I have been reading claims from manufacturers about how efficient variable refrigerant flow (VRF) systems are over chilled water systems for a few years now. I have also met with consultants recommending VRF systems saying how efficient VRF systems are compared to chilled water systems. My 40-plus years of experience in the large-tonnage chiller and energy solutions industry, combined with my extensive experience working with both chillers and VRF systems, made me think this cannot be true. I decided to investigate and research the VRF industry’s claims. And thus, most of the information I will be presenting here was taken from industry literature and studies conducted by recognised industry professionals and organisations. I could find no studies or data to support statements made by VRF manufacturers regarding their claims of superior efficiency.

Dan Mizesko

Manufacturers of VRF systems claim that the lifecycle cost of VRF technology is lower than that of chilled water-based systems, stating that VRF systems waste less energy, have higher rated efficiency and require simpler, more streamlined maintenance. Again, these industry claims are not supported by studies. ASHRAE decided to test both systems in its Atlanta headquarters and meter and measure their performance. The findings were not good for the VRF manufacturers and the consultants who recommend them. The ASHRAE study of a VRF system and a chilled water system, installed in the same building and addressing similar loads, revealed that “on an annualized basis, the VRF system had an energy consumption 57% higher in 2010 than the hydronic system, 84% higher in 2011 and 61% higher in 2012”.

I also found other issues with VRF systems, which would convince facility owners to purchase a chilled water system as opposed to a VRF system. Let’s look at occupant safety. Almost all VRF systems use R-410A, which is a colourless and odourless gas that has the potential to induce asphyxiation. Since the design of VRF systems relies on refrigerant as the working fluid, and since the system piping filled with R-410A has to be installed across the entire envelope of the building, VRF systems have hundreds, if not thousands, of feet of piping, filled with hundreds of pounds of pressurised refrigerant. In the case of a chilled water system, the refrigerant is located in the chiller, which in turn, is located in a plant room with leak-detection monitors. Building occupants are not at any risk of asphyxiation with a chilled water system. Of course, the same cannot be said for a VRF system, which lacks leak-detection monitors. In addition, long runs of piping located in occupant spaces create difficulty in locating refrigerant leaks, which leads to even higher concerns for occupant safety. ASHRAE 15 has design requirements that limit the total refrigerant volume contained within a system, based on the volume of the smallest space served by the system (more on this under ‘Codes and Standards’).

On the environmental impact of R-410A – compared to R-134A, which is used in most centrifugal chillers – while R-134A has a Global Warming Potential (GWP) of 1,430, R-410A has a GWP of 2,088, almost 50% more than that of R-134A.

Other considerations of VRF systems include:

• The need for a dedicated ventilation system to deliver the outside air to various zones
• Long refrigerant lines and a large number of branch connections, which could result in refrigerant leakage • The need for condensate drain lines for each VRF indoor unit
• Use of supplemental heat, which may be required for a quick warm-up
• Compliance with maximum allowable refrigerant quantities within a given volume

CODES AND STANDARDS

VRF systems must comply with ASHRAE Standard 15 (packaged with Standard 34). This addresses refrigerant capacities and possible leakage, especially if the system serves small rooms, which could cause oxygen depletion. Due to the ability to displace oxygen, ASHRAE Standard 34-2013 Addendum L has established the maximum refrigerant concentration limit (RCL) of 26 lbs./1,000 ft3 of room volume for occupied spaces.

According to Standard 15, a VRF system is classified as a direct system/high-probability system where a refrigerant leak can potentially enter into the occupied space.

ASHRAE Standard 15 requirements should be applied to each VRF system design in the following steps:

• Determine the occupancy classification for the rooms
• Calculate room volume
• Determine the amount of refrigerant in the system, including the outdoor unit, indoor units, and associated piping
• Verify that the room is not too small

I would like to mention one last fact on VRF R-410A systems. Considering that R-410A is a blend of refrigerants – R-32 and R-25 – in the event of any leak, the entire charge must be recovered, the leak repaired and new refrigerant charged. To elaborate, R-32 and R-25 boil and leak at different rates, so in the event of a leak, you no longer have R-410A as an entity, and thus, you would need a new charge of the refrigerant.

Let’s now look at VRF limitations to consider. Manufacturers and some consultants say that VRF systems offer such benefits as consistency of comfort, energy efficiency, zoned heating and cooling, and the ability to heat and cool simultaneously. For small building types, this might be valid. However, there are some important limitations that the VRF technology imposes on most buildings, including HVAC loads, maximum system capacity and the effects of outdoor air temperature and piping design. VRF technology places noteworthy limitations on the total length of pipe within a system. Rated capacity decreases as the piping length increases, but this actually starts at quite a short length. ASHRAE standards rate a VRF system based on 25 feet of piping and zero vertical separation between indoor and outdoor units. Rated capacity of VRF systems start to decrease when pipe length and vertical separation go beyond 25 feet and 0 feet, respectively. Capacity decreases continue as system sizes expand. Where the total piping length is 600 feet, the systems have lost 10% of total capacity. And where engineers might choose systems for larger buildings, VRF systems cannot function beyond the maximum piping length of 3,281 feet. At that stage, more than 50% capacity has been lost.

INSTALLATION COST

There are many studies that prove chilled water systems have a lower installation cost compared to VRF systems. The claims of VRF systems being lower cost to install are not based on fact.

In closing, to get back to the main point of this discussion, energy efficiency claims by VRF manufacturers have been difficult to verify, and without actual test data, it’s been difficult to determine the actual facts. The ASHRAE building comparative energy usage study proves that a VRF system is not as efficient as a chilled water system. In all cases, newer variable-speed water chillers and heat pumps outperform variable-speed VRF systems.

Dan Mizesko is with U.S. Chiller Services. He may be contacted at dmizesko@uscsny.com.