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White Powder on Furnace Exhaust After New Roof


brianross
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Hello. Looking for help on a quick question. Had my roof replaced this summer and have not run the furnace since. As it is fall, I went down to fire up the furnace but noticed a small puddle of water surrounded by white powder on the floor where the furnace vent pipe comes down from the roof. The vent goes into an elbow before it elbows again into the furnace (travelling from roof down to furnace). The section before the first elbow is nearly 80% covered with a white powdery substance that is distributed in a manner that it looks like its left over from water running down it and evaporating. The puddle is directly below this first elbow. I have lived in this house for >10 years and never seen this issue in summer or winter.

Is this definitely a case of poor sealant around the storm collar at the roof allowing water to run in, which is reacting with the galvanized coating on the flue? If so, should I have the exhaust flue inspected for damage/holes before I run the furnace? The house is 17 years old. My concern is that my roofing company may just come out and improve the seal at the storm collar but damage may already be done.

By the way, the controller is not allowing the furnace to turn on. This may be relevant but it also may not be. It could be a problem in the furnace somewhere else but I checked it out and couldn't find anything obvious. I think the furnace controller has a sensor to make sure it is getting an appropriate draft up the flue, but am not sure. Can provide more info if it helps. Right now I have turned everything off and have called an HVAC guy and my roofing company. I just want to make sure all bases are covered and I am being fair in terms of what I am going to request is done.

Worried in MN...

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More info:

Furnace is Amana Air Command 80

Controller is White Rodgers Emerson Electric Model50E47-170

Although I said nothing obvious seemed wrong in my original post, all of this white reside inside the furnace, particularly on several of the electrical connection points, may be the reason the furnace is not firing up. HVAC guy comes Friday but I hate waiting. ;-)

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Its not too cold yet, but the fleece advice is well taken. Thanks.

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Hi,

That's exhaust condensate. For some reason, the flue isn't drafting and the water that's produced by the ignition process is trapped and is draining back into the furnace and is corroding the heck out of everything. It has nothing to do with a storm collar.

ONE TEAM - ONE FIGHT!!!

Mike

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Hi Mike.

I'm new in here and have been reading your posts. Very good.

The funny thing about my issue is that the furnace has not run at all since I was down there in the spring after I shut it off and checked all the Air Conditioner connections. Its hard to see but I replaced all the putty that was sealing off the AC lines (dark grey circles on left side of first picture) at the time because the old stuff was all dried out. None of this residue was present at the time (~4 months ago). Am I missing something from your post when you refer to "ignition process"?

Thanks again!

Brian

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Hi,

It's got nothing to do with the air conditioner; it's condensate from furnace exhaust, a pilot light or the water heater. If the furnace hasn't been used, it has to be from the water heater that's sharing that vent.

When gas burns, the byproducts are mostly water, sulfur and nitrogen (acid). If that gas can't rise up out of the stack and dissipate, it condenses on the inside of the vent and then drains back to the furnace. The white residue is the mineral salts left over after the acid reacts with the zinc coating in the galvanized coating on the pipe.

Find out what's wrong with the flue and stop focusing on the AC lines because they have nothing to do with it.

ONE TEAM - ONE FIGHT!!!

Mike

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Mike.

The hot water heater does share flue, which means my original thought that water was coming in from a bad sealant around the storm collar on the roof is most likely wrong. What I tell the guys I work with is 2 degrees in engineering means I knew how to read books and take tests, not actually fix problems! I was too focused on the furnace and knew it wasn't running so I couldn't figure out how anything could be condensing. I missed the hot water heater. The point of me mentioning the A/C was just to let you know that there was NO residue in the spring (it was the last time I was down there) so this buildup has developed over the past ~4months. Sorry if this confused the situation.

It turns out that the guy I called to come on Friday doesn't know a lot about this and was just going to look at the furnace. Can anyone here recommend a good HVAC guy in Minneapolis?

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Originally posted by brianross

Hello. Looking for help on a quick question. Had my roof replaced this summer and have not run the furnace since. As it is fall, I went down to fire up the furnace but noticed a small puddle of water surrounded by white powder on the floor where the furnace vent pipe comes down from the roof. The vent goes into an elbow before it elbows again into the furnace (travelling from roof down to furnace). The section before the first elbow is nearly 80% covered with a white powdery substance that is distributed in a manner that it looks like its left over from water running down it and evaporating. The puddle is directly below this first elbow. I have lived in this house for >10 years and never seen this issue in summer or winter.

Is this definitely a case of poor sealant around the storm collar at the roof allowing water to run in, which is reacting with the galvanized coating on the flue? If so, should I have the exhaust flue inspected for damage/holes before I run the furnace? The house is 17 years old. My concern is that my roofing company may just come out and improve the seal at the storm collar but damage may already be done.

By the way, the controller is not allowing the furnace to turn on. This may be relevant but it also may not be. It could be a problem in the furnace somewhere else but I checked it out and couldn't find anything obvious. I think the furnace controller has a sensor to make sure it is getting an appropriate draft up the flue, but am not sure. Can provide more info if it helps. Right now I have turned everything off and have called an HVAC guy and my roofing company. I just want to make sure all bases are covered and I am being fair in terms of what I am going to request is done.

Worried in MN...

First, thanks for the very clear set of pictures. It's nice to have an establishing shot followed by clear detail shots.

There are several problems with that installation. First, although it's permissible to use single-wall vent connectors in a heated space, it's always better to use double-wall connectors. They provide some insulation, heat up faster and limit the amount of condensation that takes place within the pipes. (All of the exhaust pipes in your pictures are vent connectors. The actual vent is only barely visible at the very top of the second picture.)

Next, the vent connectors in your picture are installed backwards. At each connection point, the upper connector is supposed to sleeve inside the lower connector so that any condensate remains inside the pipe instead of leaking out at the joint.

Next, I don't know how large (how many BTUs) the furnace and water heater are nor do I know how tall the vent is, but I suspect that your vent connectors as well as your vent might be too large in diameter. An overly large vent connector and vent will allow more condensation and slower exhaust than a properly sized set.

If you have a blocked flue, which is certainly possible, the furnace will attempt to start but the sequence will abort before the gas flows or the flame ignites. You'll hear the draft inducer fan start but it will stop again when the pressure sensor (the disk-shaped round thing in the third picture) tells the system that there isn't negative pressure in the inducer housing. It's also possible that this sensor might have been damaged by corrosion.

Last, this isn't a new installation. If the leaks and corrosion really have appeared only within the last 4 months, something else has changed to cause these problems. As an engineer, you're probably familiar with the maxim, "There's no change without change." Besides the roof, what's changed?

- Jim Katen, Oregon

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Originally posted by Jim Katen

[Next, the vent connectors in your picture are installed backwards. At each connection point, the upper connector is supposed to sleeve inside the lower connector so that any condensate remains inside the pipe instead of leaking out at the joint.

- Jim Katen, Oregon

Are you certain, Jim? I was taught just the opposite to prevent flue gases from discharging into--in this instance--the basement.

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"Vent" pipe should be same diameter as the furnace exhaust vent. It isn't in this photo. That can lead to poor or marginal venting. As for the roofing job leading to this, perhaps you had your attic ventilation improved and that has lead to a colder attic and that could lead to colder vent in the attic and you hit the tipping-point to where the condensate shows up?

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It wouldn't hurt if you can go on roof and look down the inside of the vent with a light. Possibly see where the corrosion begins or if something is blocking the flue. I've seen things inside vents on occasion. Finding where that corrosion starts inside the flue venting is what I'd try to find first.It's usually not too hard to disconnect joints and look inside if not tape sealed.

Jim C.

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I made that mistake many years ago, Chad, when I first started in this bidness. I looked up the flue of a manufactured fireplace and thought, "Whoa, this is completely backwards."

I got my ass handed to me, deservedly.

I learned, like you said, that solid fuel pipes are installed that way so the creosote won't trickle down the outside of the flue pipe and settle on top of the firebox.

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Originally posted by Bain

Originally posted by Jim Katen

[Next, the vent connectors in your picture are installed backwards. At each connection point, the upper connector is supposed to sleeve inside the lower connector so that any condensate remains inside the pipe instead of leaking out at the joint.

- Jim Katen, Oregon

Are you certain, Jim? I was taught just the opposite to prevent flue gases from discharging into--in this instance--the basement.

So you believe that the flue gases are more likely to leak when the connection faces one direction rather than another? Do the flue gases care?

If there's negative pressure inside the vent (as there should be whenever a category I or II appliance is running), it doesn't matter which way a hole is pointing. Air will *enter* the flue, not exit it. If, on the other hand, there's positive pressure inside the vent (a bad thing), then the flue gases will escape no matter what direction the hole faces.

In contrast, the direction of the hole makes a huge difference when it comes to containing condensate since the liquid will be contained by a male-on-top connection.

If you don't believe me, look at a section of B-vent. The upper inner liner alway sleeves into the lower inner liner. This isn't immediatly apparent from the outside because the outer portion of the B-vent has the opposite configuration.

- Jim Katen, Oregon

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Originally posted by Bain

........... solid fuel pipes are installed that way so the creosote won't trickle down the outside of the flue pipe and settle on top of the firebox.

I thought it was so the creosote wouldn't go to flame when it came in contact with the room air.

I'm also a *top inserts to bottom* proponent regarding flue connectors.

Come to think of it, I kinda like top inserting into bottom with just about everything......

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Since the roof was changed, start there. I have seen several vent installations screwed up by roofers. We rarely have masonry chimneys here, B-vent typically extends through the roof and roofers disconnect vent pipes and rarely make sure everything is reconnected in the attic when the roof jack is re-installed.

My bet is blockage in the vent.

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So I spend most of my day at my job thinking about my next move. I decide I need to get on the roof to look down the vent. Hopefully my mother in law who disappeared a few weeks ago is down there and I solve two problems at the same time. Sharon if you're reading this, I kid. I kid.

The need to get at least one thing done at work today so I get home late and then I can't pass up the nice taco dinner waiting for me. The fact that its freezing rain may have played into me changing plans and deciding to not get on the roof tonight. But the rain allows me to dig further.

The attic has a few clues but I'm not sure if its a roof leak yet or condensation (or both). There appears to be signs of water being absorbed into the insulation around the point where the flue enters the attic (first pic below). However, there is evidence of the galvanic reaction present at every joint as you travel up the flue towards the roof. There is more white powder at the base (where the slow moving hot exhaust first hits a cold pipe). The attic is quite cold so at least one thing in the house works right. The hole in the roof looks bigger than I expected to see and it appears felt is folded over the hole and runs up the pipe. Its raining and I can see small streams of water running down the outside of the pipe. The furnace still doesn't turn on and the hot water heater hasn't run for a few hours so I'm pretty sure that rain water is coming in. I decide that its possible I have 2 problems at the same time (leak at the roof joint AND condensation due to low exhaust flow) but cannot tell until I get up on the roof and look down the vent and/or find an HVAC guy that is willing to take a look at this.

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Still puzzled by why the furnace won't turn on, I decide to have some fun. No luck today trying to find an HVAC guy who can do more than a "fall tuneup" on my furnace... I removed the draft inducer fan praying that a gallon of water wasn't going to come running out of it. There is quite a bit of scale but no water. I clean it all out and hook everything back up. Furnace still doesn't start. I give a couple of snaps to the pressure sensor with my index finger and thumb and the furnace fires up.

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OK. One part is solved. Guess I better put a Tridelta FS6111-504 pressure switch on my christmas list. I turn off the furnace and disassemble some of the exhaust vents so I can look inside for pieces of squirrel. There is some evidence of galvanic reaction but it isn't even half of what I see on the outside of the flue (tried but couldn't get a decent picture - sorry). I reassemble and turn on the furnace to heat the house a bit.

I don't like what I see or hear. There is a lot of hot air coming out of the slots in the top of the furnace. I am also getting hot air forced out the vent at the top of the water heater. The top surface of the water heater has a puddle of water forming. I run outside with a flashlight and the exhaust is coming out of the flue at the top of the house but it doesn't look normal to me. It looks like its just barely leaking out. I let it run for a few minutes and decide I'm not in the mood to deal with a flaming squirrel family crashing down the flue into my basement. I shut everything down and decide I could be paranoid and everything is OK but I really want to make sure nothing is occluding the flue.

I duct tape the dogs to my legs so I can keep warm as I decide one more cold night isn't all that bad and I head to bed. I really need to get a look into the flue from on top the roof tomorrow. I totally shocked that my roofing guy didn't call me right back today. I am thinking I would like to go up there with him so someone can call for help after I fall. I am also thinking I want someone to actually measure the flow rate of my draft and I should get a new pressure switch - once they start to stick its only a matter of time before they stick again. My only real concern is all the galvanic reaction on the flue in the attic. I don't want to have to replace all of it...

Thanks for all the help and feedback. This is actually quite fun. Would be nice to solve this tomorrow.

B

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Originally posted by brianross

The attic has a few clues but I'm not sure if its a roof leak yet or condensation (or both). There appears to be signs of water being absorbed into the insulation around the point where the flue enters the attic (first pic below).

Ok. You have your answer. The roofer screwed up the flashing around the B-vent where it passes through the roof. Make sure that he fixes it properly -- that means no goop.

However, there is evidence of the galvanic reaction present at every joint as you travel up the flue towards the roof. There is more white powder at the base (where the slow moving hot exhaust first hits a cold pipe). The attic is quite cold so at least one thing in the house works right.

That's a double-wall pipe and the inner wall of each section, which contains the flue gases, sleeves into the inner wall of the section below. There should be no leakage of flue gases or condensate there. What you're seeing is rainwater that's somehow getting into the annular space between the inner and outer sections of the B-vent. Since the outer sections come together with the female fitting on top, the water leaks out at every joint.

The hole in the roof looks bigger than I expected to see and it appears felt is folded over the hole and runs up the pipe.

The size of the hole looks ok to me. There's supposed to be 1" clearance between the B-vent and any combustible material. That includes the plywood roof deck and -- ARE YOU PAYING ATTENTION MR. ROOFER? -- the roofing felt. It also includes the cellulose attic insulation that used to be fire retardant before it got wet.

Its raining and I can see small streams of water running down the outside of the pipe. The furnace still doesn't turn on and the hot water heater hasn't run for a few hours so I'm pretty sure that rain water is coming in. I decide that its possible I have 2 problems at the same time (leak at the roof joint AND condensation due to low exhaust flow) but cannot tell until I get up on the roof and look down the vent and/or find an HVAC guy that is willing to take a look at this.

Start with the roofer.

Still puzzled by why the furnace won't turn on, I decide to have some fun. No luck today trying to find an HVAC guy who can do more than a "fall tuneup" on my furnace... I removed the draft inducer fan praying that a gallon of water wasn't going to come running out of it. There is quite a bit of scale but no water. I clean it all out and hook everything back up. Furnace still doesn't start. I give a couple of snaps to the pressure sensor with my index finger and thumb and the furnace fires up.

OK. One part is solved. Guess I better put a Tridelta FS6111-504 pressure switch on my christmas list. I turn off the furnace and disassemble some of the exhaust vents so I can look inside for pieces of squirrel. There is some evidence of galvanic reaction but it isn't even half of what I see on the outside of the flue (tried but couldn't get a decent picture - sorry). I reassemble and turn on the furnace to heat the house a bit.

I don't like what I see or hear. There is a lot of hot air coming out of the slots in the top of the furnace. I am also getting hot air forced out the vent at the top of the water heater. The top surface of the water heater has a puddle of water forming. I run outside with a flashlight and the exhaust is coming out of the flue at the top of the house but it doesn't look normal to me. It looks like its just barely leaking out. I let it run for a few minutes and decide I'm not in the mood to deal with a flaming squirrel family crashing down the flue into my basement. I shut everything down and decide I could be paranoid and everything is OK but I really want to make sure nothing is occluding the flue.

Good idea. Still, you might try firing it up and leaving it on for 5 minutes. It's not unusual for a system like this to backdraft for a minute or two untill the plug of cold air in the vent is exhausted. This is especially true if the vent is oversized.

Oh, and carbon monoxide alarms are cheap. Get some and install them.

I duct tape the dogs to my legs so I can keep warm as I decide one more cold night isn't all that bad and I head to bed. I really need to get a look into the flue from on top the roof tomorrow. I totally shocked that my roofing guy didn't call me right back today.

Ha! What a nice guy you are.

I am thinking I would like to go up there with him so someone can call for help after I fall. I am also thinking I want someone to actually measure the flow rate of my draft and I should get a new pressure switch - once they start to stick its only a matter of time before they stick again. My only real concern is all the galvanic reaction on the flue in the attic. I don't want to have to replace all of it...

It's just ugly. It hasn't reached the damaged stage yet. I'll bet that the flashing is messed up and that the rain cap isn't properly seated. There might also be some debris in the flue, but I think that it's more likely that the slow draft is just the natural way that this vent draws.

- Jim Katen, Oregon

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Jim

Thanks for the reply. I can't tell all of you how cool this journal page is. I've been reading a lot of the other forums. I do have a carbon monoxide detector plugged in right under the furnace. It didn't leave 0 but I didn't leave the furnace on very long.

Again - many thanks.

B

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Jim,

Today I found myself thinking about what you wrote vis a vis the male/female orientation of vent connectors. Everything you said made perfect sense, yet I've never seen the things installed the way you described.

Which I don't say to be contentious.

Maybe I've only seen them installed the way I'm familiar with because the female ends nestle nicely onto draft hoods. I dunno.

I spent a few minutes Googling different permutations of applicable words about flue connectors, but came up empty. Does the fuel gas code--which I don't have a copy of--specify a particular orientation? Or is it a best practice notion?

Your pal,

John

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Originally posted by Bain

Jim,

Today I found myself thinking about what you wrote vis a vis the male/female orientation of vent connectors. Everything you said made perfect sense, yet I've never seen the things installed the way you described.

Which I don't say to be contentious.

Oh yeah, right, sure, uh-huh!

Maybe I've only seen them installed the way I'm familiar with because the female ends nestle nicely onto draft hoods. I dunno.

That's a good observation. The draft hood outlet is sized to fit into the inside diameter of the vent connector. That happens to be the same size as the female end of a single wall vent pipe. To do it my way, you need to start with a section of pipe that has no male crimp -- essentially a fitting with two female ends, a coupler.

I spent a few minutes Googling different permutations of applicable words about flue connectors, but came up empty. Does the fuel gas code--which I don't have a copy of--specify a particular orientation? Or is it a best practice notion?

Your pal,

John

It isn't in any code and, as far as I know, single wall pipe doesn't come with any instructions. It just seems obvious to me. If you do it with the male on the bottom, the joint will leak.

- Jim Katen, Oregon

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