VRF has it’s place, but certainly not advantageous vs VAV

Letter from Thomas R. Edwards, Ruskin Group, Grandview, Mo.:
I just completed reading your two-part article on DOAS and VRF (“Combining DOAS and VRF, Part 1 of 2,” March 2014, http://bit.ly/Bowers_0314, and “Combining DOAS and VRF, Part 2 of 2” April 2014). Of course, VRF certainly makes sense in a variety of applications—most specifically, where ductwork is not available or jobs where it is not practical to install. However, the author is assuming benefits vs. ducted VAV systems, and in my experience, it doesn’t add up in both first cost and life-cycle costs in North America climates for the following reasons:
• These systems have been sold in the U.S. since 1983. The reason they have not had a lot of success is because they not only cost more, but typically are not more efficient than ducted VAV systems.
• VAV systems sometimes get a bum rap by not including the latest technology available on modern VAV and air-distribution systems—they, too, are available with modulating/variable-speed compressors and fans, along with energy-recovery wheels.

Duct systems in commercial buildings now are being installed virtually leak-tight using today’s sealing methods, providing multiple points of air distribution in each zone.
• Ducted systems can use outside-air economizers for compressor-free cooling and distribute it throughout a building. VRFs do not have economizer capability. Today’s tight commercial buildings need cooling year-round in the interior. VRF systems must run their compressors on a call for cooling—even at outside-air temperatures below 60°F. In the article, the author mentioned reheat can occur on VAV systems. However, please note that during the winter and transition months, this is compressor-free cooled air! Additionally, many VAV systems use separate perimeter heating systems, thereby eliminating any box reheat.
• Most climates in the U.S. can use economizers six months or more during the year—these climates certainly don’t make sense for VRF, if VAV with economizers can be used.
• VRF systems need DOAS to handle minimum ventilation requirements required by code in the U.S. All DOAS are sized to handle only 100 percent of the ventilation load at design conditions, whereas VAV with enthalpy comparative economizers can handle 100 percent of the total system load with a greater capacity for free cooling (no compressor or pumps necessary). On VAV AHUs, you can have both economizers and ERVsERVS (with bypass)—you can’t have this with a DOAS.
• VRF advocates imply the flexibility of their systems. Ask a contractor how easy it is to move DX fan coils and refrigerant lines in a building. Moving diffusers and VAV boxes is much easier, offering greater tenant flexibility.
• Hundreds of pounds of refrigerant running above a plenum space requires compliance with ANSI/ASHRAE Standard 15, Safety Standard for Refrigeration Systems, in a number of applications.
• As stated in the article, the industry is moving beyond minimum code-compliant systems in order to meet more aggressive targets. If we really look at optimizing the energy-saving controls and features of modern VAV systems, energy studies and real-life building applications show that VAV knocks the socks off these perceived “new VRF” technologies.
I encourage the author to do energy studies comparing modern VAV systems with enthalpy economizer to VRF systems that require year-round compressor operation. He will hear what I continually hear, “Why doesn’t this VRF system beat the energy savings of a VAV?” The answer: Because they simply don’t in most U.S. Climates!

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2 comments

  1. You CAN have a DOAS with enegy recovery. But the VRF condenser operation during cooler temperatures is a deal breaker for me! Any yes, the first time and operational costs for a VRF system are higher!!

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