4-Pipe Hydronic System (4-Pipe System)

4-Pipe Hydronic System (4-Pipe System):

A Smart Climate Control Solution for Commercial Facilities

A 4-pipe hydronic system — also known as a 4-pipe HVAC system — is a sophisticated climate control solution that uses an intermediate heat/cold carrier. With separate piping circuits for heating and cooling, the system is tailored to the specific needs of different building zones, ensuring energy efficiency and occupant comfort in commercial buildings, multi-unit residences, hotels, and other facilities.

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Fig. 1 – Schematic of a 4-pipe system

General Overview of a 4-Pipe Hydronic System

The core idea behind a 4-pipe system is the presence of two separate circuits — one for heating and one for cooling — each connected to a central boiler and chiller. These central units produce hot and chilled water on demand and distribute it throughout the building via circulating pumps.

The most common intermediate carrier is water, though water-glycol mixtures may also be used depending on the application. The fluid is delivered to indoor units — such as fan coil units (FCUs) or air handling units (AHUs) — where fans blow air across heat exchangers containing either hot or chilled water. This enables localized heating or cooling in individual zones as needed.

Fig. 2a – Two-pipe system

Fig. 2b – Four-pipe system

As illustrated in Fig. 2, the key difference lies in control flexibility. A classic two-pipe system (2a) switches between heating and cooling modes centrally, at the plant level — affecting all users simultaneously.

In contrast, a four-pipe system (2b) allows individual mode selection at the zone level by using valves on the return lines, enabling each room or area to independently activate heating or cooling based on demand.

Key Components of a 4-Pipe Hydronic System

• Boiler – The primary heat source, responsible for heating the water in the heating circuit.
• Chiller – The system that generates chilled water for cooling.
• Circulating Pumps – Ensure continuous flow of the thermal medium through the system.
• Piping System – Includes separate supply and return lines for both hot and chilled water. These pipelines distribute water throughout the building to maintain the desired indoor climate.
• Terminal Units (Indoor Units) – Typically fan coil units (FCUs) or air handling units (AHUs). These are equipped with heat exchangers that transfer heat between the water circuits and indoor air. In a 4-pipe system, each unit has separate coils for heating and cooling, which allows simultaneous heating in some zones and cooling in others.
  Figure 3 illustrates connection schemes for such terminal units in both two-pipe (3a) and four-pipe (3b) systems.
• Control System and Valves – Advanced control systems and regulating valves are essential to manage the flow of hot and chilled water to the indoor units. This ensures precise temperature control in each zone and optimal comfort for building occupants.

Fig. 3a – Two-pipe system: FCU connection diagram

Fig. 3b – Four-pipe system: FCU connection diagram

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Application Areas and Advantages of a 4-Pipe Hydronic System

The primary use case for 4-pipe systems is climate control in commercial buildings, apartment complexes, and hotels. These systems are particularly effective during shoulder seasons, when abrupt temperature fluctuations can significantly impact the thermal balance of a building and the comfort of occupants.

For example, on a sunny spring day, a modern building with extensive glazing may require active cooling. Yet, the very next overcast day may call for heating to maintain indoor comfort. A traditional 2-pipe system, unlike a 4-pipe system, does not support frequent switching between heating and cooling modes.

Figure 4 illustrates the periods with the highest potential for deploying a 4-pipe system.

In addition, 4-pipe HVAC systems offer a number of key advantages:

• Simultaneous heating and cooling makes this system ideal for buildings with varying temperature requirements across different zones.
• The use of water as a thermal medium provides high energy efficiency compared to air-based systems, due to the high heat capacity of water.
• Individual temperature control improves user comfort, allowing occupants to set their preferred conditions independently.
• Ease of maintenance for HVAC personnel: system components are standard and well-known to technicians.
• The absence of distributed refrigeration units and compressors reduces both system complexity and overall costs.
• Reliability and durability – 4-pipe systems typically have a longer service life and offer more stable performance than other HVAC configurations.

Fig. 4 – Annual graph of heating and cooling demand in a typical office or residential building. Green areas indicate transition periods where a 4-pipe system provides the highest benefit.

Limitations of a 4-Pipe Hydronic System

Despite its numerous advantages, a 4-pipe system also has certain limitations. The initial investment can be higher than that of other HVAC systems due to the larger amount of piping, insulation, supports, shut-off and control valves, heat exchanger blocks, and installation labor required. Additionally, it is necessary to allocate sufficient space for boilers, chillers, and other system components.

However, competent system design can significantly reduce unnecessary overspending on the above. We recommend consulting our technical specialists for professional system calculations, optimized piping schemes, and cost-saving recommendations for both capital and operating expenses.

The 4-Pipe Multizone Unit: A Logical Evolution of the 4-Pipe System

As discussed earlier, a traditional 4-pipe hydronic system requires a separate connection to a chiller and a boiler. However, newer integrated HVAC solutions have emerged that combine heating and cooling functions within a single unit.

From the outside, such a TRANE unit may look no different from a standard chiller or heat pump:

Fig. 5 – Example of a 4-Pipe Unit

What’s the difference between a traditional heat pump and a 4-pipe unit?

A traditional heat pump can either cool or heat, but not both simultaneously. It represents a 2-pipe system, using a single water heat exchanger to supply either chilled or heated water to the building.

A 4-pipe unit can be defined as a heat pump capable of simultaneously covering both cooling and heating demandswithin a 4-pipe hydronic system, achieving maximum operational efficiency.
Figure 6 illustrates the conceptual difference between a 4-pipe system based on a chiller and boiler (6a) and one based on a dedicated 4-pipe unit (6b).

  Fig. 6a – 4-Pipe System Based on Chiller and Boiler

  Fig. 6b – 4-Pipe System Based on Integrated 4-Pipe Unit

A typical 4-pipe HVAC unit includes three key heat exchangers:

• Condenser – heats water for the building’s heating circuit
• Evaporator – cools water for the cooling circuit
• Balancing air-cooled heat exchanger – an additional coil that helps maintain thermal balance between the heating and cooling sides in a 4-pipe setup. This component allows excess heat or cold to be discharged into the ambient air if it’s not required inside the building

The operation of a 4-pipe unit is based on heat transfer:
either extracting heat from the environment for cooling or releasing heat into the environment for heating.

  Fig. 7 – Operating Principle of a 4-Pipe HVAC Unit

Energy Savings

The unique combination of heating, cooling, and energy recovery modes allows 4-pipe multifunctional units to achieve an overall efficiency that significantly exceeds the COP or EER of conventional heat pumps. These systems are a high-performance alternative to traditional chiller + boiler configurations in 4-pipe systems, offering substantial reductions in installation and operating costs.

Fig. 8 – Typical Annual Profile of Heating Load (red), Cooling Load (blue), and Potential Recovery (green)

In Fig. 8, the green area represents the potential energy available for recovery when operating a 4-pipe unit in multizone mode under typical yearly load conditions.

Thus, 4-pipe hydronic systems with energy recovery are an excellent solution in terms of energy efficiency, user comfort, and equipment lifecycle. While the initial cost and maintenance complexity may be higher, these systems remain the preferred choice for buildings with zoned climate demands.

Additionally, for buildings located in mild or temperate climates, especially those with low-temperature heating systems, field experience shows that 4-pipe multifunctional units can meet both cooling and heating needs with significantly higher efficiency than traditional solutions based on a chiller and boiler.

The most favorable periods are typically April, May, September, and October, when the simultaneous demand for cooling and heating (high coincidence factor) is most balanced. During these transitional months, the multifunctional unit reaches its highest efficiency and energy savings.

Need Help Selecting a 4-Pipe Unit?

Contact our engineering team for technical consultation — we’ll help you choose the most suitable solution for your project.
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Installing a 4-Pipe Unit

The installation cost of a 4-pipe unit is often not higher than that of a traditional system with a separate chiller and boiler. In fact, installation is typically faster and easier, as there is no need for a dedicated boiler room. The main heating and cooling pumps are often integrated into the unit itself, further simplifying the process for HVAC contractors.

That said, as with any heat pump or chiller, it’s crucial to respect the minimum water volume required in the system, as recommended by the manufacturer.
This ensures the unit’s stable operation and protects against temperature fluctuations on the water side. The required water volume for the heating circuit is generally larger, mainly due to defrost cycles in heat pump mode.

Conclusion

Only a detailed feasibility study can determine the exact savings potential for your project, but it’s clear that the 4-pipe multifunctional unit is a technically and economically viable alternative to traditional systems.

When simultaneous heating and cooling demands are present throughout the year, a 4-pipe system enables significant energy savings.

If simultaneous loads are not typical, a conventional reversible heat pump may still offer a cost-effective solution.

Still have questions about system selection?

Reach out to EVROPROM specialists. We’ll help you choose the right solution and offer you reliable chillers from our catalog.

What You Get with EVROPROM

Partnering with EVROPROM means getting more than just equipment — you receive a turnkey engineering solutiontailored to your project:

• Professional equipment selection based on thermal loads, climate conditions, and building architecture.
• System layout optimization — with recommendations to reduce capital and operating costs through individualized project analysis.
• Supply of verified equipment — including chillers, fan coil units, and multifunctional 4-pipe systems from trusted manufacturers.
• Support at every project stage: from initial consultation and technical calculations to commissioning and system start-up.
• Expert engineering assistance throughout the entire lifecycle of the project — including design, installation, commissioning, and after-sales service.

Article by:
Andrei Kokhan, refrigeration engineer
30.07.2025