Most furnaces in your home need a pump to run efficiently. The pump connects to the water supply of the furnace to the outside air intake. This air is not circulated evenly because of the moisture in the air. It usually requires three pumps, each of which is responsible for pumping water through the furnace. Pumping water into the furnace creates a negative pressure and air will be drawn into the furnace through the drain hose. Usually you cannot notice the overflow because it just goes away when you turn the furnace off.
However, when there is overflow water, this is an important thing to look out for. Do not try to empty the water in the furnace unless you know how to do it. You might damage the pipes or break the drainage hose and end up with a small leak in the sewer line that could lead to serious plumbing problems. Some common mistakes are failing to empty the water out of the furnace properly, failing to close the drain valve properly and failing to remove any water trapped in the furnace coils. Ignoring these things can lead to a major problem with your home heating system.
You should always look out for signs of water in the furnace drain. When you feel the water rise, you should immediately check the problem with the pump and the outside air intake. If the problem is not with the pump, then the excess water could have spilled out from inside the furnace and drained down the drain hose. The cause could be as simple as a broken or missing part.