VRF Systems are Energy Efficient

Energy modeling software has been used to compare inverter-driven VRF split-systems with other types of systems, applying the design conditions for several major U.S. cities. VRF systems were consistently 25 percent or more efficient than traditional systems at start-up. There are several factors that contribute to energy efficiency. These include low capacity operation, ramp up speed, zoned capabilities, heat recovery technology and heat pump technology.

Operates at low capacity

Low capacity inverter-driven compressors permit capacity operation as low as 4 percent. Most fixed-speed compressors in traditional HVAC systems are either off or on, wasting energy when partial-load conditions prevail. Even a traditional system with two or three stages, doesn't compare to the full-range variable capacity of the inverter-driven system that fully supports part-load operation. (See sidebar Part-Load Operations above.)

Simultaneous cooling and heating (heat recovery) system includes an outdoor unit, a branch controller and indoor units. Image courtesy of Mitsubishi Electric

 

Ramps up quickly

Inverter compressors ramp up quickly, providing the energy necessary to achieve the cooling or heating demand of the zone. Working in tandem with system controls and sensors, the inverter compressor varies its speed to maintain the desired comfort level. Thus, the system only uses the amount of energy required to satisfy the cooling and heating requirements of each zone.

Zoning capabilities

Inverter-driven VRF systems save energy, while providing a high degree of comfort through their zoning capabilities. Zones or rooms can be designed to exact needs, taking into account occupancy, solar gain parameters, and diversity of usage.

Intelligent indoor units have sensors to measure room air temperature at the return. If the design requires it, air temperature can be measured at a remote controller. The ability to choose the measurement location allows for better air temperature management, maintaining the set point within one degree Fahrenheit. Some VRF system indoor units feature a special sensor accessory, which senses and compares air and floor temperatures, and then adjusts the unit output as needed to optimize the comfort within the space. All indoor units feature linear expansion valves to ensure the precise amount of refrigerant and capacity are delivered to the zone.

Typical VRF systems with multiple indoor (air-handling) evaporator units connected to an outdoor compressor unit can deliver just the right amount of refrigerant to precisely meet each zone's load. Indoor units can be controlled to operate only in those occupied areas that need conditioning; indoor units in vacant areas can be turned off. By conditioning only the occupied areas, heating or cooling capacity is not wasted.

Since conventional HVAC systems are sized depending on the cooling and heating peak loads of the building, the combined capacity of the indoor units can match, exceed, or be lower than the capacity of the connected outdoor unit. Inverter-driven VRF systems, on the other hand, can be designed and sized on a zone-by-zone basis to adjust for a building's solar gain and the changing seasons. Depending on the system selected, typically up to 50 indoor units can be connected to each outdoor unit, producing a total applied capacity of up to 150 percent of the outdoor unit's rated capacity.

Heat recovery system

Certain VRF systems have the ability to simultaneously operate in cooling and heating modes–using the building's own environment to save energy. These systems, also called heat recovery systems, include an outdoor unit, indoor units and a branch circuit (BC) controller to control the refrigerant being delivered to separate indoor units requiring cooling and heating. The BC controller facilitates removal of wasted energy from one zone, and applies it to a different zone. For example, if an indoor unit in one room is calling for cooling and an indoor unit in another room is calling for heating, the BC controller can take the heat removed from the room that is being cooled and use it to warm the room that needs heat. This reduces the cooling or heating requirements put on the compressor, saving electricity. Heat recovery systems are offered with a connected capacity of up to 150 percent of the outdoor unit's rated capacity. Typically, these systems are available up to 24 tons of capacity.

VRF manufacturers either use 2-pipe or 3-pipe technology to create a simultaneous cooling and heating system. A 2-pipe system uses a single branch controller box which distributes refrigerant to individual indoor units which operate independently. A 3-pipe system requires a branch selector box at each zone to provide independent operation. The other large difference lies in the amount of piping connections required by each type of system. As shown in the illustrations, the number of piping connections is much less for a 2-pipe system than a 3-pipe system. Since connections can be directly correlated to installation time, connections are a good estimation of installation cost.

VRF heat pump technology

A VRF heat pump system can operate in either a cooling or heating mode. They include an outdoor or water source unit, headers or branch joints, indoor units and a control system. By taking advantage of the diversity in a building, heat pump systems are capable of operating with a connected capacity of 130-plus percent. Heat pump systems are available up to 30 tons of capacity.

A 2-pipe system uses a single branch controller. Images courtesy of Mitsubishi Electric

A 3-pipe system requires a branch connector box for each zone.