Thread: White powder below flue

Today I found a white powder below the flue for a gas fired furnace. Any ideas what this might be? Would post the picture, but I'm having trouble resizing it. Any ideas on that as well?

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Originally Posted by John Thompson

Today I found a white powder below the flue for a gas fired furnace. Any ideas what this might be? Would post the picture, but I'm having trouble resizing it. Any ideas on that as well?

Jerry, Aaron and some others must have been partying over there

Originally Posted by John Thompson

Today I found a white powder below the flue for a gas fired furnace. Any ideas what this might be? Would post the picture, but I'm having trouble resizing it. Any ideas on that as well?

John,

There are utilities that you can download that will resize for you. I don't know which ones work well. Someone else will likely provide a link.

The white stuff is likely zinc oxide or aluminum oxide from the interior of the flue pipe. Is this an older furnace?

Gunnar-It's a 1983 GE furnace. The powder was piled up on top of the furnace, and running down the flue at joints along the run. Any idea how to report on it?

Originally Posted by John Thompson

Gunnar-It's a 1983 GE furnace. The powder was piled up on top of the furnace, and running down the flue at joints along the run. Any idea how to report on it?

Ohh... That sounds different. That is likely caused by moisture condensation on the interior of the flue pipe. The exhaust gases contain water vapor and this vapor is slightly acidic, which can react with the galvanized or aluminum material in the flue pipe. Another cause is when a metal flue pipe is inserted into an old transite (cement/asbestos) flue pipe. Both will end up in corroding out the flue pipe. Is this an older home (1950s) with a more current furnace? I'll see if I have any example photos.

In addition, most heating contractors around here will give between 20 and 25 years as life expectancy. A 1983 furnace is pretty old by those standards. I would note the condition and defer to a heating contractor.

As an aside, the National Association of Home Builders has a list of life expectancies for various appliances and components of homes. Someone posted this on the site a couple of years back.

Thanks Gunnar. It is a 1925 home, and the heating season here in Montana is looooong. Not sure if you're a member, by ASHI's magazine "ASHI Reporter" put an abridged version of the NAHB report in this month. Will definitely recommend, at least, servicing, and, at worst, budgeting for eventual replacement.

Originally Posted by John Thompson

Thanks Gunnar. It is a 1925 home, and the heating season here in Montana is looooong. Not sure if you're a member, by ASHI's magazine "ASHI Reporter" put an abridged version of the NAHB report in this month. Will definitely recommend, at least, servicing, and, at worst, budgeting for eventual replacement.

John,

Sorry, I was not able to find any photos of the deposits under a transite flue pipe. Did you see metal b-vent or transite in the attic.

Yes, Montana has a long heating season and it is likely that a furnace will not last as long there as it will in a milder climate like CA

Originally Posted by John Thompson

I'm having trouble resizing it.

Any ideas on that as well?

.

Google Bomes image resizer.

Several sites have a free download copy.
.

This one ( FastStone Image Viewer - Powerful and Intuitive Photo Viewer, Editor and Batch Converter ) is free and has additional excellent features - someone here told me about it and it is great.

Batch rename files.

Batch convert files.

Etc.

John, does the furnace have a humidifier? A lot of times you will get a lot of condensation from the humidifier which causes white stains and a little bit of powder.

A really 'techie' HVAC guy once told me that is a reaction between sulfur, which is inhernet to natural gas, and the zinc that the flue pipes are galvanized with. I've never researched it any further but was fairly satisfied with his conclusion.

I see it from time to time 'growing' around inducer motors and dropping out of the bottom of the flue pipe openings. It almost looks like the crusty stuff that grows on battery terminals when they're not cleaned, although not quite as severe.

Originally Posted by Matt Fellman

A really 'techie' HVAC guy once told me that is a reaction between sulfur, which is inhernet to natural gas, and the zinc that the flue pipes are galvanized with. I've never researched it any further but was fairly satisfied with his conclusion.

I see it from time to time 'growing' around inducer motors and dropping out of the bottom of the flue pipe openings. It almost looks like the crusty stuff that grows on battery terminals when they're not cleaned, although not quite as severe.

MF:

I recently read about a method of dezincifying galvanized steel scrap where the zinc metal is dissolved in an aqueous acidic bacterial liquid medium containing sulfur.

Water and sulphur dioxide are by products of the combustion of fossil fuels. So then, when you burn one of these fuels you are effectively dezincifying the vent of the appliance.

Originally Posted by Gunnar Alquist

Ohh... That sounds different. That is likely caused by moisture condensation on the interior of the flue pipe. The exhaust gases contain water vapor and this vapor is slightly acidic, which can react with the galvanized or aluminum material in the flue pipe.

You're dead on Gunnar.

It is usually accompanied by a furnace that is not firing at it's proper firing rate and could also be a sign of a venting problem or other issues.

The flue gas shouldn't be condensing in the flue.

If the flue enters a masonay flue ( and sometimes metal) it may need a liner or a draft hood adapter. A newer furnace with a inducer fan has less BTU's entering the flue and condensation can occur. OR it may be water from lack of a rain cap.

Originally Posted by A.D. Miller

MF... dissolved in an aqueous acidic bacterial liquid medium.

"Aqueous acidic bacterial liquid medium"??? Holy alkaline protozoa Batman, I don't even know what that means!

Originally Posted by Gunnar Alquist

" Holy alkaline protozoa Batman, I don't even know what that means!

A protozoa lives in the gut of a Termite. Acts like a Laxatives for the Termite
The Mama termite throws up and the baby Termite eats it and gets the protozoa so he can relax and have better bowel movements AKA loose stools
no more constipation

Best

Ron

Originally Posted by Stacey Van Houtan

If the flue enters a masonay flue ( and sometimes metal) it may need a liner or a draft hood adapter. A newer furnace with a inducer fan has less BTU's entering the flue and condensation can occur. OR it may be water from lack of a rain cap.

Actually more heat will enter the flue from an induced draft furnace, no draft hood to cause air mixing from dilution air.

The draft hood on atmospheric furnaces kept the flow of air up a flue constant which produced a drying effect and lowered the dewpoint of the flue gases.

With induced draft furnaces we took the draft hoods off and killed the flow of air up the flue.
When the inducer shuts down at the end of a heating cycle so does the flow of air up the flue.

Hey John,

Is this similar to what you saw?

Another Image

Best

Ron
Santa Rosa California Home Inspection - Exterminating & Thermal Imaging

Ok Bibler, the gloves are off now.

I'll one-up you. How about this?

Your Mama CALL:.....

Best

Ron

Pfft. Try this. Terminated within the return-air duct.

The BTU from the furnance entering the flue ARE less on a on a induced draft when compared to a gravity draft. A foraced draft even less. but this type of furnace would have a plastic vent. Usually two sources of water can cause the corrosion, condensation or rain water. (sometimes Plumbing leaks). Some condensation is normal at startup. A flue is checked with a micro-manometer after it is at a operating tempature (aprox 10 min.) At this point it should draw. If there is not a draw than there is a venting issue. It can have various reasons: Indueced draft into a masonary (usually exterior) chiminey (this is called a cold cap in some areas) , inducer fan isssues (craks or leaks), flue and/or connecter issues (bird nest, T-connector vs Y a water heater). Bottom Line- testing is beyond the visual home, But visible installation issue are within the scope of a HI , such as a induced drat into a masonary flue (wrong at some geographacial areas) , connecter issues and some inducer fan issues, these should be reporeted. And the usual comment when a HVAC issue is observed -Further evaluation needed and service by a HVAC contrator.

Originally Posted by Stacey Van Houtan

The BTU from the furnance entering the flue ARE less on a on a induced draft when compared to a gravity draft.

Curious how you've come to this conclusion Stacey.

I teach this to my new home inspector at our inspection school PITI. I know a inspector that bought a flue liner due to lack of this knowledge.

Old Gravity furnance 100K input. 80 K output 20 K up the flue
Induced draft furnace 100 k input 90 K output 10 K up the flue

While it is true that a garvity furnace had a constant chimney effect (unless the old thermal activated flue dampers were installed) this extra draft air was only of value for condensentation issues while the furnace was running. And that is why a draft hood with a barometric interlock switch is accepted as well as a flue liner when up grading to a induced draft furnance that is installed in a masonary chimney.

Can you share a make and model?

I'm guessing you are confusing terminology here:
Cat. I either draft hood equipped or fan assisted (draft induced) negative vent pressure up to 83% AFUE.
Cat IV mechanically vented positive vent pressure with AFUE>83%.

I'd like to hear your version of the reason for chimney liners before I comment Stacey. Also, no offense but could you run a little spell check? It's really tough to read and get your point.
TIA,
Bob

Originally Posted by Bob Harper

Cat. I either draft hood equipped or fan assisted (draft induced) negative vent pressure up to 83% AFUE.
Cat IV mechanically vented positive vent pressure with AFUE>83%.

Bob (or anyone),

I have always wondered; whatever happened to Cat II and Cat III?

No, really. I am curious.

Cat. II is atmospherically vented with a flue loss less than 17% (>83% AFUE). Since this would not provide enough heat to generate sufficient draft, this category has always been largely theoretical. Some would argue even 80% draft induced is too much of a problem, esp, in cold masonry chimneys.

Cat. III is positive flue gas pressure vented (mechanical vent) at less than 17% flue losses. The difference here is positive vent pressure. Usually found in the 78-83% AFUE range. Tankless water heaters and some older 'high efficiency' boilers come to mind. These are borderline condensing units. Often found with single walled galv. pipe 'vents' straight out the side of the house or up a chimney and gobs of red RTV silicone on the dripping joints. Must use listed venting (UL1738).

Cat. IV is a condensing furnace using mechanical venting with resultant positive flue gas pressure. Must use listed venting or Per Mfrs Instructions, which has largely been left veeeery loose as to which type of PVC, joint prep, etc. No testing required as with DWV pipe (guess they have their priorities straight).

HTH,
Bob

Good morning: What an amazing amount of info in this post from all of you.Most interesting.You are actually discussing 2 different issues when you bring up 'white powder' & 'white stains', though they both point to a venting issue.The issue of white powder where you actually find a build-up of it on the interior of a heat exchanger & inside vent connectors & even inside the B Vent if a B Vent is used, is exclusive to early model(usually pre-1990) gas furnaces with standing pilots.It is often that there is a problem with vent connector length/size, such as 90 degree elbow on the flue collar, too long or oversized vent connector, & other restrictions in the venting system; most often the furnace is vented into an unlined(brick only not even a clay tile liner) or clay-tile lined chimney.It has been attributed to carbon dioxide solids by some 'experts'.It can be found inside the heat exchanger forming even where there is no galvanized surface so its cause would be debatable.Suffice it to say that its exclusive to older gas furnaces/boilers or any such appliance with a standing pilot system, & that it should be replaced more due to their age & inefficiency than any other issue. As to the white stains you see at the joints of vent connectors or vents themselves that is a sign of condensing taking place inside the vent, & you are seeing the stains left when that condensate runs back down & leaks out the joints. If its an older heating system perhaps its an issue of too long a vent connector run including too many turns/dips/sags/etc.; if its a newer furnace it could be that you have (1)a furnace only or (2)water heater only connected by itself to a chimney with clay tile liner, & it could be a combination of too long a vent connector + outside chimney(meaning 3 sides exposed to out doors at end of building) + oversized chimney.I won't bore you & go on & on.The deposit you see at the joints and/or the whire powder inside heat exchanger/vent connectors or vent should be referred to a qualified gas contractor because either way it indicates a problem.if you are seeing stains either streaks indicating a leak from a joint, or rust colored stains at joint of PVC vents on condensing gas appliances then you are seeing leaking joints that should be repaired(preferrably by whomever was paid to initaially install them).Have a pleasant day.

Bob,

Thanks. I have been asked in the past about those two and had no answer.

Phillip,

I would like to introduce you to Mr. Spacebar and Mr. Enter Key. Knowing about, and using them judiciously will make it much easier to read your posts. Otherwise, I will not bother.

I'm sorry I was a not a english major.

Bottom line - PV=nRT This is the basis for all flue caculations. AND basicly liners and/or interlocked draft hoods are needed on some installations and at some areas of the US when the furnances is upgraded and it vents to a chimney clay liner or not.

Stacey please don't take this as busting on you but honestly I cannot fathom what you're talking about with regards to when listed liners are required and when they are not. You are mixing in things such as draft hoods when discussing liners. What I'm after is a simple explanation of what conditions do you perceive trigger the need for a listed liner. I'll make it easier:

Sample 1- draft hood equipped Cat I gas boiler venting into exterior masonry chimney. Has 8x13" terra cotta flue tile visible at the top of the 30 ft. tall chimney for the heater (fireplace has its own separate flue). Boiler input 100,000 BTU/hr. has a 5" draft hood with spill switch. There is also a 40,000 BTU/hr input gas water heater common vented into this flue. WH has a 3" draft hood. Location is Philadelphia, Pa. Equipment located in a basement measuring 14x30x7ft. and has a gas clothes dryer located in the same area. The home is 85 yrs old. It used to vent a huge coal fired boiler, which was replaced with an oil burner (non-flame retention head burner), which was replaced about 15 yrs ago with this boiler

Does this require a listed stainless steel liner and why or why not?

Situation 2- 2 yr. old Cat. I fan assisted gas furnace vented into exterior masonry chimney with 8x8 terra cotta flue tile visible at the top of the chimney. Furnace is 80,000 BTU/hr input rated 80% AFUE with a 4" collar. No other appliances attached to the 14 ft tall chimney. Chimney has always vented gas furnaces. Located in Philadelphia, Pa.

Does this chimney need a listed stainless steel liner and why or why not?

Originally Posted by Stacey Van Houtan

I teach this to my new home inspector at our inspection school PITI. I know a inspector that bought a flue liner due to lack of this knowledge.

Old Gravity furnance 100K input. 80 K output 20 K up the flue
Induced draft furnace 100 k input 90 K output 10 K up the flue

While it is true that a garvity furnace had a constant chimney effect (unless the old thermal activated flue dampers were installed) this extra draft air was only of value for condensentation issues while the furnace was running. And that is why a draft hood with a barometric interlock switch is accepted as well as a flue liner when up grading to a induced draft furnance that is installed in a masonary chimney.

In your example of an induced draft furnace Stacey that would have to be a condensing furnace you're referring to correct?

The draft hood on the older equipment was why the flue temperatures were lower.
The dilution air brought the temperatures down where an induced draft (80%) furnace has no drafthood and sends all the heat of combustion up the flue.

I'm a big fan of barometrics, drafthoods on the other hand should be banned.

Originally Posted by Gunnar Alquist

Phillip,

I would like to introduce you to Mr. Spacebar and Mr. Enter Key. Knowing about, and using them judiciously will make it much easier to read your posts. Otherwise, I will not bother.

Gunnar,

I TRIED to read it too, but then I had to stop as I couldn't follow it typed like that - keep either reading the same things over and over or skipping parts of it ... simply had to give up on it.

By the way, Gunnar, it is Mr. Space Bar. He and Mr. Enter Key live in the same neighborhood, not far from each other.

Originally Posted by Jerry Peck

By the way, Gunnar, it is Mr. Space Bar. He and Mr. Enter Key live in the same neighborhood, not far from each other.

JP,

Wait, are you trying to tell me that I missed an opportunity to use Mr. Space Bar?

Good morning:My apologies to all for overwriting & long-windedness, & lack of skills or ignorance of posting on internet.I'm new to all.I shall improve.Have a pleasant day.

Wow. Something isn't right there. According to the chemistry that I learned, carbon dioxide is a gas at room temperature. And I have seen acidic deposits on every brand of furnace, new and old, that use NG or LPG. It's a symptom of excessive condensation within the flue, or an improper flue, and should be addressed, according to all of the reading and instruction that I have had.