How AC works

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Last Updated: Sunday, 03 August 2025 23:00
Published: Monday, 13 June 2022 22:51
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Air Conditioner working:

Air Conditioner (AC) work on Thermodynamics principles.

This article explains it in very layman language: https://aircondlounge.com/air-conditioner-working-principle-simple-explanation-with-diagram/

HVAC School: A channel on youtube devoted to HVAC workings. Very informative. Watching first few of them will give a very good idea: https://www.youtube.com/@HVACS

AC Service Tech: This guy has a youtube channel, with over 0.5M subscribers which has lots of details on AC. Even better is the website that he has that details a lot of basics that you won't find anywhere else.

A detailed doc on basics of refrigeration cycle is in this pdf by copeland: Fundamentals of refrigeration

Minisplits: These are slightly different than std AC that you see in homes. These are ductless, and can work to both heat and cool w/o using any gas or heating element. As such, they are very efficient in heating too.

Here's a video that shows very neatly the internals of a mini split: => https://www.youtube.com/watch?v=ATk1fx_N_qc

Refrigeration cycle of mini split in cooling mode => https://www.youtube.com/watch?v=Os2WMQxGSws

Refrigeration cycle of mini split in heatlng mode => https://www.youtube.com/watch?v=8gSmOxkbpBg

 

 

 

Vapor Pressure:

This is the most important term to know before you learn AC. We learned about Vapor Pressure in "Phases of Matter". Basically, it's pressure of vapors that form above a liquid when liquid is completely enclosed in a jar, which is close to a vacuum (i.e no other gases inside). In such a case, a liquid won't remain 100% in liquid state. Some of it will vaporize and fill the space, so liquid will become partially vapor and partially liquid. How much of it will become vapor, depends on temperature. If temp is very high, 100% of it will become vapor. If temp is very low, almost all of it will be in liquid state (it will never be able to get to 100% liquid state).

The P-T chart is the most used chart in AC industry and it shows this vapor pressure at each temperature. Basically this is the pressure that the mixture of iquid + gas will have at that temp.

R410A refrigerant, the one that is most widely used, is approximately a 50/50 blend by weight of:

  • R‑32 (Difluoromethane, CH₂F₂) with a molar mass of about 52.02 g/mol

  • R‑125 (Pentafluoroethane, C₂HF₅) with a molar mass of about 120.02 g/mol

My minisplit takes about 35 oz (2 lbs 3 oz). So, R32 and R125 are both 1lb 1.5oz. However, mole wise, there are twice as much of lighter R32 molecules than those of R125 in this mixture.

The P-T chart exists separately for R32 and R125, but we don't need that. We are interested in the P-T chart of the mixture of the two. Turns out that P-T chart for each of the two refrigerants is pretty close to that of the mixture R410A. The P-T chart for R410A is here => https://www.igasusa.com/files/R410A-PT-Chart.pdf

 

Operating pressures in AC:

Non operating pressure in an AC are just the above pressure based on ambient Temp. However, when the AC is running, pressure in Vapor line (suction line) go lower (~120 psi) while pressure in Liquid line (discharge line) go higher (~350 psi). Below link talks about this

Link from AC Service Tech => https://www.acservicetech.com/post/r-22-and-r-410a-operating-pressures-charging-and-recovery-setup

 

 

Superheating and Subcooling:

Delta T => This is the temperature diff b/w the hot air from inside our house getting sucked into the blower vs cold air coming out of the blower and going back into our house to cool it. For a house sitting at 80F, and AC trying to cool it, the cold air coming out may be at 60F (assuming thermostat is set at 60F or that is what the homeowner's desired temp is). So, we get a Delta T of 20F in this case.

These are 2 other important terms to know since they are used so often. These 2 measurements determine the proper working of your system.

Superheating (SH): This is in the evaporator side of system. It is the temp difference between the temp of liquid refrigerant (LR) entering on one side of evaporator (after the EEV, see below diagram), and the temp of vapor refrigerant (VR)  before it goes to the compressor. It's the extra heat that VR has, so it's called SH. Ideally you want 100% LR to get converted to 100% VR at same temperature, so that you take out the most heat from the room which you are trying to cool. The SH is that extra heat in VR which allows for some margin. Usually, the liquid entering from EEV is 80% LR and 20% VR (not 100% LR). On a system running properly, SH is between 8F to 15F (or 5C to 10C).

One of the best SH vid by Pioneer HVAC Control => https://www.youtube.com/watch?v=As44ldnUWh8

Few scenarios here:

  • Low heat going thru the evaporator coil (underloaded system): These are couple of cases where Low heat happens => Dirty filter or closed supply vents, bad blower indoor motor, Low return temp, etc. Heat going from the blower motor onto the evaporator coil is what boils the LR into VR. If there's not enough heat, then all LR will not boil, and we will have 0 SH. This is really bad, as LR will get into the compressor (known as flooded compressor). Low SH happens when when all LR has boiled, but didn't get enough heat to get rise in VR temp.
    • Low SH => not enough LR was able to boil
    • Low side pressure => low (It's similar to Overcharged system below, but here, the pressure is low in contrast to overcharged system, where low side pressure is high). So,if you see low pressure, it doesn't always mean it's an undercharged system (since undercharged system also shows low pressure), it may also be due to dirty filter. Not sure why? FIXME
    • High side pressure => high or low? Not sure? FIXME
  • High heat load (Overloaded system): This is in cases where house is just too hot, and AC needs to throw in a lot of LR to evaporator in order to cool it. EEV will keep opening more t allow more LR to flow thru.
    • High SH => since LR just keeps on boiling completely too soon.
    • Low side pressure => high
    • High side pressure => high
  • Restriction on refrigerant flow: This happens at the filter dryer or the EEV, where something may get into that hole and restrict the flow of refrigerant. So, evaporator will be starved of refrigerant, leading to low pressure in evaporator coil (anywhere in between the EEV valve and suction line, as the pressure is the same everywhere within this section). SH will be high (as very low amount of liquid is coming thru the EEV, so boils off quickly). Pressure will be low in evaporator (low side pressure line), but it will be higher than normal in compressor (high side of pressure line), as Refrigerant was backing up in compressor.
    • High SH => very little LR
    • Low side pressure => low
    • High side pressure => high
  • Low refrigerant (undercharged or starved): This is the most common case that you will see with most AC units, as refrigerant keep leaking over time (they are not supposed to leak, but most units develop some small leak somewhere). Here, the quantity of refrigerant is low to start with.  So, all pressures will be below normal in both sections of pipe. 
    • High SH => very little LR
    • Low side pressure => low
    • High side pressure => low (different than above case of restriction). In "restriction case" it was high, due to the refrigerant backing up in compressor, 
  • High refrigerant (overcharged or flooded): This will happen only if you put more refrigerant by mistake. Here, the quantity of refrigerant is high to start with. So, all pressure go high. 
    • Low SH => since LR is not able to boil completely, SH will very low (0 or close to 0). Delta T will be high at 30F or so, and you will see good cold air coming out and cooling the house rapidly.
    • Low side pressure => high
    • High side pressure => high
  • Correct amount of refrigerant (ideally charged): Here all pressure and SH will be within spec.  

Subcooling (SC): This is in the condensor side of system. It is the temp difference between the vapor refrigerant (VR) entering on one side of condensor (after the compressor), and the liquid refrigerant (LR)  before it goes to the EEV (see below diagram). It's the extra cooling that LR has, so it's called SC (sub cooling and NOT super cooling, as temp go further below). Ideally you want 100% VR to get converted to 100% LR at same temperature, so that you give out the most heat from the refrigerant to the outside air. The SC is that extra heat that we sucked out from LR which allows for some margin. Usually SC is between 10F to 15F (or 5C to 8C). SC has slightly tighter limit than SH, not sure why ?? FIXME?

One of the best SC vid by Pioneer HVAC Control => https://www.youtube.com/watch?v=5_qgIUFt6ls

How do these numbers look like with different amount sof refrigerants in AC. Let's look at 3 cases:

Properly charged:

Undercharged:

Overcharged.

Dirty filter or bad air flow:

Very hot outside air temp:

Very cold outside air temp:

 

 

AC Fundamentals (Summarized):

Below I've tried to summarize the fundamentals in one page.

In a nutshell, this is what happens in an AC:

 

 

Here's an hand drawn diagram that summarizes it in 1 page:

AC Functions Diagram

 

 

AC Components:

Below are details of few imp components.

Thermostatic Expansion Valve (TXV): This is the component that regulate the amount of refrigerant that goes into the evaporator. If it's too hot inside the room, that more refrigerant needs to be sent to the Evaporator, so that there is more liquid to evaporate, taking in more heat and hence lowering the temp. In essence, it maintains the Superheat (SH) to a range of 8F-12F in general. TXV is used in duct AC systems that are used in central AC. Minisplits use Electronic Expansion Valve (EEV) which uses electronics to achieve the same result.

TXV working (AC service Tech) => https://www.youtube.com/watch?v=WkgshvMOx00