No, not the four legged, scurrying around vacant warehouse type.
RAT as in Return-Air-Temperature, or more specifically, low return air temperature.
How does low RAT kill a heat exchanger? The same way over-sizing does; it causes the flue gas temperature to drop below its dew point. The resulting moisture mixes with the products of combustion and forms acid.
So, what causes of low RAT?
• Leaking curb gasket (drawing in outside air)
• Economizer open (too much ventilation air?)
• Space temperature set-point too low (warehouse, unoccupied?)
In the end, if you don’t have a corrosion resistant heat exchanger (stainless steel) keep extended run return air temperatures above 65 degrees.
Stay tuned for part four.