3-tube VRF systems: performance and energy savings

3-tube VRF systems, known as "energy recovery" systems, provide both heating and cooling according to the needs of different areas. Particularly adapted to the thermal demands in half-season, they allow the user to improve energy efficiency.

Heat pump in winter, cooling unit in summer ... The VRF systems are thermodynamic devices operating in direct expansion: it is neither air nor water circulating in the ducts but refrigerant, transported to the exchanger which will serve as a condenser or exchanger depending on the chosen mode, hot or cold. The amount of fluid varies according to the needs of the indoor units. If 2-pipe systems are reversible, 3-pipe systems produce both hot and cold. An ideal solution in mid-season when, for example, premises on a facade in the shade present a demand for heating, and others, located on a sunny frontage, have cooling needs.

If, in general, the VRF market is in good shape (+ 15% according to Uniclima figures published in 2017), this almost exclusively concerns the service industry sector. These systems are still relatively rare in the residential sector. In addition, because of their additional costs related to distribution boxes and the specific design of outdoor units, 3-tube VRFs remain high-end hardware. The less expensive alternative to satisfy both the needs of hot and cold remains the implementation of several external 2-tube groups organised by facade. However, the miniVRF offer is growing to meet the air conditioning of small groups and small installations such as pavilions. This solution has another advantage: there are fewer tubes to deploy in the distribution network because the indoor units are in series and not in parallel from the outdoor unit, as is the case for a multi-split system.

Thermal transfer from one area to another

In a 3-tube VRF energy recovery solution, the indoor units being completely independent, performance and energy savings are increased: the system mainly allows a heat transfer from a hot zone to a cold zone, and vice versa. In fact, in the case where the majority of the indoor units distribute cold, the premises in heat demand are heated by the energy recovered in the air-conditioned rooms, the external unit providing the necessary additional condensation. In the case where the majority of the indoor units are set in hot mode, the energy required for heating is mainly taken from the outside air and it is the supplement that will be recovered to satisfy the cooling demand of the other rooms. In the same zone, it is possible to install between 6 and 9 indoor units, according to the manufacturers. They can be wall type, console, ceiling or connected to a ventilation network. The refrigerant is distributed in the different zones via a distribution box: it supplies the indoor units with high-pressure steam (HP) or liquid depending on the thermal needs and the set temperature.

Respecting the maximum fluid quantities

For the refrigerant circulating directly in the building in VRF systems, it is essential to comply with the standards in force, governing the maximum authorised quantities. Thus, the European standard EN378 imposes a limit of 0.44 kg / m³ of refrigerant quantity contained in the smallest closed volume containing an indoor unit. In order to respect these figures, the calculation must be carried out by the installer or the design office in charge of the design of the installation. It is recommended to create several circuits in the same building in order to limit the quantities of fluid in the same circuit. The advantage: the lengths of pipes will be reduced as will, in fact, the pressure losses. In addition, compared to a chilled water system, the pipes are in smaller sections, so easier to pass, which is an asset if you want a summer / winter comfort solution for a renovation project.

In order to guarantee and maintain the proper functioning of the system, it is recommended that you establish a maintenance contract with a specialist, especially since these systems involve electronics. Special care should also be taken in periodic cleaning of the indoor units as well as condensate removal with the installation of a gravity system preferably, without a pump when possible, to avoid additional risks.