Rusty Stove Pipe Mystery

[Flumixt]

We replaced our airtight sheet metal no-longer-legal-but superior stove with a "modern" contrivance (stove) because age took its toll ie it burned out. With the new "modern" contrivance (stove) we then got a big rust streak down each side of the roof below the stack. Oxalic acid removed the stain but we never had that happen for the 25 years we used the airtight no-longer-legal-but superior stove.

What was rusting? Detailed investigation revealed that the inside top 1/2 portion of the single walled stack between the stove and ceiling is rusty; about 2.5 feet. The triple walled pipe thru the ceiling/attic is not rusty.

Certainly one of the products of combustion is water. Of course the wood is not perfectly dry - it is dryed in 100º heat most of the summer - but absorbs some moisture from the air even tho its stored in a shed. So that's the source of the water. The said water must condense on the inside of the pipe to cause it to rust. For that to happen the temp of the pipe must be below the dewpoint of the water in the hot xost gases rising from the stove. How can that be?

I often at bedtime throw in a relatively large bunk log, get it burning then damper the stove down for overnight. In the morning the house is warm but the pipe is or may be touchable - you can lay a hand on it. The stove likewise. The log is sometimes gone and sometimes there is still some of it left.

I believe it is during this cooler operation that the condensation and rust formation occurs. I believe it did not occur with the airtight no-longer-legal-but-superior stove because it had an automatic damper which would be wide open in the morning. That is, it would open as the stove cooled, increasing air flow and preventing the relatively incomplete combustion and lower xost gas temperature which must occur in the "modern" contrivance (stove) with its non-automatic damper.

There must be a temperature gradient up the pipe. The single wall pipe radiates to the room and cools. The top 1/2 is coolest. The triple wall pipe inner surface thru the roof probly stays hotter the whole time since it is shielded by its 2 outer walls.

So it would seem that the most practicable solution to prevent rust is the use a double wall pipe from the stove to the ceiling thus shielding the inner wall and allowing it to stay (hopefully) hot enough that condesation does not occur. A caveat is that we'd not get the heat previously radiated from the 1-walled pipe.

Is my analysis valid? Is there another solution? Should I heat the upper half of the pipe ? Waddaythink?




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[imok2]

Use a double wall pipe from the stove to the ceiling that should solve the problem

 

[Flumixt]

The original pipe was double walled. I just now drug it out and looked at it and to my surprise the upper inner half is stainless steel. Perhaps this problem has been addressed before. Well between the double wall and the stainless the problem should evaporate. Thanx.

 

[moltenmetal]

Hmm: the pipe between my stove and the ceiing is single wall and carbon steel. Considering how much heat it throws, I suspect it's a major contributor to the overall efficiency of the stove. Don't worry, the insulated SS pipe extends through the ceiling using an approved separation box etc. But no problems with condensation for sure...

 

[willard3]

I will bet also that most creosote deposits at the single-wall piece.....it's better to have creosote there than above the ceiling.

 

[Compositepro]

You should also understand that the main products of combustion of wood are carbon dioxide and water. The wood can contain zero moisture and you will still get condensation if the flue is too cold. You will extract significanly more heat if you condense the water but it does cause corrosion and has to go somewhere. You can have refluxing in the flue at low burn rates. The water will condense at the top and drain down to hotter sections where it will vaporize again.

 

[moltenmetal]

The single-walled piece is trombone-style and can be replaced in about 2 minutes. Makes cleaning easy too. No creosote to speak of, since I've been burning mostly dry framing off-cuts and demolition lumber- and taking pail after pail of nails out of the ashes with a magnet...But the price is right, and it does the world more good heating my house than it does going into the local landfill!

My single-walled pipe runs way too hot to reflux for any appreciable period. You get lots of heat out of that pipe even without condensing the water of combustion. And all the required clearances from combustibles etc. are met so it's fine to use single wall in my region.

 

[Flumixt]

Hello Again-

I removed the single wall pipe. I could find absolutely zero rust inside that pipe. None in the triple wall pipe thru the attic, none in the exposed stack above the roof and none in the stove except:

1) red droplet spalsh marks and
2) red color or stain on the baffle blocks.

It would appear like rusty water precipitated out.

Beats me where the rust came from.

I put the single wall pipe back in cause it fits and there seems to be an adapter missing for the double wall pipe.

Guess there will be one more weekly chore this winter - spraying oxalic acid on the roof and washing it off. What a drag.

 

[chief]

Fit a mesh metal screen in your uptake,preferably a few inches before the end piece. This will condense any remaining vapors and prevent condensation carryover.

Offshore Engineering&Design

 

[Flumixt]

Well it would creosote up and plug the stack.

I wonder if the condensate would drip back down the pipe, revaporize and recondense with the process looping until the amount of water built up would flood the stove, put out the fire and cause a soggy mess in the living room.

Or perhaps I could fix up an oxalic acid spray entrainment into the pipe that would combine with the.........naw.