Condensation: Water dew point vs. Acid dew point

jesmed1

As a non-expert homeowner trying to learn more about oil boiler efficiency, I've been warned repeatedly about the dangers of flue gas condensation if the flue gas temperature gets too low. Experts here have cautioned that 300+ degree flue gas temps are needed to prevent harmful condensation.

That number has puzzled me for years. Water boils at 212 degrees under normal atmospheric conditions. So how can condensation be happening at flue gas temps closer to 300?

So I asked Google AI. And to my surprise, I learned something. This is a summary of what Google AI said:

"Water Dew Point vs. Acid Dew Point:

The water vapor dew point for heating oil flue gas is typically 115-125 F, while the small amount of sulfuric acid in the flue gas typically has a much higher dew point, typically from 248 to 300 F, which means corrosive liquid can form long before water vapor appears."

I had not heard that explanation before, and finally it all made sense. So it's the high acid dew point that we're trying to avoid.

The AI answer included a link to a useful paper about dew-point corrosion in boilers, which I'll include below.

https://www.nationalboard.org/PrintPage.aspx?ID=196&pageID=164

hot_rod

There are more factors involver regarding flue gas. The design and operating condition of the boiler, Co2 level ad chemistry of the fuel oil. The sulphur content for example. That is why you see different numbers and opinions publushed on acceptable return temperature. 130 for NG, 150 for fuel oil is a common range you see.

I suspect bio blends behave differently also.