Why Do You Need a Trap on an Air Conditioner Drain?
Any plumbing drain that drains in to the main sewer line has a trap on it to prevent sewer gasses from backing up in to your home. A trap on an air conditioning condensate drain is for the same basic reason. If no trap is present, the low pressure area located inside the air handler would cause the water to never drain, and the condensate pan to overflow.
Condensate is accumulated in your air conditioner as a byproduct of the cooling method. Hot indoor air is blown across cold Freon coils, which "sweat" the humidity in the air, just as a cold glass of water sweats in a hot room. This reduces the indoor humidity and makes the climate more comfortable. The water drains outside your home through a pan and length of PVC or hose. This water has to be able to drain away from the system in order for the air conditioner to function properly.
Plumbing traps are small elbow-like pieces of pipe that prevent gas and air from backing up through a drain. A drain only functions as long as there is no substantial pressure differential between both sides of the drain. Traps are used in most any draining application as a method of ensuring the drain only flows one way.
Low Pressure Air Handler
Many air handlers create a low pressure zone, or slight vacuum, inside their cases. Due to the vacuum, if no trap was present, air outside the home would be pulled in through the drain pipe. This would not allow water to drain through the condensation drain and would cause the condensation pan to overflow.
Sewer Line Hookups
In rare cases, air conditioner condensation drains may be attached to the main sewer line. An air conditioning drain trap exists in these setups to prevent sewer gas from backing up into the air handler. This would cause a number of problems, the most serious being a potential fire hazard when methane is introduced in to the electrical environment of the air handler. Additionally, sewer gas is quite unpleasant smelling and would not be enjoyed when spread throughout the home by way of the air conditioning system.