This tech tip is written by experienced tech and VRF / VRV specialist Ryan Findley. Thanks, Ryan! (Note: Ryan refers to VRV rather than VRF because he specializes in Daikin and these articles are written from a Daikin VRV perspective)
This tech tip will cover the different modes of operation of a VRV system along with some of the unique control features of the VRV machines.
First, before we discuss how the machines accomplish the actual heating and cooling of the space, let’s define a few terms that we use frequently.
Target Evap/Target Cond– VRV machines have target condensing temperatures and target evaporator temperatures that change based on load. It’s not uncommon to see some of these numbers get pretty far off what a standard a/c will run. The only reason to hook gauges to one of these machines is to verify the accuracy of the pressure transducers.
4 Way Valve– Reversing valve. There are multiple 4-way valves on these machines. VRV 3 has 2 (a heat exchanger 4-way valve and the dual pipe 4-way valve), and VRV 4 has 3 (2 heat exchanger (outdoor coils are split coils), and one dual pipe).
Thermistor– A temperature sensing device that alters its resistance based on temperature. There are multiple different types of thermistors used across the product lines. Refer to your service manual or service app for temperature->resistance charts.
A1P Board– The A1P board is the conductor of the machine. It is the one talking to all the other boards and also the board where the majority of the thermistor inputs and solenoid coil outputs come from.
Before we discuss the individual modes of operation, I’m going to cover how the bs boxes work to set the mode of operation for the fan coil.
With this diagram from the service manual, we can see how the refrigerant is routed through the bs box. The top is showing a unit in cooling mode and calling for cooling, the middle unit is in heating mode and calling for heating while the bottom unit is in heating mode, but the thermostat is satisfied.
Now that we have the foundation let’s get into the modes of operation.
Cooling Mode– No need to make it any more complicated than it is. The liquid line is always the liquid line, the dedicated suction line is active, and the dual gas line is also a suction line. EEVs in the indoor units are in are controlling to a target superheat at the outlet of the fan coil. This number is typically 9 degrees, but there are instances where that number can change. The farther off setpoint, the target superheat will be lower. The opposite is true the closer you get to setpoint. The outdoor unit(s) will leave the heat exchanger 4-way valves de-energize and energize the dual gas four-way valve to use the dual gas line as an additional suction line. Outdoor fans will modulate to maintain a steady discharge pressure. Here’s a visual aid from the VRV 3 service manual. This is taken from a heat recovery machine.
Parallel Operation– This mode of operation is used when both heating and cooling are needed. The liquid line and suction line remain dedicated to those tasks, so in this mode, the dual gas line will be a discharge line. It accomplishes this by leaving the dual gas pipe 4-way valve de-energized. Outdoor heat exchanger control is determined by the load.
Heating– During the heating operation, the system will energize the heat exchanger 4-way valve and leave the dual pipe 4-way valve de-energized. The dual gas line carries discharge gas to the bs boxes. The suction line is inactive in this mode. During this mode, the indoor EEVs control to subcooling control. Target subcooling in the indoor units is typically 9 degrees. The farther off setpoint it gets, the lower the target subcooling goes. The opposite is true as it approaches setpoint.
Defrost- Defrost is initiated based on demand. The machine is looking for at three conditions that will cause a defrost:
- Small temperature drop across the outdoor heat exchanger.
- If there is a temperature drop at the outlet of the heat exchanger.
- If the low pressure stays low enough over a 2 hour period.
Defrost will terminate if:
- The max defrost time is 5 minutes and 30 seconds or 6 minutes have elapsed (depending on model).
- Heat exchanger temp rises above 57.8 degrees for more than 90 consecutive seconds.
- Condenser pressure rises above 440.8 psi.
During defrost, the modules maintain whatever mode they were in when defrost was initiated. If this is a multi-chassis system, the units will alternate who is in defrost to continue to provide heating. As each module finishes, the next one initiates. Fan coils continue to operate during this period. Fan coils in heating mode take their fan speeds to the lowest speed, if any fan coil happens to be in the off mode then the fans will turn off. EEVs move to 160 pulses or 224 pulses depending on the model of the outdoor unit.
Oil Return– Oil return is used to bring oil back to the outdoor units. Since oil is an insulator along with a lubricant, oil return is necessary to maximize the efficiency of the indoor heat exchangers. Oil return is initiated if:
- Run time of 8 hours or 2 hours after a power cycle.
- On-demand based on target evap, target condenser, and compressor speeds.
Oil return terminates in cooling mode either after 5 minutes or if the suction line temperature minus the target evaporator becomes less than 41 degrees.
Oil return terminates in heating mode if evaporator pressure drops below 31.9 psi or 9 minutes.
Backup Operation– If a unit faults out on a compressor related fault, if the indoor units are turned off then back on by the centralized controller the machine will go into automatic backup operation. The unit will run with the offending compressor disabled for 24 hours then try to run it again. To get out of backup operation, cycle power to the outdoor units.
Emergency Operation- This is used when there is a need to lock out a compressor until you can get back to it to replace it. In VRV 3, the entire module will be locked out. With VRV 4, you can lock out individual compressors or the entire module. Emergency operation can be found in mode 2->38, 39, and 40.