Electric exhaust fans mounted on the roof are useful during the summer to help remove heat from the attic. In the winter, they can depressurize the house and actually increase heat loss. We recommend these not be used during the winter.
A power vent operating in the winter is an original arrangement or homeowner education issue.
Increased heat loss and air leakage into the attic are implications of a power vent operating in the winter.
Where you see a power vent in the attic, determine whether it's operating. Trace the wiring back to the controls to see how the system is activated (manual switch or thermostat, for example).
When the power vent is operating in the summer, good soffit venting is necessary because we want to pull outdoor air into the attic and exhaust the superheated attic air. We don't want to pull the cool air out of the house into the attic. If soffit vents are missing or obstructed, the negative pressure in the attic will naturally move more air out of the house. If the house is air conditioned, we are wasting energy by pulling cool air up out of the house and into the attic.