Condensation problem, is cold metal chimney cap the only culprit?

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Jan 29, 2021
145
VA, east central
I just bought a 2 story house with partial basement a month ago and have been using the wood stove in the basement for primary heat.
The stove is a Buck Model 81 and is connected to an external brick chimney that's approximately 27' tall from the point that the stove pipe enters it. The chimney has a terracotta liner that measures 6.5" x 11". The chimney is roughly 4' above the roof level and it has a stainless steel cap/cover.

About a week ago, I loaded the stove up for the night and had temps approaching "overheat" according to the magnetic temp gauge that's mounted above the center of the stove door. The previous owner had placed small metal tray up against the block wall right under the chimney's ash clean out door. I noticed water dripping out of the bottom of the clean out door and into the tray. Several ounces were accumulating on this occasion. It had rained the day before and temps had been freezing since then. The next day I went up on the roof and observed that the crown mortar had several cracks in it and was also eroded where it met the liner. There were also several spots several feet apart on the exterior of the chimney that were moist with some dark staining.

As a temporary measure, I wire brushed the cracks in the crown and used a rubberized roof cement to fill them and the joint around the liner. Since then, I've continued to notice moist spots on the exterior of the chimney and I've observed brown water droplets on the bottom of the chimney cap. I haven't noticed any leaking at the ash clean out door however. This may be due to the fact that I made and installed an aluminum trim piece that covered the top and sides of the terracotta liner, with the top of the trim piece sloping towards the outside of the liner in order to allow any dripping condensate from the chimney cap to run off the top of the chimney crown vs running to the interior and down the flue. The trim piece was fashioned from a heavy aluminum disposable food tray that matched the exterior liner dimensions perfectly.

Anyways, this morning I checked the chimney cap again and there was a fair amount of condensed water puddling around the aluminum foil trim piece where vapor seemed to be condensing on it and the cold lower band of the chimney cap. I didn't see condensate on the underside of the cap this time, but it's been fairly windy, so not much smoke/vapor is having the chance to hit the underside of the cap. It's mostly being blown sideways in various directions. I'm still seeing damp spots on the exterior of the chimney. I'm not seeing condensate forming on the interior of the flue liner, I peered in there briefly with a bright flashlight.

Given that I'm seeing a lot of water condensing on the cold metal of the chimney cap and it appears to be dripping back down the flue and I'm assuming leaking through liner joints (they're not the greatest in terms of being aligned perfectly) and wicking into the brick and mortar joints, I've removed the chimney cap temporarily to see if this solves the problem. Of course, when it rains, that will become another variable, but I have about 36 hours before we get any more precipitation (snow).

I've seen where people talk about keeping flue temps up to prevent condensation from occurring, but my problem seems to be with condensate forming on the cap and not on the inside of the terracotta liner itself. Maybe a 6" SS liner would prevent this, maybe not? I was thinking maybe I needed one before I noticed the condensate problem because I smoked the basement out on my first attempt to light a fire in the stove. The draft was super weak and maybe even back drafting a little and I wasn't savvy enough to use newspaper torches to heat the stove chamber and flue a bit first. Plus the newspaper wasn't burning very well, almost seemed a little high in moisture content, after the fact. Once I figured all that out though, I haven't had any problems even when doing restarts from small amounts of coals and after things have cooled down a lot. Having said that, the draft doesn't seem super strong, but it does appear to be adequate.

Thoughts? I'm wondering if there are caps out there made of material other than metal that are less likely to cause condensation. Or, maybe a larger metal cap is in order so that all of it's structure is further away from the opening of the flue and the vented gases which might allow moisture to dissipate before it has a chance to condense on the colder metal parts of the cap.
 
Last edited:

Woodsplitter67

Minister of Fire
Jan 19, 2017
1,964
Woolwich nj
I'd get the chimney inspected by a professional.. If you had water coming in from cracks there may be water still in the chimney and if its dripping back down it may take a while for it to dry up.. You may wat to get a second opinion on this also..
 

Nate R

Member
Nov 5, 2015
67
Wisconsin
I suspect you'd still get condensation with no flue cap. There's probably moisture IN the brick itself (brick holds water), and the air going into the chimney is probably releasing some moisture as it cools as well.

It's hard for me to envision the whole setup as you describe with words, but I do think a chimney inspection is in order, and I wonder if that flue is too large?
 
Jan 29, 2021
145
VA, east central
So I definitely have water condensing on the cap. The question I should have asked at the beginning is whether this is occurring because my draft is too weak and/or flue temps aren't high enough? Now that I have removed the cap, I'm not seeing any moisture or leaking at the ash clean out, and it appears that the moisture spots on the chimney have dried up, although there is staining. I have visually checked the interior of the flue from the roof and have not noticed any condensation or signs of moisture on the inside of the flue for at least 15 feet or so as that is as far as I can easily see.

The cracks on the crown were likely allowing water to seep into the brickwork and down the flue as well, but I've filled those for now. I'll redo the crown once the weather warms up a little bit. So I suppose I need to check with Buck stoves to see whether or not they think my flue is too large for the stove. The stove seems to operate well, maybe it would benefit though from a 6" liner. If condensation dripping from the cap ends up being the only source of water after properly fixing the chimney crown, I'll need to come up with something better than the current chimney cap that I have. I don't care to spend the money on a SS liner just to solve that problem.

I've uploaded pictures of my chimney, stove, and the trim piece that I temporarily installed at the top of the chimney.
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20210131_152340.jpg

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Jan 29, 2021
145
VA, east central
I'd get the chimney inspected by a professional.. If you had water coming in from cracks there may be water still in the chimney and if its dripping back down it may take a while for it to dry up.. You may wat to get a second opinion on this also..
The real question is how to avoid condensation on the cap and having it drip back down the flue. Since removing the cap and sealing the cracks on the crown, the water spotting on the exterior of the chimney seems to have stopped and be drying out. Probably premature to have someone come inspect it, but we'll see how it goes.
 
Jan 29, 2021
145
VA, east central
I suspect you'd still get condensation with no flue cap. There's probably moisture IN the brick itself (brick holds water), and the air going into the chimney is probably releasing some moisture as it cools as well.

It's hard for me to envision the whole setup as you describe with words, but I do think a chimney inspection is in order, and I wonder if that flue is too large?
I've attached some pics in my first reply. I'm not seeing condensation on the inside of the flue. I've removed the cap now. The spots seem to be drying up; I've sealed the cracks. I'll just keep an eye on it. I may have someone come inspect everything if I still have problems, but I suspect I'll need a different chimney cover that is a little bigger so that water vapor in the flue gasses has more time to disperse before hitting the cold underside of the cap and condensing.
 

peakbagger

Minister of Fire
Jul 11, 2008
6,486
Northern NH
I am surprised this question has not come up. What is your current wood supply, Since you are a new home owner, you dont have two year old stacks of wood (unless you brought the with you). This brings into question is the wood you are burning really dry? Sure it may burn but the drier the wood the less water vapor you are moving up the stack and trying to disperse. Along with this water vapor is potentially creosote. You signature is truncated so I cant see where you are located but since you started burning in relatively cold weather less than optimum wood can hide because of the temperature difference between indoors and outdoors.

If you plan to burn wood its worth getting a moisture meter and learn how to use it to check what you are buying. I think the consensus on Hearth is if you buy wood its not full seasoned, sure there may be exceptions but if in doubt its wet.
 
Jan 29, 2021
145
VA, east central
I am surprised this question has not come up. What is your current wood supply, Since you are a new home owner, you dont have two year old stacks of wood (unless you brought the with you). This brings into question is the wood you are burning really dry? Sure it may burn but the drier the wood the less water vapor you are moving up the stack and trying to disperse. Along with this water vapor is potentially creosote. You signature is truncated so I cant see where you are located but since you started burning in relatively cold weather less than optimum wood can hide because of the temperature difference between indoors and outdoors.

If you plan to burn wood its worth getting a moisture meter and learn how to use it to check what you are buying. I think the consensus on Hearth is if you buy wood its not full seasoned, sure there may be exceptions but if in doubt its wet.
Thanks, good points. Thankfully, there was a small amount of seasoned oak left on the property. Since that wasn't going to last long, I bought a ton of Liberty Bricks - compressed kiln dried hardwood sawdust and wood chip "bricks". I've been burning those along with small supplemental pieces of cedar that died many years ago. All wood I'm burning also has had a chance to dry for at least 24 hours near the stove to remove any moisture accumulated while covered outdoors. So most of the moisture going up the flu is probably from humidity in the indoor air supply. The night in question where I saw several ounce of water dripping from the ash clean out may have also been due to water that had made its way into the brickwork the previous day and frozen - only to remelt and leak down the flue when warmed by the high stove temps that night.

At this point, it's looking like the issue was a cap/cover that was relatively small, hence close to denser exiting flue gases that might be a little on the cool side. Maybe a SS liner would increase temps a little along with draft, I don't know. I wouldn't be able to insulate the liner though since the flue dimensions are 6.5 x 11. I figure I'll just monitor the situation at this point now that the crown cracks are sealed and the cap has been removed. I need to get a larger cap though to keep cold metal further away from exiting flue gases.

I also think I'll be switching to a pellet stove. With Covid and teleworking, I've been able to tend to the fire enough to keep the heat pumps from running very much, but it's taking a lot more time than I care to spend. This stove has a manual air control, so every time I load it up, I've got to make sure I go back within 20 minutes after it's come up to temperature to cut the air supply and let the stove maintain a safe temperature and run by burning volatile gases off the secondary air supply. Then I need to go down around three hours later to open the air supply to maintain heat output. :(

I'll have to fix my sig, I'm in VA, around 60 miles south of D.C.