HFO refrigerants explained

Published Oct 31, 2015

Hydrofluoroolefin, or in short HFO, is a definition that is familiar to many of us. R1234yf, R1234ze(E) are few examples of HFOs. They are used in a number of applications today, but have been barely studied just a decade ago. This article will cover some basics about HFOs.

What are the HFOs?

Hydrofluoroolefins (HFOs) are nothing new for chemistry. Much like traditional hydrofluorocarbons (HFCs), they composed from hydrogen, fluorine and carbon. The only difference is that they are unsaturated, meaning that they have at least one double bond. Such molecules are named olefins or alkenes, hence it is correct to name such refrigerants as HFC, HFA or HFO. The later name has become the most used name to refer refrigerants with carbon-carbon double bond.

The presence of the carbon-carbon double bond is not unique for HFOs as there are other unsaturated compounds to be fond, as for example unsaturated hydrocarbons (e.g. propene). In fact, HFO-1234yf is a propene molecule that is halogenated by replacing 4 atoms of hydrogen with 4 atoms of Fluorine (see Figure 1).

HFOs are relatively stable compounds, but are more reactive than HFC due to the reactivity of the carbon–carbon bond. This also reduces their global warming potential and therefore became favorable property in light of increasing concerns on climate change.

Environmental effect

The interest to HFOs as potential refrigerants arose with the adoption of European Directive 2006/40/EC on mobile air-conditioning systems (MACs) that aimed on at reducing emissions of fluorinated greenhouse gases in the automotive air-conditioning systems. It therefore, de facto, banned the long term use of R134a refrigerant, which is itself came as an environmentally friendly replacement to R22 just a dozen years before the Directive. Fluorinated propene isomer R1234yf has become a refrigerant of a choise and many cars manufacturers have adopted new refrigerant in nearly 50 vehicle models [1].

Unlike R134a, new refrigerant has very low effect on global warming. This feature is common for other HFOs, for a number of which their integrated over 100 yr. contribution to global warming is not greater than that of CO2 [2]. This is mainly due to their short lifetime that is within a couple of weeks for many HFOs (see Table 1)

Table 1 – Lifetime and global warming potential of some HFOs [2].


Chemical formula

Lifetime, days

GWP (100 yr.)


















The experience suggests that there is no single best refrigerant as, when selecting refrigerant, one normally faces trades off between different criteria. In case of HFOs the benefits on the environmental side are partly outweighed by the concerns on their safety.

The concerns on safety of HFOs were in particularly raised by Daimler AG, when the company made the decision to discontinue usage of HFO-1234yf in some of their car models [3]. The decision was followed by a number of tests that showed potential danger of flammable HFO use in MACs. The results were questioned by refrigerant manufacturer, that claims that “neither flammability nor HF formation present a significant safety concern” [4]. The potential danger of HFO-1234yf in MACs thus led a number of car manufacturer to look after alternative MAC system designs [5]. In particular, Volkswagen has already announced the use of CO2 based systems in selected models of cars [6].

It is therefore 2 main safety issues that are related to HFOs: flammability and their potential to form dangerous acids. A number of HFOs, including HFO-1234yf, HFO-1234ze(E), are flammable, while others are not, as for instance HFO 1336mzz(Z) and HFO-1233zd(E). We have discussed the flammability issues related to different refrigerants in the previous Kyla.

Like any other the halogenated hydrocarbons, HFO is easily decomposable under the influence of high temperatures to form hydrogen fluoride - the highly soluble in water gas that easily form acid,  that would cause skin irritation, eye irritation and throat irritation and could lead to death [7]. It is quite obvious that flammable refrigerants have greater probability to get under effect of high temperatures and therefore bring greater risk of formation of dangerous substances.

VIP status of HFO

HFO, being technically HFCs, are surprisingly given special rights by many. For instance, the newly adopted EU Regulation 517/2014 on fluorinated greenhouse gases excludes HFOs from the definition of ‘‘hydrofluorocarbons’’ and, therefore, do not include them in their ambitious schedule of the reduction of the quantity of hydrofluorocarbons placed on the market. Japan adopts similar behavior and removed a number of HFOs from their “the act on rational use and proper management of fluorocarbons” that has recently came into effect [8].

Applications of HFO

As of today, HFOs found in a number of applications. In refrigeration and heat pump technology HFO-1234yf has been chosen to replace R134a in MAC systems; HFO-1234ze(E) and HFO-1233zd(E) are used in chillers [9] [10] and HFO-1336mzz has suggested for high temperature heat pumps [11].

The application of new HFOs extended when they used as a component of lower GWP refrigerant mixtures. For instance, HFO-1234yf and/or HFO-1234ze(E) are components of new refrigerant mixtures (e.g. R448A, R449A, R450A, R513A) that aim to replace traditional refrigerants with high environmental impact. These new refrigerant mixtures are already supported by leading compressor manufacturers [12] [13] with some reliability concerns due to comparatively higher compressor discharge temperatures of some of the new refrigerants [14].

HFOs are new range of synthetic refrigerants that promise to be a part of solution to the environmental problems. Still, considering ongoing investigations in regard of their safety, and looking back to the previous generations of synthetic refrigerants, it is still too early to tell whether hey represent the long term solution or not.

Följ gärna våra publikationer och få vårt digitala nyhetsbrev. Anmäl dig genom att följa länken bit.ly/kth_ett.



Vehicleservicepros, "Current list of OEs that use R-1234yf refrigerant," 19 March 2015. [Online]. Available: bit.ly/r1234yf_oem.


IPCC, "Climate Change 2013: The Physical Science Basis," 2013.


Safecar, "A/C Refrigerant May Ignite In A Crash," 01 Oct 2012. [Online]. Available: bit.ly/SL_recall.


Chemours, "Global Evaluations—HF Formation," 2015. [Online]. Available: bit.ly/chemours_hf.


Everything R744, "Is Europe more open to chemical giants’ corporate concerns than safety?," 19 june 2016. [Online]. Available: bit.ly/r1234yf_MAC_safety.


Everything R744, "VW to implement CO2 MAC in two luxury models," 19 March 2015. [Online]. Available: bit.ly/vw_r1234yf_co2.


Fireworld, "Environmentally safe can be hazardous to your health!," [Online]. Available: bit.ly/1I8gtZX.


Coolingpost, "Japan excludes HFOs from new emissions law," 14 06 2015. [Online]. Available: bit.ly/Japan_act.


GEA Consulting, "MHI Chooses HFO-1233zd(E) for New Centrifugal Chillers," 18 Jun 2015. [Online]. Available: bit.ly/R1233zdE_chiller.


Airedale, "TurboChil FreeCool," 2015. [Online]. Available: bit.ly/turbochill_ze.


ECN, "R&D on industrial heat pumps," 2014.


RAC, "Emerson Climate Technologies announces qualification of HFO refrigerant blends for its compressors," 13 July 2015. [Online]. Available: bit.ly/Emerson_HFOcompressors.


Honeywell, "Honeywell Solstice® Lower-Global-Warming refrigerants approved by leading global compressor suppliers," 18 June 2015. [Online]. Available: bit.ly/1LedeTn.


Tecumseh, "Tecumseh updates position on refrigerant transition," 08 June 2015. [Online]. Available: bit.ly/Tecumseh_updates.


Title Date
Refrigerants: what to expect in the future 1497866340000 Jun 19, 2017
Standards and their role in refrigerants development 1486493580000 Feb 07, 2017
This happened in refrigerant development during the past year 1485513000000 Jan 27, 2017
Environmentally friendly refrigerants of the future 1485472260000 Jan 27, 2017
An option to replace R404A in small refrigeration systems 1479134100000 Nov 14, 2016
Highlights form the Gustav Lorentzen conference on natural fluids. 1479064620000 Nov 13, 2016
The opportunities and challenges of R152a. Part 2. 1478976660000 Nov 12, 2016
The opportunities and challenges of R152a. Part 1. 1478972820000 Nov 12, 2016
Sources of refrigerant property data 1458746090455 Mar 23, 2016
A year in low GWP refrigerants development 1458321340452 Mar 18, 2016
Reducing environmental impact of refrigeration technology 1456742106439 Feb 29, 2016
The potential dangers of TFA 1446379694788 Nov 01, 2015
Key news in low GWP refrigerants developments 1446379056093 Nov 01, 2015
HFO refrigerants explained 1446331398277 Oct 31, 2015
Understanding refrigerant flammability 1435064580000 Jun 23, 2015
New opportunities for R32 1434883631689 Jun 21, 2015
A guide to F-gas regulation guides 1429648197227 Apr 21, 2015
In short about R1234ze 1429633031450 Apr 21, 2015
Replacements for R404A 1413929083023 Oct 22, 2014
R1336mzz-Z - new generation nonflammable low GWP refrigerant 1411323643330 Sep 21, 2014
Low GWP refrigerants for high temperature heat pumps 1411301763082 Sep 21, 2014
Safety of new low GWP refrigerants 1409930678682 Sep 05, 2014
Can global temperature change potential replace GWP in upcoming regulations? 1397660808419 Apr 16, 2014
Quantifying global warming potential 1398278734277 Apr 23, 2014
New F-gas Directive, one step closer to the final decision! 1391371244346 Feb 02, 2014
A year in new low GWP refrigerants development 1391371204822 Feb 02, 2014
Defining “Low GWP” 1383523245613 Nov 04, 2013
Which refrigerant replaces next R410A? 1383498021156 Nov 03, 2013
New opportunities for natural refrigerants 1383253243874 Oct 31, 2013
Uncertain future of fluorinated refrigerants 1383040808722 Oct 29, 2013
European Commission’s feedback on MAC Directive implementation issues 1382997636808 Oct 28, 2013
R-1234yf to be used in MAC systems, despite safety concerns 1382821235278 Oct 26, 2013
Environmental metrics for evaluating refrigeration systems operation 1382688001149 Oct 25, 2013
Low GWP alternative refrigerants in heat pumps 1382688057994 Oct 25, 2013
Mercedes-Benz wishes to continue using the tried-and-tested R-134a refrigerant in passenger cars 1348830013187 Sep 28, 2012
Stability and compatibility of HFOs 1344326426533 Aug 07, 2012
Counterfeit refrigerants are on the rise 1342432820544 Jul 16, 2012
European Commission sets new deadline: manufacturers may continue to use old refrigerants 1337367617013 May 18, 2012
Sweden accelerates the transition towards HFC alternatives 1336644051890 May 10, 2012
Refrigerant effect on system performance 1336482013123 May 08, 2012
Welcome 1333112422484 Mar 30, 2012
Top page top