What are they?
Variable Refrigerant Flow or VRF systems (Also known as Variable Refrigerant Volume systems) are like a split system except that rather than having one outdoor unit for each indoor unit, they have up to 20 indoor units for each outdoor unit. This allows a higher level of technology and sophistication to be used in the outdoor unit, leading to efficiency improvements.
Advantages / Disadvantages
- Moderate-to-high install cost. VRF systems are a premium solution for small to mid scale buildings
- Flexibility. VRF systems provide a high level of flexibility and ability to control small spaces independently. However, owing to limitations on indoor unit capacity, they are poorly suited to serving large spaces such as halls
- Proprietary Systems. VRF systems are a highly integrated system in which all the components are part of an integrated single-vendor system. This creates excellent opportunity for efficiency but does lead to a high degree of dependence upon the vendor to deliver.
- Very sensitive to blocked air filters. Blocked filters lead to a reduction in airflow which will lead to excessively low and excessively high supply air temperatures which drastically reduce effectiveness and comfort.
- Refrigerant costs. VRF systems have large amounts of refrigerant. Refrigerants of all types are becoming more expensive, and as a result refrigerant replacement during maintenance can become a major cost
- Moderate (15 year) lifespan. VRF systems do not have as along a lifespan as chilled water or AHU systems.
VRF systems can be highly efficient particularly in three pipe format and with free cooling included.
The age and condition of VRF Systems will significantly affect running costs. Older systems tend to have higher rates of temperature controller failure, refrigerant loss, coil damage as well as less efficient underlying hardware, all of which can increase costs.
As with all systems, efficiency will be improved by good management, especially in relation to the calibration of temperature sensors and the maintenance of filters, coils and refrigerant charge.
Retrofit / improvement opportunities
The major areas for improvement for VRF systems re:
- Ensure filters and coils are not blocked
- Ensure control set-points are consistent and appropriate.
- Ensure that refrigerant charge is correct
- Ensure that there is good airflow around the outdoor units so that heat can be rejected and does not “pool” around the outdoor unit.
- Install free cooling so that outside air can be used for free cooling when conditions are appropriate.
VRF systems are best applied in small to medium buildings, and are commonly used in office park and industrial office park developments.
Floor plate implications
VRF system indoor units are ceiling mounted and so do not take up floor space.
Temperature control / Occupant comfort
VRF indoor units are generally quiet. As with all refrigerant based systems, VRF systems have limited control of supply air temperature which can produce occupant comfort issues.
VRF systems are reasonably low maintenance but nonetheless require regular maintenance to maintain efficient and effective operation. Filters, coils, controls and refrigerant charge must be well maintained.
VRF systems have large outdoor compressor units which are typically mounted on a roof. Indoors, these systems generally use ceiling mounted units.
Maintenance or operational staff will be able to advise whether a building uses VRF systems.
Questions to ask
- Are the VRF systems capable of providing heating and cooling simultaneously within the same system?
- How old is the VRF system?
- Do the units have free cooling?
- Is the system regularly maintained (at least annually) and seasonally commissioned?
- How often are the filters cleaned?
AIRAH Guide DA08 HVAC&R An Introduction