I live in Texas and have two electric (4 amp) Roof Ventilators on my home. One of the motors just went out after 10 years and I can replace for $35.00 - the other one went out about two years ago. My concern is whether or not the cost of removing hot air from my attic is worth the cost of powering these motors - they do have thermosats. I am considering replacing these two power units with four passive turbines. What do you think would be best ?