Is VRF right for your next project?
Variable refrigerant flow (VRF) systems can be specified into a variety of buildings, especially those that require flexibility.
- Understand where variable refrigerant flow (VRF) systems should be specified in nonresidential buildings.
- Know the codes and standards that dictate their design.
According to the 2014 HVAC and Building Automation Systems (BAS) Study conducted by Consulting-Specifying Engineer, the average annual dollar amount of HVAC and BAS products specified is $2.63 million. While this includes traditional products and systems—like air handlers, fans, and pumps—it also includes more nontraditional systems, including chilled beams and variable refrigerant flow (VRF) systems.
Building owners have the option to incorporate VRF systems into their buildings, and these “alternative” system manufacturers field many questions from interested engineers and building owners, though they may not be gaining momentum (see sidebar, “VRF market presents dilemma for engineers”). An overview of these systems and some best practices have been provided by Julianne Laue, PE, LEED AP BD+C, BEMP, senior energy engineer at the Center for Energy Performance, Mortenson, Minneapolis.
Question: Are there any best practices or tips you suggest for engineers considering VRF systems?
Laue: Analyze feasibility of VRF project selection based on energy-efficiency goals, building type, project type, climate, and building size.
Question: What types of projects lend themselves best to specifying VRF systems?
Laue: Any type of commercial construction projects that require high flexibility. Retrofit projects with low floor-to-floor heights are also prospects. Buildings with diverse, multiple zones requiring individual control—office buildings, schools, hotels—are all good candidates.
Question: What are the positive aspects of specifying VRF systems?
Laue: There are several: energy efficiency, design flexibility, quiet operation, ability for individual users to control temperatures, smaller ductwork, lightweight, and the ability to simultaneously heat and cool in zones on the same system.
Question: What are the negative aspects of specifying VRF systems?
Laue: Price per ton is higher than conventional systems, and long refrigerant lines and many fittings increase the potential for refrigerant leaks. There also is a need for supplementary heat in cold climates, and a need for a separate ventilation system.
Question: In a VRF system, how do you deal with limitations of refrigerant volume in a space, if the system circulates in so much of the building?
Laue: ASHRAE Standard 15: Safety Standard for Refrigeration Systems and Standard 34 provide safeguards for refrigerant systems. It is best to use these design standards as well as consult with your local authority having jurisdiction (AHJ) as to their interpretation of the standards.
Question: What codes/standards do engineers need to consider when specifying VRF systems?
Question: What type of commissioning/balancing is required to ensure these systems are designed and working properly?
Laue: Commissioning of VRF systems is similar to other unitary air conditioning and heat pump systems. Additional steps may include verifying the capacity of the outdoor units to provide specified heating and cooling capacities at peak loads and extreme temperatures, and verifying proper operation of electronic expansion valves. It is best to consult with manufacturers on specific requirements for their equipment.
Question: Long-term, what can engineers do to ensure facility owners and building managers have so the VRF systems run successfully in their building?
Laue: Routine maintenance, which is similar to maintaining other unitary air conditioning and heat pump systems, and usually requires changing filters and cleaning coils. Typically, VRF controls/automation are proprietary to each manufacturer and can be tied to a BAS for integration of schedules and alarms.
Question: Have you seen many VRF systems used in cold climates providing heating and cooling? Have they provided a back-up heat source for below-zero operation?
Laue: Check the heating capacity ratings of the VRF manufacturer you are designing around. As outdoor air temperature decreases, the heating capacity will decrease. You have options for heating, including a nonsimultaneous heating and cooling system; you can oversize the system by designing it for the heating load at the heating design day. Another option is to provide an auxiliary heat source.
Question: There are concerns about properly insulating the VRF system. Is insulation considered when the design work is done? Installing contractors are not using the appropriate thicknesses, as we have seen problems with lack of proper thickness to control condensation. Also, do you recommend vapor dams on these systems?
Laue: It is important to design and install any HVAC system per the manufacturer’s recommendations. Installers typically install what is specified. I recommend working with your local manufacturer’s representative for the VRF system you are specifying to ensure all design and installation conditions are met.
Question: Regarding VRF systems that have the best energy (simultaneous heating/cooling), are there difficulties finding enough vendors to comply with government project requirements for “or-equal”?
Laue: Specifying any HVAC system should be done with great care. Specifications that are copied from an individual manufacturer create increased difficulty in meeting an “or-equal” condition. Energy performance should be looked at as a minimum performance, enabling products with performance above the minimum to be allowed.
Download the 2014 HVAC and Building Automation Systems (BAS) Study: www.csemag.com/research
Watch the webcast about HVAC: Alternative heating/cooling systems: www.csemag.com/webcast