The International Building Code defines institutional occupancies under Institutional Group I and includes the use of buildings or structures where care or supervision is provided to persons who are or are not capable of self-preservation without physical assistance. It also includes facilities that house persons who are detained for penal or correctional purposes and/or those whose liberties have been restricted.
VRF systems are direct expansion heat pump systems in which multiple indoor units are connected to a common condensing unit, which can be either air or water source, via a refrigerant piping network. These systems have become popular in North America in the last 10 years and are often applied to less complex institutional occupancies. In this article, we will review the basics of VRF systems and highlight some considerations for applying these systems to Group I occupancies. Specifically, we will examine considerations for heating redundancy, the requirements necessary to meet ventilation and airflow requirements, and compliance paths for ASHRAE Standards 15 and 34.
Institutional Occupancy Overview
Institutional occupancies have several unique requirements and include facilities such as:
- Alcohol and drug centers;
- Assisted living facilities;
- Congregate care facilities;
- Halfway houses;
- Residential board and care facilities;
- Foster care facilities;
- Detox facilities;
- Nursing homes;
- Psychiatric hospitals;
- Correctional centers, jails, and prisons; and
- Day care facilities (child and adult).
Many states have specific requirements in their own building and health codes for the design and operation of these facilities, and many reference requirements found in ASHRAE Standard 170: Ventilation of Health Care Facilities and guidelines produced by the Facility Guidelines Institute.
VRF systems’ indoor units, which can be ducted or ductless, are sized for the peak cooling or heating loads of the zones they condition, while the condenser system is sized for the highest block (simultaneous) cooling or heating load.
VRF systems are available in heat pump and heat recovery applications (Figure 1). In heat pump applications, the total system will operate in a common mode of operation that is determined by a “master” indoor unit or remote controller. Each individual indoor unit can have individual set points and can be controlled to operate independently of one another based on the set points and measured room/return for that zone or room.
FIGURE 1A & B.
For buildings that require cooling and heating simultaneously, a VRF heat recovery application is available. What typically required a secondary heat source (electric/hydronic reheats or perimeter heating) can be accomplished by VRF heat recovery as it transfers heat absorbed from areas of the system operating as cooling to areas of the system operating in heating using a heat recovery unit. For detailed information on the operation of the VRF heat recovery mode, consult a specific manufacturer’s training manual or the ASHRAE Handbook of Fundamentals: Systems and Equipment 2016; variable refrigerant flow.
VRF systems utilize inverter driven compressors, which vary their speeds and refrigerant displacement based on total system operating conditions. This helps improve efficiencies compared to conventional systems and reduces energy consumption during part-load conditions. Compressor speeds are determined by the high- and low-side saturation temperature and pressure and how far they are from the predetermined target values as determined by the unit’s control board logic. In the cooling mode, the target evaporation saturation temperature is ~43ºF, and in heating mode, the target condensing temperature is ~115º. The deviation of these values from the target combined with mode of operation and several other operating parameters is what generally determines compressor speed.
Capacity is controlled at the indoor units as well. The electronic expansion valves at the indoor coil can modulate in steps between fully open and fully closed to reach the desired opening percentage. These expansion valves modulate the amount of refrigerant through the indoor unit’s coil based on the room set point, actual return or room air temperature — measured through either a return air sensor in the unit or at the remote controller, if available — superheat at the coil (if in cooling mode), or subcooling at the coil (if in heating mode).
When the indoor units are under high-load conditions, they can modulate the expansion valves near the fully open position to increase refrigerant flow and bring the room back to temperature with only a minor pull-down/warm-up period. The compressors would ramp up their speeds to make sure enough refrigerant was flowing for the indoor units to use. Conversely, as the indoor units bring their rooms closer to set point, the expansion valves will throttle closed, reducing the indoor unit’s capacity. This avoids the short cycling, temperature swings, and poor dehumidification associated with conventional equipment during lower load conditions. In general, most VRF systems can maintain a room or return air temperature of +/- 1º deviation from the set point if they’re sized, installed, and configured correctly.
Another feature of VRF systems is the ability to connect a total indoor unit capacity that exceeds 100 percent of the condenser system rating, which is known as the “connection ratio.” This can be accomplished because indoor loads in comfort cooling applications will shift throughout a building or space throughout the day. This shift is affected by changes in occupancy, orientation of the building, equipment uses, location of zones, etc. This is referred to as the “block load,” which by definition is the “largest sum of all simultaneously occurring zone loads,” or coincident load.
> FIGURE 2. The peak zone loads for each zone of a representative light commercial building located in a 4a climate zone.
As shown in Figure 2, indoor unit capacities are selected for the peak zone load of the space they serve. Using south zone as an example, the indoor unit capacities for this zone would be selected based on the peak heating load requirement of 47.6 MBH if the VRF system was to be the sole source of heating. If additional heat sources are used, the indoor units can be sized based on the lower cooling load for the zone of 37.3 MBH.
> FIGURE 3. Minimum filtration efficiencies from ASHRAE Standard 170.
Production line indoor units typically range from 0.5-8 nominal tons of cooling capacity. Indoor units typically have a sensible heat ratio between 0.7-0.85 at nominal conditions and airflows ranging between 325-450 cfm/ton. Ducted indoor units typically produce rated cfm at static pressures between 0.4-0.8 inch water column.
When required, most VRF manufacturers can allow for the connection of third-party and applied air handlers to their condensers. This is useful for applications with higher cfm or static pressure requirements, multi-zone air distribution, and when additional components need to be included within an air handler.
VRF condensers can be air-cooled or water-cooled and range in capacity from 3 to 40-plus tons. These condensers are modular in design with individual modules ranging from 3-20 tons. Multiple modules (Figure 4) can be combined to achieve larger system tonnages on a common refrigerant piping circuit. These units offer robust operating ranges. Many advertise 80 percent-plus nominal heating capacity at low ambient conditions.
ASHRAE Standard 170 defines requirements for utilities, heating and cooling systems, AHU design, air distribution, outside air intake and exhaust, filtration, and energy recovery. Using these requirements and knowledge of VRF systems characteristics, one can assess whether selecting a VRF system is appropriate for the design parameters required by different applications. For instance, VRF units are not a great idea for operating rooms but can often be applied to administrative areas, inpatient nursing, nursing facilities, some diagnostic and treatment rooms, and some support spaces.
Section 6.1.2 of ASHRAE Standard 170, Heating and Cooling Sources, requires that heat sources and essential accessories have reserve capacity to meet a facility’s needs, which includes requirements for redundancy in both heat sources and accessories to ensure continuous heating capacity in the event of equipment failure or routine maintenance. This section also includes requirements for domestic hot water for the facility. Similar requirements exist for central cooling systems over 400 tons, but as VRF is considered a decentralized system, that would not apply.
Achieving true redundancy with VRF alone might be challenging and often requires the integration of additional heat sources. Depending on climate zone and budget, some solutions include using a hot water system as the primary heating source or using electric backup heat. In cold climates, hot water with redundancy on the boilers tends to be the popular option in many facilities with electric backup heat primarily used in assisted or senior living facilities (residents often pay their own utilities in these facilities).
It’s technically possible to achieve true redundancy with VRF, but it can be very difficult. To be clear, true redundancy means no loss of capacity in the event of a failure. In my experience, many VRF reps try to pass these systems off as being inherently redundant because they utilize multiple condensers and compressors. In order to truly have redundancy it might be required that a “backup” condenser is added to the system selection, essentially increasing its capacity by 20-30 percent. This can be problematic as it results in a higher system refrigerant charge and may make complying with ASHRAE 15 more difficult.
Section 6.4 of ASHRAE Standard 170 specifies minimum filtration requirements based on space designation and can require both pre and final filters. Section 7 defines the minimum ventilation requirements of the standard and provides design parameters in Table 7.1 (Figure 4) for minimum outside air change rates, minimum total air change rates, requirements for recirculated room air, design relative humidity, and design temperature. Section 7 also allows the designer to separate the filtration requirements between outside air and recirculated air. For instance, the standard might allow a minimum filter rating of MERV 6 at a VRF indoor unit that is providing recirculation and space conditioning of the outside air if it is treated by a dedicated outside air system or central air handler with filtration as required in Section 6.4.
> FIGURE 4.
When making selections, it’s important to keep in mind some of the characteristics of VRF indoor units. One should pay close attention to the unit’s cfm/ton, latent capacity, fan speed control, and fan static pressure capabilities.
The last consideration we will examine is complying with ASHRAE Standards 15 and 34, which define requirements for safely applying refrigerant-based HVAC systems. The standards specify the maximum allowed system refrigerant charges based on the volume of the room that any part of the refrigeration system is in (coils and indoor units, piping, etc.). The standards also define requirements for the location and installation of refrigeration piping and refrigerant-containing components.
This is a particularly important consideration for institutional occupancies as Standard 15 and 34 are more stringent in the amount of refrigerant allowed compared to other occupancies. VRF systems use R-410a, which has a refrigerant concentration limit (RCL) of 26 lbs./1,000 CF of room volume in most occupancy types. For institutional occupancies, however, that amount is cut in half to 13 lbs./1,000 CF.
It’s important that you also check your local building codes, as there could be variations between model codes like the ICC, ASHRAE standards, and what is adopted by the AHJ. Standard 15 allows a designer to use an engineered solution if approved by the AHJ, and this could include refrigerant leak detectors, room pressurization control, using transfer openings to connect spaces, and other means of increasing room volumes. I emphasize could because some building departments have allowed these strategies to demonstrate compliance. In my experience, most do not, and it is strongly recommended that you check with the relevant building departments and review the interpretations provided by the ASHRAE Standard 15 Committee, which address many commonly considered alternatives.
Manufacturers and their representatives can add value here by helping to identify how to best layout a system with regards to minimizing the installed system refrigerant charge; however, I would recommend against relying on the vendors’ interpretation of the local codes or standards when it comes to determining the amount of refrigerant allowed.
In summary, institutional occupancies have many additional requirements beyond the standard building codes. VRF can be applied to these facilities, but care should be taken to ensure any application-specific requirements are met. To learn more about these requirements, I recommend checking out the Facility Guidelines Institute and ASHRAE Standards 15, 34, 62, and 170 as well as ASHRAE HVAC Design Manual for Hospitals and Clinics, second edition.