Feelin' Cold, Cold, Cold
An Air Conditioning Primer

Spring is sprung, the grass is riz, I wonder how my air conditioner is? Now that the warmer weather is upon us, it's time to spark up that mysterious box that supplies the cold, like a horse-drawn carriage delivering blocks of ice from house to house in days gone by. Just how does that finned and fanned metal cube cool the house anyway?

Two Main Parts
It is convenient to refer to the metal box in the backyard (or side yard, or roof, or somewhere else outside) as the air conditioner. It is actually one-half of the A/C system. The other half is inside, but we can't see it because it is inside the ductwork above the furnace. The device inside the ductwork is a heat transfer "coil", which is jargon for a metal thing with tubes and fins that looks like a car radiator. The grill and fins on the exterior box make up the exterior coil. Running between the coils are two long copper tubes. Like a car radiator, the coils transfer heat to air blowing by.

The Compressor
The heart of the system lives inside that exterior box. It is the compressor. A compressor is a pump that can push around gas, whereas your common-garden pump can only move liquids.

The Refrigerant
As the compressor is the heart, so the refrigerant is the blood. In the past we used Freon, but it is an ozone-depleting CFC, so we have stopped using it. Inertia being what it is many people still say Freon when in fact they mean refrigerant. They should have called the new stuff Fre-off.
The refrigerant is the magic ingredient. It is carried in the copper tubes from the inside coil to the outside coil and back again. It is like a blood circulating system for your house. The copper tubes in our analogy are the veins. As blood can be used to cool our bodies down, the refrigerant is used to cool the house down. While we could use just about any liquid to cool the house, we carefully design the refrigerant to have incredibly good heat transfer capability. It's like how in a pinch we can fill our overheating car with water, but it is best to use special radiator fluid.

Get the Heat Out
While it is easy to picture the A/C system delivering cold like blocks of ice, what it is actually doing is getting rid of the heat. It does this by using the furnace fan to suck the hot, moist air from the house into the ductwork above the furnace. This air blows past the interior coil, which has very cold refrigerant running through it. The heat from the house air is absorbed by the refrigerant. Having selflessly shed all its heat, the house air is now cold.
Of course, that means the refrigerant is hot. We have to dump this extra heat, so we flow the refrigerant back outside to the compressor, where in a fit of ill-logic, we squeeze and squeeze the stuff until it gets really hot. Paradoxically, we have to do this first, or else we won't be able to ditch the heat when we flow it through the coil. After the run through the exterior coil, the refrigerant is cool, but it's not cold, so we march it single-file through a bottleneck, which deflates the pressure and cools the jets. Now it's ready to cool the house air again, and the cycle goes on.

Figure - How the A/C system works

Let it Rain
One of the important and fortunate side benefits of blowing hot, moist air through a cold coil is that we will cause a lot of the humidity in the air to condense. We collect it in a drip pan, and direct it into a drain. All you see is the drain hose sticking out of the duct right above the furnace, going into a laundry tub or other drain. This is an important step in cooling the house. If we cooled the air but didn't de-humidify it, the house air would be like a dog's nose…cold and clammy.

Keeping Your Cool
Now that you have a basic idea of how the system works, here are a few tips for getting it up to speed, and keeping it in the zone:

Start-up

• Take the winter cover off the exterior box, if there is one. The unit cannot operate without air blowing through it.
  
• Use a garden hose to spray off any dirt, mud, and debris from the fins.
  Tip: spray the fins from the inside out to push dirt to the ground outside the unit.
• Find your electrical panel. If the circuit labelled "A/C" is off, turn it back on, then wait 24 hours before you turn the thermostat to COOL. This heats up the oil in the compressor.

Maintenance

• Check the outside box for plants and other greenery blocking air from blowing across the coil. Trim back as needed.
  
• Check the outside box for level. If it is a little off, you can shim up the bottom to get it straight. If it's really wonky, call a service contractor.
  
• On the inside, if the house is cooling down, and there is no leaking or dripping at the furnace, then everything is probably fine.
  
• Be on the lookout for reduced cooling ability, water dripping at or near the furnace, damage to the exterior unit, unusual noises, or ice on the refrigerant lines. Any of these is enough to call the service contractor.

On a final note, while timely upkeep is important, regularly scheduled maintenance by a specialist is important to ensure your A/C system keeps its cool.

Gerard Gransaull, Engineering Manager, Carson Dunlop and Associates Ltd, www.carsondunlop.com

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