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Concerns persist over R466A compatibility

USA: New tests on R466A, Honeywell’s long-awaited non-flammable lower GWP alternative air conditioning refrigerant, are said to have shown comparable performance to R410A but have again raised material compatibility concerns.  

First announced by Honeywell in June 2018, R466A is a blend of R32 and R125 – the two components of R410A – with the addition of 39.5% of trifluoroiodomethane, better known as CF3I or R13I1. Despite the initial market enthusiasm, more than three years on, R466A has still not been adopted by any OEM in a production model. And this is despite Honeywell insisting that R466A has undergone “extensive analysis” by OEMs and leading compressor manufacturers. 

The latest tests by the University of California, Davis, Western Cooling Efficiency Center (WCEC), evaluated its performance in a Trane Foundation rooftop unit. The results were described as “very successful” displaying similar capacity and COP across a wide range of outdoor air conditions. 

However, the UC Davis tests again question its material compatibility, stating that it represented a viable alternative to R410A “if the compatibility issues with certain metals can be mitigated through the use of alternative materials and appropriate additives”.

UC Davis refers to “documented concerns” around the chemical stability of the R-13I1 component which, they say, “reacts with zinc causing the molecule to break down”.

Honeywell has previously countered such claims, telling the Cooling Post that the stability and shelf-life can be addressed with standard measures practiced in the industry.


The latest tests were carried out on a new 15-ton Trane Foundation model, brought in to replace an existing 15-ton Trane R22 model on the roof of one of the UC Davis campus buildings. The RTU was initially charged with R466A and an unspecified oil additive, which the UC Davis researchers said was “to avoid compatibility issues” with R466A.

The additive used in the tests was said to have been developed by Trane Technologies specifically to improve the compatibility of R13I1 with system materials. The Trane Foundation unit was a production packaged R410A RTU, where the R410A had been recovered and replaced with R466A.

The system with R466A was monitored from July 13 to August 29, 2021. It was then replaced with R410A and monitored through to September 29. There was no charge optimisation performed for either refrigerant. In both cases, the system was charged with the nameplate quantity of 14.6lbs, which was recommended for R410A.

A previous study, by WCEC, found that a refrigerant charge optimisation for R466A required minor adjustments to the thermal expansion valve and a 7% increase in refrigerant weight compared to R410A to compensate for differences in refrigerant liquid densities. This project looked at the worst-case scenario for a drop-in replacement, where no charge optimisation was performed and the charge was simply based on manufacturer recommendations for R410A. 

The tests

In the tests, R466A appeared to deliver slightly higher capacity than R410A as outdoor temperatures increased, but, in general, the capacity differences between the two refrigerants were within the margin of error of the measurements. 

Power draw tests showed R466A to have a slightly steeper profile with lower power draw at lower outdoor air temperatures and higher power draw at higher outdoor air temperatures. However, the UC Davis engineers recognised that this increase in power draw could be attributed to the lack of a charge optimisation with R466A. In terms of COP, the refrigerants showed very similar across all outdoor air temperature conditions.

The average subcooling during the measurement period was 9.0°F for R466A and 4.1°F for R410A

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