The Naked, Cooling Truth About Air Conditioning · How Do They Work & How To Properly Size A Unit

If you plan on spending your summer in the Mexican Caribbean you’ll suddenly realize the importance of air conditioning. Let’s have a look at how they work, why they have to be cleaned and serviced and what is the right way of sizing a unit.

Willis Carrier was the one to invent modern day air-conditioning in New York, back in 1902. The electronics have changed a lot since then, but the principle remains the same.

The refrigerant circulating in the air conditioning's copper piping loop is the vehicle that transports coldness from the outdoor condenser to the indoor unit, and heat from the interior to the exterior.

Contrary to the popular belief, most air conditioning systems are not adding fresh air to the indoors, they are just recirculating existent air in order to maintain the desired temperature. A ductless mini split basically suctions the warmer air of the upper part of the room, forcing it true the cold evaporator of the indoor unit where it cools down. The evaporator also dehumidifies the air - water vapor condenses in contact with the cold evaporator (thus the need for drain pipes from the inside unit) .

Most modern AC units come with some kind of filter in order to prevent dust from being spread around when the unit is functioning. In time, dust, pollen and pet hair accumulate in filters, blowers and evaporators until the filtering capacity is exceeded. This creates a dark, dirty and humid environment where germs or mold can start developing affecting our quality of life as we inhale the air that passes through the unit.

How a dirty filter looks:

This is a dirty blower rotor:

A dirty evaporator:

Regular cleaning is recommended in order to avoid potential health hazards and maintaining the recommended air flow. Remember, dust and pet hair can dramatically reduce the efficiency of your AC unit making it run full speed with minimal effects (except raising your electricity bill).

And talking about electric bills, this is another issue when it comes to using AC. Technology has come a long way, modern units can be extremely efficient while older ones can turn into electricity black holes if left running for long hours. The part that consumes most electricity is the compressor, responsible for cooling the refrigerant. Older units are simple on and off devices.
What does this mean? Let’s say you return home and turn the AC on. Outside temperature is around 30 C, you set your unit at 17 C as you want a fast drop in temperature. Your compressor starts working at maximum speed to keep up with the demand. Presuming you have a decently insulated house, in about 1 hour the inside temperature has reached the 17 C (this is just an example, not a real-life situation). The thermostat kicks in and stops the compressor. ...Until the temperature rises. ...Then the compressor starts again at full speed. ...Then it stops, and so on. In other words, once the desired temperature is reached, the AC compressor kicks in and out trying to maintain a constant temperature, trying to save electricity as it does not have to run constantly. Remember we are talking about the older units; the problem is, whenever the compressor starts at full speed, a surge occurs, thus consuming a lot of electricity.

Newer models come with the so-called A/C inverter technology. This allows the compressor to adjust its potency to the demand. It is not an on/off situation anymore with high surges whenever the compressor starts but rather an adaptive pattern where the compressor quietly delivers what’s needed turning faster or slower, thus saving energy. However, inverter units come with a higher initial cost.

Sizing An Air Conditioning Unit

Surprisingly, bigger is not always better. On some home inspections I encounter ridiculously large units in rather small rooms installed in an attempt to save electricity. The logic behind is that when the unit is very powerful it has to work very little to cool the space thus consuming very little electricity.
Unfortunately that’s not how it works. Installing a large system in a small room will only lead to short cycling of the compressor as the exiting air stream bounces back into the returning air stream. With large units the velocity of the exiting air stream is of about 6 feet/second. If the opposite wall is less than 10 feet away, the exiting air stream travels for only 4 seconds before bouncing back to the unit. Since the temperature sensor is usually located on the cooling coil, this quick return of a cold air jet will make it think the desired temperature has been reached, therefore it will stop the compressor very soon after it started operation. Result? High energy bills as the compressor quickly cycles on/off, warm room as it never has the chance to truly reach the programmed temperature and a short life span of the cooling equipment.

It is extremely important that the compressor runs for more than 5 min when started. Why? Because this allows oil to return to the compressor (yes, your AC system runs on a mix of oil/refrigerant to provide internal lubrication). The second reason is its ability to dehumidify – the longer a unit runs the better it is at removing humidity.

So you see, properly sizing a unit has its advantages and can literally save you money.

Willis Carrier with his first A/C unit in 1902:

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