About variable refrigerant systems
Variable refrigerant flow (VRF), also known as variable refrigerant volume (VRV), is an HVAC technology invented by Daikin Industries, Ltd. in 1982. Like ductless mini-splits, VRFs use refrigerant as the cooling and heating medium. This refrigerant is conditioned by a single outdoor condensing unit and is circulated within the building to multiple indoor units.
VRF/VRV Systems are among the most flexible and efficient HVAC solutions. They require less space than many other installations, using compact refrigerant lines instead of extensive ductwork or hydronic piping, consolidating heating and cooling into a single system.
VRF/VRVs are typically installed with an Air conditioner inverter which adds a DC inverter to the compressor to support variable motor speed and thus variable refrigerant flow rather than simply perform on/off operation. By operating at varying speeds, VRF/VRV units work only at the needed rate allowing for substantial energy savings at load conditions.
Heating and cooling with the VRF/VRV systems. Heat Recovery
VRF/VRVs come in two system formats, two-pipe and three-pipe systems. In a heat pump two-pipe system, all of the zones must either be in cooling or all in heating. Heat Recovery (HR) systems can simultaneously heat certain zones while cooling others; this is usually done through a three-pipe design, except Mitsubishi and Carrier, whose systems can do this with a two-pipe system using a branch circuit (BC) controller to the individual indoor evaporator zones. While the heat recovery system has a greater initial cost, it allows for better zoned thermal control of a building and overall greater efficiencies.
Wintertime operation for the VRF/VRV systems
When a building uses a VRF/VRV system during winter time, the frost accumulation on the outdoor unit can affect the system's performance. The unit’s heating performance is gradually reduced as ice builds up on the outdoor coils, and excessive frosting can damage the system. To protect the coil the manufacturers introduced a defrost cycle that reverses the heat pump’s operation:
A heat pump in defrost mode draws heat from indoor air and releases it outside.
During the time the cycle takes place, the indoor unit releases cold air or hot air but at a lower temperature.
The heat release melts the ice accumulated on the outdoor coil.
When the outdoor unit is defrosted, the heat pump returns to heating mode.
The defrost cycle runs with automatic controls, and there are many possible configurations. For example, some VRF/VRV systems enter the defrost cycle at regular intervals based on outdoor temperature. Other defrosting controls measure the heat transfer coefficient or pressure of the outdoor coil to detect performance losses caused by ice, activating the defrost cycle when necessary.
An air source heat pump that meets the requirements described below is considered a cold climate air source heat pump (see NEEP latest specification), which makes it suitable for very low temperatures:
A SEER of 15 or more
A Coefficient of Performance (COP) of 1.75 at -15°C at maximum capacity operation
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The Defrost Cycle And How It's Effecting The Unit Performance
The key limitation of a defrost cycle: Indoor spaces are left partially or totally without heating during the cycle, and defrost cycles are required more frequently as the outdoor temperature drops. In turn, this leaves the indoor areas without heating for longer periods, precisely when it is needed the most.
Adding a backup heating system helps the VRF/VRV perform when outdoor temperatures are low. The backup system uses a resistance heater or gas burner.
The heat pump manufacturers have developed ways to optimize the defrost cycle. This makes VRF/VRV systems less dependent on backup heating.
Another solution to avoid the cold draft during the defrosting cycle is to have a sequenced defrost cycle to avoid simultaneous defrosting. This works especially when the application has multiple outdoor units.
Example: If a VRF/VRV system has six outdoor units and the defrost cycle is programmed for two at a time, the building only suffers a partial loss of heating output.
This article compares VRF/VRV heat pump systems from several manufacturers, focusing on their minimum temperature rating and their defrost cycle operation. The brands compared are Daikin and Mitsubishi.
All three manufacturers have product lines rated for temperatures of -20°C, or less.
Although the defrost cycle is unavoidable even with top brands, some technologies have been developed to mitigate its impact.
Daikin VRV Systems
The VRV systems use several methods to mitigate performance loss during the defrost cycle.
Daikin has developed a heat accumulator for its VRV systems, which continues supplying heat while the defrost cycle is active.
Some heat pumps have a dual heat exchanger outdoors, and only one coil is defrosted at a time. The VRV heating output stays above 30% with this configuration.
The minimum operating temperature of VRV systems depends on the specific model. The following table lists the minimum temperatures for the main Daikin product lines, along with the maximum duration of the defrost cycle:
The heat recovery VRV systems allow simultaneous heating and cooling for different areas.
Mitsubishi VRF Systems
Mitsubishi developed the Hyper Heat technology to minimize the need for defrosting:
With the Hyper Heat system, one of the hot refrigerant lines circulates through the outdoor VRF unit. The released heat slows down the accumulation of frost or may prevent it completely.
The normal defrost cycle only activates in extreme cases where the Hyper Heat system cannot defrost the unit by itself.
The R2-Series with Hyper Heat can operate at temperatures as low as -32°C with the corresponding accessories.
Conclusion
All VRV/VRF systems are affected by the defrosting cycle. New technologies were adopted by all manufacturers to reduce the negative effects on indoor conditions during the cycle.