2-Pipe vs 4-Pipe Fan coil unit: What's the difference?
There are three key differences between 2-pipe fan coil units (FCUs) and 4-pipe FCUs. These are:
The number of water connections (pipes) to the fan coil unit
The key difference between a 2-pipe fan coil unit and a 4-pipe FCU is, as the name suggests, the number of water supply and return pipes to which it is connected. In a 2-pipe FCU there are two pipes: one to supply either chilled or hot water to the FCU and a second one to return the water to the source. In a 4-pipe FCU there are four pipes: two supply and return only chilled water, and the other two supply and return only hot water to/from the FCU.
The number of water heater exchangers (coils) in the fan coil unit
The number of water coils in the FCU also differs between the two types. The coils are the water-to-air heat exchangers. They’re used to condition the air being drawn over the coil by the fan before it is supplied into the space. In a 2-pipe fan coil unit, there is only one coil that can be used either for heating or cooling. Its mode depends on whether it is connected to a source of chilled or hot water. A 2-pipe FCU installation cannot heat and cool at the same time. In a 4-pipe fan coil unit, there are two coils: one for heating and one for cooling, connected to two separate sources. A 4-pipe fan coil unit installation can therefore heat and cool simultaneously.
How is the hot or chilled water supplied to the fan coil unit?
The source of supply of conditioned water – or, more precisely, the number of sources –differentiates a 2-pipe from a 4-pipe FCU. In a 2-pipe FCU, water is supplied from a single source. Whether the output of a 2-pipe FCU is cooling or heating is dependent on whether the single source is a chiller or a heater. This is why a 2- pipe FCU installation cannot cool and warm at the same time.
Switching between cooling and heating is possible with a 2-pipe changeover unit because the water source can be switched over from chilled to warm or vice versa. This enables the function of the 2 pipe changeover units to be switched from supplying cool air in the hotter months to warm air during the cooler months. A changeover sensor is included in a 2-pipe FCU to determine the mode of the system – either heating or cooling.
In a 4-pipe FCU, one coil is connected to a chilled water source by supply and return pipes. The second coil is connected to a heated water source, again via a separate supply and return pipe. This means that a 4-pipe fan coil unit installation can heat and cool at the same time and does not require a changeover sensor because both hot and chilled water are available.
What are the considerations when deciding if a 2-pipe or a 4-pipe FCU is the most appropriate specification?
When choosing whether a 2-pipe or a 4-pipe FCU is the most suitable for specification in a new or existing building, four key factors must be considered:
- HVAC Requirement – Determine if the FCU is needed for heating, cooling or both.
- Application – Spatial constraints, building function and design goals can influence the choice. FCU flexibility is crucial.
- Building Type – Especially relevant in retrofits, as existing pipework may support certain systems without modifications.
- Aesthetic and Space Design – FCUs and their distribution systems should be coordinated with the overall building and service layout.
Once the criteria have been qualified, a 2-pipe system will be used for heating or cooling only. If both services are required, 4-pipe systems will likely be specified. However, in some instances, where heating or cooling requirements are likely to only be seasonal, 2-pipe changeover FCUs may be appropriate.
The system design should be discussed – including reasons for its selection, sizing and integration with other components – with a specialist. They will be best placed to advise the most appropriate solution for your project.
Can 2-pipe FCUs and 4-pipe FCUs work with heat pumps?
As heat pumps deliver lower grade heat than fossil fuel-based heaters, existing fan coil units – both of the 2-pipe and 4-pipe design – may not be able to supply the same level of heat required. However, there are now 2-pipe and 4-pipe FCUs where the coil has been redesigned to work with lower-temperature hot water sources. The increased surface area of the coil means the FCU works both with traditional fossil fuel energy sources, delivering around 60°C flow and 40°C return, and also with heat pumps supplying 45°C flow and 35°C return, or below, to the coil of the FCU.
In this way, both 2-pipe fan coil units and 4-pipe fan coil units can be fitted in new and existing buildings where heat pumps are being used now, or in preparation for heat pump installation in the future.
Where can I find more information about 2-pipe and 4-pipe FCUs?
Here, at Ability by Dimplex we have a range of both 2-pipe and 4-pipe fan coil units and chilled water cassettes suitable for both 2 and 4-piep systems. We have a vastly experienced team on hand to help you find the most appropriate fan coil solution design for your project. Please contact us and we will be happy to help.
