HVAC Refrigerant Gases and Pressure Switch Settings

Three Properties Decide Everything Downstream

Pick a refrigerant and you have already fixed most of what follows. Its operating pressures determine the pressure rating of every component in the circuit. Its safety classification determines where you are allowed to put the equipment and how much charge you can use. Its global warming potential determines whether it is still legal to install.

Those three properties are what the table below records. Everything after it deals with the part engineers most often get wrong: how a refrigerant pressure switch setting is actually arrived at.

Refrigerant Reference Table

Refrigerant	Safety class (ASHRAE 34)	GWP (AR4, 100 yr)	ODP	Status	Typical low side bar / psi	Typical high side bar / psi
R22	A1	1,810	0.055	Phased out	3–5 / 45–75	12–19 / 175–275
R410A	A1	2,088	0	Phasing down	6–9 / 87–130	20–32 / 290–460
R407C	A1	1,774	0	Transitional	3–7 / 45–100	13–25 / 190–360
R134a	A1	1,430	0	Phasing down	2–4 / 29–58	9–16 / 130–230
R404A	A1	3,922	0	Phased down	2–4 / 29–58	16–23 / 230–330
R32	A2L	675	0	Current	4–8 / 58–116	17–31 / 250–450
R1234yf	A2L	4	0	Current	2–4 / 29–58	7–18 / 100–260
R290 (propane)	A3	3	0	Current	0.5–2.5 / 7–36	10–14 / 145–200
R600a (isobutane)	A3	3	0	Current	0.2–1.5 / 3–22	4–6 / 60–87
R717 (ammonia)	B2L	0	0	Current	1–5 / 14–72	9–20 / 130–290
R744 (CO₂)	A1	1	0	Current	20–45 / 300–650	45–110 / 650–1600
R12	A1	10,900	1.0	Banned	1.5–2.5 / 22–36	9–14 / 130–200
R502	A1	4,657	0.33	Banned	1.5–2.5 / 22–36	14–22 / 200–320

Pressures are indicative operating ranges across common applications and ambient conditions, not design values. GWP figures are IPCC AR4 100-year values, which is the basis most F-Gas regulation has used; AR5 figures differ, sometimes materially, so always check which assessment report your applicable regulation specifies. Status reflects general market direction and varies by jurisdiction.

Reading the Safety Classification

The ASHRAE 34 code is the single most consequential piece of information in that table, and it is the one most often left out of comparison articles. The letter is toxicity, the number is flammability.

A1 — lower toxicity, no flame propagation. R410A, R134a, R744 and the legacy fluids.
A2L — lower toxicity, mildly flammable with a low burning velocity. R32 and R1234yf. Not inert, but far harder to ignite than an A3.
A3 — lower toxicity, higher flammability. R290 and R600a are hydrocarbons and will burn readily.
B2L — higher toxicity, mildly flammable. R717, ammonia.

This classification drives charge limits in occupied spaces, ventilation and leak detection requirements, and in many cases whether the equipment may be installed indoors at all. EN 378 and IEC 60335-2-40 set those limits as a function of room area and charge, and they are not optional.

How a Pressure Switch Setting Is Actually Determined

This is where a lot of published guidance goes wrong. A high-pressure cut-out is a safety device, and its setting is not a number you look up by refrigerant.

The order the decision is made in

Equipment manufacturer’s data first. If the unit has a specified cut-out, that is the value. Nothing overrides it.
Then the system design pressure. The cut-out must operate below the maximum allowable pressure of the weakest component in that part of the circuit.
Then the relief device. The cut-out must act before the pressure relief valve or rupture disc, with margin. A safety switch that trips after the relief has already lifted has failed at its job.
Refrigerant properties last, as a sanity check that the chosen setting sits sensibly above normal operating pressure at the worst-case condensing condition.

Pressure rating is not the same as setpoint

Two different numbers get conflated constantly. The rating is the maximum pressure the switch body can safely contain. The setpoint is the pressure at which its contacts change state. A switch rated to 45 bar set at 42 bar is fine; the same switch set at 42 bar on a system whose components are rated to 38 bar is not, because the switch will never protect anything.

Manual reset on safety duty

A pressure control that cycles a compressor and a pressure limiter that protects against a dangerous condition are different devices, even when they look identical. Safety limiters on the high side are commonly required to be manual reset, so that a system which has tripped on high pressure cannot quietly restart into the same fault. Check what your applicable standard requires for the system class and charge, because auto-reset on a safety limiter is a recurring inspection finding.

On the numbers you will find published. Cut-out values circulate widely in tables like the one this article used to carry. They are useful for recognising when something is obviously wrong — a 22 bar cut-out on an R410A system, for instance. They are not a specification, and setting a safety device from a web table rather than from the equipment documentation is not defensible if anything goes wrong.

Three Refrigerants That Need Extra Care

R290 and R600a — flammable

Hydrocarbons are excellent thermodynamically and have negligible GWP, which is why they keep gaining ground. They are also A3 flammable. Charge limits in occupied spaces are strict, electrical components in the refrigerant enclosure need appropriate protection, and service procedures differ from A1 practice. Generic components selected on pressure rating alone are not sufficient.

R717 — toxic

Ammonia plant is a specialist field with its own standards, machinery room requirements, detection and ventilation rules, and material restrictions — copper and copper alloys are attacked by ammonia, so the whole component set differs. Anything on an ammonia system should be specified for ammonia duty by someone who works on ammonia systems.

R744 — pressure

Transcritical CO₂ runs at pressures an order of magnitude above conventional systems, and its behaviour at standstill catches people out. Left to warm to ambient, a CO₂ circuit at 20 °C sits at roughly 57 bar simply from saturation, with no compressor running at all. That is why CO₂ systems carry standstill pressure management rather than relying on operating controls, and why every component is rated far higher than the running pressure alone would suggest.

What This Guide Does Not Cover

Selection of a specific switch for a specific machine — that comes from the equipment documentation
Charge limit calculations, which depend on room area, installation height and standard
Retrofit and drop-in compatibility, including oil and elastomer changes
Recovery, handling and certification requirements, which vary by jurisdiction
Current regulatory status, which moves faster than any article can track

Frequently Asked Questions

What high-pressure cut-out should I use for R410A?

There is no single correct answer by refrigerant. The setting comes from the equipment manufacturer’s documentation, and must sit below the design pressure of the components it protects and below the relief device setting. Refrigerant properties are only a sanity check that the value sits above normal operating pressure at worst-case condensing conditions.

What is the difference between a pressure switch rating and its setpoint?

The rating is the maximum pressure the switch body can safely contain. The setpoint is the pressure at which its contacts change state. A switch can be correctly rated and still set wrongly. On a safety limiter the setpoint must be below the design pressure of the part of the circuit it protects, otherwise the device protects nothing.

What does A2L mean on a refrigerant?

It is an ASHRAE 34 safety classification. The letter is toxicity and the number is flammability. A2L means lower toxicity and mildly flammable with a low burning velocity, which covers R32 and R1234yf. A2L fluids are harder to ignite than A3 hydrocarbons but are not inert, and they carry charge limits and installation requirements that A1 refrigerants do not.

Why does a CO2 system sit at high pressure when it is switched off?

Because saturation pressure for CO2 is high at ordinary ambient temperatures. A circuit allowed to warm to 20 degrees Celsius reaches roughly 57 bar with nothing running. This is why transcritical CO2 systems include standstill pressure management rather than relying on operating controls, and why every component is rated well above running pressure.

Should a high-pressure safety switch be manual or automatic reset?

A device acting as a safety limiter on the high side is commonly required to be manual reset, so that a system which has tripped cannot restart into the same fault unattended. A pressure control that simply cycles the compressor is a different device and may be automatic. Check what the applicable standard requires for the system class and charge, because auto-reset fitted on safety duty is a frequent inspection finding.

Which GWP figure should I use, AR4 or AR5?

Whichever your applicable regulation specifies. Most F-Gas legislation has been written against IPCC AR4 100-year values, which is what the table above uses, but newer instruments reference AR5. The difference is material for some fluids, so quoting the wrong basis can put a calculation on the wrong side of a threshold.

Can I use the same pressure switch on R410A and R32?

The operating pressures are broadly similar, so a switch may be pressure-compatible. That is not the whole question. R32 is A2L and carries flammability requirements that R410A does not, which can affect what components are acceptable inside the refrigerant enclosure. Compatibility must be confirmed against the equipment documentation, not inferred from pressure alone.

References

ASHRAE 34 governs refrigerant designation and safety classification. ASHRAE 15 and EN 378 govern refrigeration system safety, including pressure protection and machinery rooms. IEC 60335-2-40 covers charge limits for flammable refrigerants in appliances. Consult the current editions and your local regulations, which take precedence over anything here.