficient ζ for a T-piece depends on the direction of flow:
- Straight direction (flow through the T without branching): ζ ≈ 0.1 – 0.3
- Branching into a side branch: ζ ≈ 0.9 – 1.5
- Merging of two flows (merging into one pipe): ζ ≈ 0.3 – 1.0 (depends on the flow ratio)
This leads to a practical conclusion: the branch direction of the T-piece is hydraulically much "more expensive" than the straight direction. Therefore, the larger flow (main branch) should pass through the straight direction of the T-piece, and the smaller flow (e.g., to one radiator or a shorter loop) through the side branch. If you reverse this – sending the main loop through the side branch and the secondary flow through the straight direction – you unnecessarily increase the overall resistance of the system.
Flow velocity as a key parameter
In central heating, the flow velocity of water in the pipes should remain within the range of 0.3 – 1.0 m/s. Below 0.3 m/s, there is a risk of air bubbles and magnetite sludge settling. Above 1.0 m/s, noise (pipe noise), erosion of the inner surface, and pressure losses increase rapidly (quadratically with velocity!). The optimum for typical home heating is around 0.5 – 0.7 m/s.
This has a direct impact on the selection of the T-piece diameter. For example, if the main pipe has a diameter of 22 mm and you are branching off only one radiator with a small flow, you will use a reduced T-piece 22×22×15 (side branch 15 mm). If instead you install a symmetrical T-piece 22 × 22 × 22, the flow velocity in the 22 mm side branch would drop so low that you would have problems with proper flow and air venting in this branch.
Which diameter of T-piece to use: 15, 22 or 28 mm
The choice of T-piece diameter must be based on the diameter of the existing pipe and the required flow in the new branch. It is not a matter of taste or what happens to be in stock – it is a calculation task with a clear solution.
15 mm T-piece – for radiator branches and small circuits
15 × 15 × 15 T-piece is the smallest standard size and is mainly used for direct connection of one radiator, a bathroom towel rail or a towel heater. The internal diameter of copper pipes 15 mm is 13 mm, which at a standard flow rate of 60–100 l/h for a radiator corresponds to a speed of 0.12–0.21 m/s – this is at the lower limit of acceptable. Therefore, the 15 mm pipe is only suitable for short connections (up to 3–5 m), not for long runs.
In practice: if you have a 22 mm horizontal distribution pipe in a hallway and want to branch off to a bathroom for a towel rail, you will use a reduced T-piece 22×22×15 (the main distribution remains 22 mm, the branch is 15 mm). A symmetrical T-piece 15×15×15 is used in installations where the entire distribution is 15 mm – for example, in a smaller apartment with short distances or in systems with circulation through a manifold.
22 mm T-piece – the most commonly used size in family homes
22 × 22 × 22 T-piece is by far the most widespread size in standard apartment and family heating installations. A 22 mm pipe can handle a flow of up to 400–600 l/h at an acceptable speed, which is sufficient for most individual heating circuits in a house with 5–8 radiators.
Typical use of a symmetrical T-piece 22×22×22:
- Splitting the main distribution into two equally long and equally powerful branches (e.g., ground floor and first floor with zero flow balance)
- Connecting another heating loop of the same capacity
- Installing another radiator in a system where the branch will also be longer or serve more radiators
28 mm T-piece – for main supply and return pipes
28 × 28 × 28 T-piece appears in main distribution lines near the boiler, in the technical room, when connecting manifolds or in larger buildings with higher flows. A 28 mm pipe can handle a flow of 800–1 200 l/h, which corresponds to a boiler output of 10–15 kW at a temperature difference of 10 K.
Important: A 28 mm T-piece is mostly found in family homes only in the machine room – on the supply and return boiler headers, when connecting a water heater, when installing a hydraulic dynamic pressure equalizer (hydraulic coupling) or in systems with multiple pump circuits. It practically does not appear in distribution lines in rooms.
Orientation of the T-piece in the distribution system – the most important practical detail
This is where mistakes happen most often and where 2 minutes of installation are "saved" at the expense of weeks of diagnosing problems. The orientation of the T-piece – that is, which side is the supply, which continues further and which goes into the branch – has a decisive impact on the hydraulic behavior of the system.
Rules for correct orientation
Rule 1: The larger flow should always go in the direct direction of the T-piece. The branch (side) branch should carry a smaller part of the total flow. This reduces dynamic losses, as a change in flow direction is energy-intensive.
Rule 2: The water inlet into the T-piece should be directed away from the boiler. That is, the supply water enters from one side of the horizontal arm, continues along the other arm further into the system, and the side branch goes to the nearest radiator or sub-circuit.
Rule 3: The branches from the T-piece should have comparable hydraulic resistance (length + local losses). If one branch is significantly shorter, it will "steal" flow from the longer one. The solution is hydraulic balancing (regulating valves), not changing the position of the T-piece.
Rule 4: Never install two T-pieces directly after each other in the same place without a hydraulic calculation. Each T-piece changes the flow velocity profile, and if two T-pieces are closer than 5–10 pipe diameters, their flow fields influence each other and the actual losses are higher than you would calculate.
Distance between T-pieces and from other fittings
In practice, I come across installations where the technician has mounted a T-piece 5 cm after a bend or immediately after a ball valve. The result? Turbulent vortices behind the bend increase the effective resistance of the T-piece by 20–50 % and the system behaves unpredictably.
Minimum recommended distances:
- After a 90° bend: at least 10 times the pipe diameter before the T-piece (i.e., for a 22 mm pipe, at least 220 mm of straight pipe)
- After a pump: at least 15 times the diameter
- After a regulating valve: at least 5 times the diameter
- Between two T-pieces: at least 5 times the diameter (recommended 10 times)
These are not unnecessary formalities – they are conditions under which the tabulated resistance values of fittings in the design documentation apply. If you do not comply with them, the actual losses can be 30–80 % higher than the design assumed.
Practical scenarios from everyday customer practice
Scenario 1: Extension of a room in a family home
The customer has an existing two-pipe distribution system of 22 mm throughout the ground floor. They are building an extension – a spare room – and want one radiator of 700 W. The distance from the nearest existing T-piece is 4.5 meters.
Solution: Two reduced T-pieces 22×22×15 are inserted into the supply and return pipe. The 15 mm branch is extended by 4.5 m to the new radiator. A thermostatic valve and a closing angle valve are installed on the radiator manifold. Hydraulic balancing is performed by manually presetting the thermostatic valve according to the length of the branch relative to the rest of the system.
Critical detail: T-pieces must be installed on both the supply and return pipe. It is not acceptable for the supply branch to have a T-piece while the return is "bridged" directly through the existing pipe with only an extension – such a connection creates a hydraulic loop with an undefined flow direction.
Scenario 2: Installation of floor heating into an existing radiator system
This is a more complex case. The customer wants to add floor heating to the bathroom (where there was originally only a finned radiator). The boiler is a condensing gas boiler with an inlet temperature of 75 °C – too high for floor heating (max. 45 °C).
A T-piece alone is not sufficient here – a complete mixing unit is required. However, T-pieces play a key role: a T-piece 22×22×22 is inserted into the supply pipe (branch to the mixing valve for the floor), and after the mixing valve, a 15 mm distribution pipe continues to the floor loop distributor. The return pipe from the loops returns via a collector and another T-piece back into the main return pipe. The main fin radiator remains connected as originally.
Scenario 3: Unbalanced system – diagnosis and solution using T-pieces
The customer complains: "Radiators on the ground floor are hot, those on the upper floor are cold." The system has one vertical 22 mm riser, from which symmetrical T-pieces 22×22×22 branch off to the radiators on the ground floor and upper floor.
Cause: Ground floor radiators are hydraulically closer to the pump and have lower resistance. Water "prefers" the path of least resistance – so it mainly flows through the ground floor and the upper floor radiators receive only the remainder. The solution is not to replace the T-pieces with larger ones – the T-pieces are correct here. The solution is to preset the control valves or install balancing valves on the ground floor radiators to artificially increase their resistance and redirect the flow upwards. The T-pieces remain unchanged.
This is an important lesson: a T-piece by itself does not balance the system. It only divides the flow proportionally to the resistances of the individual branches. Balancing is done through regulation, not by the choice of T-pieces.
Soldering vs. press-fit T-pieces: difference in pressure loss
Copper T-pieces from Hepworth are designed for soldered joints (capillary soldering with soft solder L-Sn97Cu3 or hard solder). Press-fit versions are also available, but they belong to a different category. From the perspective of pressure loss, the difference between a soldered and a press-fit T-piece is minimal, provided the soldered joint is properly executed – i.e., without excess solder that could narrow the cross-section at the joint.
If the soldered joint is improperly made (too much solder, running into the inside of the fitting), the internal diameter at the joint can be reduced by 1–3 mm. In a 15 mm pipe, this represents a reduction in cross-sectional area by 15–40%, which dramatically increases local velocity and pressure loss. Therefore, careful and proper soldering is absolutely essential – more on this in the article Mounting a T-piece on a copper pipe: soldering step by step.
Calculation procedure for dimensioning a T-piece in a distribution split
For practical use in field practice, the following simplified procedure is sufficient:
Step 1: Determine the total output of the heating system behind the T-piece. For example, 3 600 W behind the T-piece in the direct direction and 1 200 W in the branch. Total output before the T-piece: 4 800 W.
Step 2: Calculate the flow rate Q [l/h] for each branch. Q [l/h] = P [W] / (1.163 × ΔT [K]). With ΔT = 20 K (e.g., supply 70 °C, return 50 °C):
Q_total = 4 800 / (1.163 × 20) = 4 800 / 23.26 ≈ 206 l/h
Q_direct = 3 600 / 23.26 ≈ 155 l/h
Q_branch = 1 200 / 23.26 ≈ 52 l/h
Step 3: Choose pipe diameters for each branch. Target velocity: 0.5 m/s. Cross-section A = Q / v. For Q = 155 l/h = 0.0000431 m³/s at v = 0.5 m/s: A = 86 mm², so d = 10.5 mm → you will use a 15 mm pipe (internal diameter 13 mm, area 132 mm², velocity ≈ 0.33 m/s – acceptable). For Q = 206 l/h before the T-piece: A = 114 mm², d = 12 mm → a 15 mm pipe would not be comfortable, so you choose 22 mm (internal diameter 19.6 mm, area 302 mm², velocity ≈ 0.19 m/s – low, but acceptable for this output).
Step 4: Choose the T-piece. Main pipe 22 mm, direct branch 22 mm (or 15 mm), branch 15 mm. In this case, the correct choice is a reduced T-piece 22×22×15, not a symmetrical T-piece 22 × 22 × 22. If only a symmetrical one is available, you can use it and reduce the branch diameter with a reducing coupling 22×15 after the T-piece – hydraulically equivalent, just slightly more expensive and longer.
Most common installation errors with T-pieces and their impact on pressure
Over the years of practice, several typical errors have emerged that repeat themselves again and again. Avoiding them will save you a lot of diagnostic work later. A more detailed discussion is also available in the article Common errors in T-piece installation and how to avoid them, but here are the most important ones from the perspective of pressure loss:
- Using a larger T-piece "just in case": Many installers believe that a larger diameter = less resistance = better. The truth is that an oversized T-piece causes low flow velocity, air problems, and an unbalanced system.
- Reversed orientation – higher flow into the branch: A classic error when installing "blindly" without considering hydraulics. Result: 30–50% higher losses in the entire system.
- Mounting a T-piece directly after a fitting: Minimum distances (described above) are ignored. Result: unpredictable flow, real losses do not match the calculation.
- Missing T-piece on the return pipe: A branch is made only on the supply, the return is connected "uncontrolled." Result: water recirculation in the wrong direction.
- Poor soldering – solder running into the inside: Cross-section narrowing at the joint, dramatic local losses, sometimes even complete blockage of small diameters (especially 15 mm).
Hydraulic balancing after installation of a T-piece
After the installation of each new T-piece and branch, hydraulic balancing of the system should follow. Without it, the flow distribution is left to "chance" – more precisely, to physics, which will not give you what you want, but what its laws allow.
Basic balancing methods for standard installations:
- Pre-setting of thermostatic valves: Each radiator has an adjustable thermostatic valve with the option to pre-set the flow (numbers 1–6 on the ring). Shorter or closer radiators are pre-set to a lower flow (higher resistance), while distant ones are set to full flow.
- Adjustable balancing valves: Installed on the return of each circuit. Using pressure difference or temperature meters, the flow is adjusted according to the design.
- Differential pressure regulators: For more complex systems with multiple circuits. They maintain a constant pressure difference regardless of how many thermostatic valves are open.
Without balancing, even the best selected and installed T-piece will not ensure that the new branch functions as it should. This is a system property of the entire piping, not a property of a single fitting.
Common questions (FAQ)
Can I use a T-piece with a larger diameter than the existing pipe if I want to increase capacity?
Not directly – the T-piece must match the diameter of the connected pipe. If you want to increase the capacity of the piping, you need to replace the entire section with a larger diameter and then use a T-piece of the corresponding larger size. Otherwise, you would need to use reducing couplings, which also increases the number of joints and local resistances – which is hydraulically disadvantageous.
How many T-pieces can I use in a row in one piping system?
Theoretically unlimited, but in practice it depends on the total available pressure head (pump). Each T-piece in the branch direction adds an equivalent length of 1–3 meters of straight pipe of the same diameter. If you have a series of 5–6 T-pieces in a row (for example, at a manifold), their cumulative resistance is the sum of the resistances of all branches the flow passes through. The designer must consider this when dimensioning the pump.
Can I install a T-piece horizontally with the branch pointing downward?
Yes, the Hepworth T-piece is symmetrical and hydraulically functions the same in any spatial orientation. A downward branch is even suitable for bottom-fed radiators. The only thing to consider when orienting downward is air venting – air accumulates at the highest points of the system, so a downward branch complicates rather than simplifies venting. Air vents must be located at the highest points of the branches.
How can I tell that a T-piece is causing pressure loss beyond the normal range?
The symptoms are: unevenly heated radiators in the system where everything used to work fine; increased pipe or pump noise after installing a new branch; the pump running at maximum speed without the system reaching the designed temperatures. Diagnosis is done by measuring the pressure difference before and after the T-piece at standard flow – if the loss significantly exceeds tabulated values for the given diameter and flow, the T-piece is likely incorrectly oriented or has a narrowed cross-section (poor soldering).
What is the difference between a symmetrical T-piece 15×15×15 and a reduced T-piece with a side branch of 15 mm?
A symmetrical T-piece 15 × 15 × 15 has all three ports of the same diameter. A reduced T-piece has larger main arms (e.g. 22 mm) and a smaller side branch (15 mm). A reduced T-piece is hydraulically more advantageous when the flow in the branch is significantly lower than the flow in the main piping – which is the case in most situations. A symmetrical T-piece 15×15×15 is suitable when all three branches have similar flow, for example in manifolds with equal loops.
Do I need to perform pressure testing of the system after installing a T-piece and a new branch?
Yes, recommended and in many cases also required by the STN standard for new piping. After installing a T-piece, before covering the piping, a pressure test (hydraulic pressure test) is carried out at 1.5 times the working pressure, but at least at 6 bar, for 30 minutes without visible pressure drop. For minor repairs to an existing system, at least a visual check of the tightness of the joints is performed after heating the system to operating temperature.
Conclusion: A T-piece is a simple component, but its correct use requires a thoughtful approach
The T-piece is one of the most commonly used components in heating piping installations. Its apparent simplicity is misleading – correct selection of diameter, correct orientation in the piping, adherence to distances from other fittings, and properly executed soldered joints are conditions under which the T-piece will fulfill its function without unnecessary pressure loss and without disturbing the hydraulics of the entire system. On the contrary, neglecting these parameters leads to problems that are much harder to diagnose and eliminate than to prevent.
If you are unsure about selection or dimensioning, we recommend also reading related articles in our Knowledge Center: How to choose the right T-piece for copper pipe: diameters 15, 22 and 28 mm, What do the dimensions of a T-piece mean and how to read them correctly, or Same vs. reduced T-piece: when to use which type. A comprehensive view of the topic including long-term maintenance of the connections can be found in the article Maintenance and inspection of T-piece connections in central heating systems.
Do you have a question about this topic?
Not sure or dealing with a specific situation in your home? Write to us – we are happy to help.