When an air conditioning system is at rest (eg before starting) how is the liquid refrigerant distributed throughout the system? I would think that the expansion valve would remain slightly open, allowing the liquid part of the refrigerant to flow at the bottom of the system and the vapor at the top. If this is true, how liquid refrigerant is prevented to flow into the compressor?

Ref pic below: Do we all agree that the gauges on the manifold set are indicating a relationship between gas T and P and that this relationship would only be true if vapor pressure exists (liquid phase in the system)? The gas pressure in the system (no liquid phase) will not provide a correct T for the P indicated.

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  • Welcome to Motor Vehicle Maintenance & Repair! Commented Oct 15, 2018 at 23:13
  • The refrigerant should equalize through the system and reach ambient temps. I'm not sure if at that point whether any is left in a liquid form or not, but would assume it wouldn't be. Leaving this as a comment, because I really don't know for sure. Commented Oct 15, 2018 at 23:42
  • @Pᴀᴜʟsᴛᴇʀ2: There's definitely some liquid left. Think about: If it were all gas, pressure depends on temperature and amount of gas left. If there's liquid, pressure depends on temperature only. (Compare nitrogen and liquid gas bottle)
    – sweber
    Commented Oct 21, 2018 at 12:10
  • The only way to prevent liquid from flowing into the compressor with system turned off is fitting a liquid line solenoid valve (LLSV) right after the condenser. This is done with bus HVAC systems, since, apparently, liquid refrigerant inside the sump of a transit compressor (google for Carrier 05G if you want an example) at startup is a very bad thing (the refrigerant displaces the compressor's oil). The TXV can definitely open up with an inactive system, since the suction line is warm when no refrigerant is flowing through.
    – Al_
    Commented Oct 30, 2018 at 14:15

3 Answers 3


First of all, yes, there is always liquid refrigerant in the system at idle.
Looking through tables, the minimum amount of R134a in car ACs is about 500g, some cars have twice or more.
[chemistry] 500g R134a is about 5mol, which has a volume of 115l (30gallons) at 25°C (77F).[/chemistry]
Now, we know that at that temperature, the AC system has a pressure of about 5Bar (72psi), which by the way is the pressure shown on the gauges on the picture.
Applying the ideal gas equation V1/V2=p2/p1 and the given amount, at that pressure, the gas (and so the AC system) should have a volume of 115l/5=23l (6gallons).
Since a cars AC system for sure does not have this volume (may be larger cars, but they also have more refrigerant), there must be liquid R134 in the system! (Keep in mind, the ideal gas equations are only valid for ideal gases. Many gases can be treated ideal, but only when far away from their boiling point)

When sitting idle, it is very likely that there will be liquid refrigerant around the compressor, since it often is the lowest mounted part of the system.

I can't back it up, but I think liquid refrigerant in the compressor is not as bad as water is for a combustion motor. In a combustion motor, cylinder valves are opened into the inside, and higher pressure inside closes the valves even harder. But in a compressor, the outlet valve opens outwards when the pressure is high enough. So, liquid in the cylinder (or what ever it is) can escape and does not cause hydrolock. This does put stress on the compressor, but as long as this only happens when switching on the AC, and not always, the compressor can take it.

  • A/C compressors are not designed to pump liquids...
    – Solar Mike
    Commented Oct 21, 2018 at 14:28
  • Thanks Sweber for your explanation, what you are saying makes perfectly sense and unswer thoroughly my question.
    – Simone
    Commented Oct 21, 2018 at 20:23
  • @SolarMike: I'd say it's not good for them. But It doesn't destroy them immediately.
    – sweber
    Commented Oct 23, 2018 at 19:34
  • That said, liquid refrigerant harms reed valves. That's a matter of fact. But the real harm comes from a continuous liquid slugging condition, caused, for example, by a TXV whose bulb lost physical contact with the suction line. Not the 1-2 secs maximum of getting rid of the minimal liquid refrigerant slug that made its way inside the compressor (the coldest part in the system when the liquid prefers migrating to it) overnight.
    – Al_
    Commented Oct 30, 2018 at 14:28
  • A variable displacement compressor usually starts pumping in the minimum displacement swashplate angle so it's the kind of compressor which gets harmed less. And automotive compressors usually don't come with an oil sump, so, anyway, there's not so much oil that can be displaced by the liquid refrigerant like with a, automotively speaking, Harrison A6 compressor. Placing the compressor high in the system (i.e. on top of the engine) can prevent liquid slugging but, at the same time, makes the compressor more vulnerable to the reduced lubricant return rate caused by a low refrigerant charge.
    – Al_
    Commented Oct 30, 2018 at 14:30

There shouldn't be any liquid in the "intake" side of the system. This is largely prevented by the Thermal Expansion Valve (TXP), or on older vehicles, an orifice tube. These parts reside in, or just before, the evaporator - the heat exchanger internal to the vehicle in the HVAC housing under the dashboard.

At normal "at rest" temperatures and pressure, all the refrigerant will be in a gas phase. Exacly like Paulster opined.

Provided the system is not overfilled, the refrigerant will remain a dense gas until compressed and the temperature lowered - this pressure comes from the compressor, and the gas-to-liquid phase change will occur in the condenser (another heat exchanger, usually in front of the radiator).

If liquid were allowed to reach the intake side of the compressor, the unit would "hydrolock" and likely destroy the compressor if you weren't lucky enough to burn or break the belt first.

Answer to comment:

@Simone I'm not sure exactly what you are saying, but most gases have a fairly high boiling point compared to refrigerant. (R134a is about -15 degrees F). I'd rather call it "gas pressure" than vapor pressure, but in any case you can put thousands of psi of oxygen, helium, argon, and carbon dioxide in a tank, but none of it will be in liquid phase at normal "human" temperatures.

It all follows the Ideal Gas Law, PV=nRT, where pressure * volume is proportional to the moles (amount) * temperature * a constant (measurement units). The liquid phase (if any) is comparitively un-compressible (which is why it would break the compressor) and would not affect the pressure of a manifold gauge set reading. The gauges have no clue about temperature, only gas pressure. (There are fancy manifold sets with thermometers built in, but the gauges are still ignorant of temperature.) A chart is needed to determine if the observed pressures are appropriate for the specific refrigerant type and operating temperatures.

At rest, since I'm fairly certain all [R134a] refrigerant will be well above -15 degrees F, there should be no liquid anywhere in the system. As temperature increases, so will gas pressure. If pressure decreases suddenly, as with a small orifice discharging into the evaporator, a severe pressure drop must beget a severe temperature drop, since volume, moles, and the constant are not changing. Viola! Enjoy the "air conditioning"...

  • Makes sense what you are saying, however the test and charging manifold gauges used for servicing the ac system have a direct correlation between pressure and temperature. This correlation exists for vapor pressure, and vapor pressure exists only if we have a liquid state in the system. Since you said that there is not a liquid state at rest, we will not have a correlation between temperature and pressure while the system is not running. Am I right?
    – Simone
    Commented Oct 16, 2018 at 14:55
  • What @Simone means: liquid R134a exposed to room temperature will for sure start to boil. But if it is enclosed inside a containment like the AC system, it will boil until the gas/vapor reaches a pressure of 5bar, and then stop. I mean, that's exactly how an ac works: by altering the pressure and make the 134a evaporate and condense. Now, assume a very low temperature and lots of liquid 134a in the evaporator in the car. When parking the car in the sun, the evaporator becomes very hot, much hotter than the AC pipe under the hood. The 134a will condense there, right before the compressor.
    – sweber
    Commented Oct 18, 2018 at 5:07
  • @Sweber Nah, not a chance. There's no impediment to flow between the evaporator and any "AC pipe under the hood". Unless the interior of the car falls well below -15 degrees F, there won't be any liquid in the evaporator. The orifice tube/TXV is pretty meaningless in this scenario as well. At rest, there's no compression, and the system will equalize to one uniform pressure.
    – SteveRacer
    Commented Oct 18, 2018 at 8:54
  • Even if the interior was -20 degrees F, and the engine compartment 200 degrees F, the system will flow in both directions until the gas pressure is equal. You will never see a major difference on a manifold gauge set with the system at rest for any period of time. Any theoretical "liquid" in the evaporator would seize the compressor. It doesn't happen, because it can't. There's no way to thermally isolate liquid from the gas, as there is no "valve" that sections off one part of the system from the other. Even a TXV is never fully closed.
    – SteveRacer
    Commented Oct 18, 2018 at 8:57
  • I don't know where "5 bar" came from, but I assure you that even a cool fall day, typical "at rest" system pressure is probably 60-80 psi, or very close to 5 bar. NONE of that is liquid. I suppose if the windows were open in Alaska, there might be an issue. Again, the Ideal Gas Law defines this behavior, and you can't have a pool of -15 degree F liquid next to anything gaseous at a higher temperature. The system will thermally equalize, ALWAYS, and unless the entire system is kept below the [very high] boiling point, there is no liquid phase that can be sustained.
    – SteveRacer
    Commented Oct 18, 2018 at 9:08

The volume of liquid refrigerant remains nearly the same regardless of if the system is running or not. An A/C system is an oddly shaped container but it is still a container.If you have a 30 pound cylinder with one pound of refrigerant in it you can shake it and feel the liquid. A small amount of refrigerant evaporates and that gas pressurizes the cylinder until the remaining liquid will no longer evaporate. There is no compressor involved. The liquid remains in the cylinder. The same thing happens in the A/C system. As the compressor pressurizes the gas to a liquid an equal amount is evaporating on the suction side. There is always a small amount of refrigerant gas pressurizing the container preventing the evaporation of the remaining liquid. The compressor could only compress the small amount that has evaporated but as it does low pressure evaporates an equal amount. The volume of the liquid refrigerant would only change as much and the same as it does in a dial a charge.

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