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White Powder around Gas Vent

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What is this powdery substance, that is coming out of these elbows of a mid-efficiency furnace? There is none on top of the furnace or water heater (located right by the furnace) . Only on these elbows.

Any ideas?

Frank

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Looks like anhydrous salts left behind by evaporating water. Moisture is likely entering around the storm collar atop the roof.

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It's from the water vapor in the combustion gas that is condensing inside the vent. Happens in cold weather and indicates improper venting details and/or burner adjustment. It's very common in our area. If you look in the attic you will not see these deposits around the storm collar or flashing. They originate from the vent joints and seams.

Spend some time explaining what's going on to the client because chances are high that the HVAC contractor is going to blame the condition on the roof details.

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I see this on just about every furnace I inspect. So . . . does that mean that every furnace vent configuration is bad? Dunno.

Certainly makes my head hurt and opens up the complex world of what constitutes proper venting.

I'd really love to hear from a true expert on why this is so frequent - at least in our area.

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Every now and then I see something I've seen hundreds of time before and decided to figure it out. This powder is one example. I began to study the codes that relate to venting. Now, when I see the condition it's easy to document the conditions that contribute to the occurrence. Simply telling the client to have a HVAC or plumbing contractor to correct it is of little help. They are the same guys who screwed it up.

So, does anyone recommend CO testing once the corrections have been made?

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Originally posted by Bain

Looks like anhydrous salts left behind by evaporating water. Moisture is likely entering around the storm collar atop the roof.

Is this salt the correct answer?

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Hi,

I suppose a chemist might call it some kind of salt but it's what you get when you mix nitric acid with the zink in the galvanizing used to coat the steel. The byproducts of that burning gas are mostly water, nitrogen, sulphur and various other crud - basically nitric acid.

Ever see the stuff that builds up on the zinc battery posts of a car? Same stuff.

How much you get depends on how well the furnace is venting. If it vents well, you'll get almost none; if it's not venting well, you'll get a lot because there's more condensation occurring inside the vent and you've got more acid there. That looks like a single-walled vent. Is it? If so, is it located in an unheated area or inside. If it's located inside in conditioned airspace and you're getting that much condensate residue you've definitely got some venting issues.

ONE TEAM - ONE FIGHT!!!

Mike

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Wow, thanks for the fast responses, especially over a holiday weekend.

This is the first time I've seen this residue. This is a "utility room" which is outside of the house and drywalled but most likely uninsulated. Very rare for this area, so maybe cold weather in this room might be attributed to it.

It is 4 inch single wall right there, and connected into a 5 inch B-Vent which connects the WH vent. There is some rust on the B-Vent right after the connection of the WH vent.

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Frank;

What you’re seeing is the result of condensation (improper venting). There are many variables to the possible cause, let’s look at one scenario:

A newer, fan assisted furnace or boiler has been installed. This vent, along with an existing atmospheric draft water heater, are still connected to a masonry chimney. The location of the chimney is also very important. If it’s an exterior chimney, it absolutely should be re-lined, maybe even abandoned. If it’s an interior chimney, re-lining is probably required.

Due to the cooler furnace flue temperatures, it’s not a buoyant as the water heater exhaust, therefore, the water heater exhaust will control.

The water heater connector may have to increase in size (it cannot increase more than 2 sizes) to cool down quicker AND the furnace connector may have to be replaced with a B vent.

There’s just too, too many variables for a home inspector to think he can solve the problem. Here in NJ, almost every house I inspect has a venting issue.

I did one last week where the furnace and WH connected to an 8â€

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Good discussion, but I had a thought.

What's the worse case than can happen in a poorly vented system? Rust? Corrosion? Have to replace it sooner than later?

Other than the extreme of killing someone (which I doubt anyone could ever cite a case - I could be wrong, though), really. . . what could happen?

Not arguing; just thinking and playing devil's advocate.

If there's so many vents incorrectly installed, there's something in play here that things aren't conforming to the letter of the law of venting rules.

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Clarification by the way: when I say "killing someone", I'm not talking about the obvious disconnected vent or other egregious error.

I'm talking about long runs, slightly reverse-sloped sections, single vs. double wall, too many ells, etc.

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Originally posted by randynavarro

What's the worse case than can happen in a poorly vented system? Rust? Corrosion? Have to replace it sooner than later?

Other than the extreme of killing someone (which I doubt anyone could ever cite a case - I could be wrong, though), really. . . what could happen?

With weak buoyancy caused by a cold or deteriorated vent, exhaust gases can under certain conditions backdraft into the home, establish a draft into the home instead of up the flue, and kill people.

Read the article that I emailed to you separately.

ONE TEAM - ONE FIGHT!!!

Mike

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Three year old T.H.

1) In utility room

2) At furnace

3) HVAC bubba's receipt that says it's normal.

Around here it is normal - but wrong. Contractor says he'll warrant his evaluation. I told client that next winter when the leaks start again to call the contractor. As I said before, explain to client what's happening because sure as heck the contractor is not going to understand what's happening. Is this a safety issue? Yes. I would not ignore it.

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Originally posted by randynavarro

Good discussion, but I had a thought.

What's the worse case than can happen in a poorly vented system? Rust? Corrosion? Have to replace it sooner than later?

Other than the extreme of killing someone (which I doubt anyone could ever cite a case - I could be wrong, though), really. . . what could happen?

Not arguing; just thinking and playing devil's advocate.

If there's so many vents incorrectly installed, there's something in play here that things aren't conforming to the letter of the law of venting rules.

The condensation that forms in the vent system is a mixture of nitric and sulfuric acid. It will eat holes through the metal vent. If it drains back to the heat exchanger, it will eat holes in the exchanger. It will turn masonry chimneys to mush, like it did to this one:

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The mortar had turned to loose sand and the bricks crumbled from gentle pressure applied by my fingers.

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I have a white powder substance all over the bathroom vent fan. Has anyone seen this before? It seems to be similar to the substance in the gas vent talked about in this thread. I haven't turned the fan on in the 6 months I have lived here, but the substance is falling out of the vent onto the rug below.

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I have a white powder substance all over the bathroom vent fan. Has anyone seen this before? It seems to be similar to the substance in the gas vent talked about in this thread. I haven't turned the fan on in the 6 months I have lived here, but the substance is falling out of the vent onto the rug below.

Toilet paper dust?

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I have a white powder substance all over the bathroom vent fan. Has anyone seen this before? It seems to be similar to the substance in the gas vent talked about in this thread. I haven't turned the fan on in the 6 months I have lived here, but the substance is falling out of the vent onto the rug below.

Where does the exhaust vent lead to? Normally it would go directly thru the roof and discharge outside.

Marc

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This white residue is nothing uncommon or out of the ordinary whatsoever. It does NOT indicate IMPROPER VENTING at all PERIOD. As other members have stated, it is what is left behind after the condensate has evaporated. Used to see it form in new furnaces back in the 1980s. Any appliance that uses a draft inducer fan like the one in the photo above, will have an afue of 80% and does not have a draft hood to introduce dilution into the venting system. This means the flue gasses will contain a higher concentration of water vapor which tend to condense inside the vent until the vent gets up to temperature. In short, any appliance which uses an inducer fan will eventually develop this white powder inside the vent connector and will show up around the fittings just like in the photo above. Older furnaces with standing pilot used a draft hood to assist venting, this introduced an equal volume of air to flue gas thus less humidity going through the vent connector, we would still see a bit of this white residue over time. So..... this residue is NORMAL for the type of heater in question. I would recommend replacing the single wall vent connector with b vent though, as the original vent is likely to corrode through eventually as the condensate and residue is highly corrosive.

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I have often thought part of the cause is where the galvanized steel single wall pipe is connected to the aluminum inner pipe of the B-vent.  A chemical reaction if you will.  I have had this problem on water heaters and changed the single wall pipe to aluminum and the problem went away.  I haven't been brave enough to try the thin aluminum pipe on a furnace yet so I still fight the "salt" on those and it does, eventually rot through the pipe.  Before that happens, I usually have to take the vent apart and dump all of that stuff out of the horizontal pipe and vacuum it out of the draft inducer part of the furnace.  I've had it almost completely block the opening in the furnace.  This usually manifests itself with a code about the draft inducer or inadequate vent flow

Edited by Brad Burke

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Hi,

The white powdery substance you are seeing is mineral salts caused by a chemical reaction between the acids in the exhaust gases and the zinc contained in the galvanized pipe used for the single-walled vent connector. To make that relatable, think battery post corrosion in your engine compartment.

That length of single-walled pipe from the collar allows those exhaust gases to cool, get heavier and slow down way too early. When that happens, the gas cools to dewpoint and condenses on the inner walls of the vent somewhere above the connection between the single-walled connector and double-walled vent pipes. Wherever the acids in that condensation comes into contact with galvanized pipe, that reaction occurs. Then, when all of that excess condensation drains back to the joint between the vent and connector and leaks out of the joint, the water evaporates and leaves the salts behind.

That's why gas exhaust vents and connectors should be double-walled all the way from the collar of the appliance to terminus. The fuel-gas code used to require only B vents to the collar in attics and concealed spaces and anywhere considered to be cold, but it seemed like the majority of HVAC installers ignored that rule anyway if the furnace or water heater was installed where it's readily viewable in unheated areas like garages. Somewhere back around the 2003 - 2005 timeframe that requirement disappeared from the code and they allowed installation of single-walled connectors in areas other than the attic or concealed spaces as long as the area wasn't any colder than 5-deg. F. in winter. This despite the fact that a lot of condensation can occur even in vents that are luke-warm to the touch in such areas like garages. I'll bet you a box of donuts that most HVAC installers still couldn't care less and are still using as much single-walled material as they can, in order to squeeze as much profit as possible out of every job.

Too much salt indicates an issue with the exhaust vent system. A little bit of salts isn't really too much of a concern at a single-walled to double-walled connection, if there isn't any issue at the vent terminus (Excessive rusting, cracks, holes due to condensation and acidic vapor lingering around the vent once it leaves the pipe), but a lot of salt is telling you there's an issue with exhaust gases cooling too rapidly - especially if there is a lot of salt sifting back down from the vent connector at the collar and ending up in/on the furnace. Cause could be the single-walled connector from the collar, if it's too long and it's in an unheated (cold) area. The fix could be to replace the single-walled section with a double-walled section, but a lot of the time that doesn't necessarily stop it if the installer didn't properly calculate the vent size requirements using the vent sizing tables in the code or if he/she didn't pay attention to the rule about limits on number and degree of bends in the vent between collar and terminus - bends that violate the 'no bends greater than 45-deg but one bend of no more than 60-deg. rule, slow those gases. The more bends, the slower the gas moves - even if the furnace has an exhaust gas inducer. Around here those numbskulls will use two to four 90-deg. bends to detour around furnace filter access doors instead of using 45-deg. vents to go around. Another thing to look at is whether or not the appliance has enough makeup air. If they've installed it in confined space with insufficient makeup air, or in a laundry room where there's a clothes dryer and no makeup air openings to the outside, those gases will stall - even with an inducer integral to the appliance. There have been thousands of times I've stepped out of my vehicle, glanced up at a rooftop, saw a heavily-rusted vent terminus and knew right away that as soon as I checked out the furnace I was going to find issues with stalling exhaust gases. Sometimes, the issue was so bad that condensation would literally leak out of the vent connector connection at the collar and drip onto components in the controller bay - eating holes through the floor of the bay and sometimes causing electrical connections to severely corrode. So, when someone tells you, like the gent above, that the salts mean nothing, take it with a grain of salt and thoroughly inspect that vent installation anyway.

It's good that you're asking questions about this. I've always said that inspectors need, more than anything else, to understand basic building sciences, and need to look at the whole picture, not just the tick marks their report format asks them to check, that are taught by a lot of so-called inspection schools, in order to really do this job well and be able to make the client fully comprehend the implications of issues found.

ONE TEAM - ONE FIGHT!!!

Mike 

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On 3/24/2020 at 3:58 PM, Brad Burke said:

I have often thought part of the cause is where the galvanized steel single wall pipe is connected to the aluminum inner pipe of the B-vent.  A chemical reaction if you will.  I have had this problem on water heaters and changed the single wall pipe to aluminum and the problem went away.  I haven't been brave enough to try the thin aluminum pipe on a furnace yet so I still fight the "salt" on those and it does, eventually rot through the pipe.  Before that happens, I usually have to take the vent apart and dump all of that stuff out of the horizontal pipe and vacuum it out of the draft inducer part of the furnace.  I've had it almost completely block the opening in the furnace.  This usually manifests itself with a code about the draft inducer or inadequate vent flow

 

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