January 13, 2017 - HVAC
By Thomas Conn
Variable Refrigerant Flow (VRF) is a method of heating and cooling that is quickly gaining acceptance and popularity in institutional and commercial facilities in the United States.
Though VRF, which uses refrigerant in either a subcooled liquid or superheated vapor state to heat and cool spaces, was invented in Japan in 1982, it was not introduced to the United States until the 2000s.
The technology spread quickly across several large markets and is now a viable option for maintenance and engineering managers to consider for heating and cooling in many applications. VRF is a simple expansion of the refrigeration cycle; instead of the evaporator being a heat exchanger, where the refrigerant cools down water to be pumped through terminal units to cool a space, the refrigerant flows through coils inside an indoor unit, cooling the air and heating the refrigerant.
The refrigerant travels back up to the outdoor unit to be pressurized by the compressor and reject heat to the atmosphere, taking the place of a condenser in a traditional system. This subcooled liquid refrigerant travels back to the coils to further cool the space. When heating, the process is reversed. The coils receive superheated, vaporized refrigerant cooled by the air in the space served by the indoor unit, and then sent to the outdoor unit to be vaporized by absorbing ambient heat from the atmosphere and sent back through the system.
Choosing systems
Without nitpicking the differences between each major manufacturer’s product line, the major difference between the two most common types of VRF systems should be understood.
Three-pipe systems employ some method of heat recovery between indoor units. A branch controller, centric to all of the indoor units, sends either vaporized refrigerant, which has just left a unit in cooling mode, to a unit that is in heating mode, or liquid refrigerant, coming from a unit in heating mode, to a unit in cooling mode. The three pipes that give this system its name are the liquid pipe, vapor pipe and suction pipe that are connected to each indoor unit. Solenoid valves that isolate each indoor unit from the branch controller allow the heat recovery to take place, with the branch controller directing the flow of refrigerant between units. Indoor units that are in cooling mode will have their liquid line and suction lines open, while a unit in heating mode will have its vapor and liquid lines open. Two-pipe systems, with liquid and vapor pipe connections at each indoor unit, do not recover heat between units in different modes. All indoor units must be in the same mode, with the refrigerant flowing from the outdoor unit with no heat exchanger in between.
Although the versatility of this system is not questionable, there are applications where VRF systems are not clearly the best choice. If the conditioned space is large and will not be divided into multiple zones, a VRF system will not be the ideal choice. If VRF is being considered for a building that already has a large chilled water/hot water or chilled water/steam system, adding a VRF system may be more trouble than it’s worth, unless the space to employ the VRF system needs to be isolated from the rest of the building’s utilities and heating/cooling plant. This may be the case of multiple tenants occupying a single building and having the ability to choose whether or not they want to use the heating/cooling system that serves the rest of the building.
Billing is also a factor that cannot be overlooked. Developers and building owners enjoy the simplicity of billing tenants for using VRF systems because no fees need dividing among the tenants or paid for by building ownership. In the case of a central heating/cooling plant, the tenants pay for the electricity used to power their fan coil units, PTACs or heat pumps, but the building owner pays to power and maintain the chillers, cooling towers, filters, boilers and pumps. In a building with multiple VRF systems, the power for the indoor units is paid for along with the rest of the tenant’s electricity bill. If the outdoor units do not receive their power from the same breaker panel serving the indoor units and the rest of the tenant space, the power supply to all of the building’s outdoor units can be easily divided among the active tenants by being worked into the maintenance or utility fees for the active tenants.
VRF systems also are appealing to architects and designers. Due to the lack of a compressor inside the indoor unit, VRF systems produce very little noise inside the spaces they serve. Most VRF Indoor Units are compact enough to fit inside the ceiling plenum, which doesn’t compromise a space’s square footage. This appeals to developers who are trying to maximize the square footage of their listings.
A downside to VRF is the performance of the system during extremely hot or cold outdoor air conditions. The operation of any VRF system is greatly hampered by the ambient conditions in the space where its outdoor unit is located, because the outdoor unit must be able to draw thermal energy from the ambient air into the refrigerant during heating mode, and heat must be rejected from the refrigerant into the atmosphere in cooling mode. This becomes difficult during extreme temperatures on both ends of the operating spectrum, and the unit’s heating capability decreases as temperatures get colder, and the cooling capability is lessened as the ambient air around the outdoor unit’s coils gets warmer. Aftermarket kits are available from several manufacturers which increase the unit’s cooling capability during periods of very low outdoor air temperatures, but there are no dependable options which increase the unit’s heating capability during these low outdoor temperatures. This is why VRF systems should not be the only source of heat for spaces where winter temperatures can get below the unit’s minimum ambient temperature rating.
Commissioning concerns
Commissioning and working with different systems in a changing world requires engineers and technicians to have open minds about new technology. Commissioning companies need to be informed of how a VRF system should be set up, and what to look for when testing these systems.
During commissioning, it must be confirmed that the number of the branch controller port that is actually piped to the indoor unit matches the dial’s setting. Close attention must also be paid to the addresses of all units that are connected to the same monitoring system, whether it is just a single apartment, an entire floor of a building, or the whole VRF system for the entire building.
Duplicate addresses cannot exist on two indoor units that are connected to the same headend monitoring system. DIP switches on each indoor unit’s control board are factory-configured to determine the capacity of each unit, allowing the same printed circuit board to be used in VRF indoor units of all capacities. The actual positions of these DIP switches on each unit must be compared to the correct DIP switch positions for that unit, which can be found in the manufacturer’s O&M manuals.
Each indoor unit has the capability to measure one of several different temperatures, and the commissioning engineers are responsible for making sure that the units are operating per design intent. Indoor units can be set to measure temperatures at the unit’s thermostat, at the unit’s return air intake, or at a remotely located temperature sensor. Changing the position of one or more specific DIP switches changes the location of where the unit is actively measuring temperature. It is always good practice for the commissioning agent to have as much supporting literature as possible so that all field-configurable settings such as these can be easily understood, as the commissioning agent must determine if design requirements are being met. Of course, functional testing of VRF units includes measuring the discharge temperature of the unit in both modes, making sure no strange noises are produced during fan speed changes, and completing all tests standard to any piece of air conditioning equipment.
Early-stage planning
It is optimal if there is the opportunity for the commissioning agent to become involved with a VRF system during the project’s design phase. Because of the dependence on VRF systems to be able to draw heat from or reject heat to the atmosphere, spaces may be improperly cooled or heated during peak days.
Cities like New York have extremely wide temperature ranges and a VRF system alone may not be able to make spaces comfortable on these peak days. If design review is in the scope of work for commissioning, the space temperature created by each VRF unit should be calculated for days of both peak heating and peak cooling, so that the comfort level of the space can be estimated on an extremely cold or hot day.
Although HEA has yet to encounter a situation where the cooling load on a VRF system is not being met, it is common for systems in New York City to underperform under heavy heating loads. If it is possible for the weather to be colder than the outdoor air temperature that the system was designed to operate in, a supplemental heating system should be strongly recommended by the design engineer or the commissioning engineer. Supplemental heating systems, such as electric radiators, can be configured to operate in sync with the VRF system so that if a space is a certain number of degrees below its heating setpoint, the supplemental heat will come on and assist the VRF system in bringing the space up to temperature quickly.
Another issue that plagues VRF indoor units is that of access to the units. VRF indoor units are appealing because they can be concealed within the space of a ceiling, but there are still dynamic components inside which need to be accessed for service. The most important components to reach are the unit’s control panel and the filter, but the condensate pump, flow switch and expansion valve are also very important and access to these parts of the unit should be considered before the ceilings go up.
The VRF systems that are out today will undoubtedly be improved as the popularity of this system continues to increase and competition exists between manufacturers to produce the best product. In the meantime, it is best to study the operating principles of what’s already out there so that the performance of these installed systems can be optimized through proper design, installation and commissioning.
Thomas Conn is a Field Engineer with Horizon Engineering Associates, LLP working out of their NYC office. In addition to providing building commissioning services, his background also includes serving as an Engine Cadet for various government and commercial ships.
