I have a ducane model number mpga100b4a and the heater will run for 5 min then flame goes out saying pressure switch is open ,the flu pipe is 3 inch off heater then goes to 4 inch with approx 5 90s and a 30 foot run before it hits chimney do you feel flupipe is too long and if so what would be a easy fix I can not relocate heater so flu pie must be that long I was told to boot a booster fan inline what do you think