VRF
Variable Refrigerant Flow
A typical Heat Recovery type Variable Refrigerant Flow (VRF) system consists of one outdoor unit and multiple indoor units. The compressor in this system is variable speed and controls the flow of refrigerant to the indoor units. A heat recovery VRF system has three modes of operation - Cooling only, Simultaneous heating and cooling and Heating only.
Indoor Unit
The indoor unit is essentially a DX fan coil. It consists of one DX coil and a three speed fan. Fan speeds are designated as H (100% airflow), L (70% airflow) and LL (10% airflow). The fan operates at minimum speed during the heating mode. Indoor units are typically unducted however. There are two models that can handle small duct systems (up to 300 Pa). The indoor unit is piped to a branch selector. This branch selector controls the flow of refrigerant to the fan coil unit. If the fan coil calls for cooling, the branch selector sends liquid refrigerant to the fan coil and the fan coil acts like an evaporator. If the fan coil calls for heating, then the branch selector sends hot gas to the fan coil and the fan coil acts like a condenser.
Since the branch selector controls the refrigerant flow and it always has access to both hot gas (for heating) and liquid refrigerant (for cooling) the fan coil is able to switch between heating and cooling mode very quickly. Indoor units can handle up to 10% outside air. Any additional ventilation requirements must be handled by an optional ventilation unit.
Each indoor unit has its own thermostat to control space conditions. Typical supply air temperatures for the indoor unit are 50°F in cooling and 100°F in heating. An optional electric or hot water coil can be added to the unit.
Airside simulation
When the room drift temperature rises above the cooling thermostat, the cooling coil is engaged at a constant cooling supply air temperature for the a percentage of the hour that it takes to bring the room temperature down to the cooling thermostat temperature. This heat is rejected to the refrigerant condenser loop. For the portion of the hour that the cooling coil is de-energized, the supply air will remain at the return/outside air dry bulb temperature (plus fan heat).
When the room drift temperature drops below the heating thermostat, the heating coil is engaged at a constant heating supply air temperature for the a percentage of the hour that it takes to bring the room temperature up to the heating thermostat temperature. The indoor unit will remove heat from the refrigerant condenser loop. For the portion of the hour that the cooling coil is de-energized, the supply air will remain at the return/outside air dry bulb temperature (plus fan heat).
Outdoor Unit
The outdoor unit consists of two condenser coils, two condenser fans, a variable speed compressor, and a reversing valve. The compressor operation is controlled by suction pressure. When operating in Cooling mode, the compressor speed varies to maintain a constant evaporator pressure. When operating in Heating mode, the compressor speed varies to maintain constant condensing pressure.
The operating mode of the condensing unit is controlled by the net load on the system. For example, if the net load on the system is cooling (cooling load + heating load = net cooling load) the unit operates in cooling mode and follows the primary unloading curve. Similarly, if the net load on the system is heating (cooling load + heating load = net heating load) the unit operates in heating mode and follows the secondary unloading curve. In all cases, the compressor capacity will modulate to exactly match the load on the system.