Condensation

Condensation occurs because of the exhaust that most furnaces produce. The heat that the exhaust carries allows the surrounding air to hold a large amount of moisture. As the air cools, the moisture condenses, usually on vents and ductwork. This leads to at least some condensation even with older furnaces. Condensation can also form on heat pump evaporators and heat exchangers for similar reasons.

High Efficiency Furnace

For high-efficiency furnaces, the exhaust is processed and the moisture is removed all at one time, which can create a constant flow of water when the furnace is working. These furnaces harvest heat from the exhaust in order to become even more efficient, but they need drain lines in order to work properly. These drain lines channel the moisture down and away from the furnace. It is normal and common for such furnaces to produce water while they operate.

Clogs

If you are noticing a sudden and usually large amount of water coming from your heating system, the reason could be a clog in the drain line. Sediment collected from dust in the air can eventually block the furnace drains. The water dripping from the condensate line then has nowhere to go, and can start flooding across the floor or inside the furnace.

Dust, Mold and Other Issues

Even if you do not have a clog in your drain, the condensation that furnaces produce can create other issues. If the condensation escapes and lines vents with a layer of moisture, it can trap dust and form areas for mold to grow. If you have a heat pump, the heat exchanger fins can become coated with dust for similar reasons, which will inhibit heat transfer and reduce efficiency. Regularly cleaning can help prevent these issues.