A Practical Guide to Multi-split Systems and Variable Refrigerant Volume (VRV) Systems
by Mike Hardy
There are many manufacturers of multi-split systems and VRV systems throughout the world and it is important that the designer / specifier / building owner has some practical understanding of their uses and limitations. These type of systems vary considerably from manufacturer to manufacturer particularly with reference to noise levels and the type of indoor units used. This is particularly the case with VRV systems where it is often assumed, quite erroneously, that there are few manufacturers of these systems and they are all similar - this is not the case.
The above systems have come of age in the last 5-10 years and are particularly popular because they require less outdoor plant space than conventional systems, are less disruptive to fit in existing buildings (particularly when occupied), are able to cool and heat through common pipework and in the case of VRV systems have inherent heat recovery.
These systems all use refrigerant as the cooling / heating medium rather than chilled water / hot water as is used in conventional hydraulic systems circulated by pumps.
Condensing units are used externally when cooling only is required and heat pumps are used externally when both cooling and heating are required.
The traditional ‘split system’ is also known colloquially and more descriptively as a 'one to one split system', meaning one external condensing unit / heat pump is connected by refrigerant pipework to one indoor cooling / cooling and heating unit.
The multi-split system uses one external unit which is connected to several indoor units. The multi-split system takes a number of different forms and it is essential the designer / specifier understands the limitations of each type of system.
- Master and slave system
One off external condensing unit / heat pump unit is connected to several indoor units as is typical for a mult-split system. One of the indoor units is provided with temperature controller / sensor and acts as master and the other unit(s) acts as slaves. All indoor units will therefore function as the master setting. Master and slave units are suitable for single areas, single rooms or even multiple rooms with very similar heat gains / losses. They are not suitable for individual areas / rooms which have different heat gain / loss characteristics because the master control will sense air temperature for one area / room only and the areas / rooms will overcool or overheat.
- Zoned control units
As previously one off external / heat pump unit is connected to several indoor unit. With these systems each indoor unit has its own individual temperature controller and thus each unit functions as required to maintain the individual room temperature. With these systems whilst there is individual control the limitation is that if cooling is required in one area it is not possible to provide heating in a different area served by the same system because the compressors will function in only cooling mode or heating mode.
- Variable refrigerant volume (VRV) systems
Again one off condensing unit/heat pump is connected to several indoor units. VRV systems are able to provide total versatility and each indoor unit may cool / heat independently of each other. In fact, if part of a building requires cooling and other areas require heating the heat rejected for the required cooling contributes or is recovered to provide heating in the other area.
Refrigerant Pipework Limitations
The maximum lengths of pipework it is possible to use for all mass produced refrigeration equipment is determined by the the compressor. All ‘split’ systems therefore have a maximum vertical and total refrigeration pipework length allowable. This is a considerable disadvantage compared with hydraulic systems which are pumped and as the pump may be sized to suit the system, then theoretically, the hydraulic pipework may be run almost infinite distances. It is important the designer / building owner is aware of these limitations. Each manufacturer specifies both the size of the pipework required for their system and the maximum permissible vertical and total refrigerant pipework runs.
Moreover it may not be assumed that these distances will be similar between manufacturers for similar capacity equipment this is often not the case. However a nominal guideline is as follows:
- Up to 5 kW of cooling maximum of 25 metres
- Up to 7 kW of cooling maximum of 25 metres to 50 metres (varies widely between manufacturers)
- Up to 15 kW of cooling maximum of 30 metres to 50 metres
VRV systems and zoned systems generally up to 50 metres vertically and 100mm overall. Typically one outdoor unit may be connected to up to a maximum of 8 indoor units.
(Some manufacturers produces a zoned system which allows up to 16 indoor units to be connected to one outdoor unit.)
All indoor units used with multi-split systems provide air distribution by mixed flow and come in various forms:
- Floor standing units
- Units mounted on walls at high level
- Unit mounted below a ceiling
- Cassette units which recess within a false ceiling and terminate with a grille flush with the false ceiling through which air is supplied and exhausted to the space.
- Ducted units or ‘void pack’ units mounted above a false ceiling and are generally connected to ductwork terminating in the ceiling with supply and exhaust grilles to the space. These ducted units may take the form of ‘low static’ pressure units or ‘high static’ pressure units.
With reference to (e), the size and type of the fan motors in these units vary considerably from manufacturer to manufacturer and this determines the extent of the ductwork allowable which is a function of the static pressure of the fan. Silencers may also be provided within the ductwork for very low noise levels required in such spaces as recording studios, but again the fan static pressure available has to be checked carefully so the pressure drop through the silencers may be overcome by the fan.
The introduction of outside air to all ‘split’ systems is often a problem (typically 8 L/S per person is required). To overcome this some manufacturers provide a heat recovery unit which provides outside air to the air conditioned space independently of the indoor units. With these systems an equal quantity of outside air and exhaust air is supplied and then exhausted from the air conditioned space. The supply and exhaust air passes over a heat exchanger so heat is recovered from the exhaust air and used to heat or cool the outside air. This solution has the limitation that air is introduced to the space at two different temperatures, i.e. that of the indoor unit and that of the heat recovery unit. If possible it is always ideal to introduce outside air to the indoor unit. It is possible to introduce outside air to the following indoor units:
- Cassette units
Some manufacturers provide cut outs on the side of the unit so outside air may be ducted into the side of the unit above the false ceiling. The length of the outside duct has to be carefully considered and if over a certain length may have to be fan assisted. Most manufacturers publish maximum permissible lengths and / or pressure drops. As the outside air will bypass the cassette filter the ductwork should also be provided with a filter.
- Ducted unit
Provision may always be made for introducing outside air into the return air ductwork.
The application of multi-split, zoned and VRV systems should be carefully considered. Whilst the VRV systems are the most versatile the capital outlay for the equipment is far higher than for the other systems.
- Multi-split and zoned systems
With most of these systems generally it is possible to provide individual control to each indoor unit (the exception being the master and slave system previously described). The sensor will provide cooling or heating as and when required to maintain the set point temperature selected.
The systems are therefore applicable when there is a clearly defined heating and cooling season i.e. cooling only required throughout the building or heating only required throughout the building. With the increased prevalence of heat producing I.T. equipment within buildings this needs some consideration because if one particular part of a building requires cooling 12 months of the year this system would not be appropriate. The solution in this case would be either to use a V.R.V heat recovery system or have an independent system dedicated to that part of the building.
With the larger zoned systems typically it would be possible to connect up to 16 indoor units to one external unit. With most manufacturers the units may be mixed i.e. an assortment of ducted, cassette, wall, floor, and ceiling units connected to the one outdoor unit.
- VRV systems
These systems are the most versatile of the multi-split systems as the indoor units may function individually and will heat or cool individually. These systems are widely used in commercial offices.