Technical Information on Variable Refrigerant Flow Systems
Adam Allington and Chris Zimbelman for Zondits, December 17, 2015
Answer: A few months ago we asked the question when does VRF make sense? In short, the answer was that these systems are ideal in buildings with different rooms or zones that have varying occupancy schedules, activities, and temperature setpoints, such as hotels, hospitals, nursing facilities, strip malls, schools, offices, and historical buildings.
Now that we answered the when, we thought we would follow up with some additional technical information on these types of systems. To recap: VRF systems are a sophisticated type of air-source (or occasionally water-source) heat pump. They have the ability to vary the flow of the refrigerant through the use of an inverter-driven compressor in order to match the refrigerant flow more closely to the heating and cooling load at part-load conditions, as opposed to a more traditional heat pump where the compressor operates at a single speed and is either on or off. This ability can improve the system’s efficiency when it is operating at these part loads. Some VRF systems can also use the refrigerant as a cooling and heating medium while still being served by a single outdoor inverter-driven condensing unit. If VRF systems are installed in an application that can take advantage of this ability, they can be very energy efficient and cost-effective.
There are two types of VRF systems. The first type only allows cooling or heating, not both, and the second type is a heat recovery system that allows simultaneous heating and cooling of various zones. Which type of VRF system to install depends on the space. Simple single-zone spaces do not need the heat recovery option, while complex multi-zone spaces that have interior and exterior spaces on the same system would be better served with a heat recovery VRF system. In large systems the condensing unit could consist of multiple compressors. The refrigerant is routed individually to multiple terminal fan coil units that can be in heating and cooling mode at the same time.
One advantage VRF systems have over the traditional air-to-air heat pump system is that most VRF systems will not need electric resistance as back-up heat, since they are capable of providing up to 100% of their capacity down to -4°F outside air temperature. However, one of the first factors to consider before installing a VRF system is the utility rates during the heating season. A VRF system is 100% electric and even though it can have a coefficient of performance (COP) in the 2 to 5 range, electricity is typically the most expensive heat source type. If the system is going to be installed in a heating-dominated climate the local electric rates and heating fuel rate alternatives need to be compared, looking at the dollars per therm. Depending on the utility rates, the higher efficiency of the VRF system will often be less expensive to heat with than the traditional heating fuel appliance.
In summary, an outline of the pros and cons for these types of systems is as follows.
- They offer excellent part-load efficiency.
- They offer design flexibility.
- They are ideal in historical buildings.
- They are quiet.
- They are compact and do not require installation of duct work, with the exception of a central ventilation system. Therefore, in new construction projects they can result in significant savings from overall reduction in the weight of the installed HVAC system components.
- They can maintain precise space temperature.
- They have reduced maintenance costs compared to central chilled and hot water systems.
- Savings of 30% to 40% can be achieved with these systems compared to rooftop variable air volume systems or central chilled water systems.
- VRF systems are all electric cooling and heating systems. This means that even though they offer better operating efficiencies than most traditional heating and cooling systems, depending on the price of fuel their operating cost especially during the heating months could be greater than a natural-fired heating system.
- Their first costs are variable and reported to be 5% to 20% greater than chilled water systems
- It could be cost-prohibitive to retrofit large central chilled water systems.
- They are not well-suited to large, low-rise buildings such as big box retail stores that feature traditional rooftop DX systems with gas heat.
- Although VRF systems operate at low ambient temperature down to -4°F, their heating capacity is considerably reduced at low ambient temperatures, and so they tend to be less cost-effective than gas heating in very cold climates.
- Some contractors are concerned about long refrigerant piping runs, which can make certain safety compliance requirements difficult to meet.
- Integration with outdoor air or ventilation air needs to be considered; due to the smaller ventilation systems, the opportunity of air-side economizing is reduced.
- Each indoor unit needs to have a condensate drain.
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