Implementing variable refrigerant flow systems

Several challenges and opportunities face HVAC engineers when implementing variable refrigerant flow (VRF) systems.

11/30/2011


Figure 1: Water-cooled heat pump condensing units connected to a geothermal system. Courtesy: Daikin ACVariable refrigerant flow (VRF) systems are gaining popularity in the U.S. for cooling and heating in the built environment. These systems have real potential for contributing to the successful implementation of the Architecture 2030 challenge. The purpose of this article is to summarize the challenges and opportunities the consulting engineer faces when implementing VRF systems. The topics of design, ownership, and energy will be presented from the point of view of an engineering professional.

VRF systems combine a series of terminal units that come in many shapes and sizes with one condensing unit to serve the occupied space. These systems provide heating and cooling by circulating refrigerant between the terminal units and condensing unit. A device commonly referred to as a selector valve is used to manage refrigerant flow. An analogy is a water-source heat pump system. With a VRF system, there is refrigerant rather than water, and a selector valve dedicated device to direct the refrigerant. Condensing units can be air-cooled or water-cooled. Water-cooled systems can be coupled to a geothermal system to further improve performance. VRF systems, more than other HVAC systems, require interaction with manufacturer representatives to understand system capacities, specialties, and particulars related to system layout.

Design

VRF systems are filling a growing niche in renovation projects. The advantages inherent to VRF systems for renovations include smaller refrigerant pipelines and decorative indoor terminal units that can be installed in the occupied space. This allows the main components to fit in tight spaces common to renovation projects. For most projects, a lifecycle cost analysis is commonly used for guidance on selecting HVAC systems. The challenge for the design professional is to research the available simulation programs that can simulate VRF systems. On a level playing field, HVAC systems that cannot be readily compared to other HVAC systems are rarely selected for projects.

System layout for construction documents can be a challenge for design professionals. Most of the time, we are familiar with selecting components that can be built with minimal variation for the system intent. The manufacturers of VRF systems have many subtle (and some not so subtle) differences that lead to creative opportunities for presenting information. Differences like maximum pipeline length, maximum refrigerant lift, and unit capacities are manageable.

The challenge for engineers preparing bid documents include pipeline configurations (two or three pipes) and selector valve types that are critical to system operation. The objective of selector valves is to enable the terminal units to be in heating or cooling mode. Ultimately, maximum system operating flexibility occurs when one selector valve or circuit of a large selector assembly is matched with one terminal unit. Each terminal unit can provide heating or cooling without dependence on other zones. The electrical and condensate connections to the terminal units, selector valves, and condensing units should be coordinated in the design documents.

Figure 2: Heat pump units can be coupled with numerous fan coil varieties. The units can use up to 500 ft of refrigerant piping between the heat pump and farthest fan coil. Courtesy: Daikin ACThere are infinite strategies for combining multiple terminal units with selector valves to reduce the cost and maintain the performance of the system. In this arrangement, multiple terminal units connected to a single selector valve can only provide heating or cooling. VRF system information typically found on drawings includes terminal unit types, location, and capacity; other information must include system zoning with particular attention to selector valve and condensing unit relationships along with pathways for refrigerant piping and condensate. The rising cost of copper can be a driving force in the pipeline configuration that is a function of system zoning and is a critical element of the design and budget.

VRF systems are capable of managing most building HVAC loads. VRF equipment is available for dealing with building ventilation loads, but most projects are using a standard dedicated outside air system (DOAS) for access to features like energy recovery, humidity control, and other desired HVAC specialties. VRF systems typically perform well serving sensible loads but have minimal latent cooling capabilities. VRF systems, particularly air-cooled models, occasionally struggle to provide heat, especially in extreme winter conditions. To overcome this challenge, the DOAS also can be used as a supplemental source of heat to the building. The benefits of DOAS, including conditioning the ventilation air, controlling building humidity, and building pressurization, provide a good partnership with the VRF system. Some VRF systems also have the capability to capture refrigerant heat and direct it to a zone needing heat.

VRF control

There are many options to consider when selecting and interfacing building management systems (BMS) with VRF systems. Stand-alone systems with an interface station provided by the VRF manufacturer are popular, especially for smaller projects. There are multiple options and details with individual temperature control devices that should be investigated. Larger projects may use a BMS to monitor and provide some control of the VRF system. An analogy commonly used is VRF systems speak, which is a completely different language from most BMS systems used in the United States, so translators are used to aid this communication.

Figure 3: This system concept diagram demonstrates how refrigerant piping, wiring, fan coils, controls, and compressors work together to form a complete system. Courtesy: Daikin ACWhen a BMS is used with a VRF system, the manufacturer of the VRF system must provide a control system that actively manages all system components and translates information for the BMS. The BMS can be used for temperature setpoint adjustment, schedule, and other necessary activities communicating through the translator. Most VRF manufacturers do not permit control of the system by a third-party BMS. There can be two distinct control systems on a project, one provided by the VRF manufacturer and another by the BMS supplier, so it important to clearly define options and interfaces so all of the intended operations are met.

A tricky issue when implementing VRF systems is understanding the amount of refrigerant that ultimately exists in the system. It can be painful to learn that your building will have to be treated like a refrigerant machine room, according to ASHRAE 15, halfway through the construction process. The engineer should be aware of this issue when laying out the components and work with the manufacturer to estimate refrigerant quantities. The U.S. Green Building Council LEED Credit EA4 “Enhanced Refrigerant Management” and IEQ5 “Indoor Chemical and Pollutant Source Control” are usually not available when a VRF system is applied. Not to worry—there are lots of energy optimization points and other points available for projects seeking LEED certification.

Ownership

VRF systems have many technology features and specialties. There are a few simple guidelines for the engineer to enhance the value of the system for your client.

It is important to require specialized training for the installing contractor. Training should focus on installation details specific to the actual equipment being installed. There are a variety of differences among the VRF manufacturers. The equipment manufacturers of VRF systems understand the benefit of a well-trained installer since they typically require training and certification before the equipment can be purchased.

Commissioning to optimize the performance and reliability of the system is vital. Maintenance technicians trained in and experienced with refrigeration systems are key to long-term, successful operation of these systems. The backbone of these systems is refrigerant and controls. A technician with these skill sets can effectively troubleshoot and solve operational issues.

There are many terminal unit options for VRF systems. The most common arrangement has the terminal located in the occupied space. One benefit of this design is that the equipment is easily accessible. The risk is that room occupants may need to relocate for a period of time when system maintenance is performed.

Energy

Figure 4: This fan coil unit normally is intended for a concealed location. Courtesy: Daikin ACThe growing interest in building Energy Use Index (EUI), popularity of U.S. Environmental Protection Agency Energy Star, and emergence of the Architecture 2030 challenge proves that energy conservation and environmental stewardship are mainstream issues. VRF systems have much to contribute in the energy conservation category. While one of the challenges of VRF systems is developing an accurate energy simulation, this equipment has advertised energy efficiency ratios (EER) in excess of 20. VRF systems also promote diversity in the refrigerant system and sharing of energy between the terminal units. Space-by-space equipment distribution and nearly infinite compressor control promotes thermal comfort, and control features, such as enabling terminal units based on occupancy, promote efficiency.

Another way to determine energy consumption of VRF systems is to use an energy benchmarking protocol. Energy benchmarking is gathering actual energy use by building type, system type, and region. This energy data should be an integral part of the forecasting process used for many functions, including tuning the computer simulations.

VRF systems use electricity to provide heating and cooling so the energy ratio of your facility will be biased toward electricity. Impressive site EUIs can be expected with VRF systems, but depending on the region, the source EUI and resultant Energy Star score can suffer. Similar to other electric-intensive HVAC systems, control sequences should consider the utility rate structure.

VRF systems offer excellent opportunities for energy performance, system flexibility, and low noise dissipation. The design documents tend to be different, primarily due to the variety of technologies used by manufacturers. VRF systems are a legitimate option for an engineer’s high-performance tool kit. Hopefully, the issues raised by this article are useful to avoid common pitfalls and lead to successful projects.

Oathout is director of engineering for Durrant. He is an energy thought leader with a passion for collaboration, integrated design, and sustainability in pursuit of achieving the 2030 Challenge for the built environment. 


VRF system toolkit

Design

  • Research energy simulation software suitable for VRF systems.
  • Understand pipeline and selector valve options offered by different manufacturers.
  • Clearly define zones and operating intent of the terminal units. The decision will have a substantial impact on system cost.
  • DOAS usually make a good partner for the VRF system.
  • How is the system going to be controlled—temperature set-back, ventilation, building pressurization, etc.?
  • Define interface relationship for the VRF and BMS, if BMS is included.
  • What is the correct amount of refrigerant in the building?
  • Identify terminal unit style and capacity.
  • Terminal units are not a good application for large volumes like gymnasiums.
  • How will the condensate from the terminal units be collected?
  • Certified Standard Ratings for equipment performance.

Ownership

  • Require installer training and certification specific to actual equipment.
  • Commissioning is vital.
  • Maintenance technicians should be familiar with refrigerant components and systems.
  • Terminal units are normally maintainable from the occupied space.

Energy

  • Computer energy simulations can be a challenge.
  • Consider using actual energy data from similar facilities/systems in your analysis.
  • VRF systems only use electricity to provide heating and cooling.

 



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