Thread: Furnance Vent

I have noticed this in several attics, appears to be moisture from Vent. 1) Is it a problem that should be noted? 2) What is the cause?

Similar Threads:

• Direct Vent Pipe Clearance to Combustibles - SLP and Direct Vent Pro
• Height of Vent Pipe from a Vent hood used in a Laboratory
• Furnance closet door sealing
• Single wall pipe to b-vent vent question
• Plastic power vent to metal vent at brick chimney

Most likely condensation on inside of vent pipe, running down till it gets to connection, then runs out of connector and down vent pipe.
If that is it, this is very common. By itself, does not mean that there is actually anything wrong. Make sure attic is properly vented. Check for draft, and length of run.
Also could be leaking at roof flashing.

Can happen from too much run, not enough rise (note elbow) and long run barely visable in the 2nd photo bottom edge, too many changes of direction; combustion air not hot enough to exhaust all products of combustion properly allowing condensation (swapping out to a higher efficiency appliance and not adapting venting); a connection end getting buggered and not locked in right with the B-vent (or an errant penetration); and as Rick said, in this case appears more likely a case of a leak from the roof penetration, flashing, boot, hood cap, etc. or a blockage (bird's nest, leaf debris partially obstructing exhaust, etc.) which allows fluid water to run along the outside of the vent and either dripping off or evaporating along the way due to the heat and air movement. View from the roof could be further telling.

Scale, salts, acidic exhaust, flowing water erroding the zinc, plus heat, over time active rust could (soon) follow.

Helps to have an idea of where in the world you're located. You might consider adding location information to your profile.

These were actually two different locations (both in St. Louis, Mo) one was completely vertical. Stains started midstream of flue. I suspect from reading the post it is propable condensation. Report issue.......No report issue
Thanks

Looks like it's coming from around the vent where it passes through the flashing. It's not quite leakproof, so the rain is hitting the vent, bypassing the flashing and coming on down the vent.

Good morning: As the B vents appear to be running through what is technically an unheated space, & as the white stains appear to be from the joints I would guess you are seeing the result of condensing of flue gases inside the vent(there may be several issues re:the venting,as the photo doesn't necrssarily tell me the whole story).Therefore the stains are probably from the acid formed during that process(sulfur content of the fuel + water vapor content).As the acid is corrosive & will eventually result in damage which can result in leakage of products of combustion, such as CO, the issue should be referred to a competent gas contractor. It appears in one photo that you may even be seeing a hole in the sidewall of the b vent(the spot that appears to show build-up of the white substance).Any problem with venting is a definite issue to be dealt with asap.

Gary, I see it on almost every inspection I do. I still do not know what it is. If you figure it out please let me know. I look very closely at the roof and the flashing and the attic venting when I do see it and if everything looks good I do not report it.

GB: Thanks for updating profile with location info.

Were these vents dedicated to one appliance, or serving more than one (ex. sharing with a gas-fired WH).

Furnace?

Standing pilot?

Appliance(s) newer than home?

It should be noted, and recommended to investigate. Improperly vented (condensation in interior of Bvent) exhausted gas fired appliances are a bad thing.

Both shared gas fired furnace and water heater, both units newer than the homes.

Combustion analysis (possibly cleaning, tuning, servicing the gas firing appliances) and resizing/rerouting or correction of vents likely needed.

More than likely the WH cannot adequately heat up and overcome the long runs, bends, offsets, length and size of the shared Bvent during the non-heating season.

Ex. home AC running tall hot attic and likely not quite as hot outdoors (say as the roof and attic still retaining heat of the day but the late PM or evening outdoor air starting to cool) and the low BTU output from the water heater compared to that of the furnace venting from BVent sized for Furnace plus WH.

The termination being too close to a wall, down drafts, etc. might also be of issue, there can be other contributing causes.

Whatever the deficiency/deficiencies in design/sizing are determined, it is a symptom (condensation) of improper venting of the products of combustion. A properly designed and sized gas vent will exhaust the products of combustion (including water VAPOR) in a gaseous state completely. Granted there may be some slight condensation at the beginning of a firing cycle, it too should be re-vaporized and exhausted before the conclusion of the firing cycle.

Although NFPA 211 can offer some tables, etc. the specifics of the appliances and home have to be applied to the situation.

Anytime symptoms of improper or incomplete venting are present, incomplete combustion is likewise suspect. It is not only a concern of corrosion/useful life of the BVent as far as replacement $$ it is a concern over Life Safety (CO poisioning, fire, etc.) First, appliance damage, property damage, etc. second.

DavidR and Bob Harper post on similar topics here frequently. You might see what they have posted on the subject if you need some phrasing for your report.

As said previously, yes report the white streaking, signs of condensation occuring in the BVent and refer to a pro.

HG
So you are saying that the statement I made is incorrect.
"By itself, does not mean that there is actually anything wrong"

Thank this help alot

You are welcome.

Originally Posted by Rick Cantrell

HG
So you are saying that the statement I made is incorrect.
"By itself, does not mean that there is actually anything wrong"

Incomplete combustion of NG = CO +water vapor
Complete combustion of NG is exothermic = CO2 + water vapor + HEAT.

I wasn't being personal, I was addressing the question. Since you asked, I don't agree with your statement; I would say by itself it is an indication that something, somewhere, at some time, is/was less than right.

The acidic nature of flue gas has little to do with the sulfur content of the gas but is more directly related to the chemical reaction in the combustion process itself.

Most combustion reactions shown stop at CO2 and H2O being produced. It would be nice if that was the end of it but it continues.
The CO2 and H2O continue to react to form H2CO3 which is the chemical formula for carbonic acid having a PH of 4.
This is a normal chemical reaction and doesn't cause any issues as long as the furnaces are venting properly.
Induced draft furnaces by design actually trap the flue gases in the vent and the heat exchanger. This is where the problems begin and the cause of the streaking that is noticed in the pictures.

Depending on different variables the condensation and rusting will either be accelerated or slow.

Thank you DavidR.

Gary,
A couple things to consider. After the B vent passes through the roof jack a storm collar is installed above the roof jack and sealed, often with silicone. If that seal leaks any rain that hits the vent will run down the pipe into the attic. That's if the moisture is coming from outside. If moisture is coming from inside the vent, could be due to causes already mentioned. I would add another potential issue. Many furnaces, especially those under 75,000 btu input require a double wall vent connector rather than the single wall that most installers want to use. (Vent connector is the pipe that connects the appliance to the flue) You can consult the IFGC vent chart to see it that is the case with yours. The reason some 80+ furnaces will require double wall connectors is to avoid heat loss needed to dry out the condensate that forms inside the flue. Heat loss from single wall is 7 times greater than double wall.

I cannot comment for sure on what I'm seeing since I ain't there but......
If this was flue gas condensate, how did it leak out? B-vent has an inner liner with the male end pointing down to contain condensate within the liner. The outer sections are male end up to prevent rain or attic moisture condensate on the outer skin from entering the pipe. Note the stains pointing to the unlisted homemade support that is screwed into the vent. This constitutes a breech of the pipe's insulation, which in turn leads to condensation. It can also push your clearance out to 6" just like unlisted single walled pipe.

Yes, you need to have a pro inspect the furnace, perform combustion analysis and tweak it. If this furnace does not already have a post-purge, they should add one. They can be adjusted for up to 5 min. but 45-120 seconds is usually enough to flush it out and lower the dewpoint. A pro would perform a comprehensive check including measuring delta T and fan speed, ESP, etc. to really do it correctly.

That vent may be oversized for what it's serving--we don't know here. It is improperly and inadequately supported and at least some of it needs to be replaced. If this is a fan assisted furnace alone, it may not have enough heat in it to generate sufficient draft. Also, too, an orphanned water heater can be a problem. Consider a WH trying to vent ~10K BTU/hr when firing but only 600-800 BTU/hr off the pilot. Now, try this in August when it's a 90/90 day----90 degrees at 90%Rh. It ain't gonna vent. Even a fan assisted furnace can fail to prime a vent if it has a short pre-purge. You have to push that slug of cold dense air out the top before it can generate lift by natural draft. Long setback on the T'stat may also lead to long wet times in a vent.

There are barely detectable levels of sulfur on most NG and LPG. Now, heating oil is another matter altogether. The crappy #2 we've been getting the last few years is higher in sulfur, which raises the dewpoint.

HTH,
Bob

"I wasn't being personal,..."
I did not take it as personal

"Since you asked, I don't agree with your statement; I would say by itself it is an indication that something, somewhere, at some time, is/was less than right."

Thanks for the responce

The general rule of thumb for offsets in attics is 1 1/2 times the diameter from inches to feet. In other words if you have a 6 inch B vent, you can offset it by 9 feet. That is the number one problem I find in most townhouses. The furnace is located in the front of the unit, the B vent goes straight up then offsets about 20 feet in the attic so it terminates on the rear of the roof.

Here's my marco for most stainig I see:

Stains observed on exterior walls of B-vent pipe in the ^. This condition is a direct result of improper venting due to incorrect installation or design of one or more of the following (height of main vent, number of bends, size of main vent, height of vent connector, size of vent connector or distance between appliance and main vent or chimney). A F.I.R.E. certified chimney contractor should review and perform the appropriate repairs.

I too see these condensate stains on flue pipes regurlarly and recommend having the systems serviced by an HVAC professional every time.

There is only one true fix for this and most HVAC guys have never had the training to perform the procedures.

This happens often in cold attics, Moister builds until B-Vent becomes hot enough to evaporate it. If you see this near the appliance it could be a concern, if only in the attic, not so much. B-Vent often needs to be screwed together to support it and is not water tight to begin with.