I have 2 heaters in my new house.. upstairs and down.The up stairs one is far at the end of the house and not "centered" to get proper flow. Since I moved in 11-08, I've had to change the Air Flow filters 4 times, including the original. I purchase good ones and still had to change them. Between the guy who installed the heater and the builder, the duck vents had been "opened" for over 1 1/2 years. Could the dirt build up be the problem? Or the place where the heater is placed in the ceiling. I freeze in the open living/kitchen area and had to purchase a portable heater just to warm up that room. Plus I have a wood stove in there and use it, too. Can anyone give ideas, please on what I can have done..Thanks..