Nothing says winter has arrived like a failed heat exchanger…- Part 2; the RAT.

No, not the four legged, scurrying around a vacant warehouse type rat. RAT as in Return-Air-Temperature, or more specifically, low return air temperature.

How does low RAT kill a heat exchanger? It cools the heat exchanger, causing the flue gas temperature to drop below its dew point.

Sound familiar yet?

The moisture mixes with the flue gas and forms acid, and the acid causes corrosion. Yup, you got it; it’s just like the oversized unit from the first post.

So, what causes of low RAT?  I’m glad you asked!

  • 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 return air temperatures above 65 degrees during extended run times.

Stay tuned for part three.

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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

Patrick’s Likes- His Wife, kids and dog. Old pickup trucks. Hiking. The industrial Revolution.

Patrick’s Dislikes- Taking work too seriously. Anything unintuitive. Emoticons :(

Patrick’s Favorite famous person- Theodore Roosevelt “I am only an average man, But I work harder at it than the average man”

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

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