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Three-pipe FCUs: the smart approach to heating and cooling?

Fan coil units (FCUs) have a proven track record and are the mainstay of many HVAC systems around the world, providing an inexpensive and efficient solution for cooling or heating. Two and four-pipe systems have been the norm but now there’s a smarter alternative, a three-pipe system developed by fan coil
manufacturer Aermec. David Evans, general sales manager, Aermec UK elaborates

FCUs are often used as a secondary HVAC system in many buildings but also used as a primary system for smaller applications such as small offices. They are simple to operate, easy to maintain and provide flexibility. 

Two and four-pipe systems can be found in many HVAC projects. A two-pipe system can be cost-effective, but heating and cooling cannot be delivered at the same time. Either the building is heated or it is cooled. 

Four-pipe systems on the other hand consist of two separate cooling and heating water coils, each having its own dedicated set of pipes enabling the system to handle a variety of environments and load changes, providing simultaneous heating and cooling. 

But now the FCU landscape is changing opening up opportunities to drive down costs, reduce emissions and lower TCOs with a three-pipe hydronic system – the 3WP, that combines the advantages of a refrigerant-based VRF system with a four-pipe hydronic system. 

Buildings are high energy users. Ambitious energy targets are being set world-wide to drive down energy consumption and reduce emissions. In the UK, the Government is engaged in consultations to revise Part L and F as the country moves towards less reliance on fossil fuels and zero energy buildings.

Heating and cooling loads vary in different environments and for variable hours per year, depending on the building and its usage but other factors need to be considered such as personal settings, insulation, shading, mass, location and age as well as heating patterns, daily, weekly and 24/7. Daily heat/cool reversals are the main reason for a more complex HVAC system than a traditional 2-pipe hydronic system.

The three-pipe offers a global solution. According to Aermec, the 3WP offers a significant reduction in installation costs and pipework compared with traditional four-pipe systems and can provide full heating and/or cooling to any room at any time.

At the heart of the three-pipe are optimised circuit heat exchangers, designed to operate at extra elevated Delta T = Kelvin and much lower water mass flow. The much lower water mass flow significantly reduces the pressure drops and the thermal properties ensure the 3WP operates with efficiency.

The three-pipe’s new coil design ensures that the system out performs in thermal heat exchange. The 3WP’s circuits are fed with warm water at a much lower temperature (28°C~35°C) than before whilst the heating capacity is maintained and improves both the COP and EER rates on a 24/7 basis. The chilled water is supplied at a moderately higher temperature (9°C~16°C). 


The smart use of a three-pipe system uses less pipework, requires less investment, reduces installation costs, is easy to design and repair which reduces overall costs and can make a significant contribution towards near zero energy buildings.  

The 3WP system ranges from 40kW to 400kW in a single unit and up to 950kW in a cascade. Compared with previous technologies the performance and efficiency has significantly increased without compromising the thermal capacity. The 3WP’s lower water flow results in further savings as well as pumping energy delivering further benefits. 

3WP leads to savings for Milan project

The three-pipe technology was trialled at an office and a four-star 150-room hotel in Milan, where the latter achieved 29% lower investment and €4010 less annual energy consumption. In addition, a continuously higher COP and EER value is achieved.

The installation costs of a three-pipe system are lower compared to a four-pipe design and economically provides a cost-effective solution that offers the possibility of re-investing the initial savings in other efficiency improvements, which can further increase the overall efficiency of buildings further.

Aermec sees the three-pipe FCU is an attractive alternative to traditional two and four-pipe systems, offering excellent advantages for installers (reduced investment) and for the user (more efficient operation). It combines all their advantages whilst helping to achieve sustainability targets.

21 July 2021


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