Accurate Temperature Control in KNX-Based VRF-Home Automation Integration

Integrating VRF systems using KNX can be daunting and complex. The good news, however, is that with CoolAutomation solutions, it doesn't have to be. Nevertheless, integrators still face challenges, including ensuring accurate temperature control. The following article explores how to overcome this issue and ensure a seamless integration.

Getting Started

As always, we must begin the process by identifying and mapping the relevant challenges, exploring possible solutions, and determining what works best in each particular case.

Our starting point: we have a VRF system and want to control it with a home automation platform via KNX.

In such a scenario, the original VRF thermostat would usually be eliminated and replaced with a KNX device that will provide local climate control and must include the following functionality:

• Operation status (on/off)
• Operation state (cool/heat/fan)
• Set point temperature
• Fan speed (usually high/medium/auto)

Our first challenge is, therefore, achieving this functionality as part of a KNX-based VRF integration.

HVAC Integration

Traditionally, this integration process can be lengthy and complex, with extensive development efforts required to bridge the gap between proprietary communication protocols used by VRF systems and the KNX-based automation platform. CoolAutomation’s smart gateways help streamline the process, natively communicating with VRF HVAC systems of all major brands, as well as KNX interfaces. 

In our particular scenario, we can achieve integration in two ways with CoolAutomation’s solutions:

1. Installing a local integration gateway for each indoor unit (CoolPlugs + CooLinkHub)
2. Installing a central integration device, as shown in the diagram below (i.e., a CoolMaster device)

This is when the second challenge presents itself.

The Challenge

VRF systems have two temperature measurement sensors: the first is in the return airflow intake of each indoor unit, and the second is in the wired panel typically mounted on a nearby wall.

During KNX-based integrations, the original VRF wired panel is usually removed, eliminating one of the temperature sensors. Here lies the problem.

While not critical for cooling, this sensor is vital during heating. As hot air always rises, the VRF internal unit can “feel” it has fulfilled the user’s requirements and start decreasing the heating capacity when, in fact, the room temperature has not reached the desired setpoint. This is where the second temperature sensor becomes vitally important, letting the HVAC system know that its setpoint targets have not been met, thereby preventing it from reducing its heating output.

The Solution

The key to overcoming this challenge lies in using the KNX temperature sensor as a reference for indoor unit operation. With CoolAutomation’s solutions, the way to achieve this depends on the selected integration method: a central gateway or dedicated gateways for each indoor unit. Let’s break down the solution for each approach.

Option 1: Central Integration Gateway

For this option, we will use one central gateway. It will be connected at a single point to the VRF system on one side and to KNX on the other.

Advantages

• Cost optimization (fewer gateways)
• Streamlined troubleshooting (only one connection point)
• Increased reliability (fewer gateways = less failure points)

Disadvantages:

• Inability to share the reference temperature recorded by the KNX sensor with the VRF indoor unit- the very issue we are trying to resolve.

It might seem as though we've reached a dead end, but happily, several simple workarounds are available. The solution does not lie in the integration process, but in HVAC design and implementation. Two approaches can be used to address this challenge, and they can be used independently or in conjunction:

1. Designing the airflow ductwork so that the return air is taken from the bottom. Thus, the built-in sensor inside the indoor units will get a more accurate input that’s indicative of the room’s actual temperature.
2. Removing the return air sensor from the indoor unit and mounting it where the control panels are located. All VRF manufacturers offer a remote sensor kit designed for this purpose.

Option 2: Dedicated Gateways for Each Indoor Unit

In this configuration, each indoor unit will have a dedicated gateway that’s integrated with the KNX system. Presumably, this option’s advantages and disadvantages would mirror those of option 1, with the advantages there becoming disadvantages here. However, the challenge we faced in option 1 (sharing the KNX temperature reference with the indoor units) is, in fact, no longer an issue in this use case. This is because the CoolPlugs are connected to the indoor-outdoor communication line (as opposed to the central line when using the CoolMaster), allowing them to easily communicate KNX temperature sensor data to each indoor unit: providing the desired second reference point. 

Conclusion

Integrating VRF HVAC systems with home automation using KNX can be tricky, but the process becomes much simpler with the right approach and solutions.

By leveraging CoolAutomation's integration gateways and addressing the challenge posed by temperature sensor placement, home automation integrators can achieve seamless functionality and precise climate control. Whether you opt for a central integration gateway or dedicated gateways for each indoor unit, understanding the advantages and considerations of each approach is key to making the best decision for your specific project. If you’d like more information on KNX-based VRF system integration or need guidance on design and integration considerations, feel free to contact us.