Furnace condensation is typically a buildup of water around the unit's base. This can be caused by a few different phenomena but doesn't necessarily indicate a problem with the furnace.
Certain types of furnaces naturally produce condensation over time. This is not an issue. These furnaces often have a specialized drain used to collect the condensation that should prevent you from ever seeing it. If you notice your furnace is producing more condensation than normal, check to make sure nothing is obstructing the condensation drain and preventing it from emptying.
A series of pipes used for various purposes is connected to your furnace. These pipes run throughout your house, and certain pipes even run outside. If the furnace is producing condensation during the winter, the problem could be that one of these pipes has frozen and is backing up all the way to the main furnace unit. The condensation is then draining onto or around the area of your house where the furnace is.
Heating, Ventilation and Air Conditioning
An HVAC unit is a furnace that also incorporates vents and a central air conditioner into your home. If the furnace connected to the HVAC system is producing condensation, the problem could just as easily be with one of the other pieces of equipment as with the furnace unit. Check your central air conditioning unit to make sure it isn't producing condensation that is traveling through the ventilation system into the furnace area.
A series of hoses will be connected to your furnace, whether you have a standalone furnace or one that is connected to an air conditioner as well. If these hoses become blocked up, knotted or obstructed in any way, it could cause the natural flow of water to back up onto the furnace and produce the appearance of condensation. Check all these hoses and replace any that are damaged, worn or torn.