How VRF Systems' Defrost Cycle Operates in Winter

Variable refrigerant flow systems (VRF) are among the most efficient and flexible HVAC solutions, using heat pumps to deliver both heating and cooling. VRF systems also require less space than many other installations, using compact refrigerant lines instead of extensive ductwork or hydronic piping. They also consolidate heating and cooling into a single system.

When a building uses a VRF system during winter, frost accumulation on the outdoor unit can hinder performance. The unit’s heating capacity is gradually reduced as ice builds up on the outdoor coils, and excessive frosting can damage the system. To address this issue, manufacturers use a defrost cycle that reverses the heat pump’s operation:

• A heat pump in defrost mode operates similar to an air conditioner, drawing heat from indoor air and releasing it outside.
• 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 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.

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An air-source heat pump (ASHP) that meets the requirements below is considered a ccASHP (Cold Climate ASHP), which makes it suitable for very low temperatures:

• A Heating Seasonal Performance Factor (HSPF) of at least 10.
• A Coefficient of Performance (COP) of 1.75 at 5°F.

How the Defrost Cycle Limits Heating Capacity

While the defrost cycle is convenient, it has a key limitation: Indoor spaces are left without heating while the process lasts, and defrost cycles are required more frequently as the outdoor temperature drops. This leaves indoor areas without heating for longer periods, precisely when it is needed the most.

• When outdoor temperatures limit the performance of a VRF heat pump, a common solution is adding a backup heating system. Typically, the backup system uses a resistance heater or gas burner.
• However, some heat pump manufacturers have developed ways to optimize the defrost cycle. This makes VRF systems less dependant on backup heating.

The heating loss caused by the defrost cycle can be mitigated by having multiple outdoor units, and programming them to avoid simultaneous defrosting. For example, if a VRF system has four 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 heat pump systems from several leading manufacturers, focusing on their minimum temperature rating and their defrost cycle operation. The brands compared are Daikin, Mitsubishi and Panasonic.

• All three manufacturers have product lines rated for temperatures of -4°F, or less.
• Although the defrost cycle is unavoidable even with top brands, some technologies have been developed to mitigate its impact.

Daikin VRF Systems

Daikin invented the variable refrigerant flow technology, and other manufacturers then developed competing products. Daikin uses the term Variable Refrigerant Volume (VRV), which is trademarked. Therefore, you will find that their products are marketed as VRV systems, while those from other manufacturers use the term VRF.

Daikin VRV systems use several methods to mitigate the performance loss caused by the defrost cycle. Note that the specific defrost configuration changes by product.

• 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:

Heat pump VRF systems are those that provide either cooling or heating for all zones, while heat recovery VRF systems allow simultaneous heating and cooling for different areas.

Mitsubishi VRF Systems

The VRF product line from Mitsubishi is called CITY MULTI, and it is rated for temperatures down to -13°F. 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 CITY MULTI product line is classified as follows:

All five product series can be equipped with low temperature accessories, which bring their operating range down to -18.7°F. In particular, the R2-Series with Hyper Heat can operate at temperatures as low as -25°F with the corresponding accessories.

Panasonic VRF Systems

The VRF product line from Panasonic is called ECOi, and it includes three main series:

• MF2 Series: 3-way VRF heat recovery
• ME2 Series: 2-way VRF heat pump
• Mini ECOi LE Series: 2-way VRF for residential and light commercial use

The minimum operating temperatures and defrosting conditions for Panasonic VRF systems are summarized in the following table:

Conclusion

There is a wide range of VRF systems in the market, and there are important differences in their operating temperature range. The design and specification process is very important when dealing with VRF systems, since even a high-quality product may underperform when it doesn’t match the application. Also, the installation should be carried out by a qualified contractor, since many VRF performance issues can be attributed to deficient installation procedures.

With an adequate design and installation process, VRF systems are among the top performing HVAC installations. They can achieve significant air conditioning savings during summer, and can provide heating without local emissions during winter.