How Air Conditioning Works.
Air Conditioning 101
This page is for the more curious among us who wonder how this magic box in my backyard keeps my house cool. The
best way to understand something complex, such as air conditioning, is to understand the problem(s) we are trying
to overcome, and break things down into simple components. As a side note, the principles laid out below, also
apply to refrigeration.
Problem: My house is hot and I would like it cooler.
Heat = Energy
Heat is defined as the form of energy that is transferred between two systems (or a system and its surroundings) by virtue of a temperature difference. What this means in plain English is that heat is energy, and energy is not created or destroyed, but instead transferred from something warmer to something else that is cooler. How this applies to us is that if we want our house cooler, then we need to transfer the heat out of our house to the outside surroundings.
How do we transfer the heat from my hot house to the even hotter outside air?
Heat Transfer
We know that heat flows from warmer objects to cooler objects, such as a roast in a oven, or a cool drink that eventually warms up to room temperature. What we need is an object inside the house that is cooler than the inside air and can absorb a lot of heat from the inside air, and another object outside that we can transfer this heat to, which will be warmer than the outside air.
How do we make one object cool, one hot, and then get heat to flow backwards?
Pressure Changes
If we think way back to high school science classes (way, way back for some of us), we will recall that there is a direct relationship between pressure and temperature in gases. As temperature goes up, pressure goes up. Think about what would happen if you threw an aerosol can on a fire. As the temperature in the can raises, so does the pressure until it explodes due to the high pressure inside the can. The opposite also holds true, as temperature decreases, pressure decreases. Since we can't destroy the heat we absorbed from the inside air, we will have to resort to pressure change trickery to raise or lower the temperature between our two objects.
Now that we know the process, what are the components involved?
Air Conditioning Components
The indoor components, evaporator coil and expansion valve (TXV), typically come welded together from the factory. In the few cases that they do not, it is left to the contractor to select the appropriate expansion valve to match the condenser being installed. Outside, the compressor always comes welded to the condenser coil from the factory, and is typically just referred to as the "condenser." This type of system shown is referred to as a split system, because half of it is inside and half of it is outside. The two components are connected by a couple lengths of copper refrigerant tubing. Due to the nearly infinite possible combinations one could achieve of evaporators to condensers, it is impossible for the manufacturers to accurately advertise the efficiency rating of a condenser, known as SEER rating (Seasonal Energy Efficiency Ratio).
How do we know the efficiency, or SEER rating, of our system?
ARI Numbers
The Air Conditioning, Heating, and Refrigeration Institute (ARI) is an organization that tracks most of the likely possible combinations and their SEER ratings. ARI numbers are the industry standard for efficiency verification. Equipment manufacturers test likely combinations according to a standardized testing procedure, and then submit the SEER rating to ARI. ARI then assigns a number, which can be used to look up and verify a split system combination and its corresponding efficiency. You should always ask for this number and verification of the efficiency as stated by the contractor before purchasing your system. Example: the manufacturer's brochure and yellow "Energy Guide" sticker on a condenser both say 15 SEER, but a careless choice of evaporator coil and furnace yield an efficiency as low as 13 SEER. On the other hand, a well researched choice of evaporator coil and furnace, could boost the SEER rating as high as 17+ SEER.
Where does my furnace fit into all of this, and can it affect the SEER too?
Furnaces
The choice of furnace can play a major role in the SEER rating of your air conditioning system. Since we need an indoor fan to circulate air for the air conditioning, as well as the heating, it makes sense to have only one fan for both. During the air conditioning mode, the furnace leaves it's gas heating off, and turns on it's fan to circulate the air. The SEER rating is based on the entire system, which includes the furnace fan's energy consumption. Therefore, if you buy a furnace with a more efficient fan motor, known as a variable speed, you can increase your SEER rating buy up to one full SEER in most cases. Example: a 15 SEER condenser, paired with a well researched choice of evaporator coil, and a variable speed furnace, could boost the SEER rating as high as 17+ SEER.