A condensate drain is a very fancy term for a simple idea. One very common application of a condensate drain is on furnaces. There is a tube near the bottom of the unit from where water drips.
So, where does this water come from? Well, it comes from the water in the air and the process is called condensation, hence the name “condensate” drain. Whenever it is humid, the air is full of water and we call it humidity on the weather reports. Usually there is more humidity in the air during warmer weather and that is when we use our air conditioners. So the warm humid air comes into contact with the cold parts of the air conditioner. At that point, the water in the air changes to liquid water on these cold parts. You have seen this happen with your glass with cold drinks on a hot summer day, but did you notice that the glass sweats and leaves a ring of water on the table?
Well it isn't really sweating and the process is called condensation. The same thing happens inside your air conditioner that pulls warm air from inside your home and passes it over the cooling coils to cool the air. Warm air in, cool air out and back to the house. In this process, water forming on the cold parts in your air conditioner needs to go somewhere. Gravity pulls the water to a collector pan which feeds into the drain tube, known as the condensate drain.
In the condensate system there can be several issues, including the following ones.
Some part of it might leak and the water take a different path to the ground. When the air conditioner is installed, care should be taken to make sure the water drains to a safe area, such as the drain pan. However, a leak can go anywhere.
As part of the air conditioning system, there is a trap in the drain. This trap works just like the trap under your sink. These are shaped so that water in the bottom of the U-shaped pipe acts like a plug for noxious odors that exist further down the system. If the air conditioner hasn't run for a while, this plug of water could evaporate, resulting in an unusual odor near the unit.
Mould in the condensate drain system can infect the cold airflow that circulates back into your home. Thus, you may experience symptoms of allergies.
As part of some air conditioning systems, there is an overflow sensor. If the condensate drain system gets plugged and the water overflows, the sensor is triggered resulting in system shut down.
So you can see that the condensate drain system is a critical part of any appliance in which it is installed. Heat pumps and furnaces often incorporate this type of system.