Clogged and dirty filters are perhaps the most common cause of furnace issues. As a rule of thumb, you should aim to replace filters roughly once every 3 months. This, of course, is only a guideline, and depending on your lifestyle or equipment it may be advisable to do so more often. If you find your house becoming dusty quickly after cleaning, or that your furnace is running more than usual, clogged and ineffective filters could be at fault.