The 12 VOLT FAN is meant to accelerate airflow in all non-electric units. It comes pre-installed in an 11" piece of 4" diameter vent pipe so that it will fit inline in your vent stack. It also protects against downdraft and increases the evaporative performance of your non-electric unit by about 10 to 15% (still not enough to evaporate all liquids on an NE). The 2 watt, 12V fan option increases capacity and evaporation for heavy or residential use. Fan option uses 1.4 watts. Its purchase is optional with an Excel NE, a Centrex 2000 NE, or a Centrex 1000 NE.