Having both a power vent and ridge vent on your roof can short-circuit the attic ventilation system. When the power vent goes on, it can pull air from the ridge vent, which can cause an imbalance of airflow along the underside of the roof deck. When the power vent goes off, it acts like a roof louver (an opening on the roof without a motor); in this case, the ridge vent pulls its intake air from the power fan which can lead to “weather infiltration” and poor ventilation along the underside of the roof deck.
In most cases, it’s best to have only the ridge vent and remove the fan.
If you have a hip roof with very little ridge, a power fan with both a thermostat and humidistat (for both temperature and humidity) should be installed, with no ridge vent.
With any attic ventilation system, the attic can be 20 degrees hotter than outside. Attic ventilation should protect the roof sheathing, insulation, and shingles from temperature and moisture extremes. However, many variables can affect the attic temperature, such as shingle color (black shingles absorb more heat and can make the attic hotter than white shingles). Other factors are the geographical location, sun intensity, orientation of the primary roof plane, and amount of total ventilation.