Choosing and specifying VRF systems
In your client’s quest for energy efficiency, determining if a variable refrigerant flow system is right requires careful consideration.
- Consider morning warm-up and cool-down strategies when specifying VRF systems.
- Know that the zoning of the indoor units and their respective refrigerant boxes must be coordinated to prevent issues related to thermal comfort.
- Understand that VRF systems may not be appropriate for spaces with high latent loads.
Variable refrigerant flow systems are currently a hot trend in commercial heating, ventilation and air conditioning systems design. Available in two forms — heat pump and heat recovery — the VRF system is specified based on the facility’s unique heating/cooling needs and anticipated load. The question is: Are VRFs ideal for your client’s building?
Used in similar fashion as water-sourced heat pumps or two- and four-pipe chilled/hot water fan coil systems, VRF units are most often sized to handle a building’s interior and perimeter loads while a separate unit provides conditioned ventilation air, with its own proprietary controls and smaller heat rejection units. Unlike hydronic systems, VRF units use refrigerant directly to reject or absorb heat from the recirculated air. Because the refrigerant flow can be controlled in these systems, several indoor VRF units can use the same refrigerant piping to reject or absorb heat from a single condensing unit.
VRF systems have several clear benefits. For one, because refrigerant operates with a larger specific heat range, it requires smaller infrastructure than both water and air cooling units. It takes about the same amount of energy to condition a space using a 1 ½-inch VRF refrigerant pipe, a 2 ½-inch water pipe and a 36-inch by 36-inch air duct.
Secondly, because less equipment is needed, there’s potentially a lower cost to installing air-cooled VRF systems over traditional water-cooled or chilled water systems. A single condensing unit can serve multiple indoor VRF units, while larger AHUs, chillers, heating boilers, cooling towers or fluid coolers may be required for other applications. When a VRF system is designed and installed correctly, it also can be a fit for retrofit applications that lack room for additional air ducts or large cooling units.
Why doesn’t every building have a VRF system? Simply put, there are several pitfalls to their design and operation. When specified and installed correctly, buildings with VRF systems can reap the abovementioned benefits. When design and installation fall short of the desired goals, the results can be as simple as high humidity or noise pollution, to gas contamination or even a fire event. Proprietary VRF controls and systems design also lock out competition if small revisions are required.
Consider the following pitfalls/best practices to designing and installing a VRF system.
Specify the VRF to meet building load demands. Like a conventional chiller/boiler system where both the chiller and boilers run more efficiently at partial-load, VRF outdoor units operate best at part-load, and more efficiently where there is both a heating and cooling load requirement. Heat recovery VRFs transfer heat from a space in cooling, to a space in heating and vice versa, further reducing compressor energy at the heat rejection unit.
In contrast, both a chiller and boiler must continually operate its compressors and burners anytime both cooling and heating are required. Conventional chiller/boiler systems, whether using air handling units or VRF systems, should be appropriately sized based on estimated interior equipment and ventilation loads using ASHRAE 62.1: Ventilation for Acceptable Indoor Air Quality.
On the other hand, indoor VRF units only need to be sized to account for the building envelope and match the interior cooling and heating load, while AHUs need to be sized to handle the additional ventilation load while also accounting for diversity of the space(s) they serve.
Note: A means to provide outside air will need to be provided with a VRF unit. Similarly, remember that a dedicated outside air system or energy recovery ventilation and corresponding duct work will typically be much smaller than a conventional AHU.
One major, but often overlooked, point in operating VRFs is the system’s ability to provide morning warm-up or cool-down as compared to a central AHU providing ventilation air. The traditional central AHU systems will use their additional available capacity to ramp temperatures up or down quickly when ventilation air is not required, a task the standard VRF system can’t compete with alone. VRF systems may need to be upsized to deliver morning warm-up/cool-down in an appropriate timeframe, or an alternate warm-up/cool-down system may be provided.
Additionally, the temperature difference across the coil for indoor VRF units are typically around 15 F – 20 F, whereas the temperature difference across the coil for conventional AHUs can be anywhere from 30 F to 40 F. Heating the building starting with 95 F supply air is much faster than starting to heat a building with 75 F as an increase in temperature difference decreases the time it takes to warm or cool the air.
Likewise, cooling a building in the summer starting with 55 F supply air is much faster than starting with 70 F air. Understanding that warm-up and cool-down are affected by both the rate at which energy can be added or removed from the space, and the loss through the windows, walls, etc., it’s possible to roughly calculate the time it takes for warm-up or cool-down. This is, of course, ignoring any thermal mass or electrical loads.
A trusted energy transfer equation can be expanded to the following:
The VRF outdoor unit needs to be sized to handle the total heating load on a design day, so assume the outdoor unit and indoor units have been sized to meet the load. If this is the case, qnet would then be small value — and the room would take much longer to heat up than a conventional AHU where we can often take advantage of the system and close the outside air damper. During morning warm-up, the system is fighting a temperature setpoint change (often from 65 F to 72 F) in addition to external heat loss.
Adding a safety factor of 10 percent to the size of the units would help bring the building back into tolerance, but with such a limited net input into the space, VRF systems need to be designed properly, just as any other system. Summer design day conditions can be met as energy from lighting and heat-producing sources will be near minimum. Therefore, total time for morning cool-down will then be a function of the compressor and the excess capacity available.
What are some options to provide supplemental heat to warm-up the building quickly? The latest energy codes, 2018 International Energy Conservation Code, require ERV systems to collect the general exhaust back at the DOAS unit. This way, it’s possible to use this “return” air and recirculate it back into the system, similar to morning warm-up sequences in conventional AHUs to provide additional heat. In colder climates such as the upper Midwest, this can be equivalent to twice as much heating or cooling capacity available for morning warm-up or cool-down.
Zone the building carefully with VRFs. One of the greatest benefits of the VRF system is its ability to provide differential heating/cooling to various areas of a building as needed. While the heat pump VRF system can only operate in either heating or cooling, the heat recovery VRF system can operate in both heating and cooling modes simultaneously. It is good practice to zone heat pump VRF units that are part of the same building orientation and/or are subject to common building load profiles. Interior spaces should not be on the same heat pump system as an exterior zone.
Heat recovery VRFs should be zoned similarly to VRF heat pump systems with the addition of a refrigerant boxes providing zone separation. Zones are differentiated by a refrigerant box that can communicate with other refrigerant boxes to determine how to strategically move refrigerant from one box to another through a third refrigerant pipe. These refrigerant boxes should be on the same outdoor heat recovery unit to transfer heat from one box to the other, allowing for optimal energy savings
For example, a refrigerant box serving indoor VRF units for a zone of west-facing offices should be connected to the same outdoor heat recovery unit as a refrigerant box serving indoor units of east-facing offices. During the shoulder season, heat can be recovered through the refrigerant from the west-facing offices and transferred to the east-facing offices.
Using refrigerant to move hot/cold air may be more efficient and economical; it also can be toxic and flammable. When refrigerant leaks, it becomes an invisible gas. Local and national codes already have requirements for refrigeration machines, and those same rules apply for VRF systems.
In general, these rules require the specifying engineer to calculate the concentration of refrigerant in a given space assuming catastrophic failure. Therefore, local and national codes often require rooms with VRF piping to have a way for the refrigerant to dissipate so the room does not become contaminated in the event of a leak. The specifying engineer is required to provide calculations assuming the entire refrigerant system were to leak into the smallest available room served by the system.
Applying ASHRAE Standards 15 and 34: Safety Standard for Refrigeration Systems and Designation and Classification of Refrigerants (see sidebar, “Don’t forget the codes”), the designer must show calculations indicating that the refrigerant concentration limit will not exceed levels outlined by the standard to “… reduce the risks of acute toxicity asphyxiation and flammability hazards in normally occupied, enclosed spaces.”
Regardless of size, ASHRAE requires designers to limit the concentration of refrigerant in a building’s smallest space to prevent contamination. The standard dictates that the smallest room in the building served by the VRF system must be made “bigger” by allowing it to “communicate” with another space until the maximum concentration of potential refrigerant that could possibly leak into the room meets the standard.
International Building Code limits flammable refrigerants (refrigerant groups A2, B2, A3 and B3) to 1,100 pounds for non-institutional occupancies, and 550 pounds for institutional occupancies. Most commercially available VRF systems use non-flammable, class A2 refrigerants. However, local authorities having jurisdiction may impose total charge limits on flammable and non-flammable refrigerants that are more stringent than those found in IBC and ASHRAE Standard 15.
As an example, when designing a small, sound-tight room in a commercial building in which doors also were designed to be airtight and minimize noise transfer, McGuire Engineers designed an air transfer path between a neighboring room to minimize the potential refrigerant concentration over a greater volume of space. Although this particular space required a transfer duct between spaces to limit sound transfer, a less sensitive space may use a door undercut to serve as a permanent opening.
This modification also must be accepted by the authority having jurisdiction. Although all refrigerant-based systems must follow this code, typical systems, such as rooftop units, do not usually have this issue, as the refrigerant can be diluted throughout the entire space served by the equipment.
High-latent loads must be weighed. Gymnasiums, or other buildings with high interior latent loads, may not be ideal for VRF treatment, as VRF systems aren’t designed to extract large amounts of moisture from the air. Instead, conventional AHUs may be better equipped to dehumidify a large space.
When employing a VRF system, codes still require outside air and exhaust to remove the carbon dioxide from the space and bring in additional outside or ventilation air. This often can introduce higher latent loads into the building. An AHU can more easily accommodate this feature by integrating it into the system with larger coils.
When using a decoupled ventilation system with a VRF system, a DOAS unit can address the latent load while the indoor units can meet the sensible load requirements so the entire system to meet the total load. Buildings with spaces that require high outside air requirements based on their space classification, such as labs, may not be good candidates for VRF systems, as space loads can usually be met by the outside air system.
A DOAS unit with additional rows of coil can be used to dehumidify the air, past the dew point to account for the additional latent load from the space.
Calculations showed that the Columbia College Chicago Getz Theater Center (see sidebar, “Case study: Theater renovation includes VRF”) needed to provide air at a decreased, 50 F dew point from the DOAS unit to offset the higher latent load from the performers and audience members. The result was a space temperature of 75 F with a relative humidity of 60 percent to stay out of moisture related issues as recommended by ASHRAE Standard 55: Thermal Environmental Conditions for Human Occupancy, and ASHRAE 62.1: Ventilation for Acceptable Indoor Air Quality (62.1 outlines “… occupied-space relative humidity shall be limited to 65 percent or less …”).
VRFs may be quiet, but condensate pumps are not. VRF units are known to be especially quiet. But, where minimizing noise is a priority, it’s important to account for the condensate coming off the VRF coils, which is significantly louder if condensate pumps are required. Condensate pumps are required if gravity draining is not possible.
Even after calculations have been completed and it has been determined that the latent load can be accommodated by an ERV, for example, the condensate pump may be in continuous operation during high latent days. The rushing sound of water from the condensate pump may not be an issue in some locations like janitor’s closets, but if used in an office above personnel, it may be distracting and is likely to cause occupant complaints.
This can be shown in two examples:
- Working on a museum in the Midwest with noise-sensitive rooms meant considering multiple alternatives. The result was specifying a ducted VRF unit for the space, and locating the VRF equipment itself — including the condensate pump — in an adjacent room. Adjacent VRF units also were gravity drained and collected at a condensate pump where pumping sound is acceptable.
- At the Columbia College Chicago Getz Theater Center, careful coordination with the architect allowed McGuire Engineers to locate the indoor VRF units above adjacent toilet rooms where the neighboring wall is acoustically isolated from the theater.
Gas-driven VRFs also may be considered. In addition to water- and air-cooled VRF units, gas-driven VRF systems have made their way into the market recently.
For example, on a recent project at a facility experimenting on the differences between electric- and gas-driven VRF systems, both systems operated equivalently during the cooling season, even though the gas-driven compressors ejected more condensate due to the natural byproducts of combustion.
Heat season operation is another consideration for the gas-driven VRF. On the abovementioned project, due to the byproducts of combustion, condensate at the outdoor unit was witnessed year-round. In a cold climate susceptible to freezing, not only can this be a hazard near the unit, condensate freezing inside the drain pipe is a larger operational issue. Significantly reduced heating output during winter operation was also a large issue with the gas-driven unit. Where an eight-ton electric VRF unit was used, a gas-driven unit required closer to a 12-ton unit to provide the same heating capacity for a winter design condition of minus 4 F.
Building needs will dictate use
Each building will have its own heating/cooling demands as well as size and applicable codes that will influence on-site engineers and facilities personnel in determining if a VRF is a good fit for the building — and, if so, which form is best. Understanding the full picture of benefits and also the pitfalls and best practices for designing a VRF system, will be key to its specification.