Edited by Eamonn Ryan

The following article is derived from an ASHRAE Journal public podcast delving into the intricacies of Variable Refrigerant Flow (VRF) systems, presented by Kelley Cramm, an ASHRAE Life Member and mechanical engineer with Henderson Engineers.

Multiple VRF heat recovery systems with a central control. Multiple Light Commercial cassettes units, high wall units and ducted units which are also connected to the central controller.

Multiple VRF heat recovery systems with a central control. Multiple Light Commercial cassettes units, high wall units and ducted units which are also connected to the central controller. Supplied by AHI Carriers

The presentation explores VRF systems, proper installation techniques, and addresses common misconceptions surrounding compressor failures.

VRF systems represent an innovative approach to all-electric heat pumps, providing remarkable flexibility. These systems may incorporate heat recovery, offering diverse indoor fan coil unit options, from drop-in ceiling cassettes to compact wall-mounted units and concealed ducted units. The unique feature of VRF is the ability to pipe multiple indoor units to a single set of outdoor condensing units, reducing the overall footprint.

They are highly cost-effective, particularly for spaces demanding individual temperature control, such as buildings with multiple enclosed offices. In comparison to hydronic fan coil systems or classic variable air volume systems with reheat, VRF proves to be a more economical solution. Moreover, its capacity for heat recovery makes it an energy-efficient choice. By utilising heat rejection from cooling zones to provide heat to zones in need, VRF ensures optimal performance and resource utilisation.

Numerous Teraco projects ranging from pre-treating fresh air to humidity control hybrid units. These projects utilised Light Commercial outdoor units and VRF outdoor units connected to custom built AHUs.

Numerous Teraco projects ranging from pre-treating fresh air to humidity control hybrid units. These projects utilised Light Commercial outdoor units and VRF outdoor units connected to custom built AHUs. Supplied by AHI Carriers

Common challenges and misconceptions

While VRF systems offer numerous advantages, the industry has faced challenges, primarily related to compressor failures. Routine compressor replacements have contributed to a negative perception of VRF in the market. Manufacturers, keen on maintaining customer relationships, often replace failed compressors under warranty without delving into the root cause of the failures. This practice has led to a persistent issue without a comprehensive resolution.

To mitigate the challenges associated with VRF, proper installation techniques are crucial. It is important to perform a thorough assessment during installation, ensuring that system components are correctly matched and that the system design aligns with the building’s specific requirements. Adequate training for installers and ongoing maintenance are essential components of ensuring the longevity and efficiency of VRF systems.

On the maintenance side, filters on indoor units need to be changed regularly. This can be challenging for buildings with many indoor fan coil units, especially if the maintenance staff is stretched thin. Failure to change filters can reduce airflow at the indoor unit, potentially leading to compressor failure. Additionally, condensing unit coils, the outdoor units, need regular cleaning at least twice a year. VRF systems often use micro fin condensers, which readily trap dirt and debris. Neglecting this maintenance can result in condenser failures due to reduced airflow across the fouled coil. Since these are heat pumps, condensing units run year-round, requiring more frequent maintenance compared to traditional DX cooling units. If the owner lacks the staff for routine maintenance, engaging a qualified service contractor is essential.

VRF Packaged unit.

VRF Packaged unit. Supplied by RPM Engineering Group

Installation practices that are important for VRF

The number one item is purging the pipe with an inert gas, usually argon or nitrogen, during brazing to prevent slag and poor joints. The brazers must be trained and certified by the VRF manufacturer. Proper pipe installation, following the manufacturer’s shop drawing, is crucial. If there are changes in the field, an as-built piping drawing must be created, and the manufacturer’s software rerun to determine the correct refrigerant charge.

VRF compressors are routinely operated at high speeds. If a compressor fails, and the owner neglects error code checks, they may not realise they’ve lost a compressor, as the remaining compressors will speed up to compensate. Overspeeding compressors for extended periods can cause additional failures. It is crucial for owners to diligently check error codes and address any issues promptly.

Installing suction line side stream filter assemblies with fine mesh filter elements and isolation valves is critical. These are used during startup to filter out fine particles generated during brazing. If not used, debris can migrate to a compressor, causing premature failure. Once the system is operational, these valves can be closed.

Refrigerant line lengths and sizes inform the refrigerant charge, and any field changes require a new charge calculation.

Engineers must ensure that the design complies with ASHRAE Standard 15 and the International Mechanical Code, considering limitations on refrigerant pipe placement, installation, and potential openings between spaces.

Understanding limitations in refrigerant pipe placement

There are several limitations, one being that refrigerant pipe installation is not allowed in egress corridors or paths of egress due to potential leakage concerns. For example, if there’s an egress corridor without a ceiling, and you need to cross it with refrigerant pipe, finding a way to enclose that pipe is necessary to prevent exposure in the egress corridor.

During the design phase, engineers should be cautious about relying on VRF systems to condition ventilation air, particularly in humid climates. VRF fan coil units have limited capacity to handle latent loads. Therefore, decoupling ventilation air from VRF units, especially in humid climates, is advisable. Utilising dedicated outdoor air systems for delivering conditioned ventilation air directly to the space is a recommended practice. This approach allows for proper sizing and dehumidification of outdoor air, transforming the VRF system into a sensible heat unit. Engineers should also specify that installing contractors provide proof of factory training and certification from the VRF manufacturer for their installers. Regular site visits during construction are essential to assess the quality of the installation. Engineers must cross-check the installation against the manufacturer’s shop drawing, ensuring adherence to the design specifications.

Contractors play a vital role in ensuring that the onsite labourers are qualified, trained and certified to install VRF piping and equipment. The responsibility falls on both mechanical contractors and general contractors or construction managers to guarantee that qualified labour is on site for VRF installations. It’s essential to ensure that labourers have the necessary expertise, especially since one designated installer may not always be present on-site due to simultaneous ongoing projects.

System integration, particularly connecting the VRF controller to the building management system (BMS), remains a challenge. Each VRF manufacturer has its proprietary software and control systems, creating what we call ‘black boxes’. These closed systems are not easily accessible or compatible with other manufacturers’ equipment. While progress is being made, achieving true integration with the BMS across different VRF manufacturers is still an ongoing challenge.


Climate impact on VRF

It is essential to recognise that VRF systems operate as heat pumps, and their heating capacity significantly decreases as the outside air temperature falls. This poses a challenge in cold climates where the demand for heat is high, but the heat pump’s efficiency is reduced. Depending on the project’s location and climate, it’s advisable to consider incorporating a source of backup heat. A practical approach is to integrate backup heat in dedicated outdoor air systems, offering additional heating capacity when needed in extremely cold weather. Additionally, manufacturers provide wind guards for outdoor condensing units, particularly vital in cold climates to counteract capacity reduction caused by cold winds.

VRF systems stand as a cost-effective solution for achieving individual temperature control in multiple spaces. Design engineers should invest time in educating themselves about these systems due to their complexity. Utilising dedicated outdoor air systems for ventilation air is recommended, coupled with adhering to ASHRAE 15 calculations and refrigerant volume regulations in occupied spaces. Rigorous attention to code requirements on refrigerant pipe placement, especially in egress paths, is crucial.

In summary, during construction, vigilant oversight of installations, adherence to shop drawings, and scepticism towards manufacturer-published heating capacities in low ambient temperatures are vital. Providing backup heat in cold climates is wise due to potential overstatement of heating capacities. Moreover, engineers should acknowledge the proprietary nature of VRF systems, differing among manufacturers, and consider bid implications in open-market scenarios. Specification requirements should ensure that successful bidders address any necessary changes to guarantee a seamless installation.