Variable-refrigerant systems

Our participants discuss the applications, benefits, and challenges of variable-refrigerant air conditioning systems in commercial buildings.

By Michael Ivanovich, Editor-in-Chief, Patrick Lynch, Associate Editor January 1, 2009

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Our participants discuss the applications, benefits, and challenges of variable-refrigerant air conditioning systems in commercial buildings.

CSE: What makes VR systems worth considering at the start of a project? What factors should engineers look for that scream VR?

John Brantley : VR systems are primarily a zoning system that can economically provide precise individual comfort control to multiple spaces. VR systems can be applied in any situation where the client is looking to minimize energy consumption. I have performed energy models comparing many types of systems in every portion of the United States and have found that VRFZ systems are consistently 25% or more efficient than the best alternative.

Lee Smith : VRV/VRF systems offer many attributes more commonly associated with chiller type systems. They offer a very high degree of comfort, have extremely quiet operation, offer the ability to serve multiple zones, have high efficiency, can be integrated to centralized control solutions, and are able to serve large capacity applications with their modular approach. Basically any application where the optimum solution would be a chiller is where VRV/VRF systems should be seriously considered. A common mistake is thinking of VRV/VRF as a “big mini-split or ductless” system, which is absolutely not this kind of system by design, application, or attributes. The original VRV/VRF system was actually invented by a Chiller engineering team.

Christian Agulles : If a project requires lots of zones for individual temperature control, energy efficiencies that can equal or exceed central chilled water systems, and very good acoustical performance within the conditioned space, VR systems are worth looking at. VR systems can also solve some problems where individual zone sub-metering is required for tenant billing and in projects where there is not much room for large duct runs. We have designed VR systems for medium and large office buildings, meeting/banquet facilities, hotels, and technology room spaces that require 24/7 cooling. They neatly and efficiently address many of the typical design challenges associated with these types of projects. VR systems easily accommodate many individual temperature zones; keep noisy compressors out of the living spaces, and the packaged control systems offer sub-metering of energy usage for each zone. With the recent availability of water-cooled VR systems and vertical indoor DX fan coil units, VR systems can be used with ground loop geo-exchange systems for an incredibly efficient system.

Sunondo Roy : Projects that lend themselves to VR applications include retrofits of large loft-type spaces to individual offices, office clusters within large warehouses, renovation of older buildings with natural ventilation for all offices, and open office areas. Spaces with varying loads can easily be accommodated with evaporator units sized for the space load while the condensing unit can be much smaller than the sum of the individual evaporator units due to nature conditioning load diversity.

CSE: What are the safety considerations for circulating refrigerant through occupied spaces, relative to ASHRAE Standard 15?

Brantley : Refrigerant safety codes, such as ASHRAE Standard 15, provide guidance on how to prevent life safety issues associated with potential refrigerant leaks by defining the refrigerant concentration limit (RCL) per unit volume associated with each type of refrigerant and where refrigerant piping should not be routed. Design professionals may not fully understand how this standard relates to VRF systems and may misapply the refrigerant codes by not correctly calculating the available exposure volume associated with determining the maximum refrigerant charge per circuit. I have reviewed multiple projects, initially deemed problematic, in which the RCL is ultimately a non-issue.

Smith : There has been a variety of scare-mongering about refrigerant safety associated with VRV/VRF systems, as the perception is that they offer a greater risk than any other system. In reality, ASHRAE 15 applies to all systems. Also, similarly stringent standards exist in Asia and Europe where VRV/VRF systems are well established and are applied with sound engineering. In addressing ASHRAE 15 specifically, it is important to ensure the refrigerant volume of a system doesn’t exceed the RCL (25lbs/1000cu. ft as defined in ASHRAE Standard 34) in the smallest occupied space. It is also important to review the classification of occupancy and the routing of the pipe work so that it does not hinder occupants’ exit from a building.

Roy : The primary design consideration is to ensure the refrigerant charge of the system as a function of the floor area served is within ASHRAE 15 limits. Where the limit becomes an issue, ducting the cooling to a number of common heating/cooling profile spaces can improve the charge-to-sq-ft ratio.

Agulles : Certainly, the refrigerant charge for each circuit of the system is something that needs to be looked at on every VR project, but ASHRAE Standard 15 compliance is the same as it would be for any DX system. From a design standpoint, you need to make sure that if there was a refrigerant leak, the concentration of refrigerant in the smallest occupied space, by volume, will not harm anyone. All of the major VR equipment manufacturers use refrigerant R410-A in their equipment, which is an A1 safety group refrigerant, the safest category of refrigerant per ASHRAE Standard 34-2007.

CSE: In what applications or scenarios are VR systems probably not a good choice?

Brantley : Properly designed VR systems work in most applications. Laboratories and hospital areas with high filtration efficiency requirements and increased ventilation rates, however, are not the best application of this type of system. VR indoor units are designed to utilize minimal energy at a lower external static pressure than typically associated with those types of filters.

Roy : Spaces that require extensive outside air requirements are always problematic as the VR systems have limited outside air capability, and having to provide a secondary direct outside air (DOA) system can become prohibitively expensive.

Agulles : VR systems start to lose some of their efficiency in designs where long refrigerant piping runs are required. In applications where the condensers cannot be located in relatively close proximity to the indoor units, VR would not be a good fit. Also, like any air-cooled heat pump, the heating capacity and energy efficiency of the system drops off significantly as the outdoor temperature decreases below 40 to 50 F. In heat dominant climates, air-cooled VR systems may not make as much sense. Water-cooled VR systems, however, can be set up the same way as a typical heat pump project, with a boiler to inject heat, as needed, into the condenser water loop to maintain a minimum condenser water temperature. Projects with high ventilation loads and a high percentage of outside air will often require a separate HVAC system to address the ventilation loads. Depending on the project, it may not make sense from a first cost or operational cost standpoint to install the central ventilation system plus a VR system.

Smith : VRV/VRF systems have many positive attributes that make them a good solution for many applications; however, there are applications where they might not be the most cost-effective solution. These types of applications include those where zoning is not important, applications where there are large open areas where the part load characteristics of VRV/VRF are not an advantage, or applications that are not specifically comfort cooling or heating applications and the VRV/VRF system is utilized as the primary solution such as data rooms, etc. (VRV/VRF systems are designed for comfort cooling.)

CSE: What about ventilation? Do VR systems require separate ventilation systems to meet ASHRAE 62.1? How are ventilation and VR systems psychometrically integrated?

Brantley : VR systems do not automatically require a dedicated outside air system to properly ventilate the zone. VR systems serving low-occupant-density spaces can directly introduce ventilation air to the zone via a unit-mounted intake. VR systems with larger ventilation rates can be interlocked with a total energy (sensible and latent) recovery ventilator (ERV) to provide the needed ventilation air at inlet conditions that a VR system can accommodate.

Agulles : For ductless systems, if operable windows cannot be used to meet the space ventilation requirements, then some type of mechanical ventilation system will be required. For ducted systems, ventilation requirements can be met in the same way you might address them with ducted conventional heat pumps or chilled water fan coil units. Outside air can be ducted into a mixed air plenum on the back of each horizontal indoor unit. Check with each individual manufacturer for specific requirements, but a good rule of thumb is to limit the mixed air temperature entering the indoor evaporator units to below 90 F dry bulb.

Smith : Ventilation requirements are application driven and as such there is no standard practice for meeting the ventilation requirements. Every solution is different, but with VRV/VRF systems, there are plenty of options even before separate systems are considered. For example, many of the indoor fan coil units can easily accept outside air and deal with the requirements of the space, and manufacturers offer OA processing units or ERV that tie in seamlessly to the VRV/VRF systems. If separate ventilation systems are incorporated, there are also options as to how much or how little they actually tie into the VRV/VRF system, with a solution being to use the VRV/VRF system to condition the space and the separate ventilation system just providing constant ventilation air.

CSE: How easy are VR systems to install, operate, and maintain? What makes them harder or easier to maintain than chilled water (CHW) or rooftop unit (RTU) systems?

Agulles : Some people claim that VR systems are no more difficult to install than a typical DX split system. For the systems installed without heat recovery capability, that is a fair statement, except that you need to pay more attention to how the joints and connections are piped. On the refrigerant piping side, VR systems can be a little less forgiving than conventional systems. For systems with heat recovery, some contractors do not have much experience dealing with the branch controller/selector component of the system.

There are also some nuances regarding how different VR manufacturers put their systems together that can be tricky for an installing contractor. For example, some of the heat recovery systems are three-pipe systems; others are two-pipe systems. Some controllers require a condensate drain connection; others do not.

I think that the manufacturers realize that the success and growth of VR systems, in terms of proliferation into the U.S. market, is partially dependent on good support for the contractor community. As far as maintenance, with VR systems, the user has distributed equipment with multiple, smaller compressors to maintain, rather than few, large centralized compressors in a chiller plant. With VR systems, there are no pumps in the system to be maintained. In comparison to a RTU system, VR systems have distributed fan coil units to maintain, including filters and fans rather than a series of VAV boxes within the space.

From an ongoing operations standpoint, there are two important factors that come into play with VR systems. One is equipment life; the second is replacement equipment and parts. With good maintenance, central chiller plants can have an operational life of 25 years or longer. VR systems have a shorter operational life, closer to 15 to 20 years. Unlike central chilled water systems, with VR systems, you are making a long-term commitment to one manufacturer. Components are not interchangeable among different manufacturers. After a system is installed, parts and replacement equipment must come from the same manufacturer.

Brantley : VR systems utilize modular outdoor units, unlike large package equipment. The systems do not require balancing like chilled water or VAV systems. Soft copper can be used throughout the system for easier installation. The entire control system is a simple two-wire installation. Finally, VR manufacturers offer system installation and startup training to ensure the highest level of quality from the very beginning.

Smith : A VRV/VRF system is delivered to site with the installation 80% complete. The installing contractor takes the factory engineered components and places them in the necessary locations and joins everything together from a piping, electrical, and communications perspective. To simplify the piping process, pipe sizes are provided with drawings furnished for the equipment, and special refrigerant fittings are also provided to reduce the amount of field piping manipulation required on-site. Minimal test and balancing is required, and commissioning is commonly an automated function built into the VRV/VRF system. Operation is simple in that each zone can be in control of its own mode and temperature via a simple-to-use LCD controller. As the system comprises DX type equipment, the normal maintenance cycle consists of a simple cleaning of filters and condensing unit heat exchangers.

Roy : I agree that the primary advantage for these systems is the ease of installation. The evaporator units are typically mounted in a lay-in ceiling with a cassette-type unit or mounted side wall in the space. The only connections are the two or three refrigerant pipes, depending on cooling only or heat/cool units, a condensate drain, and a power feed from a nominal 208V/1 phase circuit. The primary disadvantage of VR systems is substantially higher maintenance resources for the distributed evaporator units in terms of filter changes, blocked condensate drains, etc. However, the distributed units are also one of the many benefits of the system in that system failure is usually limited to the area served by the evaporator units. Failure of the condensing unit is an equal burden to centralized systems such as at an RTU or chiller unit. In virtually every maintenance aspect, the modular design of a VR system allows for quick and efficient replacement of component failures.

** CSE: How are VR systems controlled, and how do these controls integrate with building automation systems ?
Agulles: At the zone level, each indoor unit is provided with some kind of thermostat in the space for space temperature control. There are many options available for how multiple zones can be controlled. A zone could be provided with a stand-alone, seven-day programmable thermostat or it could be tied to a multizone controller that allows control of a number of zones from a central control panel. These controllers can typically handle up to 16 different condenser units. The multizone controller is ideal for smaller projects that are not provided with a central building management controls system. The next step involves a touchscreen system controller that can be either stand-alone or networked. In stand-alone installations, these controllers act like a mini-building management system. Multiple system controllers can be tied together with a networked PC to create a pretty robust central controls system for all of the VR system components, complete with remote operation capabilities through a Web browser. For large projects that have a central building management controls system, the VR controls can fully integrate into the BMCS via LonWorks or a BACnet gateway.
Brantley: VR systems have built-in controls that automatically control the equipment to provide independent zone control based on user setpoints. A variety of remote controllers are available to suit the needs of the application.
Roy: VR systems have wired or wireless remote controllers for local occupant comfort conditions and wired central controls that integrate the demand load from multiple evaporator units with the central condensing unit. Typically, these controls are integral to the condensing unit, and the only fieldwork is to connect remote evaporators to the central condensing unit. These systems are meant to operate in a stand-alone fashion with no separate BAS programming or integration.
Smith: VRV/VRF systems can be controlled in numerous ways and offer an extremely flexible approach to meet the wide application scope of the technology.heduling, and general operating parameters to be set as required. For integration to BAS applications, most VRV/VRF manufacturers offer open protocol gateways that connect directly to a third-party BAS and allow two-way control and feedback from the VRV/VRF systems.

**CSE: How much can VR systems modulate for part-load conditions?
The diversity of the distributed evaporator units allows for significant part-load modulation of the condensing unit compressors. Also, larger VR systems will have multiple compressors which allow for stepped loading with one variable speed compressor to smooth out the final loading step to precisely match the actual load.
Brantley: VRFZ units are available which can operate at 4% of total capacity. A common minimum low operating capacity is 19%. The linear expansion valves associated with each indoor unit precisely monitor refrigerant flow to each coil to accurately meet the load within each zone. In terms of operation, a 30-ton VRFZ system has the ability to modulate down to meet a 2-ton space load.
Agulles: Part-load performance and energy efficiency are two of the strongest attributes of the VR system. Since VR systems are modular by nature, they inherently have very good turndown for part load operation. VR systems use multiple smaller scroll-type compressors, typically in the 4- or 5-ton range. In each unit, some of the compressors are driven by an inverter or variable speed drives, to provide tight temperature control and efficient part load performance. Single compressors can generally turn down to around 15% of full capacity. On manifold systems with multiple compressors, you can see turndown of 6% to 8% of full capacity depending on the manufacturer. Since most buildings normally operate between 40% and 70% of the design load, the high-part load efficiency and SEER ratings can dramatically reduce energy consumption and costs.
Smith: This varies from manufacturer to manufacturer, but basically most VRV/VRF systems will modulate from 10% to 100% of the condensing unit rated capacity and do this with up to 50 steps of capacity control. Systems do go as low as a 6% turndown. This modulation is possible due to the use of variable speed compressors and electronic expansion valves on fan coil units, offering superior part load operation of the system.

**CSE: Are there risks for condensation forming on the piping and causing moisture problems, for example, in interstitial spaces?
A properly installed and insulated refrigerant piping system should cause no problems. A two-pipe simultaneous cooling and heating system requires that both the high-pressure and low-pressure lines be insulated due to the state of the refrigerant within the lines. Packaged linesets, which come pre-insulated with a minimum of 0.5-in. of closed cell foam insulation on each line, are available to facilitate proper insulation.
Agulles: There is no more risk of condensation forming on piping in a VR system than in a conventional DX split system. Of course, all refrigerant piping should be properly insulated.
Roy: This is a legitimate concern, but one that is also present in any other competing application. Whether with refrigerant liquid piping, chilled water piping, or cold air ducts, they all have to be insulated to avoid condensation and they all have the possibility of moisture in concealed spaces. The advantage of VR systems is that the piping is smaller and the insulation is only on one pipe versus an equal chilled water solution that has two larger insulated.
Smith: These kind of issues are uncommon with VRV/VRF systems as all manufacturers insist that both liquid and gas lines are insulated. As VRV/VRF systems serve long pipe runs, most systems will aggressively sub-cool the liquid line to ensure cooling capacity performance. As such it is essential to insulate both lines to maintain the integrity of the liquid or gas refrigerant, and it also serves to help prevent moisture issues in interstitial spaces.

**CSE: Do VR systems deal with load diversity differently than a CHW system or a standard RTU?
Yes, they approach load diversity by utilizing the built-in zoning functions inherent in the system and by controlling refrigerant flow to each zone.s.
Agulles: Yes, they can. First, you can connect between 130% and 200% of the condensing unit capacity to each condensing unit, depending on the manufacturer. A 20-ton condensing unit can, in some cases, have up to 40 tons of indoor units connected to it. This allows a designer to take advantage of system diversity in a way that is not available with conventional DX split systems.refrigeration cycle without the condenser energy because you are using the refrigerant to transfer heat. With VR systems, you need to pay close attention to system layout and design in such a way that you can share heating or cooling loads between fan coil units to take advantage of these efficiencies and reduce the overall load on the condensing unit.
Smith: The load diversity is a factor of the building load calculations rather than the system. VRV/VRF systems provide capacity to areas where there is a load, and as load varies depending on occupancy, time of the day, etc., the VRV/VRF system will adjust accordingly. The considerations for applying a VRV/VRF system are very similar to that of a chiller. For example, block loads are determined, and then the optimum fan coil units are selected and connected to the optimum condensing unit to satisfy the block loads rather than the total loads. By using the available building diversity, and considering the design and selection appropriately, it is possible to connect 200% of fan coil unit capacity to the rating of the condensing unit without any operational impact to the system. This helps minimize the overall mechanical cost to the building, can help improve the overall efficiency of the building, and can better use the available energy in the building, especially in a simultaneous cooling and heating operation.
Roy: At a conceptual level, a VR system offers load diversity similar to a chilled water system or variable air volume RTU or AHU. Condensing units, chillers, and RTU condensers are not sized as the sum of the individual distributed cooling units, evaporator units, cooling coils, or VAV boxes, but rather as a fraction of the total based on coincident loads of the individual zones. At a practical level, however, the VR system provides greater system energy efficiency since it eliminates the energy load of the cooling medium distribution device of chilled water pumps. For air system fans, the advantage is minimal since a central fan with VFD is typically more efficient than the average efficiency of the distributed evaporator fans. However, the air system has to be sized to distribute air to the various zones and also return the air. A detailed calculation of the energy impact of the VR and air system would be of substantial value in establishing the overall efficiency comparison.


Christian Agulles , LEED AP Vice President

John Brantley , LEED AP CITY MULTI Mechanical Engineer,

Sunondo Roy , PE, LEED AP Vice President

Lee Smith , Dir. of Product, Engineering and Applications