Heat Exchanger Failure #3 – Low RAT.

ratIn part 2 we told you that over-sizing is the number one killer of gas heat exchangers, so what’s number two?

The RAT!
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.

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Patrick is Zen HVAC’s diagnostic and training guy.

Patrick started in the trade the day he left technical school and never looked back.

He’s served in various technical and training roles in the HVAC industry but specializes in system troubleshooting and diagnostics, retro commissioning, and technical training.

His moto: If I can understand it, anybody can.

Patrick uses the Zen common sense approach to teach

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Famous Patrick Quote- “Well, that was stupid of me”

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