An efficiency comparison of VRF & hydronic systems
Since Variable Refrigerant Flow (VRF) first was introduced to the U.S. 20 years ago, there have been concerns about the safety and viability of the technology in commercial building HVAC systems.
In 2011, the Air Conditioning, Heating and Refrigeration Institute (AHRI) developed and approved AHRI Standard 1230, the official testing and rating standard for VRF systems. As with most industry standards, it is common to review them on a regular basis to ensure they are on target and evolving with technological advances. Recently, the U.S. Department of Energy (DOE) decided to review the accuracy of AHRI’s test protocols and manufacturers’ stated efficiency ratings. The DOE formed a working group to discuss and develop proposed test procedures and amended energy conservation standards for VRF systems.
At the center of this review is whether test methods currently accepted under AHRI 1230 yield materially inaccurate or unrepresentative energy-use data. Field tests performed by a major utility company in 2018 found that efficiency results were 40% to 50% lower than VRF manufacturers claim. Overstating energy efficiencies makes it difficult for consumers to make reliable system purchase decisions and does not provide the relief to the energy grid that utilities depend on.
As the industry continues to rely on sustainable building practices to maximize building performance and minimize environmental impact, the type of HVAC system specified for a facility plays a key role in realizing energy-efficiency targets.
VRF & Hydronic
Two heating and cooling methods often compared in terms of energy consumption and system performance are hydronic systems and VRF systems. Hydronic systems provide water-based heating and cooling through pipes, ductwork and other components such as pumps, drives, controls, heat exchangers and valves. VRF systems use refrigerant as their primary heating and cooling medium, comprised of a main compressor unit connected through refrigerant lines to multiple indoor cassette units that can be individually controlled.
While each has its place in commercial building HVAC systems, specifying engineers must carefully review project parameters and the applicable safety codes of ASHRAE Standards 15 and 34 to ensure they are making suitable selections. To understand which system is right for a particular building, a closer look at key factors such as safety, compliance, serviceability and lifecycle costs are important to the evaluation process.
It is possible for leaks to develop in both hydronic and VRF systems, but a leak in a VRF system potentially is hazardous. Refrigerant leaks cannot be detected by sight or smell, making them hard to find and difficult to repair.
Hydronic systems with cooling units also require refrigerant to operate, but the average system uses 66% to 75% less than a VRF system of the same size, according to the Hydronics Industry Alliance. Hydronic systems are not exempt from ASHRAE and IAPMO codes. However, the refrigerant typically is contained within a mechanical room, which is required to have the proper ventilation to manage a potential leak.
Aside from the small amount of refrigerant associated with cooling units in a hydronic system, the water running through the system and in its pipes over the life of the system poses no safety or environmental risks.
The same cannot be said for VRF refrigerants. In late 2016, the U.S. EPA issued major changes to the Section 608 rules of the Clean Air Act, which govern the handling, use and sale of refrigerants. Most notable is the regulation banning the use of hydrofluorocarbons, like R-410A, in new chillers, rooftop units and VRF systems beginning in 2024.
Phasing out refrigerants during the next 10 years could have a costly impact on existing VRF installations because the entire system likely will need to be replaced. Uncertain life cycle costs of VRF systems, as well as their proprietary nature and safety concerns over refrigerants and the serviceability of long refrigerant runs, prompted the U.S. Department of Defense to issue a directive in 2017 prohibiting VRF systems at U.S. Air Force facilities and strongly discouraging their use at U.S. Army facilities.
Most VRF systems are manufactured in Asia, and many use proprietary components. The availability of these specialized components may be more limited, especially in emergency situations when they are needed quickly. Proprietary VRF systems also require specialized technicians for installation and maintenance, which can drive up costs.
Conversely, the piping, valves, circulators and terminal units required in most hydronic systems are universal components that easily can be sourced from companies with national distribution networks. This provides options when the system initially is designed and installed, as well as when maintenance or replacement parts are needed in the future. The initial cost of a hydronic system generally is lower, and systems offer a range of flexibility for components, operation and maintenance, both in terms of parts and service.
According to the ASHRAE Equipment Life Expectancy chart, hydronic systems typically last 20 to 25 years, while VRF systems could need replacing within 10 to 15 years. The compressor in a VRF system is forced to work harder during heating cycles, reducing the life of the bearings and the compressor.
When specifying a system, it is important to consider not only building size, but also the size of the HVAC system itself. Hydronic systems are better suited to handle buildings that require 50 to 100 tons of cooling capacity or more. Hydronic systems also have the capacity to pump water efficiently over long distances, such as a college campus or a high-rise office tower.
VRF performance and energy consumption is directly tied to refrigerant piping length. Extended refrigerant pipe runs adversely impact system efficiency. Typically, refrigerant charge in a VRF system is 4 to 6 lb of refrigerant per ton of cooling. To adhere to ASHRAE 15 requirements, the VRF system may need to be broken down into smaller refrigerant circuits, thus compromising the benefits of diverting loads.
VRF systems generally have been limited to buildings less than 10 stories because the length of piping runs must be limited in order to carry refrigerants and oil through the building in accordance with manufacturer guidelines. Long lengths of piping can jeopardize performance of the unit if oil or refrigerant accumulates in the piping or migrates back to the unit.
With heating and cooling among the largest costs in commercial buildings, HVAC system selection remains an important factor in both new construction and retrofit projects to keep costs in line and realize energy-efficiency targets.
For now, DOE is continuing to review and consider proposed updates to ensure the accuracy of AHRI Standard 1230 to actual field operations. Until VRF energy efficiency ratings are verified, system designers must remain diligent in their analysis to ensure HVAC systems are code compliant, energy efficient and adaptable to future energy sources.