How does air conditioning work?

An air conditioner is basically a refrigerator minus the insulated box. Its function - in addition to cleaning and dehumidifying the air entering the vehicle - is to maintain the air at a selected temperature.

So, how does the system keep the car interior just the way we want it? Most people think that an air conditioner simply adds cold air to the interior of the car. Actually, there is no such thing as cold, rather there is an absence of heat. The job of the air conditioner is to remove the heat and humidity that we find uncomfortable. A compressor circulates a liquid refrigerant, compressing the liquid into a hot, high pressure gas. This hot gas is then run through coils so the heat can be dissipated and condensed into a liquid. The liquid then goes through an expansion valve and evaporates to become a cold, low pressure gas. This cold gas then runs through a set of coils which allow the gas to absorb heat and, in so doing, cool the interior of the vehicle.

My air conditioning unit isn't cooling. What's wrong with it?

Poor performance could be due to one or more factors:

Low refrigerant:- An air conditioning system requires a minimum amount of refrigerant to cool properly. If the refrigerant level is low, it will work less efficiently and will not cool well.

Dirty condenser:- The condenser is the heat exchanger which cools the hot high pressure refrigerant after it exits the compressor so that it can condense into a liquid. If the condenser is full of leaves, insects or other road debris, air flow through the unit may be impeded to the point where little cooling can take place. Cleaning the condenser should cure this problem.

Condenser cooling fan not working:- The condenser often has its own separate electric cooling fan. This fan should come on, and remain on, when the air conditioning system is operating. If the wiring, fan motor, or motor relay is defective, the fan may not work.

Internal blockages:- Any type of debris, e.g. rust, in the system may block the tube or the metering valve through which refrigerant flows into the evaporator. This will cause a loss of cooling, and potentially cause damage to the compressor as the system probably relies on oil circulating with the refrigerant for lubrication.

Air or moisture contamination:- The refrigerant inside the system, which must remain free of moisture, can freeze and form ice that will cause blockages. Contamination can result from leaks in the system, or failure to vacuum purge the system prior to recharging it with refrigerant.

Other electrical or mechanical problems:- These would include compressor wear, the compressor clutch failing to engage, metering valve failures, inoperative pressure switches, etc. Precise diagnosis will require a technician who has all the necessary tools and equipment, and who has been trained in this highly specialised process.

How can I tell if my air conditioning system needs refrigerant?

Most air conditioning systems lose a little refrigerant over time. Newer systems are likely to leak less than older ones, which may lose several ounces of refrigerant each year. It is therefore not unusual for the system to need re-charging.

My A/C system is leaking refrigerant. How can I find the leak?

The two most common ways to find a leak are to:

Visually inspect the system for telltale oil stains

When refrigerant leaks from the system, it immediately evaporates into thin air leaving no trace except possibly the compressor oil residue that leaks out with it. Wet oily areas around hose connections and fittings and/or greasy streaks radiating outward around the compressor clutch or on the underside of the bonnet just above the compressor are good visual clues to the location of the leak.

Flourescent Dye is added to the system, the presence of the coloured dye around hose connections and fittings after the system has been run for a period of time are indications of escape. Leak detecting dyes will often reveal tiny leaks that might escape visual detection. Some are fluorescent and require illumination with a special light before you can see them. Despite this, even dyes can fail to determine the location of a leak if the leak is in the evaporator (located inside the heater/defroster plenium under the dash) or in a hard-to-observe or hidden location.

Use an electronic leak detector

Most professionals use an "electronic" leak detector that reacts to the presence of refrigerant in the air. Such detectors are extremely sensitive and can detect leaks as small as 1/4 oz. of refrigerant per year!

How can I fix a leak?

Secondly, it may seem cheaper to keep recharging your system with additional refrigerant instead of having the leak fixed. To knowingly do this is now illegal with heavy penalties
(see legal declaration).

What does the filter/dryer do?

The filter/dryer is filled with desiccant; this absorbs harmful moisture and acids from the a/c system and acts as a filter similarly to an oil filter on a car engine.

Why change the filter/dryer?

After approximately 2 years the filter/dryer becomes saturated with moisture and the a/c system becomes less efficient and the cooling effect inside the vehicle decreases; there is also a risk that the desiccant can be dispersed into the a/c system causing system failure.

Why flush the system if the compressor has seized?

When the compressor has seized metal particles/filings can be deposited in the system, these must be flushed out before fitting a new compressor and the filter dryer and block/expansion valve should also be replaced. This is comparable with fitting a new engine and changing the oil filter at the same time.

Why should the a/c system be used even during the winter months?

Using the a/c system during winter months will prevent the windows from misting inside the vehicle as the air is dehumidified, and regular use will help prevent oil seals drying out thus preventing refrigerant loss or compressor failure.

What is the evaporator?

This is usually situated behind the dashboard with a blower, which draws the warm air from the vehicle over the cooled evaporator and back to the vehicle interior.

What does the compressor do?

This is the heart of the a/c system and pumps refrigerant around the various components.

Why does the compressor seize?

Primarily failure is caused through lack of lubrication, it is important to ensure that oil levels are kept topped up and that the compressor is used regularly throughout the year.

Why change the expansion/block valve if the compressor has seized?

Metal particulates/filings can block the valve and cause system failure.

Are there different types of refrigerant?

Yes, Isceon 49 or RS-24 (r12 was used up to October 2000 but its use is now banned) is used for vehicles up to 1993, later vehicle use R134. Refrigerants must never be mixed or cross-contaminated.

Why is oil added to the system when the refrigerant has escaped?

When refrigerant escapes from the system oil is also lost, if this is not replaced the compressor will seize.

What type of oil is used in an a/c system?

This is a special oil that is matched to the refrigerant type, these oils must not be mixed, no other oil should be used in Air conditioning systems.

Should I have my Air Conditioning Serviced?

Yes, at least every two years. Vehicles loose about 15% of their refrigerant per year. Refrigerant and oil level checks form part of the National Aircon Ltd service plan.

There are a number of ways of converting a system to accept the new type refrigerant, without going to the expense of replacing major components, contact us for details.

National Aircon are trialing an injected small leak sealer. Trials at this stage are promising. Please call for details.