Chimney Cleanout - Part III

So the current theory being peddled by the local chimney maintenance people to explain why moisture is forming in the bottom of the shaft goes like this:
They claim that the modern higher efficiency furnaces emit exhaust at a lower temperature than the older furnaces do. On a long vertical run (in this case, something on the order of 25-35 feet or so), the exhaust cools enough that - before it can exit the flue - it hits the relatively colder air toward the top of the chimney and causes condensation.
The claimed fix is to either insulate the flue itself with material made for this purpose, OR to reduce the inner dimension of the chimney to reduce the amount of free air that surrounds the furnace flue.
This seems like nonsense to me, but what do I know. The chimney in question does have a cap on it, though the critter screen that the top is probably compromised. So, I don't think it is direct precipitation getting in. Moreover, the bottom of the cleanout manifests dampness not rivers of water.
Many thanks to all taking the time to comment here ...
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wrote:

Depends on how efficient it is. Some of the highest efficiency ones don't have a flue at all, they have to be vented with a plastic or other corrosion resistant pipe that is sloped in such a way that it drains outside.

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On 3/1/2019 10:57 PM, Tim Daneliuk wrote:

No, it makes perfect sense. One of the byproducts of combustion is water. While the cap on top keeps critters out, it also makes a cold spot for the vapor to hit, condense, fall to the bottom in the form of water.
Burn wood too cool and you can get puddles of water as it has a higher water content.
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rec.woodworking the following:

    Burn wood too cool and you get creosote build up. Chimney fires are "exciting" I'm told.
--
pyotr filipivich
Next month's Panel: Graft - Boon or blessing?
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On Friday, March 1, 2019 at 11:01:04 PM UTC-5, Tim Daneliuk wrote:

If you don't know, how can you say it seems like nonsense?
I mentioned this exact issue in part 2 when I said:
"If it's not rain, it could be condensation caused by the flue, which is also not a good thing."

Moreover, condensation would cause dampness, not rivers of water.

BTW...why do you keep starting new threads instead of keeping this single subject contained in one so that it is easier to follow?
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wrote:

Well if you replace the furnace you can vent via schedule 40 PVC pipe, then you can abandon the chimney altogether. You can also not worry about the dampness in the chimney and let the next home owner worry about it's effects on the mortar and bricks.
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On 3/1/2019 9:57 PM, Tim Daneliuk wrote:

Is this an open flue with the HE furnace just dumped into it? If that's the case, it's perfectly reasonable (and, at least here, not allowed for anything that isn't a natural draft furnace).
It's a bad idea even if it is/was allowed there at the time it was done...using a double-walled riser would solve the problem as they say it 'twould.

If it's just a single-wall ducting inside the existing flue, then it's true will likely be getting condensation before it all gets out the top, but most of that's going to run back down the flue itself because the air still hasn't exited.
=
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