Central Control Capabilities Of VRF Systems
Understand the differences that can exist among various options so you can avoid a design that inadvertently paints the owner (and yourself) into a corner.
Designing a VRF system for competitive bid is challenging due to each manufacturer’s unique implementation of the VRF system technology. Each has its own set of requirements and restrictions that must be considered. However, if the system is designed solely for a particular manufacturer’s requirements, the design will be hard to bid by others. In particular, the capabilities of the proprietary central controls systems are vastly different. We recently designed a large VRF system that required controls integration with a central BMS, rooftop units, and indoor humidification control by 3rd-party duct humidifiers and humidistats. As we talked with different VRF manufacturers, we learned that it wouldn’t be as simple as we initially thought.
This article will share what we learned by answering a handful of questions related to connection capabilities of three VRF manufacturers to demonstrate the magnitude of difference in their controls platforms. The VRF manufacturers were arbitrarily selected and have been left anonymous in this article. The purpose of this article is not to “rank” the VRF manufacturers, so their position in each question slot has been randomized.
Can the VRF controller connect to a BMS, and how? What level of control and monitoring can be performed at the BMS? Any other options for higher level control?
Manufacturer 1. Manufacturer 1 offers varying levels of controllers. Their cheapest/simplest controller is designed to operate standalone, and does not offer any connection to a higher-level system other than a single hardwire forced-off safety input. Their more advanced controller can connect to a BMS through BACnet or LonWorks communications via an optional gateway. The BMS can perform status monitoring, alarm monitoring, and start/stop through this method. It is necessary to call out this additional gateway accessory on the controls drawings or specifications, as it does not come standard. The more advanced controller also allows for monitoring through a proprietary web-based PC application, which comes at an additional cost to the owner.
Manufacturer 2. Manufacturer 2 has a few different options for connection to a higher-level controls platform. One option is through BACnet communications via an optional gateway. This gateway is offered as an additional accessory. Other options include web-based PC monitoring and a proprietary software interface, both of which are sold separately. All options allow for remote status monitoring, alarm monitoring, and start/stop commands. This manufacturer has an option that forgoes the touchscreen interface on the central controller if the user is planning on monitoring through BACnet communications only.
Manufacturer 3. The VRF controls platform offered by Manufacturer 3 is primarily designed to rely on the use of their PC applications via ethernet connection for larger VRF systems. They offer a proprietary software and a web-based client. Monitoring is possible from a BMS through BACnet and LonWorks communications, but the enduser would probably end up using one of the PC applications for all monitoring/managing functions. Smaller VRF systems could forgo the PC application if that is desired. In this case, the manufacturer offers an on/off controller and a touchscreen central controller, neither of which are capable of remote monitoring at the BMS.
Author recap. Most, if not all, of the VRF manufacturers in today’s market will have some option to connect a central controller to BMS for status monitoring, alarm monitoring, and start/stop. Any requirements further than that would need some custom coding solutions. Notice, however, that this communication often comes as an add-on accessory or a particular type of central controller is required. It is critical for the design engineer to detail any BMS connection needs or else they will not be picked up in the bid. The web-based PC monitoring is also an option, but will incur additional costs to the owner, particularly because these software packages are often sold on a subscription service. We recommend that this decision be brought up to the owner prior to the project bid.
Does the VRF controller have any standard connections to other packaged equipment like a rooftop unit? What kind of information is shared in this exchange?
Manufacturer 1. The VRF system can be paired with any of their packaged rooftop unit options via a Modbus communication gateway. This gateway is treated by the central VRF controller in the same manner as the VRF outdoor units. The VRF controller is capable of performing the following RTU functions: start/stop, set point adjustment, alarm monitoring, and status monitoring. Larger systems need to consider the bandwidth limitations of the Modbus line.
Manufacturer 2. Manufacturer 2 does not manufacture packaged rooftop units, but their VRF controllers have the capability to connect to third-party units through an additional controller. The standard option assumes the rooftop unit will use refrigerant from the VRF outdoor unit, as shown in the diagram below. The AHU controller can perform functions that are typical for a packaged rooftop unit. A benefit to this system is that it eliminates a compressor in the rooftop unit, but the designer needs to account for this load when sizing and selecting the VRF outdoor unit capacities.
Manufacturer 2 also offers a standard option of connecting their energy recovery ventilators. This is done by adding the ventilator to the modbus line along with the VRF indoor units. In this case, a separate controller is required for all integrated equipment. Limited communication options are available for other products made by this manufacturer, like packaged air conditioners, but this communication is limited to digital I/O such as start/stop or status monitoring signals.
Manufacturer 3. Manufacturer 3 does not manufacture packaged rooftop units, but their VRF controllers have the capability to connect to third-party units through an additional controller. The standard option assumes the rooftop unit will use refrigerant from the VRF outdoor unit, as shown in Figure 7. The controller has limited standard control function, and can only control the unit based on discharge air temperature. This option could be useful for very simple constant volume scenarios, but more complex control requirements for the rooftop unit would be much better served by an independent controller. Much like Manufacturer 2, this design eliminates the compressor in the rooftop unit, but the designer needs to account for this load when sizing and selecting the VRF outdoor unit capacities.
Author recap. VRF and rooftop unit integration is becoming a standard in the industry, but the control capabilities of these RTUs are very limited. The integrated controllers that are usually offered are small application-specific controllers. In a scenario where the goal is to keep the system simple and cheap, this type of VRF/RTU integration makes sense. However, if the RTU needs to perform any complex control schemes outside of typical RTU operation, a more custom solution will be necessary. Some of the VRF manufacturers can offer more complex solutions, but others cannot.
Careful consideration must be made when sizing the VRF outdoor unit capacity, particularly when RTU coils are added to the system. Be careful of claims that “the outdoor units can support up to 130% of their listed capacity” or similar statements. These claims are based from assumptions that the sum of the connected load will only operate in block loading conditions. It does not change anything regarding the actual capacity of the system. Most of the time, this statement comes from a sales rep trying to undercut the other bids by decreasing their outdoor unit capacity. Do not let them trick you into arbitrarily decreasing the outdoor unit capacity — the design engineer should be solely responsible for block loading design.
Can the VRF controller connect to other non-proprietary 3rd-party auxiliary equipment and sensors? If so, what level of control can be performed?
Manufacturer 1. For auxiliary equipment, Manufacturer 1 offers a modular, fieldbus independent I/O system that can be added to the standard VRF controller. This add-on can be used for both digital and analog inputs and outputs. It has a select few standard application sequences, but custom sequences can be written for an additional design service charge. There are limitations on how much outside equipment can be added in this way. Since this is a modular system, each additional control point requires an additional module. The current limitations are set by the physical size of the controller enclosure, which can fit 64 of the smaller modules or less if a larger module size is required. It is important to also note that there are software limitations that can decrease this amount.
Manufacturer 2. Manufacturer 2 offers digital I/O integration through a few of their controller options. The controllers have a limited number of input and output slots depending on which controller model is used. Analog control does not come standard and would require custom coding.
Manufacturer 3. Manufacturer 3 can connect to 3rd-party auxiliary equipment and sensors via an external contact signal. The communication is limited to digital I/O, so you can only perform functions like start/stop or status monitoring. Analog I/O is not possible at this time. The communication can either be used to enable/disable an indoor unit or can be used to send an enable/disable command to other equipment. Notice that this communication exchange happens at the VRF indoor unit as opposed to the central controller (like Manufacturer 1).
Author recap. VRF manufacturers vary greatly on connection capability to third-party equipment and sensors. In general, many will offer some sort of simple digital I/O integration. Analog I/O integration is rare, and will require some custom coding even when it is possible. Even 3rd-party thermostats are not accepted by many VRF systems.
In our design case that required integration with 3rd-party humidifiers, we specified an enable/disable signal to the humidifiers to match the run status of the associated indoor unit fan. The humidifier then cycled and modulated itself based on a room humidistat through its on-board controller. This method allowed us to keep multiple manufacturers in the specification. Any analog integration would unintentionally rule out the majority of the field as it is not yet offered by most manufacturers.
What is the main takeaway from all this?
VRF system manufacturers in today’s market implement the VRF technology in vastly different ways. Therefore, each VRF system will take a unique approach to meet certain design requirements. As long as the design engineer is aware of these subtle, but important differences, the design will be able to go through a fair and competitive bid. If, for instance, a design relied too heavily upon the capabilities and approaches of a certain manufacturer, it could unintentionally discourage other manufacturers bidding from the project.