Regulating HVAC with VRF systems

Variable refrigerant flow systems have a lot to offer as an alternative HVAC system

By Pieter de Bod July 25, 2022
Courtesy: WSP

 

Learning Objectives

  • Learn about the benefits using a heat recovery variable refrigerant flow system.
  • Understand how ASHRAE 15 and 34 influences the VRF system design.
  • Recognize the benefits of using an advanced centralized multizone system controller.

 

Variable refrigerant flow systems use a refrigerant as the working fluid in the refrigeration cycle in direct expansion air conditioning systems. Leading VRF manufacturers have labeled their systems as variable refrigerant volume, or VRV.

VRF systems comprise several components:

  • Direct expansion outdoor unit(s).
  • Indoor unit(s) with a coil.
  • Filter (ducted and ductless) and fan.
  • Refrigeration piping with refrigerant.
  • Thermostats.
  • Condensate drainage.
  • Power wiring connections.

Ensure that the refrigerant piping systems meet the installation requirements as listed in ASHRAE Standard 15 : Safety Standard for Refrigeration Systems. For example, piping is not permitted to be installed in enclosed stairways, landings of egress, elevator shafts or any shaft with moving objects or elevations lower than 7 feet 3 inches above the floor.

Outdoor VRF units: air- or water-source

Air-source outdoor units need to be installed in well-ventilated areas to prevent or reduce the possibility of recirculating of air through the outdoor unit because it reduces the system performance efficiency, however there were cases when installation in well-ventilated areas was not possible for air-sourced VRF outdoor units because of limits of refrigerant piping length.

VRF outdoor units were traditionally only air-sourced. In 1990, water-cooled outdoor units were introduced to overcome the challenges with maximum refrigerant pipe length and provided more flexibility. Water-cooled VRF units use condenser water for heat exchange, therefore they can be installed in enclosed environments like underground mechanical rooms. Condenser water can be made by cooling towers, a campus loop or geothermal systems.

Figure 1: The energy efficiency of variable refrigerant flow systems have been significantly bolstered specifying the heat recovery VRF system that uses the waste heat generated during the cooling process of direct expansion systems and reuse it for heating, thereby providing independent cooling and heating to different zones from the same outdoor system. Courtesy: WSP

Energy efficiency

The system energy efficiency in air conditioning systems that use the principle to cool air and then heat it up (like VAV terminals) is poor compared to air conditioning systems that do not rely on this principle. VRF systems do not waste energy by cooling air and heating it up again to control the supply air temperature. Instead, a VRF system carefully regulates the temperature and amount of refrigerant flow in the indoor unit to achieve the desired leaving air temperature.

The power consumption for an air-sourced heat recovery VRF system ranges between 0.5 and 0.8 kilowatts/ton compared to 0.7 to 1.0kilowatts/ton conventional air-sourced chillers with VAV terminal units. Leading VRF manufacturers have established a unique refrigerant circuit that closely regulates the refrigerant flow for both heating and cooling to achieve individual air conditioning, even for buildings with both cooling and heating needs.

The overall fan energy for VRF systems is much less compared to central ducted systems like central or rooftop air handling units. The overall annual energy consumed by fans are wide-line underestimated in buildings and should get more focus in building energy consumption.

The third fan law stipulates that the electrical power consumed by a fan change with the cube of the airflow. For example, this means that a 10% increase in fan airflow results in a 33% increase in electrical power to do that work. The cubic nature of this relationship between power and the airflow or rotational speed shows how, even for small performance gains, large amounts of additional power are needed.

Ducted or ductless indoor VRF units have low-pressure fans that uses a fraction of the power compared to a central air conditioning system like VAV boxes and medium-pressure fans.

Heat recovery VRF

The energy efficiency of VRF systems has been significantly bolstered with the heat recovery VRF system that uses the waste heat generated during the cooling process of direct expansion systems and reuses it for heating, thereby providing independent cooling and heating to each zone from the same outdoor system. Heat recovery systems are identified by either using refrigerant branch controllers connected to two refrigeration pipes in the system or by using three refrigeration pipes with special y-pipe joints or headers.

Three-pipe technology comprise dedicated refrigerant pipes for suction gas, liquid and discharge gas. The dedicated refrigerant pipes provide smooth and efficient refrigerant flow during all main modes of operation and aid with the heating performance of the system.

In a two-pipe heat recovery system, where the gas and liquid travel as a mixture in the refrigerant pipes, the condensing temperature needs to be higher to separate the mixed gas and refrigerant. The higher condensing temperature that is needed means that the compressor has to work harder. In addition, the disturbed refrigerant flow in large pipes on a two-pipe system results in extra pressure drop, which can negatively impact the system capacity and efficiency.

Cooling only or heat pump VRF

VRF systems are offered as cooling only systems, heat pump systems and heat recovery systems. Cooling only systems should only be specified when all the internal zones require cooling all the time. Heat pump systems should only be specified when all of the zones are in either all heating or all cooling.

Because the indoor units of cooling only and heat pump VRF systems cannot independently cool or heat from the same system, it should not be considered when the internal zones require both cooling and heating, even if offered as value engineering alternative.

Occupancy comfort is vitally important and will be compromised if cooling only or heat pump systems are installed in climate zones where simultaneous cooling and heating is needed.

Variable refrigerant temperature control

Some of the latest VRF systems include variable refrigerant temperature control, which is a revolutionary method that adjusts not only the refrigerant flow, but also the temperature in relation to the space load and the ambient weather. This results in even higher energy efficiency and better comfort control.

The evaporating temperature (in cooling) and condensing temperature (in heating) are automatically adjusted to minimize the difference with the condensing temperature and the evaporation temperature, respectively. This makes the compressors work less and also enables the system to always maintain the ideal compressor speed so that the VRF system can deliver the optimum efficiency. The user has the option to select three different variable refrigerant temperature modes:

  • Basic mode: This mode is selected to maintain optimal occupancy comfort and the refrigerant temperature is fixed.
  • Auto mode: This mode allows the VRF system to automatically optimize for either energy efficiency mode (reaction speed is slow), rapid cooling (powerful mode, reaction speed is very fast) or occupancy comfort (priority is given to keep the room temperature constant) and the refrigeration temperature is not fixed, but floating. This mode also allows the system to boost cooling capacity above 100% if needed.
  • High sensible mode: Energy saving mode and the refrigeration temperature is not fixed, but floating.

Refrigeration concentration limit

The refrigerant in VRF systems migrated from R-22 to R-410a, which is a more environmentally friendly refrigerant. Refrigerants used in VRF systems are normally heavier than air and pose a health hazard to the occupants of a building if not carefully designed.

The refrigeration concentration limit of every VRF system should be carefully calculated according to ASHRAE Standard 15 and ASHRAE Standard 34: Designation and Safety Classification of Refrigerants. The concentration limit shall not be exceeded as dictated by the ASHRAE standards and local or international building codes. The concentration limit is the total VRF system refrigerant charge (in pounds or kilograms) divided by the allowable internal space volume (cubic feet or cubic meter). The space volume is the floor area x false ceiling height.

ASHRAE Standard 15 classifies VRF systems as direct systems and high-probability systems, which means the indoor unit evaporator coils are in direct contact with the conditioned air stream and have a high potential to leak refrigerant into the occupied space. Most VRF systems sold in the U.S. market use refrigerant R-410A and ASHRAE Standard 34 lists R-410A as a safety classification group A1 are labelled as nontoxic and nonflammable.

Refrigerant R-410A is heavier than air and will displace oxygen, hence Standard 34 dictates the maximum refrigerant concentration limit of 26 pounds/1,000 cubic feet of room volume for occupied spaces. Also refer to ASHRAE Guideline 41: Design, Installation and Commissioning of Variable Refrigerant Flow Systems for additional information.

ASHRAE 15 and 34 should be used when designing VRF systems. The following procedure can be followed to ensure compliance:

  1. Schematic design: Once the heating and cooling loads have been finalized, prepare a schematic design showing the various VRF systems, including the location of the outdoor units, indoor units, piping layout and circuit controllers. Consideration should be given to the correct zoning of the heat pump systems in various spaces to avoid combining spaces with different thermal load profiles (e.g., avoid serving a perimeter office from the same indoor unit as an indoor office and instead serve various indoor spaces from the same indoor unit and serve perimeter units from another indoor unit). A VRF energy recovery system that serves both perimeter spaces on the north and south of a building (provided it is practically possible) from the same outdoor unit is typically more efficient than a VRF unit that only serves perimeter spaces on the north of the building because the heat that is extracted from the south perimeter offices that needs cooling can be efficiently recovered and piped to the north perimeter office that may need heating. The overall system efficiency is higher. Ensure all the indoor units and branch controller have adequate access.
  2. Calculate total refrigerant charge: Calculate the total refrigerant charge in each VRF system by adding the refrigerant quantity of the VRF equipment and the piping.
  3. Create room list: Prepare a list of rooms served by each VRF system. List the floor area, ceiling height, volume and occupancy classification. Use ASHRAE Standard 15 to find the refrigerant concentration limit for each room.
  4. Calculate: Calculate the minimum allowed room volume by first calculating the minimum allowed floor area based on the formula:
    • Minimum allowed floor area (square feet) = [Total system refrigerant charge (pounds)] / [(refrigerant concentration limit (pounds/1,000 cubic feet) x Ceiling height (feet)] x 1,000.
    • Compare the room volume with the calculated minimum allowed room volume. Verify that the room volume is more than the calculated minimum room volume. Mark rooms that do not meet this requirement for corrective action.
  5. Corrective action: There are various corrective actions that can be taken to ensure compliance, which includes (but is not limited to):
    • Increase the room volume by connecting it to other rooms and increase the room size.
    • Increase the room volume by raising the ceiling height.
    • Reduce the refrigerant charge by reducing piping length.
    • Reduce the refrigerant charge by serving less rooms from the VRF system and increase the amount of VRF outdoor units.
    • Serve very small rooms from a standalone direct expansion mini-split system.

Figure 2: Installation of the indoor units and the branch controller (circuit controller) in progress. Courtesy: WSP

VRF scalability

VRF systems are inherently scalable and can be built one system at a time or installed separately one floor at a time. This makes a VRF system ideal for large-scale construction projects, older buildings undergoing extensive renovations and hotels or apartment buildings with multiple tenants.

VRF manufacturers offer an advanced centralized multizone system controller that provides the most cost-effective way to control and monitor the complete system. The multizone controller features an LCD touch screen. Up to 128 indoor units can be monitored and controlled with individual cooling and heat setpoints, setpoint range limitation, setback setpoints and auto changeover. Up to 1,024 indoor units can be monitored and controlled with the addition of adaptors. The multizone system controller has the ability to store the following data for a few days:

  • Unit operation data.
  • BACnet client objects.
  • Input/output system data.

The operation data can be exported through a USB drive or through the web browser remotely. The building automation system can monitor the BACnet objects of indoor and outdoor unit operation data with the BACnet server gateway option.

Ancillary equipment like interlock sensors, switches, dampers, fans, pumps and lighting can be integrated into the multizone system controller.

With the addition of the tenant billing module, the multizone controller determines the energy consumption of shared condensing units based upon tenant indoor unit demand.

Other functions of the multizone controller includes the following (but not limited to):

  • Optimum start and timed override
  • Advanced Auto changeover with fixed, individual, average and vote methods
  • Web accessibility and alert email (standardized): all screen views and configuration menus can be accessed through web.
  • I/O: monitor and control third party equipment with digital input, digital output, analog input and analog output) signals; up to 512 management points; interlock function with indoor units and ancillary equipment.
  • Power proportional distribution option: calculates apportionment of outdoor unit’s total power consumption to individual units on the system.
  • BACnet client option: enables the controller to use the BACnet/internet protocol; allows for full monitoring and control of third-party BACnet capable equipment.

VRF installation

Like most HVAC systems, VRF systems needs to be installed correctly and comply with the manufacturer’s installation requirements. The installation of refrigeration piping systems for VRF systems are more sophisticated compared to chilled and heating water systems and therefore requires specialized training and skills.

VRF manufacturers offer quality training and certification programs. The manufacturer should always inspect the finished VRF system installation and confirm compliance to its installation guidelines. VRF systems also require regular maintenance to ensure optimal functionality and performance, and past experience demonstrates that maintenance intensity is less compared to chilled and heating water VAV air handling unit systems.

Based on popularity of VRF systems worldwide, coupled with the improved energy efficiency and competitive or lower first cost, VRF systems are here to stay for the foreseeable future.


Pieter de Bod
Author Bio: Pieter de Bod is a senior mechanical engineer at WSP.