Author: Ryan Findley

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:

  1. Small temperature drop across the outdoor heat exchanger.
  2. If there is a temperature drop at the outlet of the heat exchanger.
  3. If the low pressure stays low enough over a 2 hour period.

Defrost will terminate if:

  1. The max defrost time is 5 minutes and 30 seconds or 6 minutes have elapsed (depending on model).
  2. Heat exchanger temp rises above 57.8 degrees for more than 90 consecutive seconds.
  3. 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:

  1. Run time of 8 hours or 2 hours after a power cycle.
  2. 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.

— Ryan

This tech tip is written by experienced tech and VRV specialist Ryan Findley. Thanks Ryan.


This is a quick overview, not a substitute for taking proper manufacturer training

 

This tech tip is geared towards the mechanic who is fairly new to VRV systems.  As with anything, the ability to install or service anything we first must understand the basic fundamentals.  Even though some VRV systems might feel like a bit of overwhelming, but they still function the same as a standard heat pump.

Note: VRV refers specifically to Daikin and this tech tip is written primarily from a Daikin point of view

Let’s start at the beginning.  There are two product lines in VRV, heat pump and heat recovery.  The heat pump is simply the same setup as your residential heat pump running 2 pipes from the indoor units to the outdoor units.  Heat recovery is running 3 pipes from the outdoor units to the indoor units allowing the machine the ability to run heat and cooling simultaneously. Daikin Heat pump model #’s are RXYQ where heat recovery will be REMQ or REYQ.

 

First, let’s identify some components.  In a VRV system we have the outdoor units, which are also commonly referred to as modules.  Modules come in various sizes ranging from 6-12 ton in heat pump and 6-10 ton in heat recovery in the VRV 3 line.  VRV 4 ranges from 6-14 tons.  Modules can be combined in tandem up to a total of 3 outdoor modules.

 

Indoor units

Indoor units (or fan coils) come in many different types, but the most common ones used are the ceiling cassette and the ducted units.

Refnets

Refnets are a Y type fitting designed to provide equal flow to both the main piping continuing on and also the branch piping that is taking off of the main.  Refnet installation is critical.  Follow manufacturers recommended install practices of keeping the angle of the refnet below 15 degrees for outdoor unit piping and 30 degrees for indoor piping.  To visually show what the adverse effects of improper installation, see this video.

Branch selector boxes

Branch selector boxes or BS boxes for short are only used in heat recovery applications.  BS boxes will have the 3 pipes coming from the outdoor units piped directly to them.  They are made up of solenoids and EEVS.  The fan coils determine the mode of operation but the actual change of the mode of operation occurs in the bs box.

 

Communication wiring

VRV systems are basically one giant communicating residential variable speed heat pump.  Information is shared from the fan coils to the outdoor units across a daisy chain of communication wiring.  The wire should be 18/2 non-shielded stranded.  The system should also be wired as shown in your submittal documents from your sales engineer.

 

PVE Oil

Polyvinylether oil is used in the VRV product line.  It’s used because of its outstanding miscibility at low refrigerant velocities.  It’s noteworthy that PVE is more hygroscopic than POE oil is. The big difference between the two is that PVE can be dehydrated by pulling a vacuum on it whereas POE will not. Another difference is when PVE interacts with moisture, it doesn’t produce acids via hydrolysis.  If PVE is exposed to moisture, it turns the oil to sludge.

 

Heat pump vs Heat Recovery

A heat pump system operates with only 2 pipes running between the outdoor and indoor units.  One line is always a liquid line whereas the other line is either hot gas or suction gas.  Mode of operation is determined by a master stat (designated at startup).  The master stat is the only one in the system who has the ability to change the mode of operation.  There’s a more complicated way to control these if you have an iTouch Manager, which we will discuss later.  Heat recovery uses a 3 pipe setup and bs boxes.  The 3 pipes consist of a liquid line, suction line, and a dual high pressure/low-pressure line.  When the machine is in full heating mode, the dual gas line will have discharge gas going down it.  The liquid line remains the liquid line and the suction pipe is not being used.  In full cooling mode, the dual gas line turns into an additional suction line.  The suction line and liquid line act as it would in a standard a/c. Parallel operation is when there is a demand for heating and cooling at the same time.  In this mode, the dual gas line will be in heating mode.

 

Differences between VRV 3 and VRV 4

There are a few big differences between the two product lines.  First is VRV 3 uses a crossover line that runs between all of the modules, if there are more than one.  VRV 4 does not have that.  VRV 3 has 1 inverter compressor and 1 standard compressor (in modules that have more than 1 compressor).  VRV 4 has 2 inverter compressors, again if it’s a module that has more than one compressor. Both product lines have different inverter boards and different cabinets.  VRV 4 also has a split outdoor heat exchanger that allows ½ of the coil to be in one mode of operation while the other ½ could be in another mode of operation.  VRV  4 inverter boards are cooled by subcooled refrigerant that runs on the back of the heat sync.  VRV 3 inverter boards are air cooled from the inside of the cabinet near the outdoor fan motor.

 

Filter Driers

Filter driers are not used unless there is burnout of a compressor. If a burnout cleanup is required, follow manufacturers recommended clean up procedure.

–Ryan

 

 

 

 

 

 

 

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