Carl-ola Danielsson
2023-08-17 01:28

Active chilled beams and fan coils are two different types of terminal units that are used to ventilate and control the temperature in a room. Both active chilled beams and fan coils are equipped with a coil that exchanges energy between the room air and water flowing through the coil. The water flowing through the coil can either be warmer than the room air to deliver heat to the room or it can be colder and thus cool the room. 

Fan-coil units (FCU) can be connected to the ventilation duct and used to distribute primary air in rooms, however, it is not necessary for controlling the room temperature. In a FCU air from the room is forced to circulate through the coil using a fan. Whereas an active chilled beam (ACB) needs to be connected to a ventilation system and relies on the fan in the air handling unit to build up pressure in the duct system. The air handling unit Supply air is distributed through small openings in the pressure chamber of the active chilled beam into the mixing chamber which creates a region of low pressure that sucks room air through the coil. This effect is also referred to as induction and works without any moving parts in the ACB.  

Active chilled beams are often considered to be an energy-efficient way to provide heating and cooling to the room. The main reason for this is that they are designed to operate with dry cooling i.e. the supply water temperature during cooling is always kept above the dew point in the room. This makes it possible to operate the chiller at a very high efficiency. Traditional installations of chilled beams have relied on a constant flow of Supply Air to contribute to good indoor air quality. The latest generation of ACB has been developed to operate with Variable Air Volume (VAV)/Demand Control Ventilation (DCV) functionality to be even more energy efficient. FCU on the other hand are often designed for wet cooling i.e. operating with much lower supply water temperatures. Due to the lower supply water temperature and the built-in fan that can circulate a lot of room air, FCU can provide a lot of cooling capacity from a compact unit but at a higher energy cost compared to an ACB. It is possible to operate FCU under dry cooling conditions. However, then the fan must run at higher fan speeds and resulting problems with noise and air velocities in the occupied space are more likely to reduce the quality of the indoor environment. Modern fans are relatively energy efficient so the added energy consumption of an individual FCU is relatively moderate but with FCU in every room, the energy consumption adds up. Fans aren’t perfectly efficient, so some fan energy is turned into heat that needs to be removed during the cooling season.  New and modern fans are also relatively quiet when they are new and especially when they operate at lower fan speeds. However, fans that run at full speed and especially fans that the end of their life generate noise which is undesirable in many settings such as a hotel room or office space.  

Active chilled beams normally have no moving parts and thus very little need for service. It is recommended to vacuum the coil to remove dust that can build up on the coil’s surface over time. Depending on the environment where the ACB is installed this needs to be done according to building conditions. Some buildings require quarterly cleaning, others only need cleaning every other year. An active chilled beam has a very long lifetime and very little need for service making them a cost-effective solution over the product lifetime. More advanced active chilled beams are adjustable so that the amount of supply air provided to a room can be controlled according to changing usage of the space reducing the need to change the equipment when refurbishing the building and improving the embodied carbon over the building life cycle. FCU operating under wet cooling conditions are equipped with filters to prevent dust from sticking to the wet coil. They are also equipped with drain pans and a drainage system to get rid of the water condensed out of the recirculated room air. Besides the fans, both the filter and the drainage system are service points that will require service. A drainage system that is not taken care of can lead to the growth of mold and bacteria which in turn can lead to odors and health hazards.  

ACB needs to be connected to the ventilation duct whereas this is optional for an FCU installation because ventilation can optionally be distributed through separate diffusers instead. The installation of piping is relatively similar for active chilled beams and fan coils. It can be either two- or four-pipes connected to the coil in the terminal unit. The main difference is that the fan-coil units intended to operate under wet conditions need to have insulated pipes on the cooling side for both supply and return. Otherwise, an active chilled beam installation will only require the installation of a control system to control the flow of water through the coil and possibly VAV supply air. A fan-coil unit will also require a connection to a control system to control water flow and fan-speed. However, a fan-coil unit will also require high voltage power to the fan and installation of a drainage system that adds to the installation cost. FCU often has a lower first, in terms of cost per BTU of cooling, however, due to lower operating and maintenance costs, chilled beams usually have a favorable life cycle cost. 

In the event that ventilation is not required active chilled beams becomes a very expensive solution. Another advantage of fan-coil units is that it is relatively easy to design a system and select terminal units. Designing an active chilled beam solution requires more attention to handling humidity and controlling supply water temperature. The high heating and cooling capacity of a fan-coil unit is often considered an advantage and seen as a possibility to save energy by letting the room temperature drift further away from the desired set-point during periods of unoccupancy. This then comes at the cost of higher power consumption, noise and risk of drought. It might save energy, especially in a building with a lightweight building with less thermal inertia. In solutions where the requirements for the indoor air are high, active chilled beams are often a preferred solution, offering excellent thermal comfort, good indoor air quality, low sound levels and low energy consumption.  

It is clear that both active chilled beams and fan-coil units have their pros and cons and there will always be cases where one or the other will be the preferred choice of terminal unit. At Swegon we are happy to help you find the best solution for your project.