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Why Does My A/C Need Regassing

Picture of A/C controls in a vehicle

Do you often find yourself in the car cranking the air-conditioning on to full blast just to bring the temperature down from pizza furnace to bread oven? Better yet, does your A/C sometimes feel like an asthmatic old man is gently blowing hot air in your direction?

Lately, we've been suffering this exact problem. In fact, it's so bad that sometimes the work ute hasn't even cooled down by the time we make it to the next job - 40 minutes away.

Now, everyone knows (ot thinks they know) the solution to this, an A/C regas. But why is that the cure all? How does a little bit of gas help deal with the heat?

Well, A/C is actually a relatively simple system overall. In fact, air conditioning actually only relies on one main principal to function properly, the change in temperature relative to pressure.

This is probably looking back to high school science a little too much, but you may remember a set of experiments where you would change the pressure around water and see the difference in the boiling point. When the pressure goes up, the boiling point does as well and vice versa. In fact, changing the pressure has such an effect that on top of mount Everest, water boils at just 68C.

In much the same way, if you rapidly change the pressure of a liquid or gas, the temperature reflects this. i.e. If the pressure goes up so does the temperature, and if the pressure drops suddenly, the temperature does as well.

Actually, the most simple example of this is deodorant cans. When deodorant is inside the can, the pressure is much higher than the outside world and when you open the valve and let the deodorant out, it very quickly changes from a high pressure to low pressure state and becomes very cold.

This is basically how A/C works. A small valve (called a fixed orifice or TX valve) uses this principle to change a high pressure to a low pressure which cools a gas significantly. This cold gas is then sent through a heat exchange unit (called the evaporator - basically a radiator in reverse) which fresh air is blown past to be cooled down.

The most effective compounds to do this are called refrigerants. And while they often have really stupid names like R12, R134 and R1234YF (better known as Tetrafluoropropene), they're all basically gasses that show a massive temperature change when going from high to low pressure.

So, is this why you need to regas your car? Is there a fuel tank like device in the back of the car that needs to be topped up every couple of years?

Well, not really. See there's really two main problems. In the old days, these refrigerants were super harmful. R12 is actually a chemical called Dichlorodifluoromethane and that was considered an improvement over other options such as ammonia gas.

In addition to being super harmful to humans, they're also incredibly bad for the environment.

However, none of this really effects the situation as much as one thing, the fact that these gases are somewhat hard to manufacture.

Now, if the gas that was needed for refrigerant was cheap, readily available, non-toxic and required little to no work to prepare for the system, we might live in a very different world. However, with all of these restrictions in place, engineers had to come up with a way to reuse the gas in the system over and over again.

Because of this, most A/C systems should "theoretically" be filled for life. This means that the same gas is used over and over again.

How does the A/C system recycle refrigerant? Can you even reuse a gas?

Well, actually yes. It's fairly simple in essence and it utilises a couple of key components, the compressor and condenser. After the gas has been changed from high pressure to low pressure and has gone through the evaporator, the compressors job is to return the gas to high pressure again.

After doing this, the gas has usually become a lot hotter overall so it then travels through the condenser (another radiator) to bring it closer to ambient temperature. This is all getting a little complex now and, in actual fact there are still a couple of components we haven't talked about, but you get the gist. High pressure turns to low pressure and then the compressor returns it to high pressure so it can repeat the process for ever and ever, never losing any gas.

So why would you need a regas? Well, hear is the interesting bit. While we like to think of these systems as perfectly sealed, they are mechanical systems and they utilise tiny, little rubber o-rings where the components join up. It's not uncommon for these o-rings to become brittle or weak with age and to slowly let the most minute amount of gas past.

On top of this, as vehicles age, it's not uncommon for the components themselves to leak slightly as well. All this leads up to a situation where there is not enough gas in the system to effectively cool down the car.

Then a regas is the obvious answer right?

In the most simple terms yes. However, this does come with a couple of caveats. Because of the toxic nature of the refrigerant, it's meant to be illegal to refill an A/C system that is leaking. Because of this, it's meant to be illegal to regas a system that is leaking. In addition to this, workshops should (from a legal standpoint) be testing the systems for leaks before regassing.

All in all, A/C regassing is a great way to make an older A/C system cold again so long as the system itself is in good health.


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