no i dont think tannis would cause faster water evaporation is your house getting warmer or drier from the furnace being on becauce water will evaporate fast if the air in your house is dry...that or have you increased the temperature in the tank a few degrees makes it evaporate fast also do you have a lid on your tank i know when i dont close the lid on my tank the water evaporates very fast almost 10x faster then when it is shut