Mystery–case of the dripping chimney
We recently installed a Heatilator brand fireplace insert and ran their version of Metalbestos chimney up through the roof. Guys installing said it was much flimsier than Metalbestos brand, but same idea. Location is Maine coast.
The fireplace is installed on the first floor of a 1-1/2 story cape-style house and exits just below the ridge. The chimney bends more or less perpendicular to the roof just before it exits to keep it away from the ridge beam. It runs in a chase through the cathedral-ceilinged 2nd floor, but is exposed metal outside.
The house is newly construction with closed-cell foam insulation. Humidity is a bit on the high side but not excessive, +-40%.
Here’s the problem: There is a persistant drip. Last week I cut open an access hole just above the second floor, expecting to see some water damage. Bone dry. I asked the owner to keep an eye on it and record what was going on when he heard the drip. Here’s his email to me this morning:
OK.<!—-><!—-><!—->
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6:45am Sunday. <!—-><!—->
Overcast; 16 degrees with desultory, light snow and no wind whatsoever. Inside it’s 62 degrees; humidity c. 40%. <!—-><!—->
Mechanicals quiet or phasing in and out with no discernable effect. <!—-><!—->
Exterior check shows no ice or snow anywhere near chimney.<!—-> <!—->
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7:15am. Plink!…<!—-><!—->
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Two narrow rivulets run down east side of chimney; they’re visible right up to offset elbow under roof… don’t know where it begins above that. I felt all around chimney and found no other moisture lines.<!—-><!—->
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An occasional drop will fall from offset, but most gather and drip from junction seam just above the wall opening you made. Water doesn’t pool on fire stop; it hits then continues down chimney side and disappears. It must follow pipe to offset over fire box – probably dripping to box itself and floor beneath. I haven’t yet found evidence of moisture anywhere, including basement ceiling beneath fireplace.<!—-><!—->
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Dripping lasted 60-90 minutes, so there is a fair amount of water involved (I’ll guess a pint). Random, widely spaced drops continue even now (11:30); perhaps 7-9/hour.<!—-><!—->
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It’s possible that water in chimney area has always been intruding, but went unnoticed until – maybe – its course changed: allowing it to free-fall and create the noise that alerted us.<!—-> <!—->
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None of this explains its source, however. (Is condensation within chimney possible? An ice cake reservoir?) The phenomenon has occurred under all sorts of atmospheric and environmental conditions. There seems to be no relationship between the dripping and precipitation, external temperature, sun, lighting a fire, snow accumulation or anything else we’ve observed. <!—-><!—->
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I’ve concluded this must result from the hydrogen fuel-cell residue of UFO; I believe they’re off-loading on our roof.
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(He’s got a good sense of humor)–
From inside the chase I could see a couple tiny points of daylight. Could be condensation but the chase was sealed up pretty well; I don’t know how much warm moist air could get in there. What is going on here?
Replies
You have condensation in the triple wall pipe layers. Normally this runs straight back down into the seat and appliance unit and no body ever notices.
But the elbow creates a place where gravity diverts its run down and some of it comes into the space.
If he could borrow a small antigravity device from his UFO visitors or trade them for a picture of strange lifeforms like say, Madonna kissing Britanny, he could keep that moisture moving up and out. 'Course if the aliens like that sort of thing - for their archives and research of course, he might find more heavy breathing around the chimney adding to his moisture problems.
Otherwise, keeping the flue warm by burning a fire in the appliance should take care of it.
Since that won;t always happen, some thermo foil duct tape might seal the joint better.
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Also, would help if the firebox is sealed off. Closing the flue damper will help some but not a lot. aS long as warm wet house air can be pulled up to where it is cold at the top of the stack, frost will be forming there.Not a solution, but as a way of confirming that I am right, tape off plastic over the fireplace front for a few days to prevent that exfiltration.
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LOL Aliens....
I wish I'd worked with metal insulated fluepipe before.... Is it usually installed with overlaps directing drips back inside the pipe, as with plain old stove pipe?
I'll try plastic over the firebox.
One detail I didn't like was the the chase is not insulated. It's a hot roof, and originally the spray foam guys sprayed tight to the chimney. Then they had to come back and scrape out at least 2" around the pipe. From my point of view, with a flashlight 15' below, it looked more like 3-5" around the chimney was insulation free.
I don't know how much an uninsulated chase would add to the condensation problem since it is relatively airtight. At least it was before I cut a 2' x 3' access hole into it.
With those conditions, every bit of moisture in the house is venting up into that rooftop chase to condense AROUND the pipe also. I would say to make up a sheet metal firestop spacer at the plane were the insulation is and use a 2000° GE caulk or other firecaulk to seal it to the metal chimney to prevent drafting up there. That will help a lot.
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That sounds like a good plan. Yesterday I confirmed with the foreman that each pipe is set to drain into the pipe below, but that there are drain holes or slots of some sort at each connection. Because of our odd situation of two joints just under the roof, creating an offset, any condensation that forms inside the pipe can drain out at that spot.
Attached is a drawing that shows what I think you are suggesting. Putting the new firestop any higher would be difficult due to access. Because condensation will still occur, I wonder about running a condensate line down from the upper firestop? I also wonder about the pros and cons of insulating the chase. Is it better to keep it warm or cold?
we're getting into opinion now, but I'd have the fs spacer at bottom of rafters and I prefer insulatring a chase, but theoretically might cause more condensation drip. I'd not use condensate drain, but woulds sewal joints in the pipe
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I like the idea of firestop at the underside of the rafters, I just don't think we can reach that high. I'll check on it though. Just regular foil tape for the joints?
Are you saying that indoor air is convecting up the inside of the chimney and condensing on the inside of the pipe? Or that interior air is condensing on the outside of the pipe up within the rafter bay?
I had a conversation with Metalbestos yesterday re a leak I have (had, hopefully) on a woodstove chimney. We talked about their universal sloped ceiling trim, which allow a huge air gap around the pipe as it exits the ceiling. I told her that if I installed that piece I would be concerned not just about air leaking but about humid indoor air condensing on the inside of the roof jack and dripping back down into the ceiling. She told me that it would be permissible to custom-make a ceiling trim (16ga stainless) that was tight to the pipe (Class A) and use high temp silicone between the trim and pipe.
There's usually some sort of problem when you go poking a hole in a perfectly good roof.
In mike's case, I suspect both. It is a fireplace iunit which is always leakier than an airtight wood stove. But also, his is a triple wall pipe and yours is a mineral pack double wall which will see slightly less of this on the inside.You can use what you have if it fits within 1/8" of the pipe. Put it up to ceiling, and run masking tape around just a hair below it, then pull it down again and run a generous bead of high tempo aulk there. Now push the trim up into the caulk bead to set it there and fasten in place.
Maybe another fine bead of caulk on the outside tooled off. Then remove the tape
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Update: We did everything suggested. Plastic over the face of the firebox showed little to no air movement. Chimney was OK for two or three days. Then the owner saw drips. Captured almost a cup of water he said. Clear, dry, and moderate temps outside the whole time.
We're thinking about asking the stove company to tear down the chimney, re-do it, and have them own it. Don't know what else to do.
Does the venting follow manufacturer guidelines?
Many have a limit to the number, length and degrees of offset bends.
Is the chase insulated?
Condensation could be forming at the roof flashing interface. Maybe stuffing some rock wool insulation up there will help- but that is just a guess.
I would make absolutely sure that the venting follows manufacturers specs first.
groan
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