By Kevin Angle, PE
If you’re anything like me, you are constantly making lists, whether it be a grocery list, materials list, or to-do list to make sure everything gets done. In my opinion, lists are one of those “necessary evils.” They can seem aggravating at times because, let’s face it, the entire list rarely gets checked off!
As VRF systems are continuing to grow in the marketplace across the globe, as well as here in the United States, it is perhaps time to create a checklist for VRF systems. These systems are being installed in numerous buildings throughout the country. In many cases, manufacturers are directly designing these systems for contractors to install. While this may be an effective approach for some projects, there are significant benefits to having an engineer, working with the manufacturer, to review the design criteria, design the system, and develop sealed and signed construction documents in accordance with the building codes.
A Checklist for VRF Systems
By no means is this list inclusive, but it can be viewed as a baseline to incorporate the fundamental elements required for your VRF system to meet those baseline expectations, and you can “check the boxes.”
■ Ventilation
Couple VRF system with a ventilation system, whether it be natural or mechanical. Natural ventilation would typically include operable windows in accordance with building code requirements. Mechanical ventilation can be accomplished through various methods, including direct mechanical ventilation to the space or common ventilation ducted to each air handler or fan coil unit.
■ Refrigerant Piping Location
Do not locate refrigerant piping in an elevator, enclosed public stairway, stair landing, or means of egress. Install refrigerant piping a minimum of 7.25 feet above the finished floor.
■ ASHRAE Compliance
Comply with ASHRAE 15 and ASHRAE 34, as both standards are referenced in the International Mechanical Code. If you are utilizing a VRF system and are not aware of these standards, you should get up to speed quickly. ASHRAE 15 was put in place around 1930, while VRF systems were first installed around 1980. Therefore, some of the information within ASHRAE 15 does not specifically address VRF systems. In future updates, however, I predict this standard will address VRF systems. Until then we must comply with the current standard.
The ASHRAE standard considers the ability of people to respond to potential exposure to refrigerants, which are classified within ASHRAE 34 with an RCL (Refrigeration Concentration Limit) designation. The standard relates to a leak of refrigerant and its ability to displace oxygen and affect the personal safety of occupants. A leak may occur in any space, and the entire amount of refrigerant within the system or circuit should be considered. An example would be R-410A, which has a maximum RCL of 26 lbs. per 1000 ft3 of room volume for occupied spaces. Most manufacturers calculate the pounds of refrigerant based on the line lengths of refrigerant piping. Smaller rooms with refrigerant-based evaporators may pose issues when considering RCL; therefore, each space volume should be evaluated to make sure the refrigerant concentration limit is not exceeded.
There are some strategies to yield refrigeration concentrations below the limits, like utilizing door undercuts or transfer grilles to increase the volume of rooms. In my opinion, these approaches tend to be ineffective and should not be used, as most refrigerants are heavier than air, and transferred air will not be effective in dispersing the refrigerant.
Many hotel brand standards and medical facility standards require that designers subtract the bed and other furniture in the rooms from the total volume, as the refrigerant will disperse around the bed. Certain school districts do not allow designers to utilize the full height of a room, as the student breathing zones are lower than full height. These items must be considered by designers, who in turn must utilize their engineering judgement to protect the occupants.
■ System Selection
Select the appropriate system type. There are multiple types of VRF systems in the marketplace, including the following air-cooled outdoor units:
- Heat pumps
- Heat pumps with heat recovery
- Heat pumps with auxiliary heating or hyper-heat
Heat pump systems are traditionally rated at 47° F and heating capacity is reduced below 47° F. Most manufacturers’ heat pump VRF systems can only supply 60% of heating capacity at 5° F. In climate zones with winter design temps below 47° F, the hyper-heat option or auxiliary heating methods should be implemented to provide adequate heating capacity. Heat recovery options provide the system the ability to simultaneously heat and cool for temperature controls zones that have differing needs.
■ Line Lengths
Verify that line lengths of refrigerant are within manufacturers’ requirements. Limitations of the refrigerant line lengths vary among manufacturers.
■ Qualifications
Engage a qualified engineer and contractor with the experience and certification to design and install VRF systems.
In summary, VRF systems have many advantages over traditional HVAC systems in certain applications, and I predict they will continue to increase their market share within the HVAC industry. However, using proper precaution when designing and installing VRF systems is critical to providing a system that is both code-compliant and highly effective.
Considering VRF as an option on your next project? Interested in our AIA CES program on VRF applications? Contact:
Kevin G. Angle, PE
Mechanical Engineer
717.434.1562
Thomas E. Leary, PE
Executive Vice President
717.434.1555
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