The push to reduce energy consumption is changing the way buildings are designed, constructed and maintained, as every aspect is evaluated to realize maximum efficiencies, including equipment and systems.
HVAC systems, in particular, have become a source of debate as to what system is best with regard to cost, performance and efficiency.
Hydronic systems and variable-refrigerant flow systems are two heating and cooling systems often compared for their efficiency performance. While VRF systems are attractive for their ductless technology and claims of superior energy efficiency, industry studies demonstrate how hydronic systems outperform VRF systems in terms of energy performance.
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 the primary cooling/heating medium, comprising a main compressor unit connected through refrigerant lines to multiple indoor cassette units that can be individually controlled.
Compared to hydronic systems, VRF systems are expensive in terms of first costs – in many cases 5 to 20% more because of their complex refrigerant management system and controls. VRF systems usually require a dedicated outdoor air system to meet ASHRAE 62 ventilation codes, which also adds to the first costs.
In addition to higher first costs, VRF systems have a shorter life expectancy. Hydronic systems have been known to last 25 to 30 years, while VRF systems may need to be replaced 10 to 15 years after installation. This is because the compressor in a VRF system operates during the heating and cooling cycles, which reduces the life of the compressor.
When selecting HVAC systems for new construction and retrofits, it’s important mechanical contractors and specifying engineers explore the operational performance and efficiency disparities between VRF and hydronic systems. Some key differences are detailed below.
Hydronic systems can take advantage of a wide range of energy sources from natural gas and propane to renewable energy sources such as solar-thermal collectors and biomass boilers, making hydronic heating and cooling systems easily adaptable to a wide variety of current and future energy sources. In contrast, VRF systems are solely powered by electricity.
While extremely hot or cold air conditions can impact the performance of both hydronic and VRF systems, hydronic systems are less affected by temperature changes. The efficiency of a VRF system is reduced as the ambient temperature increases in the cooling mode and decreases in the heating mode. Additionally, a VRF system may require a supplementary heat source in cold climates, such as hot water from a boiler, which may negate the energy efficiency of the system.
A VRF system can provide zoned comfort, recovering heat from one zone for use in another. This is effective in buildings such as hotels and schools, which run both heating and air conditioning at the same time. However, a VRF system does not have thermal energy storage capabilities. Water-based hydronic systems, such as a water-sourced heat pump system, can draw the heat or chill from a room and carry that energy back to the system for later use. This reduces energy consumption and costs.
It’s important to consider the size of the building when selecting an HVAC system. Hydronic systems are better-suited to handle buildings that require 50 to 100 tons of cooling capacity or more. They also are advantageous because they have the ability to pump chilled or heated water efficiently and effectively over very long distances to service a sprawling campus or high-rise office buildings.
VRF systems are suited for buildings that are 10 stories or less because the length of piping that can be run in order to carry refrigerants and oil through the building is limited by 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. Because of these restrictions, good applications for VRF systems are historic buildings and facilities that are three stories or under. A larger project could include multiple VRF systems, but that significantly increases costs.
VRF systems require specialized technicians for the life of the system – installation, adjustments and repairs. This adds to the lifecycle costs of the system.
Since VRF piping requires mechanical fittings brazing and soldering on site, the quality of installation depends on the installer’s level of expertise. Installers must be qualified to work with refrigerants under extremely high pressure and be knowledgeable about refrigerant piping locations under their state’s International Mechanical Code, as well as leak detection and ventilation requirements per ASHRAE Standard 15.
Hydronic systems are designed with universal components that can be installed and serviced by any trained HVAC service technician.
In late 2016, the Environmental Protection Agency extended its requirements related to ozone-depleting substances to hydroflourocarbons (HFCs) such as R-410A, which is the current refrigerant choice in the HVAC industry.
Taking effect Jan. 1, 2019, the updates include:
More stringent requirements for repairing leaks in larger appliances;
New recordkeeping for the disposal of appliances containing 5 to 50 lb. of refrigerant;
New reporting requirements for chronically leaking equipment; and
Restricting the sale of HFC refrigerant to technicians certified under sections 608 or 609 of the Clean Air Act.
It is possible for leaks to develop in both hydronic and VRF systems, but a leak in a VRF system is potentially catastrophic. Refrigerant leaks cannot be detected by sight or smell, making them hard to find and difficult to repair. In spaces with minimal ventilation, refrigerant concentrations could possibly reach levels that put people at risk of losing consciousness or even suffocation.
Under the EPA’s updated refrigerant management requirements, the leak rate must be calculated every time substitute refrigerant is added to a piece of equipment (unless the addition is made immediately following a retrofit, installation of a new unit or qualifies as a seasonal variance).
The final rule also includes the following provisions for owners and operators of refrigeration and air-conditioning systems:
Quarterly or annual leak inspections or continuous monitoring devices for refrigeration and air-conditioning equipment that has exceeded the threshold leak rate which was recently lowered from 10 to 15% of total refrigerate charge annually;
Owners or operators must submit reports to the EPA if systems containing 50 or more lb. of refrigerant leak 125% or more of their full charge in one calendar year;
Sales restriction extended to HFCs and other nonexempt substitutes with the exception of small cans (2 lb. or less) of nonexempt substitutes for motor vehicle air-conditioner services; and
Technicians are required to keep a record of refrigerant recovered during system disposal from systems with a 5- to 50-lb. charge size.
The phasing out of refrigerants such as R-410A over the next 10 years could have a costly impact on existing VRF installations because the entire system including piping, outdoor units and fan coils will need to be replaced.
While VRF systems are advantageous in small buildings, hydronic systems will continue to be the dominant HVAC technology for commercial applications because of their ability to adapt to changing system demands and proven environmental and economic benefits.
This article was originally titled “Tech Topic: Hydronic vs. VRF systems” in the September 2017 print edition of PM Engineer.