Ice on furnace flues

Why exactly?

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I am already in the repeat mode. If I haven’t explained to your satisfaction, then the threshold for your satisfaction is beyond my abilities.

Can not, or should not? :wink:
Don’t forget the power of construction adhesive.

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I see pictures like that and I realize how much I love living in San Diego. Me and winter don’t get along very well.


If you don’t mind this happening…
Be my guest! :wink:


Sorry, I’m just waiting for you to deliver data, rather than opinions…

You know, why and when combustion gas goes through the sublimation or deposition process as it moves through the flue.

Never mind, I was just trying to understand your thought process. Was confusing to readers.

Dave, I know you know a lot about HVAC. You gave me some valuable insights into inspecting heat pumps which I rarely see. But your comments on this topic have me scratching my head.

Where do you see an extreme? Surely, not 23F. I reviewed my comments here and no where did I say that the combustion gas temp is wrong. In fact, I said that the flue temps are what they are. Sizing and designing the flue correctly so that the flue gases stay warm enough to evacuate most of the moisture in the gases is what I claim is important. Here is a photo of a flue on an 80% furnace (with water heater tie-in) where excessive condensation dissolved the flue on a five year-old system.

Then your next sentence in that post is…

You seem to say that there is a problem in the original post, but then say ice has nothing to do with it. What design flaws do you see? Would the ice still be there if there were no design flaws? Maybe we disagree about what is excessive condensation. I think when the condensate becomes free flowing that is bad. I claim that it can (and often does) “eat” a hole in the flue which can leak combustion gases into a house. My conclusion is that since 23F is a common winter temp, then if free flowing condensate is occurring at that temp, then there is a high likelihood of damage occurring to the furnace flue in an 80% furnace.
BTW, here is a photo of those “white marks” that you mentioned earlier.

I don’t like to see that either, because in my opinion, that is an indicator of excessive condensation.
And finally, since your comments have confused me, maybe we are somehow talking past each other. You have made comments and even posted a photo of the damage from excessive condensation where a galvanized flue was used on a 90% furnace. So, why wouldn’t similar excessive condensation be a concern on an 80% furnace? Isn’t free flowing condensate excessive? Do we really disagree that a gas-fired furnace should be able to evacuate the combustion gases sufficiently that the humidity suspended in the gases does not condense enough to free flow condensate back down the flue at a mere 23F?

@dandersen I did not mean to imply that I would vent a condensing furnace this way.
I meant to leave the impression that Murphy’s Law applies: if it can be done wrong, it will be done wrong, and it’s up to the HI to catch it.

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What an arrogant worthless response and frankly not one that I expected from you. Luckily, for all home inspectors they don’t have to misspell psychrometrics to understand that excessive uncontrolled condensation in a gas furnace is bad.