This article is sponsored by Trane. In this Voices interview, Consulting Specifying Engineer spoke with Joseph Guerin, Ductless Sales Specialist at Trane, to discuss how A2L refrigerants are changing the way engineers think about variable refrigerant flow (VRF) systems as part of HVAC design.
Consulting Specifying Engineer: The HVAC industry is amid a major shift toward low global warming potential (GWP) refrigerants. Are building codes and safety standards, such as ASHRAE® 15, evolving quickly to accommodate these new A2L refrigerants in VRF applications?
Joseph Guerin: Yes, and it’s been quite a ride. There’s a lot that went into ASHRAE 15 Safety Standard for Refrigerant Systems. In summary, ASHRAE 15 is the national application safety standard for refrigeration systems. Like all ASHRAE standards it is intentionally product agnostic. You have to dissect what’s being referred to in each section. Whether you’re running piping or using a certain kind of cooling and heating system, the Standard answers the question, “How do we put that into a building while maintaining occupant safety?” ASHRAE 15 provides enforceable language, which is then picked up by the model building codes for adoption by states and municipalities.
To give you a sense of the pace: ASHRAE 15 ballooned from 41 pages in the 2019 edition to 81 pages in 2024 version. This is thanks to the new requirements for A2L refrigerants. We’re still in the ASHRAE 15 adoption phase, but the codes and local jurisdictions are watching closely. HVAC units with A2Ls are already going in today, and VRF installations could begin as early as January 1, 2026.
How do you anticipate the use of A2L refrigerants will influence VRF system designs in the future?
The main things we have to pay attention to with VRF design and A2L refrigerants is where we’re running piping, whether we have leak detection, and making sure we have a means of circulating air in occupied spaces. When refrigerant leaks, we don’t want it concentrating in a small area. We want to dilute any potential leak over as many cubic feet as possible.
Mitigation controls are newer additions. If we sense a leak, we can isolate a unit. Refrigerant pipe shafts are another big consideration because they’re like arteries in a building. We need to be cognizant of what we’re putting in those shafts, even if it is only low flammable.
These details require the engineer’s attention, but it’s manageable.
Leak detection and safety shutoff valves are likely becoming standard components in VRF equipment. How do you foresee this impacting not only safety and code compliance but also system reliability, diagnostics and the peace of mind of owners and occupants?
VRF is a refrigerant-intensive system. There’s a lot of refrigerant per ton of cooling compared to other HVAC systems. There’s always been a little misgiving about a hundred pounds of refrigerant pulsing through a building. But as contractors and owners have gotten familiar with VRF, those worries have faded.
Today, with release mitigation controls, we are entering a new era of transparency and innovation. Integrated leak detectors and mitigation controls mean, if there’s a leak, you don’t just discover it from a low system charge or a stained ceiling tile—it gets flagged immediately. The system isolates the defective fan coil, but the rest of the system still operates. You know immediately where to start your investigation, and you’re not taking down the whole system. How great is that?
From an engineer’s perspective, what are the most significant challenges — and opportunities — presented by the A2L refrigerant shift for VRF systems?
There are challenges, but these are hurdles we can clear. For example, branch controllers posed a bit of a question mark—does ASHRAE 15 treat them as continuous piping, or something else? Recent interpretations clarified you have to do a calculation for the branch controller. This drives important design considerations like using ventilated enclosures and making sure we have enough dispersal volume.
Another challenge is refrigerant pipe shafts. Earlier this year, ASHRAE Std-15 changed language so that piping shafts were no longer required per the standard. That was a helpful provision for engineers. However, whether shafts are required or optional, you still need ventilation – an inlet and outlet air so the refrigerant gas has somewhere to go and doesn’t build up inside the shaft in case there is a leak. There’s a proposal to remove the ventilation requirement. It’s still being debated, but the ventilation requirements aren’t bad — a 4-inch outlet at the bottom of the shaft opening to the outside, with an inlet at the top. Alternatively, a detector tied to an exhaust fan is also a common solution.
Like this proposed addendum discussing ventilation, one of the biggest challenges for everyone in the design world right now is timing. The code lags ASHRAE by three years. You’re not going to get an engineer saying to a code official, “I’m designing to an old safety standard.” You want to go by the newest safety standard.
I was just talking to an engineer last week. The code official saw an A2L system in the building and said, “We don’t have any provisions for this in the code — you can’t do this.” And the engineer said, “Well, we can’t get anything else in the building, so are you telling me we can’t install HVAC in this county for the next three years?” Of course not. Keeping the conversations going with code officials, so they know what the industry is doing and how we’re designing to the latest safety standard— that’s important.
Those are the main challenges. Of course, there are also lots of opportunities. If something’s not in the code — say I’m in a municipality that has IMC 2021 but I’m trying to cite ASHRAE 15-2024 — you can file a variance citing that IMC 2027 will reference ASHRAE 15-2024.
A coincidental benefit of needing to evaluate branch controllers now, is that this drives towards smaller systems. Lower tonnages can actually benefit an owner. Lower tonnages typically have higher efficiencies and higher heating output per ton. So a nice side benefit of designing around smaller systems is that you can get more out of your equipment efficiency-wise, heating-wise and electrification-wise.
Are there applications or building types where the requirements may differ for A2L refrigerants and VRF?
Definitely. Requirements differ in institutional spaces where occupants can’t exit readily without assistance — nursing homes, hospitals, prisons, sometimes daycares. In these spaces, the ASHRAE safety standard says a few things. One, you must have leak detection. With VRF, you’re probably including leak detection anyway, so that’s not a big deal.
Two, you can’t use safety shutoff valves or mitigation controls as a means of institutional compliance. You can put them in there, but it doesn’t help you toward your ASHRAE 15 calculations for volume dispersal.
If I have an office building where I’m allowed X pounds per thousand cubic feet, the allowance for institutional gets cut in half. You’re really restricted on institutional applications with these A2L refrigerants, and rightly so. If occupants can’t exit without help, you’re bound by stricter rules.
To tackle these sensitive environments, ducted air distribution systems are a great option to connect spaces through ductwork or natural ventilation openings. When that’s not feasible, HVRF, or Hybrid VRF system really shines. It was specifically invented for this refrigerant transition.
So how do we get a VRF system but have hydronic — water — serving the occupants? HVRF can keep refrigerant between the outdoor unit and the branch controller, and then all the zones are served by water. Since ASHRAE puts no limit on water volume in spaces, HVRF opens up possibilities for tougher institutional applications, and beyond.
For engineers who are just beginning to specify A2L-based VRF systems, what resources, training or industry guidance have you found most valuable? Additionally, what further support is needed to facilitate this transition smoothly?
First and foremost, it starts with a conversation. ASHRAE has local chapters—join, meet others in the industry. I’ve learned a ton by bouncing questions off engineers in Maryland and Pennsylvania. “How are you handling it? What questions are you getting from code officials? What projects are you trying to do?”
At Trane, we have developed updated materials that cover ASHRAE 15 and VRF basics. You can reach out to your Trane account manager, and they can provide it as a lunch-and-learn. Trane also has educational resources on our site, covering the basics.
Lastly, ASHRAE.org let’s you read the standard for free on their site. If you have an older version of the safety standard, you see the new changes by downloading the official approved addenda and interpretations for free.
Between the resources and continuing the conversation, that’s the best way to get a handle on this. And something to keep in mind, especially for newer engineers. Don’t be intimidated, even if you’re new to all this. It might seem daunting, but it’s really a cool time to be in the industry. For decades, we’ve understood chillers inside and out. A2Ls and VRF? Only three years old, and we’re in the early innings of this change. The landscape’s shifting, but working together, it’s something fun we all get to be a part of. For more information, visit trane.com/VRF.
